Schematic of a sacrificial anode on a hot water tank (C) Carson Dunlop AssociatesHot Water Heater Selection, Inspection, Installation, Diagnosis, & Repair Guide

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Water heater inspection, diagnosis and repair home page.

Guide to inspecting & fixing electric, gas or oil fired hot water heaters. Guide to tankless coils, indirect fired water heaters, range boilers. No Hot water? how to get more hot water flow, quantity, performance from your water heater.

Water heater synonyms: geyser (non-potable water), calorifier (potable water). Water heater tank synonyms: hot water tank, water heater tank, calorifier tank, geyser tank, water kettle, water cauldron, water pot, water copper, domestic hot water.

This page contains links to in-depth articles on inspecting, testing, and repairing problems residential hot water heaters of all types, including their parts, controls, and alternative sources for hot water as well as tips for improving hot water temperature, hot water pressure, and hot water quantity.

Articles listed here will answer just about any question about domestic water heaters, water heater selection, installation, problem diagnosis, repair, operating cost, or performance. Page top sketch provided courtesy of Carson Dunlop Associates.

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Water Heater Inspection, Installation, Diagnosis, & Repair Guide

Flue gas spillage at gas fired water heater (C) Daniel FriedmanWhile an electric, gas-fired, or oil-fired water heaters, along with tankless coils on heating boilers are among the most common methods for producing domestic or residential hot water for washing and bathing, there are plenty of other ways that people obtain hot water.

All hot water supply systems use some energy source (electricity, oil, gas, solar energy) to heat either a reservoir of hot water stored in a hot water tank, or to heat water as it is used (such as tankless coils and Thermar™ type instantaneous water heaters.)

An exception is geo-thermal hot water (such as is readily available in Iceland and in more localized areas in other countries). And the most common hot water complaint we hear is "how can I get more hot water quantity - I keep running out of hot water" or "how can I get better hot water pressure?"

Our photo shows clear evidence of flue gas spillage from a gas fired water heater. In articles linked from this page we discuss this and other water heater diagnosis and repair topics.

The articles listed below describe the inspection, operation, diagnosis, repair, and improvement of each type of water heater, and we include detailed advice about how to improve hot water supply in buildings as well as inspection and correction of leaks, high hot water costs, odors, noises, and safety problems.

Note that in some parts of the world, such as South Africa, folks use the term geyser for water heater or hot water maker.

Common Water Heater Problems

Not Enough Hot Water, or Water Not Hot Enough - what to do about it

This is the most frequent question people ask about hot water heating systems. See these detailed hot water diagnosis and repair or improvement articles:

Water Heater Basics: Types, Properties, & Inspection Topics for Home Water Heaters

Please see the articles listed at page top or at the MORE READING links at the bottom of this article under HOT WATER HEATERS for our complete list of water heater articles.

Have you already checked our Hot Water Trouble Diagnostic Guide?

Before you start fixing or buying stuff to fix a hot water problem hot water problems and diagnostic guides for all kinds of hot water troubles are summarized at WATER HEATER PROBLEM DIAGNOSIS. You might want to check there to be sure you're fixing the right problem. Those questions & suggestions can help point you to the shortest route to troubleshooting hot water complaints like "no hot water" or "not enough hot water".

The following information about residential water heaters is adapted, illustrated, and expanded from the Carson Dunlop Home Reference Book, with generous permission of Carson Dunlop Associates.

Domestic water heaters provide hot water to the faucets and appliances. Most water heaters are conventional storage-type heaters, where heated water is stored in a large tank

. There are also tankless systems, where water is heated on demand, either by a boiler, or a dedicated water heater. Indirect water heaters are a third type. They typically have a large tank, with the heat being provided by the boiler that heats the house.

Water may be heated by gas, propane, oil or electricity. Solar water heaters are also available.

Water heaters may be used to heat all, or part of a house, through the use of fan-coil units, or radiant heating. This is called a combination heating system because the water heater provides domestic hot water and it heats the home.

The text below discusses residential electric, gas, and oil fired water heaters and tankless water heaters. Also see

What Are the Properties of Conventional Water Heaters? Gas, Oil, Electric Water Heaters

Whether heated by gas, propane, oil or electricity, all conventional water heaters work the same way. Cold water enters the tank, and heated water leaves the tank. The heated water temperature is typically 120 to 140° F.

When a fixture runs hot water, the heated water leaves the tank and cold water enters, triggering the thermostat and turning on the burner or element. If heated water flows out faster than the incoming cool water can be heated, we will run out of hot water. The larger the tank, the longer it takes to run out of hot water.

Water Heater Details ((C) Carson Dunlop Associates Water Heater Details (C) Carson Dunlop Associates

- Courtesy Carson Dunlop Associates, used with permission

How Big Should My Water Heater Be - What Size?

Water heater details Carson Dunlop Associates


Water heaters should be big enough to satisfy the needs of the house. A family of four will often find a 30-gallon gas or oil system or a 40-gallon electric system satisfactory.

Electric water heater drawing (left) courtesy Carson Dunlop Associates, used with permission

Details about water heater size, hot water heating capacities or properties, recovery rates, water heater sizes, are in articles listed beginning

If you do not have enough hot water quantity, that is if you run out of hot water, or the hot water is not hot enough, see these diagnosis and repair articles:

What is Water Heater Recovery Rate?

Water heater details Carson Dunlop Associates

When the hot water is depleted, that is, when you are running hot water long enough that you run out of hot water, the recovery rate becomes important.

Water heater recovery rate drawing (left) courtesy Carson Dunlop Associates, used with permission

Generally speaking, oil fired hot water heating has the fastest rate of recovery, with gas fired water heaters and electric water heaters third. If water is drawn off slowly, the recovery rate may be such that the tank can be kept filled with hot water.

Faster water heater recovery rates allow more water to be drawn off without running out of hot water.

Should I Insulate My Water Heater?

Modern water heater tanks are insulated to slow the heat loss from the tank. Energy-efficient tanks have better insulation. Some people also insulate their hot water piping.

But you should not add water heater insulation yourself without first reading the installation manual for your water heater. You may make the water heater unsafe and/or you may void the water heater warranty.


and INSULATE HOT WATER TANK? for details

Water Heater Temperature Settings

Thermostats control the water temperature inside the water heater. There are some conflicting issues around appropriate water temperature. We don’t want the water so hot that it scalds people, but we want it hot enough to prevent bacteria like Legionnaires disease from growing in the water heater.

Also, dishwasher manufacturers often recommend that the water be 140° F, since some dishwashing detergents will not dissolve completely at lower temperatures.

Many dishwashers have internal heaters to bring cooler water up to appropriate temperatures for washing dishes.

See WATER HEATER SAFETY for details.

Anti-Scald Valves and Mixing Valves at Water Heaters

Some jurisdictions require tempering valves on water heaters, so water in the tank is at 140°, but as it leaves the tank, cold water is mixed in to deliver 115° to 120° water. These tempering valves may be installed at the water heater, or at individual fixtures.

Details are at ANTI SCALD VALVES

Water Heater Fuel Problems: Oil, Gas, Electric

Malfunctioning burners, electric elements, sensors or controls will cause poor operation or may result in the system not working at all, meaning no hot water. See our separate articles on electric water heaters, gas water heaters, and oil fired water heaters for details.

Also see the Home Reference Book Heating chapter for more information about fuel systems, burners and electric elements.

Water Heater Exhaust Venting Problems for gas or oil fired hot water heating systems

[Illustration: Vent Connector Length for a Water Heater]

To be safe and to work properly, most gas and oil water heaters have to be vented into a chimney with adequate draft. [An exception are electric water heaters and direct-vent water heaters.] Poorly arranged or disconnected vents are safety hazards, which should be corrected promptly.

Aluminum vents are not permitted. Vent sections should be as short as possible, screwed together, and should slope up 1/4 inch per foot, minimum. Vents should extend two feet above the roof and should be two feet above anything within ten feet horizontally.

Vents should extend at least five feet above the draft hood. Exhaust gases spilling out at the draft hood or burner may present a life-threatening situation.

This problem requires immediate action. Some modern gas water heaters employ induced draft fans and high-temperature plastic venting that discharges out through the house wall.

The vent materials were originally PVC, CPVC or ABS. In some areas these are replaced with special plastic vent pipes rated for the high exhaust gas temperatures.



Water Heater Location

Gas or oil water heaters should not be in sleeping areas. This is a safety issue.

Gas-fired heaters in garages should be 18 inches above floor level to reduce the risk of the heater igniting gasoline fumes, and should be protected from mechanical damage. Some jurisdictions call for electric heaters in garages to be similarly elevated.

Water Heater Noises

A snapping, hissing, crackling, or popping sound coming from the water heater tank when the heater is "on" may indicate a scale problem that is reducing hot water temperature, quantity, and water heater life.


Why Bad Electric Water Heater Elements Mean Tepid Hot Water or Not Enough Hot Water or No Hot Water

It is not unusual to find one of the two elements in electric water heaters burned out. Replacing an element is not expensive. Most heaters are arranged so that both elements cannot be on at the same time – the elements operate in a sequence.

Depending on which element fails, there may be some hot water, or none.


Also see our complete article on electric water heater properties, inspection and diagnostic checklists, maintenance and repair procedures beginning at ELECTRIC WATER HEATERS

Water Heater Leaks & Leak damage

Water heaters can, of course, leak, and the tanks can be mechanically damaged.


Sludge in the Water Heater

Where sludge has accumulated in the bottom of the tank, water pressure from the hot water system may be limited.

When water pressure problems are experienced on the hot water system only, it makes sense to drain the water heater to ensure that sludge accumulation is not the problem.

Some experts recommend draining one or two gallons out of the bottom of the tank monthly to prevent sludge build-up.

Water Heater Relief Valve Safety Warnings

LARGER VIEW of a heating boiler cad cell relay switch

The temperature/pressure relief (TP or TPR) valve lets water escape if the temperature or pressure is too high.

This valve should be connected to a tube that discharges no more than six inches above floor level so hot water is not sprayed on to anyone nearby. Some areas require that the tube discharge outside the building.

The tube should be as large as the tank fitting and the tube end should never be threaded, capped or plugged.

The tube diameter should be at least as large as the TPR valve fitting. The tube should be able to withstand 250°F temperatures, should have no shut-off valve, and should be as short and as straight as possible.

An alternative to the high temperature function of the relief valve is a high temperature shutoff in the tank.


Water Heater Age: When is the Water Heater Near End of Life?

Typical water heater life expectancy is 10-12 years, though there are exceptions with heaters that last a shorter period and others that we sometimes find last much longer.

The life that your water heater manufacturer expects for the unit is reflected in the water heater warranty period.


Water Heater Installation Hookup: Piping Arrangements: single, serial, parallel installations

Details about water heater piping are at WATER HEATER PIPING - cascaded, ganged, in series, in parallel

Reader Question: in a parallel water heater hookup do both heaters work at once?

My boyfriend is a long time plumber in a small town here in Northern Calif. He wanted me to look up Parallel plumbing on two water heaters for opening a new bar. I found two sites with the same hookup Cold water to Cold Water and Hot to HoT on cold water.

He took the diagram and all the wholesalers in The Chico calif. were perplexed as to why this was shown this way.

Is this a mistake? Or is there an advantage to taking water from bottom of tank. Does this work both at same time or just one at a time. I found this to be a challenge to see if this was something very ingenious and would like to make sure and understand and share that maybe you are getting more hot water this way or it's a mistake is labeling.

Note: see how the cold and hot are hooked up. Thanks for your time in this matter, my boyfriend hooked up the units as always but, my question wants to see if this is more efficient in a bar setting needing more hot water. - C.B. 8/5/2013

Reply: Piping Connections for Individual & Ganged Water Heater

I'm not sure I have a clear picture of the question but it seems to me that

1. For all conventional vertical reservoir-type tank-type water heaters, we always take hot water from water that is near the top of the tank interior ( hot water rises to the top of the tank interior regardless of how the water is being heated; cold water flowing into the tank to be then heated is delivered to the tank bottom) when delivering hot water to the building

2. There are very different reasons for hooking up multiple water heaters in series versus in parallel as I outline just below

Hooking Up Water Heaters in Series - cascaded water heaters to handle varying demand

In series hookups the hot water out of heater 1 is taken into the cold inlet of heater 2 and the hot water outlet of heater #2 then feeds the building.

This approach is often used for both heating and hot water heating in large buildings and is sometimes called a cascade approach; with proper heater control settings it allows very economical heater operation - we just run one smaller heater when demand is low, but we can run two or more heaters when demand is greater;

In a cascade arrangement the heaters downstream from the first one act as boosters and turn on only as needed.

A variation of the cascade approach is to install a simple water storage tank indoors ahead of the heater; water in the storage tank absorbs heat from the ambient indoor environment before feeding the water heater - reducing the heater's workload.

Hooking Up Water Heaters in Parallel - Case 1 parallel water heaters provide individual building area hot water OR Case 2 parallel water heater hookups provide high constant hot water output volume to a single user

Parallel water heater hookups (Which I think you are describing) basically are feeding cold water in parallel to multiple water heaters (i.e. simultaneously) and the output from each of the heaters (the hot out) feeds either different building areas, apartments, or users (case Parallel 1) , or feeds a manifold that then joins the output from all of the heaters to feed a single hot water line feeding a large building (case Parallel 2).

Parallel 1 is what we would expect to see in a small apartment building or multifamily house - essentially each tenant has their own water heater - common cold water in but individual hot water out is fed to each tenant or apartment or building area. This approach is economical and allows each tenant to be charged for their individual water heater use (if metering is installed).

Parallel 2, which I've heard-of but never seen and which I think has less application, is in my OPINON an inefficient variation on the cascade water heater approach designed to give a high hot water output quantity to a single destination.

Watch out: keep in mind that there is good reason that the incoming cold water must be connected to the "COLD" marked inlet on the water heater, as the manufacturer specifies - a dip tube is delivering cold water to the heater tank bottom. Hooking up a water heater backwards gives bad results.

For your boyfriend's case, hooking up hot water supply for a bar, to decide how to hook up his two water heaters depends on what problem he's solving. if the problem is adequate total hot water quantity when hot water demands vary significantly over time then he'd want to use the cascade approach - hook up the heaters in series.

If for some reason he has no room for a larger capacity single water heater but needs a large quantity of hot water always available then a parallel hookup (Parallel hookup case 1 above) might be usable.

You didn't say what energy source these heaters use - I'm guessing they are electrical, but the parallel / series hookup question and answer remains the same for all energy sources. At the end of the day I wonder if we were not a bit confused about "parallel" vs "series" hookups of hot water sources. If I've misunderstood your situation or question please let me know.

Watch out: your boyfriend, being a plumber, will doubtless confirm another little installation detail that we mention for other readers: the water heater shutoff valve should be only on the cold inlet side of the heater. A shutoff valve on the outgoing hot side of the heater - right at the heater, invites a disaster.

Now what happens if we hook up two or more water heaters in series? The hot out of heater

1 enters the cold inlet to heater

2. But I would not install a shutoff between the two water heaters - doing so creates the same unsafe condition.

What does the plumber say about shutoff valve locations on a multi-heater installation in series?

Water Tank Piping Connections & Number of Pipes Can Identify the Original Use of Storage Tanks

The following piping arrangements are discussed in more detail

Water Heater Maintenance Procedures

Proper water heater maintenance - such as draining sludge out of the tank or removing lime and scale can significantly increase water heater life. See these water heater maintenance articles:

Tankless Water Heaters: Properties of, Common Problems With

Details about tankless water heaters are found at TANKLESS WATER HEATERS. Tankless water heater basics are given just below.

As the name suggests, tankless water heaters have no storage capacity. Tankless heaters are typically gas or propane fired and have a burner, heat exchanger, venting system, and controls.

When the faucets and fixtures in the home are idle, the water heater is dormant. When there is a call for hot water, the heater detects the water flow and ignites the burners.

These powerful burners quickly heat the water inside the small heat exchanger. As hot water leaves, fresh cold water is drawn in and heated as it passes through. An advantage of this system is that you can’t empty all of the hot water out of the tank because there is no tank – just continuous hot water.

Tankless Water Heaters do not "Store Hot Water"

The other major advantage over conventional water heaters is energy savings. Tankless water Water heaters have no reservoir of hot water sitting idle. It takes energy to keep the tank of water hot all the time for when it’s needed.

Small Size of Tankless Heaters

Tankless water heaters are much smaller than conventional heaters with storage tanks, and are usually wall-mounted. They do not take up much space.

Tankless water hater fuel and combustion gas venting

Most tankless water heaters are fueled by natural gas or propane and are vented through a side wall of the house.

Efficiency of Tankless Water Heaters

Tankless water heaters are often more efficient than conventional water heaters, using modulating burners, direct venting and/or condensing combustion systems.

Mixing Valves on Tankless Water Heaters - Anti Scald Devices

Most systems include a mixing (tempering) valve and a means of setting a maximum water temperature to avoid scalding. This tempering valve mixes some cold water with the hot water leaving the unit to reduce the temperature.

Remote Controls on Tankless Water Heaters

Some tankless systems include a remote control, which can be used to monitor the performance of the system, display error codes or change the desired water temperature.

Other Uses of Tankless Water Heaters

Tankless water heaters may also be used to heat the home, either as part of a forced air combination system, or a radiant hot water system.

Watch out: The duty cycle of a conventional water heater or even a tankless water heater may not support home heating applications.
See WATER HEATERS for HOME HEATING USE? for some warnings about using water heaters for conventional heating.

Common Tankless Water Heater Problems

Fuel Supply Troubles

The water heater must have a continuous fuel supply. Malfunctioning burners, sensors or controls will cause poor operation or may result in the system not working at all, meaning no hot water.

Scale Build-up

The small diameter of the heat exchangers means that these units are susceptible to clogging with scale, especially in areas with hard water. In hard water areas, annual de-scaling is recommended.

Longer Wait Times

When a hot water faucet is turned on, it may take longer to get hot water with a tankless heater than a conventional system. The delay between opening the faucet and getting hot water can be longer with tankless heaters than conventional tank heaters. Better tank locations and multiple tanks can help with issue.

Maximum Flow Rate for Tankless Water Heaters

The hot water flow rate is not only dependent on the heating capacity of the water heater and the output water temperature, but also on the inlet water temperature. Homes in northern climates draw water from colder sources, and since it takes longer to heat up colder water, tankless water heaters installed in these homes have lower hot water flow rates.

Minimum Flow Rate for Tankless Water Heaters

The burners are triggered by sensors that detect the flow of water. If the flow rate is less than 1/2 gallon per minute, the burners may not turn on and no hot water will be delivered. Water-saving shower heads, for example, may not flow enough to turn the water heater on, especially if the water heater needs a high flow rate before it will come on.

Tankless Water Heater Temperature & Pressure Safety Relief Valves

The temperature/pressure relief (TPR) valve lets water escape if the temperature or pressure is too high. This valve should be connected to a tube that discharges no more than six inches above floor level so hot water won’t scald anyone nearby. Some codes require that the tube discharge outside the building.

The tube should be as large as the tank fitting and the tube end should never be threaded, capped or plugged. The tube should be able to withstand 250°F temperatures, should have no shut-off valve, and should be as short and as straight as possible.

High Cost for Tankless Water Heaters

Tankless water heaters are considerably more expensive than conventional tank-type heaters, and although tankless units are more energy efficient, it may take a long time to recover the extra investment.

High Maintenance for Tankless Water Heaters

Conventional water heaters are relatively inexpensive due to their simplicity. Tankless water heaters are more expensive and more complex. Their complexity also means that maintenance and repairs can be more expensive. Isolating valves help simplify draining and other regular maintenance.

Dirty Water Filter on Tankless Water Heaters

If the heater is equipped with a water filter, this should be checked and cleaned monthly, or performance will suffer.

Details about Tankless Water Heaters

See our article series on tankless water heaters or instant water heaters, point of use water heaters, and demand water heaters beginning

- Adapted from the Home Reference Book, courtesy of Carson Dunlop Associates.


Continue reading at HOT WATER IMPROVEMENTS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see AGE of WATER HEATERS - data tag decoding, contact information for water heater manufacturers, water heater manuals


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