Warning about improper relief valve installation at a water heater (C) Carson Dunlop AssociatesHot Water Thermal Expansion Tanks & Valves

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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.

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

Hot water system expansion tank from Falcon Stainless, Inc. (C) InspectApedia.copmWater 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 accomodate 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)

Article Contents

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.

Hot Water Expansion Tank Installation Instructions

Potable hot water heating expansion tank installation instructions and manual from Watts Corporation cited in this article (C) InspectApedia.comHot 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


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

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


For a useful example of the complete installation instructions for this device see

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.


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: []

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

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.


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.


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


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


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 at the end of this article.

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.

Water Heater Expansion Tank Resources


Continue reading at RELIEF VALVE LEAKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see EXPANSION TANKS, BOILERS expansion tanks used on hot water (hydronic) heating systems

Or see ATTIC EXPANSION TANKS, HEATING used with some older hot water heating systems.

Or see HOT WATER PRESSURE EXPANSION RATE - how much does water expand when you heat it?

Or see THERMAL EXPANSION TPR VALVE LEAKS - why the water heater, cylinder, or calorifier relief valve leaks when heating wat

Or see WATER TANK SIZE & VOLUME - water pressure tank or water storage tank size requirements

Suggested citation for this web page

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

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