Sketch of a tankless coil tempering valve or anti scald valve (C)Carson Dunlop Associates Sudden Loss of Hot Water Pressure
Lost hot water diagnosis, & repair procedures

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Hot water supply troubleshooting - no hot water:

This article explains how to diagnose the sudden loss of hot water pressure, quantity, or flow in a building.

We explain the difference between slowing or poor hot water supply and a hot water supply that suddenly is not there at all, and we give a diagnostic procedure to find and fix the trouble.

First we outline things to check if you have no hot water at all. Then we describe things to check if you have hot water but the pressure or flow rate is drastically reduced or just too darned weak.

We discuss the following: Possible places where hot water supply piping may be clogged. Hot water valve clogging troubles: Three Types of Plumbing Valves Used on Water Piping: Globe Valve, Gate Valve, Ball Valve Compared. Draining the Hot Water Heater Tank to Fix Poor Hot Water Pressure? Clues Pointing to Mineral Debris Clogging in Hot Water Piping.

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How to Diagnose Sudden Loss of Hot Water Pressure & Flow in a Building

Water heater leak diagnosis & repair (C) Daniel FriedmanBefore 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".

Things to check if there is NO Hot Water Whatsoever

Check the Water Heater Energy Source

Check that the water heater has fuel (oil or gas) or if it's electric, that it has electrical power.

If yours is a gas fired water heater and IF the heater uses a temperature-operated automatic gas shutoff valve, that safety device could have shut down your system.

Details about the installation, testing, and functions of gas automatic shutoffs are at AUTOMATIC GAS WATER HEATER SHUTOFF

Check the Incoming Cold Water Supply

Is there cold water flowing into the water heater? Do you have cold water pressure?

Is the cold water inlet valve at your water heater open?

Check that the Water Heater Burner or Heat Source Operates

Now we can move along into approaches to "no hot water" for each type of water heating system - for which water heater, calorifier, geyser, and hot water cylinder are all synonyms.

Water heater manuals:

Before we start: do you have the installation and operation or owner's manual for your water heater? Usually there is a nice troubleshooting table near the back of each heater manual.

If you don't have the instructions for your water heater you can still follow the advice given in more-detailed articles at but you might want also to see WATER HEATER MANUALS to find your equipment guide.

Here are the places to start troubleshooting for each type of hot water system that you might have in your building:

No Hot Water from an Electric Water Heater

See ELECTRIC WATER HEATER REPAIR GUIDE where we discuss checking for power, checking the thermostat, the reset button, the temperature control, and then we move on to describing how to test electric water heater elements and controls.

If you don't know how to even find the controls on an electric water heating system see WATER HEATER TEMPERATURE CONTROLS

No Hot Water from a Gas Fired Water Heater

Is there fuel in your LP gas tank? Or if your system uses piped-in natural gas, is the gas valve open?


Also see WATER HEATER LEAK REPAIR since water leaks can shut down a gas burner or its pilot


No Hot Water from an Oil Fired Water Heaters:

Is there oil in the oil tank? If yes then see OIL BURNER WONT RUN


No Hot Water from an Indirect Water Heater

An indirect water heater is one whose hot water tank is heated by a physically-separate heating boiler.

So first confirm that the heating boiler will turn on in a response to a call for heat in your building. If that works then the problem is probably with the circulator or control that sends boiler water through the heating coil inside the indirect water heater.


No Hot Water from a Solar Water Heater

Most solar water heating systems use an electric, gas, or oil backup system so you may have two systems to diagnose.

Check that there is water in your system and that the necessary pumps are operating.

See SOLAR HOT WATER HEATERS for more about these systems.

No Hot Water from a Tankless or Instant Water Heater

Tankless or Demand or "Instant" water heaters may be electric, gas, or possibly even oil-fired devices that do not store any volume of hot water.

When you turn on hot water at the tap, water flowing through the system activates a control to turn on the heater that in turn heats water as it passes through the device.

So if you have no hot water then your heater is either out of fuel or is not turning on. But these devices can be tricky: you need a fast-enough water flow through the heater to activate it. So running the hot water very slowly or very weak building water pressure could be the problem.


Things to Check if there is Suddenly Weak Hot Water Pressure

Question about diagnosing the cause of sudden loss of hot water pressure:

We suddenly lost hot water pressure [only] in the whole house, a week ago (7 years old townhouse: basement, main floor and upper floor). We drained the hot water tank, checked the various valves in the house but still same weak pressure.

Do you think it could be mineral deposits in the pipes? Or a clogged pipe? What is the most common cause for it? My plumber doesn’t seem to find out the cause of the sudden lost of Hot Water Pressure in the whole house.

Are there specialized plumbers for this situation? I assume not all plumbers have the training. I am kind of confused. - R. S.

Answer: things to check if hot water pressure is poor

We assume you mean that hot water pressure and flow are suddenly badly diminished in a building, not that pressure or flow have stopped entirely.

We also note that you refer to a hot water system that uses a tank, not a tankless coil to produce the building's hot water. Tankless coil clogging is discussed separately

If mineral clogging of the building plumbing system is a problem, usually we notice that hot water pressure is worse than cold water pressure, but also that the hot water pressure declines over time, rather than appearing to suddenly diminish severely. So our diagnosis here is going to look for things that might produce a sudden severe reduction in hot water pressure - some sort of blockage or valve problem in the building water piping.

If hot water pressure suddenly stops entirely but cold water flow continues just fine, then a control valve may have been left closed, or a valve may have failed internally (appearing to be turned on but actually "off" internally due to a broken valve stem part), or a valve or pipe elbow may have become partly blocked by a chunk of mineral scale or by other debris. Here are some hot water pressure diagnosis suggestions:

Often the problem with loss of both hot and cold water pressure at plumbing fixtures is local clogging at a fixture such as faucet strainer and shower head. But if if you SUDDENLY lost hot water pressure everywhere in the building we have some different ideas:

It is unlikely that fixture or strainer or shower head clogging would explain a hot water pressure sudden loss at all plumbing fixtures; usually that clogging builds up over time. Indeed a scrap of mineral crud could break loose and suddenly clog an individual fixture strainer, but the chances of that happening at the same time at all individual fixtures just doesn't seem likely.

Possible Point Stoppages or Clogs in Building Hot Water Pipes, Valves, or Heaters

But a similar problem could have occurred right at the water heater, or in water heater piping near the point of hot water origin. For example, a failed, or clogged control valve, pipe section, pipe elbow, or even the outlet at the hot water tank itself could be come blocked. Clogged piping diagnosis is discussed in more general terms (besides a clogged tankless coil)


and repairs at WATER PIPE CLOG REPAIR.

But let's distinguish between clogging of runs of water supply piping in a building, and a specific point clog or blockage. Point clogs or stoppages in water piping can occur anywhere in the water pipe system, but are more common at these points:

Three Types of Plumbing Valves Used on Water Piping: Globe Valve, Gate Valve, Ball Valve Compared

As Carson Dunlop Associates' illustrations below show, a broken water control valve stem can obstruct or block water flow through the valve. Globe valves (below left) tend to most restrict water flow compared with the gate valve (center) and ball valve (right). But a broken stem on a gate valve can also leave the valve stuck partly or even fully closed (or open), regardless of how you may be able to turn the valve handle.

Water control valve type (C) Carson Dunlop Associates Water control valve type (C) Carson Dunlop Associates Water control valve type (C) Carson Dunlop Associates

Draining the Hot Water Heater Tank to Fix Poor Hot Water Pressure?

Hot water tank control valves Carson Dunlop Associates

Simply draining the hot water tank might not fix the problem. For example if a chunk of mineral debris has escaped the tank and is clogging a pipe, elbow, or valve. But if during draining you found clots of mineral scale, that adds to the suspicion of a scale problem and blockage.

For example, if a chunk of scale happened to flow into the piping leaving the hot water heater tank, or into a valve in the piping system, that could cause a blockage.

Watch out: as Carson Dunlop Associates' illustration (left) shows, for safety reasons we normally expect to see a shutoff valve only on the cold water line into the water heater, not on the hot water heater tank outlet side.

Clues Pointing to Mineral Debris Clogging in Hot Water Piping

Also, WATER HEATER FLUSH PROCEDURE discusses how to flush scale and debris - which you may have already tried, BUT maybe without recalling the extent of scale you found you may need to take another look at the chance of scale or debris clogging a valve or pipe or pipe elbow at the water heater.

Also check cold water supply INTO the water heater. If you open the water heater drain and turn back on the cold water supply to the water heater, and if you do NOT see powerful water flow out of the heater tank drain, then the drain valve could itself be blocked by debris (most common) or alternately you might have poor water flow INTO the water heater.

Check all of the plumbing control valves feeding the water heater to be sure no one closed one of them, and if there are shutoff valves on the hot water piping system ahead of most of the building plumbing fixtures, check those for operation and blockage too.

Thanks to Carson Dunlop Associates, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article.


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

Or see HOT WATER PRESSURE LOSS FAQs - diagnostic questions & answers on lost hot water pressure, flow, temperature

Or see these

Hot Water Quantity & Flow Improvement Articles

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