Domestic hot water thermal expansion tanks: this article describes both thermal expansion tanks and relief valves used on domestic hot water heating systems.

These expansion tanks are required in some areas when the customer is on a “closed” water system: when water is heated in a closed water system, thermal expansion can cause rapid increases and decreases in system pressure which can in turn cause spillage at pressure/temperature relief valves (and related building water, flood, or mold damage), or damage to other plumbing components.

Thermal Expansion Tanks & Thermal Expansion Control Valves for water heater systems

Water heater (hot water cylinder) expansion tanks such as the 2 1/2 gallon expansion tank shown here from Falcon Stainless, Inc., is sold by Amazon and is intended to accommodate expansion in potable water or domestic hot water supply heating systems.

[Click to enlarge any image]

This expansion tank is made of stainless steel and is designed to maintain safe pressure in the domestic hot water system.

It uses a butyl ruybber internal diaphragm combined with a plastic liner to keep the hot water separated from the air charge. That prevents loss of the air charge by absorption into the water supply.

The size of water heater expansion tank required depends on the volume of the water heater or water cylincer (calorifier), the incoming water su7pply pressure, the water heater recovery rate or temperature and the highest hot water temperature that will be produced in the system.

Example: a 50 gallon water heater that heats water to 140 F, with incoming water supply pressure of 60 psig and a water heater rated recovery of 40 F will require about a 5-gallon expansion tank.

Water heater manufacturers and plumbing codes require the installation of an expansion control device if a backflow preventor, pressure reducing valve or check valve is installed in the domestic supply line.

Therm-X-Trol satisfies all manufacturers' requirements. - Amtrol (2014)

Amtrol Inc. provides the THERM-X-TROL® line of thermal expansion shock absorbers - expansion tanks for domestic hot water systems. Quoting from the company's product literature:

Thermal expansion occurs as domestic water is heated. Left unchecked, the resulting pressure increase can cause relief valve discharge and other potentially unsafe conditions. Therm-X-Trol® expansion tanks absorb this expanded water, keeping water pressure at a safe level. - Amtrol (2014)

An expansion tank or additional pressure/temperature relief valves can prevent these problems and may also extend the life of the water heater.

Cause of Thermal Expansion Problems in Water Heaters & Water Pipes

To protect the sanitation of public water supplies, plumbing codes require a dual check valve or equivalent to prevent back-flowing water out of a building's water supply system into the municipal supply system. Such backflow can occur should the municipal water supply pressure be interrupted or reduced below water pressure in the building.

The use of a back-flow preventer check valve at the building's water entry point indeed protects the municipal supply. But in the building's plumbing system the check valve also creates a closed system.

When water is heated in the building's water heater the pressure in the water system increases.

For buildings connected to a private well and sing a water pressure tank, an increase in water pressure caused by heating pushes some water back into the pressure tank, thus absorbing the pressure increase and the increase in water volume by compressing air in the pressure tank.

But for buildings connected to a municipal supply and equipped with a backflow preventer check valve, any increase in water pressure within the building has no such easy pressure-absorbing device.

Instead, increases in water pressure can cause potentially dangerous leaks at pressure relief valves on heating boilers or water heaters. The danger is that repeated leaks at a temperature/pressure relief valve ultimately cause it to clog or jam, risking

a BLEVE EXPLOSION at the water heater or heating boiler.

That's why modern plumbing codes require provision of a thermal expansion tank on hot water supply systems.

Hot Water Expansion Tank Installation Instructions

Hot Water Expansion Tank Installation Instructions are of course provided by the manufacturer and for safety as well as warranty protection need to be read, not just used to kneel on while sweating in your piping connections.

The illustration showing possible positions of the expansion tank in a potable hot water thermal expansion system is adapted from installation instructions from Watts for their DET series potable hot water expansion tank cited at the end of this article. There we give a link to the installation manual for that expansion tank.

Watch out: be sure to follow the specific hot water heating expansion tank installation instructions from the manufacturer of your expansion tank brand and model. Improper installation is dangerous. The warning below is quoted from information provided by the Watts Corporation:

Improper installation, adjustment, alteration, service or maintenance can cause property damage, serious bodily injury or death. Read instructions completely before proceeding with installation.

Only qualified personnel may install or service this equipment in accordance with local codes and ordinances.

Do not exceed 80psi (5.5 bar) air charge. Air charge pressure exceeding 80psi (5.5 bar) could become hazardous and will void any and all warranties, either written or implied.

Failure to follow these instructions will result in the possibility of property damage, serious bodily injury or death.

This Expansion Tank is designed and intended for water storage at a maximum pressure of 150psi (10.3 bar) and a maximum temperature of 200°F (93ºC). Any use other than for potable water or a sustained or instantaneous pressure in excess of 150psi (10.3 bar) or 200°F (93ºC) is UNSAFE and can cause property damage, serious bodily injury or result in death. - source: Op. cit. see citation at end of this article.

Be sure you choose the proper size water heater expansion tank. Amtrol, for example, offers eleven different expansion tank sizes in volumes from 2 to 86 gallons to handle water heaters in sizes from 40 to 120 gallons and at static water supply pressures between 40 psig and 80 psig and at three different maximum water heater temperature settings of 140°F, 160°F or 180°F.

Watch out: also, to understand why water heaters are set to above-scalding temperatures see Legionella BACTERIA in WATER HEATERS and to prevent dangerous or even fatal scalding burns

Also see ANTI SCALD VALVES / MIXING VALVES.

Excerpting from Amtrol's Therm-X-Trol installation instructions we see that installation is simple but has several key steps:

• Install the expansion tank on the cold water supply line to the water heater at a point between the water heater and backflow preventer or other one-way device (see Amtrol's instructions Figure 2.)
• Mount [hot water thermal expansion] tank vertically only
• Once the expansion tank is installed, check the cold water supply line for any leakage. Make repairs if necessary.
• Before the initial firing of the water heater, open any hot water fixture and draw water until all air is removed from the system. Turn the water heater temperature control to desired ending temperature level, (see water heater instructions).
• Do not set temperature higher than that which the expansion tank was originally sized.
• To relieve initial thermal expansion, slightly open a hot water faucet. Continue until water heater aquastat temperature is satisfied. Once heater is at its operating range, no further bleeding of expanded water

The water heater and its thermal expansion tank will now be operational. The hot water thermal expansion tank will control pressure increases caused by thermal expansion to a level below the water heater relief valve setting. So if you see leaks at the water heater's relief valve see our diagnostics

at RELIEF VALVE LEAKS

and at RELIEF VALVE, WATER HEATER.

Thermal Expansion Valves for Domestic Hot Water Heater Systems

Question: should I replace the water heater thermal expansion valve on the cold water inlet side of my water heater with a water heater thermal expansion tank?

I have a thermal expansion valve that is installed on my inlet line (cold water line going into my hot water tank) that is leaking, now the question that I have is, is it required to be on the inlet line . Talking to a few friends that do plumbing and they are saying that it is not required, but they recommend that I install a expansion tank.

So, another question is if the system was installed without a expansion tank is it a good idea to install one are just replace the thermal expansion valve that was installed when the hot water tank was installed, are just remove the old system and install a new hot water tank system per new system directions ? So many questions, but can't find all the answers! - C.D. 12/23/12

Reply: what's the difference between an expansion control valve and a water heater thermal expansion tank & how are they installed & used?

A thermal expansion control valve is essentially a pressure relief valve installed at a water heater to prevent forming excessive pressures in the closed loop of water supply piping and water pressure tank when water is heated at the water heater.

A thermal expansion valve is then in essence a safety device that will open and spill water and pressure to prevent plumbing equipment or piping damage or worse, unsafe conditions.

Expansion control valves are produced by a variety of companies including Watts & Wilkins-Zurn who produce a variety of thermal expansion control products including valves and pressure tanks.

We see thermal expansion control valves installed as an "extra" pressure relief valve in buildings installed on the water piping itself where the water supply piping is not rated as capable of withstanding the normal pressures that may otherwise develop in a hot water system. The thermal expansion control valve installed on water piping at the water heater is typically rated to open at 100 psi.

See BLEVE EXPLOSIONS

Watch out: As Carson Dunlop Associates point out in our text earlier in this article, a thermal expansion control valve can be located on building water supply piping apart from mounting on the water heater itself.

However, the thermal expansion control valve cannot be located where it is separated from the water heater by a stop valve nor by a pressure reducing valve, as doing so can prevent the thermal expansion control valve from safely sensing increased water pressure created by the water heater.

Watch out: where you see an "extra" 100-psi-rated thermal expansion control valve at a water heater, you should understand its presence to also mean that the water supply piping in the building might be a plastic product that is less strong (and potentially less reliable against leaks and breakages) than copper water supply piping.

An example of piping where a thermal expansion control valve is often found is Flow-Guard Gold® CPVC water supply piping produced by Charlette pipe, rated for 100 psi at 180 degF.

Note that the rated working pressure and temperature ratings of different pipe products from different manufacturers varies. For example, quoting from Georg Fischer GF Harvel CPVC pipe

CPVC pressure pipe has an upper working temperature limit of 200°F (93°C), or approximately 60°F (15°C) above that of Type I Grade I PVC. As with all thermoplastic piping systems, CPVC’s ability to withstand pressure varies with pipe diameter, wall thickness, and temperature.

For pressure piping applications it is recommended for temperatures as high as 200°F (93°C) when appropriate temperature de-rating factors are applied.

As the pipe diameter and temperature increases, the pressure rating of the product decreases. For example, 1/2" Schedule 80 CPVC pipe is engineered for continuous service of 850 psi @ 73°F (23°C).

The same pipe in service @ 180°F (82°C) carries a maximum working pressure rating of 210 psi at this temperature. 6" Schedule 80 CPVC pipe is designed for 270 psi @ 73°F, and 54 psi @ 200°F. [1]

Normal pressures in a hot water heater and piping can reach levels a bit below 150 psi. Above that 150 psi level, water will spill from the standard pressure & temperature relief valve installed on the water heater itself.

See RELIEF VALVE, WATER HEATER

You should not normally see water spilling from the thermal expansion control valve - if you do, the system is not operating properly and inspection/adjustments/corrections are needed.

We are researching water expansion control valves further, as we noted in 2011 that Zurn had discontinued production of the type of thermal expansion control valve found installed directly on water supply piping at the water heater.[3]

Water heater expansion tanks, or HOT WATER EXPANSION TANKS in contrast, are intended to absorb water piping system and water tank pressure increases without causing inappropriate spillage from pressure/temperature relief valves attached to the system, so long as water pressure and temperature at the water heater remain in the safe range specified by the manufacturer.

We recommend water heater expansion tanks in many installations, as you can read above. But these are not an "either-or" solution. If your water piping is plastic CPVC it may require the added protection of a secondary lower-pressure pressure relief valve.

Water Heater Thermal Expansion Tank Location

Question:

I'm getting ready to replace the electric water heater in my 2006 mobile home. The replacement will be another HUD-approved mobile home water heater. Our water meter has a check valve in it, so I know a standard hot water would need an expansion tank.

However, the mobile home didn't come with an expansion tank, at least in the space where the water heater is. Do mobile homes or mobile home water heaters have some other provision for this? It seemed odd to me that the factory setup of a mobile home would have to be modified to have it work correctly with certain water meters. - Anonymous by private email 2017/12/29

Reply:

Thanks for the interesting question. Typically a water heater expansion tank is pretty small and can be connected to piping at a nearby location where you can find space for it. It does not need to be immediately adjacent to the water heater.

The following advice about how to locate the expansion tank is excerpted from information

in WATER HEATER THERMAL EXPANSION TANKS, OWNER'S MANUAL

provided by State Water Heaters and cited below.

1. The location selected should be indoors in an area not subject to freezing.

2. The items which should be installed in sequence in the cold water line are

(1) the EXPANSION TANK must be installed at least 18 inches away from the cold water inlet fitting on the water heater,

(2) the PRESSURE GAUGE, and

(3) the PRESSURE REDUCING VALVE, if required, see Figure 1.

3. The expansion tank is designed to be supported by the system piping in the vertical position.

4. The expansion tank, pipes and your connections may, in time leak. Put the expansion tank in a place where a water leak will not damage anything.

The expansion tank should be located in an area where water leakage from the tank or connections will not result in damage to the area around the expansion tank or to the lower floors of the structure.

Hot Water Heater Thermal Expansion Tank Size Chart

Notes to the table above

1. The table above assumes that water temperature will not exceed 140 F
2. The table above assumes that the water pressure in the system will not exceed 150 psi.
3. At pressures ove 60 psi you should install a pressure reducing valve to keep water pressure at 60 psi or below.
4. Adapted from the WS Tanks Expansion Tank Manual cited below.
5. Watch out: check the specifications and sizing chart for your specific brand of water heater thermal expansion tank as tank capacities and size recommendations may vary from the table above.

Water Heater Expansion Tank Air Pressure Pre-Charge Test & Set

Why might you want to check or adjust the thermal expansion tank's air pre-charge?

• To confirm that the air charge is properly-matched to the building's water pressure as delivered to the water heater
• To check for a ruptured expansion tank bladder. Loss of air charge due to a ruptured tank bladder will ultimately leave the pressure in the expansion tank about the same as building water supply pressure.

This procedure for checking the expansion tank air charge is adapted from the WS Tanks Expansion Tank Manual cited below.

1. Turn off the water heater and cool down the water heating system.
   
   W.S. Tanks warns that if you measure expansion tank pressure when the domestic hot water system is already hot (and therefore "expanded") you will not be able to check nor set the pressure in the expansion tank correctly. You need to make this measurement when the water heating system is cold.
   
   Therefore a best practice might be to turn off the water heater and open one or more hot water faucets, running cold water through the water heating system until you have cooled down the hot water piping and cooled-down the water heater itself.
   
   More details on this point are explained below.
2. Shut off cold water:
   
   Turn off the building cold water supply valve or turn off the cold water supply valve that is ahead of (feeding into) the piping that supplies the expansion tank and water heater that the expansion tank serves.
   
3. Relieve all pressure from the hot water supply piping (or from the entire system)
   
   Open a nearby hot water faucet long enough that water stops flowing. At this point you have relieved all water pressure from at least the hot side of the building piping system.
4. Measure / adjust expansion tank air pressure:
   
   Use an air pressure gauge, such as a tire gauge, to check the expansion tank's pressure by removing the cap on the valve stem found on the expansion tank and pressing the gauge on to the open valve stem.
   
   Watch out: if water squirts out at this valve stem most likely we already know that the tank's bladder is torn and leaky and the expansion tank needs replacement.
   
   Standard or factory pressure pre-set by the expansion tank supplier may vary but is typically set to 38 psi. Check the manual for your water heater expansion tank to be sure that's the factory pressure on your unit.
   
   Typical "adjusted" air pressure at the expansion tank will be set to equal the incoming water supply pressure. On a pump and well system where water pressure may vary between a CUT-IN / CUT-OUT pressure (for example 20/40 psi) set the tank pre-charge pressure to no less than 20 psi and no more than 40 psi.
   
   If necessary you can release air from the tank by pressing on the center pin in the Schrader valve stem, or you can add air to the tank using a simple bicycle pump.
5. Restore water pressure & flow:
   
   Turn on the cold water supply that you shut off at step 2 above.
6. Remove any air from the water heater ?
   
   Watch out: While you probably won't have this problem, particularly if your water heater is electric, never turn on the water heater before running cold water through the hot water system until you are certain that all air has been purged from the water heater tank itself. Turning on an electric water heater that contains air is going to burn up the water heater elements in just a second.
7. Turn on the water heater
   
   See the operation manual for your water heater to get details about how to turn it on. For example some gas fueled water heaters may need a pilot to be lit.
8. Periodic water heater thermal expansion tank re-test?
   
   Some manufacturers advise testing the expansion tan every six months to assure that the air pre-charge pressure is either as specified on the pressure tank's data label (the factory pre-set pressure) OR as the customer's own air pressure pre-charge if the pressure was adjusted.
   
   Really? In over five decades of working in and on buildings and their mechanical systems, I [DF] have never once met a homeowner who performed this test.

When to measure water heater expansion tank pressure: water heater should be cold

Watch Out: it is important that you read your city system pressure correctly. If you read it when it is already hot and expanded you will get false readings.

If you have a pressure gauge in your line, you should open one hot water valve and let the water run for 15 minutes to reduce the line pressure, then shut the valve off and then read the pressure immediately.

If you don’t have a pressure gauge in your line you can easily measure system water pressure by purchasing an inexpensive pressure gauge with a fitting that will connect to a hose bibb or washing machine connection, as shown in our photo.

If you follow the above procedure with your tank installed and read the air pressure from your tank you will have an approximate system pressure. (In this case read the pressure with the tank full of water).

If the air pressure reads 50 PSI you need to shut-off your main water line, open a valve let the water out of the tank, and pre-charge the expansion tank with 50 PSI of air pressure.

Question: should I adjust the pressure in my water heater expansion tank?

The InspectAPedia site has plenty of information on setting pressures for well systems with a bladder, but I can't find anything relating to recommendations on how the small (1 and 2 gallon) water heater-type expansion safety tanks should be pressurized. The manufacturer typically ships these with 40 psi--is that good for all conditions, or should the tank be pressurized to average line pressure on a municipal system?

For example, average pressure delivered to my residence by the city is 70 psi. So I'm wondering what normal practice is--leave the tank at the pressure as the manufacturer shipped it, or pressurize to average incoming water line pressure when one is on a municipal system? - W.F.

Reply: select the correct water heater expansion tank size, then leave the factory expansion tank set where found at 40 psi under normal circumstances, and never exceed 80 psi

Watch out: We agree that it's important to get the size and pre-charge pressure of a water heater expansion tank right, as while the tank is preventing inappropriate relief valve overpressure spillage under normal water heating conditions, the expansion tank also needs to allow the relief valve to spill if an unsafe temperature or pressure occurs at the water heater.

See BLEVE EXPLOSIONS

The correct size (and pressure) for an expansion tank used on a domestic water heater (potable water or hot water tank) is determined based on the incoming water supply pressure, the water heater tank size, and the spill pressure set on the water heater tank's pressure/temperature relief valve.

Watts® offers some guidance

at WATER HEATER EXPANSION TANK MANUALS

There you'll see a table that matches water heater size and water supply pressure to expansion tank SIZE. If you pick the proper expansion tank size, using it at the factory pre-charged pressure should be exactly correct.

For your example and using the sizing table, a Watts® expansion tank type "A" (PLT-5 or 2.1 gallons) would work for a supply pressure up to 70 PSI IF your water heater tank is no larger than 50 gallons. If your water heater tank is 80 or 100 gallons you'd need a PLT-12 (4.5 gallons), and a 120 gallon tank would need a PLT-20 (8.5 gallons) - all at 70 psi incoming water pressure and all with the factory pre-charge expansion tank pressure of 40 psi.

Older Watts "DET" expansion tanks are obsolete but may still be in use on some water heaters. Currently the company sells hot water expansion tanks in models PLT-5 (2.1 gallons), PLT-12 (4.5 gallons), PLT-20 (8.5 gallons), and PLT-35 (14-gallons) sizes.

Other manufacturers offer similar sizes and models. Quoting from Watts: [http://www.watts.com/pages/_products_details.asp?pid=852]

Series PLT Potable Water Expansion Tanks are designed to absorb the increased volume of water created by thermal expansion and to maintain balanced pressure throughout the potable water supply system.

Series PLT tanks feature a pre-pressurized steel tank with an expansion membrane that prevents contact of the water with the air in the tank. This prevents loss of air to the water and ensures long and trouble-free life for the system.

These tanks may be used with all types of Direct Fired Hot Water Heaters (gas, oil or electric) and hot water storage tanks. IAPMO listed and ANSI/NSF 61 certified. Maximum Pressure: 150psi (10 bar).

Series PLT Potable Water Expansion Tanks are designed to absorb the increased volume of water created by thermal expansion and to maintain balanced pressure throughout the potable water supply system.

Heated water expands, and in a domestic hot water system, the system may be closed when the potable water system is isolated from the public water supply by a one-way valve such as pressure reducing valve, backflow preventer or check valve.

Provisions must be made for this expansion. Series PLT expansion tanks absorb the increased volume of water created when the hot water storage tank is heated and keeps the system pressure below the relief setting of the T&P relief valve.

It is a pre-pressurized steel tank with an expansion membrane that prevents contact of the water with the air in the tank. This prevents loss of air to the water and insures long and trouble free life for the system. These tanks may be used with all types of Direct Fired Hot Water Heaters (gas, oil or electric) and hot water storage tanks.

Details on how much water expands when it is heated are found

at HOT WATER PRESSURE EXPANSION RATE.

Water Heater Expansion Tank Manuals & Resources

Above: example sizing charts for water heater thermal expansion tanks from Amtrol, cited just below.

• Amtrol, THERMXTROL EXPANSION TANKS SIZING CHARTS [PDF] (2019) Amtrol Water Heater Expansion Tanks, Amtrol, - retrieved 2023/08/10, original source: amtrol.com/wp-content/uploads/2019/02/MC10230-01_19-TXT-Handout-Brochure.pdf
• AMTROL THERM-X-TROL® THERMAL EXPANSION ABSORBERS INSTALLATION & OPERATION INSTRUCTIONS [PDF] Models ST-5C through ST-210VC, Amtrol Corporation, 1400 Division Road, West Warwick, RI 02893 USA Tel: 800.426.8765 Website: www.amtrol.com retrieved 2017/11/02, original source: http://www.amtrol.com/media/documents/thermxtrol_asme/9017091_02_15_TXT_ST5C_IO.pdf
• AMTROL ENGINEERING HANBDBOOK [PDF] (1977, reprinted 2011) Chapter Two, Section B, “Hydro-pneumatics in Hot Water Heating Systems”. The complete handbook covers the application of hydro-pneumatics in heating, plumbing, cooling, water supply and commercial water heating systems.
  
• H2 PRO THERMAL EXPANSION TANKS INSTALLATION INSTRUCTIONS [PDF] (2004), Flexcon Industries, 300 Pond St., Randolph MA 02368 USA, Tel: 781-986-2424 retrieved 2017/11/02, original source: https://www.flexconind.com/pdf/wh-whv_install.pdf
  
  Includes a sizing chart for hot water expansion tanks from this manufacturer.
  
  Excerpt:
  WH & WHV thermal expansion tanks eliminate the problem of nuisance cycling of the safety relief valve when a backflow preventer or check valve is present in a domestic hot water system.
  
  Utilizing Flexcon’s patented double diaphragm system, they provide a reservoir for expanded water volume and help to prevent dangerous over pressurization. The diaphragm is permanently separated from the tank’s air charge.
  
  The tanks also feature a brass or stainless steel system connection for maximum corrosion resistance.
• Chicago plumbing code:
  
  CHICAGO CODE 2 BACKFLOW PREVENTION DEVICE OR CHECK VALVE (§ 18-29-607.3)
  
  Where a backflow prevention device, check valve or other device is installed on a water supply system utilizing storage water heating equipment such that thermal expansion causes an increase in pressure, a device for controlling pressure shall be installed. - retrieved 2018/02/15, original source: https://chicagocode.org/18-29-607.3.txt
• Durango, Colorado, PROTECT YOUR WATER HEATER FROM THERMAL EXPANSION [PDF] (2012) Durango, CO 949 East Second Avenue, Durango, CO 81301 UDA, Tel: (970) 375-4882 Email: cccp@ci.durango.co.us, retrieved 2018/02/15, original source: https://www.durangogov.org/DocumentCenter/View/161
• National Plumbing Code, 607.3 THERMAL EXPANSION CONTROL, [PDF] (1996 - 2009) as adopted by Mecklenburg County North Carolina Building Department, retrieved 2018/02/15, original source: https://www.mecknc.gov/LUESA/CodeEnforcement/Tools/CodeInterpretations/CodeInterpretations/Thermal%20Expansion%20Control.pdf
  
  [This example illustrates plumbing code on thermal expansion control as widely adopted by many U.S. jurisdictions:
  
  
  Excerpt from the example cited above:
  
  607.3 Thermal expansion control. A means of controlling increased pressure caused by thermal expansion shall be provided where required in accordance with Sections 607.3.1 and
  
  607.3.2. 607.3.1 Pressure-reducing valve. For water service system sizes up to and including 2 inches (51 mm), a device for controlling pressure shall be installed where, because of thermal expansion, the pressure on the downstream side of a pressure-reducing valve exceeds the pressurereducing valve setting.
  
  607.3.2 Backflow prevention device or check valve. Where a backflow prevention device, check valve or other device is installed on awater supply system utilizing storage water heating equipment such that thermal expansion causes an increase in pressure, a device for controlling pressure shall be installed.
  
  Interpretation:
  
  Thermal expansion devices (tanks or valves) shall be installed at each water heater whenever the building service is protected by a backflow device.
  
  The owner or his agent (contractor) may elect to provide a single tank for each structure by having an engineer registered in the State of North Carolina calculate the total thermal expansion load (total accumulated amount of water displaced) of all the water heating devices and provide a sealed letter of certification.
  
  Drain allowances (for waste water discharged from the expansion device) shall be provided for thermal expansion valves. Thermal expansion tanks must be rated for 80 psi or higher. T & P VALVES ARE NOT APPROVED FOR THERMAL EXPANSION CONTROL.
• WATER HEATER THERMAL EXPANSION TANKS OWNERS MANUAL [PDF] State Water Heaters, retrieved 2017/12/29, original source: http://www.statewaterheaters.com/lit/im/tanks/317825-000.pdf,
  
  Excerpts:
  
  Many local codes require back flow preventers. Since this seals off the system an expansion tank is required or the relief valve will discharge on every heating cycle. The thermal expansion tanks eliminate this wasteful, costly and dangerous practice.
  
  The thermal expansion tank is certified to ANSI/NSF 61 DHot, but are suitable for temperatures up to of 200° F and pressures to 150 PSI.
  
  DO NOT USE A STANDARD TANK. The sizing chart will allow you to choose the correct size expansion tank for your application.
  
  PURPOSE OF EXPANSION TANKS
  
  When water is heated it expands. Provision must be made for this expansion in a closed hot water system.
  
  This precharged diaphragm tank stores the expanded hot water then returns water to the system when the temperature drops
• Washington State DOH, TROUBLESHOOTING BLADDER PRESSURE TANKS, [PDF], DOH 331-342, retrieved 2017/11/02, original source: https://www.doh.wa.gov/portals/1/Documents/pubs/331-342.pdf
• WATER HEATER THERMAL EXPANSION TANKS, OWNER'S MANUAL - WS Tanks [PDF] retrieved 2017/11/02, original source: http://www.wstanks.com/permatank/pdfs/332845-000.pdf
• WATTS BACKFLOW REGULATOR APPLICATIONS in the 2000 UNIFORM PLUMBING CODE [PDF] retrieveed 2018/02/15, original source: http://media.wattswater.com/f-ipc.pdff
  This document provides the most-extensive listing of all of the plumbing devices & fixtures for which backflow prevention devices are required, applied, or used.
• WATTS® POTABLE HOT WATER EXPANSION TANK MANUAL, [PDF] Models: DET-5-M1, DET-12-M1, DET-20-M1, Water Safety & Flow Control Products,
  USA: 815 Chestnut St., No. Andover, MA 01845-6098; www.watts.com
  Canada: 5435 North Service Rd., Burlington, ONT. L7L 5H7; www.wattscanada.ca retrieved 2017/11/02, original source: http://media3.wattswater.com/1915323.pdf
• ZILMET USA EXPANSION TANK INSTALLATION & OPERATION MANUAL [PDF] (2016) ZILMET USA Corp. , 400 Frenchtown Rd. | East Greenwich, RI 02818 USA Tel. 401-884-4943 | Fax 401-287-4951 (p) 401 884 4943 Websote: http://zilmetusa.com/ Email: info@zilmetusa.com
  
  Notice that Zilmet combines installation instructions for both thypes of expansion tanks: expansion tanks on hydronic (hot water) theating systems and expansion tanks on potable domestic hot water heaters. Instructions are included for installation & operastion of Zilmet USA Expansion Tanks on solar hot water systems as well.
  
  Excerpts:
  
  INSTALLATION IN CLOSED HYDRAULIC HEATING (HYDRONIC) SYSTEMS
  
  The tank must be installed on the runback piping, please see the following installation scheme. An alternative location may be on the supply side, provided that the maximum working pressure and temperature limits are kept
  
  INSTALLATION OF THERMAL EXPANSION TANKS IN SYSTEMS FOR THE PRODUCTION OF SANITARY HOT WATEr
  
  
  How to install Thermal Expansion Tanks Proper thermal expansion tank sizing depends upon the volume of water being heated, the temperature and the incoming water pressure – please refer to the Quick Sizing Chart here below to properly size your thermal expansion tank.
  
  When in doubt always go to the next larger size as you can never oversize an expansion tank, but can easily undersize the tank. Adjust pre-charge to be equivalent to the incoming system water pressure (adjust the pre-charge while the tank is empty).
  
  Do not set a pre-charge higher than 80 psi; if necessary, install a pressure reducing valve. Installation Location
  
  Expansion tanks and water heaters may leak over time Install on the cold water side after the pressure reducing valve and between the backflow preventer and the water heater. Do not install in an area where leakage could cause property damage The tank can be installed either vertically or horizontally.
  
  However, if the tank is to be installed horizontally, then it must be fully supported independent from the piping.
  
  Failure to do so will void the warranty. Maintenance Pre-charge control and visual inspection every 6 months
  
  INSTALLATION of HOT WATER THERMAL EXPANSION TANKS IN SOLAR SYSTEMS
  
  For installation and operation review the system norms. Solar vessel must be installed in the supply line of the solar collector through use of a suitable temperature-resistant solar ball valve with a shut off to the system.
  
  The maximum working temperature in the membrane may not exceed 212 °F. This can be achieved by installing a temperature control system or using a vessel for compensating temperatures (type ZILMET VSG).
  
  The solar vessel must be installed in frost-free areas and have no direct heat. During the start-up, the pre-charge of the vessel must be adjusted according to the system data and must be written on the label.
  
  When checking the systems pressure, the maximum working pressure of the solar vessel must be considered and, if necessary, the solar vessel must be separated from the system.
  
  The solar vessel is resistant to 70% antifreeze. The use of proper anti-scale and corrosion inhibitors additives is required in solar systems.

• ZILMET FLAT HYDRONIC WALL HUNG EXPANSION TANKS PRODUCT LITERATURE [PDF] (2015) [Illustrated above ]

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: where do I locate the thermal expansion tank for a DHW tank?

2020/11/25 KC Massachusetts said:

I might have posted this in a wrong other place as a comment under Expansion Tanks, sa discussion of someone else's question.
2nd hopefully right place here ...

A) Topic: Location point to install Thermal Expansion Tank (ET)

B) Question:

1) Must ET go in/on the potable cold supply BRANCH for the DHW-tank or can it go before that branch, such as on the main house incoming supply trunk before other branches? (In the later case, it seems it would act as a "Whole House cold ET" which I've never seen.)

2) Does the boiler-to-tank heat-exchanger loop need an expansion tank too?

C) Details:

C.1) Job:

Installing ExpTank (ET) to protect DHW tank (70 gal), as part of the bigger job of replacing DHW tank with failed heat exhanger coil. ET=4.5 gal. There is an ET already at boiler hot out to house heat main line, after which is a branch over to DHW tank heat exchange coil.

C.2) System detail:

Single family cape 1850 sq ft, 4 bed 2 bath, oil heat boiler, hydronic baseboards, indirect fired DHW tank, closed system because of Pressure Reduction Valve near main street meter, 70 gal with priority call, set to 125°F, main supply metered from street (100psi), Pressure Reduce Valve after main meter, set to 40psi.

3 branches off main cold supply from street:

1) cold to all house cold faucets (main supply direct to, 40psi);

2a) boiler cold refill branch has dedicated flow check valve also PReduceV set to 12psi and PReliefV at boiler set to 30psi;

2b) boiler to tank heat exchanger has flow direct valve, return from exchanger to boiler has Taco circulator to pull water back for reheat;

3) DHW hot water tank cold potable supply without dedicated check valve (PReduceV is near main supply meter), without branch PReduceV, or expansion tank, has dedicated PReliefV at tank

Whew!

This Q&A were posted originally

at HEATING SYSTEMS and also at the bottom of this very page.

Moderator reply:

KC

There's no law at InspectApedia about right or wrong place to post, though if we use the on page search box we can often find the article on the topic or our question: that's a good place to post, but ultimately I might move a discussion to a page where it fits better, so as to invite more-helpful comments.

A search for "hot water expansion tanks" finds several articles including

inspectapedia.com/plumbing/Hot_Water_Expansion_Tanks.php HOT WATER EXPANSION TANKS

That includes a section titled "Water Heater Thermal Expansion Tank Location"

and where in a section titled "Water Heater Expansion Tank Manuals & Resources" we provide a listt of water heater expansion tank installation manuals, each of which will include the manufacturer's suggestions for where such tanks should be installed.

The thermal expansion tank on a hydronic heating system (a "hot water" boiler used to heat a building) is physically independent from and has little to do with the expansion tank that we might install at a domestic hot water heater (hot water for washing and bathing).

A home water heater will experience thermal expansion that can cause relief valve drips - a condition that over time can make the heater unsafe.

So where does the water heater expansion tank go? From the page above we have

2. The items which should be installed in sequence in the cold water line are

(1) the EXPANSION TANK must be installed at least 18 inches away from the cold water inlet fitting on the water heater, ...

Notice that the recommendations say "at least" and don't give a maximum distance as long as the thermal expansion tank is located on the cold side of the water heater.

Please see the water heater expansion tank location details and the illustrations showing where it goes, found above on this page. Don't hesitate to ask follow-up questions and also to use the "add image" button to post a photo of your water heater set-up.

On 2020-08-16 by (mod) signs of a failed water heater expansion tank

John, as most water heater expansion tanks use an internal bladder, if the bladder ruptures eventually air from the tank may escape into the building hot water piping system and the expansion tank will become waterlogged.

As a result the tank will sound "full" when gently tapped, and also more important,

as a result the water heater's temperature/pressure relief valve may begin to leak or drip at each water heater "on" cycle.

You can also check the pressure in the thermal expansion tank itself. If the tank has lost its air charge its pressure will be just about identical to the water pressure in the water supply system.

See HOT WATER EXPANSION TANK AIR CHARGE TEST

On 2020-08-16 by john - how can you tell of the expansion tank is no good?

how can you tell of the expansion tank is no good?

On 2020-08-11 by (mod) - water hammer from the sprinkler system after new water heater installed

Pam: Please see the diagnosic and repair steps

at WATER HAMMER NOISE DIAGNOSE & CURE

On 2020-08-11 by Pam C.

Had new hot water heater with an expansion tank installed. Whenever my sprinkler system runs and moves to the next watering zone there is one loud bang coming from the expansion tank when the water starts in the new zone.

That's the only time this ever happens and there is no "hammering" in the pipes and no loud bangs from the expansion tank when dishwasher, washing machine, etc. runs-- just the sprinkler system, and only one big bang when water "starts" in new zone. not when water stops before moving to next watering zone.

Any ideas on why this happens and what I can do to stop it? There's no damage to anything, just the one loud bang and one jolt from the expansion tank. Thanks for any help you can give me!

On 2020-06-20 - by (mod) - best location for water heater expansion tank

Stefan

Thanks for the water heater expansion tank location question - that's helpful. I've see expansion tanks on either hot or cold side, as is shown in one of the illustrations on the page above.

But you're right that typical water heater expansion tank instructions say to put it on the cold side.

I am looking further for the science behind that advice and will comment here. My guess is that the greater expansion occurs in colder water that is heated rather than in hot water that is thus already heated and expanded.

The excerpt below is from a State water heater expansion tank installation guide cited in the document above. It specifically says, agreeing with you - put it on the cold side.

[Click to enlarge any image]

Quoting:

FACTS TO CONSIDER ABOUT THE LOCATION of a Water Heater Expansion Tankl

1. The location selected should be indoors in an area not subject to freezing.

2. The items which should be installed in sequence in the cold water line are

(1) the EXPANSION TANK must be installed at least 18 inches away from the cold water inlet fitting on the water heater,

(2) the PRESSURE GAUGE, and (3) thePRESSURE REDUCING VALVE, if required, see Figure 1.

3. The expansion tank is designed to be supported by the systempiping in the vertical position.

4. The expansion tank, pipes and your connections may, in timeleak. Put the expansion tank in a place where a water leak willnot damage anything. The expansion tank should be locatedin an area where water leakage from the tank or connections will not result in damage to the area around the expansion tank or to the lower floors of the structure.

On 2020-06-20 by Stefan

Hy, why the expansion tank is tipically installed on the cold water supply and not on the hot water supply? Thank you in advance.

On 2019-05-19 by Home inspector - are water heater (calorifier, geyser, hot water cylinder) expansion tanks always required?

Re-posting from private email:

Home inspector wrote

On an inspection I noticed there was a PRV installed but no expansion tank in the whole house, which is what I usually see when these are installed to allow for hot water expansion.

The plumber that I asked was very non-committal about this.
I was advised by a fellow inspector to check with the AHJ if its required but I would really like to advise the buyer if its recommended or not.

I was told that you are very helpful and I should contact you for what my report should say and for the proper verbiage.

Moderator reply: water heater expansion tank home inspection report language

Hot water expansion tanks are not required in all installations - though are recommended by manufacturers. In my (limited as I'm just one old guy) experience, at least at oil and gas fired water heaters in the eastern US expansion tanks are rare.

IF you saw evidence of TPR leaks AND system temperatures and operating pressure range were generally not diagnostic of the problem (ok so typing while thinking ...) THEN an expansion tank might solve the problem. I'd point out that a leaky TPR is very dangerous - eventually they stop - and we risk a BLEVE.

If no leaks, no observed issues, probably ok.

So you'd say:

I noticed that no thermal expansion tank is installed on your water heating system.
I did (or did not) see evidence of a history of temperature and pressure relief valve(s) on the water heater.

Such leaks *might* mean that an expansion tank is needed if there is no other more-immediate (and more dangerous) cause.
In sum, if in the future you see leaks at a pressure/temperature relief valve on any water heater (or heating boiler) the system is unsafe and needs prompt attention.

What I'd have explained but most inspectors don't care to include:

If you find leaks at the relief valve on water heating equipment the valve may be doing its job of protecting you but the leak needs to be found and fixed promptly as the risk if left un-attended is a dangerous exploding water heater.

In cases where the leaking TPR valve is traced ONLY to pressure surges caused by thermal expansion in the hot water system then a thermal expansion tank would be needed. But keep in mind that there are other more-common causes of such leaks like abnormally high water pressure, water hammer, abnormally high heater operating temperatures - all of which are unsafe.

On 2017-11-27 - by (mod) -

DO

If you are not seeing a problem with leaks at the pressure temperature relief valve on your water heater then you probably don't need a thermal expansion tank on the system.

Location of the expansion tank goes on the hot side of the water heater or Clara fire but is not always physically above the top of that Appliance.

On 2017-11-27 by DearOldDrDad

If I have a pressure regulator valve, do I still need the expansion tank? And must it be above the hot water supply?

On 2017-04-28 - by (mod) -

Perhaps the expansion tank was not properly air-charged, or the TP valve developed a leak during the prior tenure of the bad old expansion tank.

Take a look at the system pressure: if it's at or above the rated opening pressure of the TP valve, then the valve is doing what it should and your system pressure is too high. In that case check the water heater temperature and check the building supply water pressure - its regulator may be allowing too much pressure on the cold side.

Also search InspectApedia (Use the on-page search box at the top or bottom of this page ) for WATER HAMMER that can also cause relief valves to leak.

On 2017-04-27 by Walt

I had my expansion tank replaced recently and now my temp pressure relief valve leaks what could be the problem

...

Continue reading  at RELIEF VALVE LEAKS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• ATTIC & ANTIQUE EXPANSION TANKS, HEATING- used with some older hot water heating systems
• BANGING BOOMING NOISE DIAGNOSIS & CURE
• EXPANSION TANKS, BOILERS - expansion tanks used on hot water (hydronic) heating systems
• EXPANSION TANK IO MANUALS & SIZING GUIDES
• HOT WATER EXPANSION TANKS
• HOT WATER PRESSURE EXPANSION RATE - how much does water expand when you heat it?
• RELIEF VALVE LEAKS
• THERMAL EXPANSION TPR VALVE LEAKS - why the water heater, cylinder, or calorifier relief valve leaks when heating wat
• WATER TANK SIZE & VOLUME - water pressure tank or water storage tank size requirements

Suggested citation for this web page

HOT WATER EXPANSION TANKS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to T&P RELIEF VALVES

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