Hot Water Suddenly Very Hot? Why and What to Do

Why is My Hot Water Suddenly Very Hot?

Your water heater is usually set to operate at 120 to 130 degrees Fahrenheit. At that temperature, the water is hot enough for your basic needs and to kill all the pathogens in the tank but will not scald.

The temperature of water inside an electric water heater is controlled by a thermostat, which turns the heating element on and off. When something is wrong with either the thermostat or heating element, you will have either no hot water or scalding hot water.

A gas water heater uses a burner which is lit by the pilot. When the water temperature inside the tank reaches that set by the thermostat, the burner will shut off and only comes off when the temperature falls.

If your hot water is suddenly very hot, your water heater is overheating. This happens when the thermostat fails to turn off the heating element or when the element is shorted. It could also be caused by sediment buildup or a blocked temperature and pressure relief valve.

When your hot water is suddenly scalding hot and you use a gas water heater, you may have sediment build up in the tank or a most likely a faulty gas control valve. with a faulty gas valve, the burner will continue heating the water even set thermostat temperature is exceeded.

Although this problem is most of the time caused by issues with the water heater, sometimes it could be caused by faucet valves. This is usually the case if say you are taking a shower, then suddenly the water becomes very hot after someone flushes the toilet.

If your shower water becomes scalding hot after someone flushes the toilet, you are using a shower mixing valve which is affected by the sudden drop in cold water pressure, forcing more hot water to flow to the shower head. Replacing the valve with a pressure-balancing valve should fix that problem.

When your hot water is suddenly too hot, replacing the thermostat, heating element or gas control valve will in most cases fix the problem. Sometimes you will only need to adjust the thermostat temperature of flush the water heater.

How Do I know if My Water Heater is Overheating?

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The best way to know if your water heater is overheating is by testing the heating element and thermostat for continuity. You cannot tell if there is sediment build up at the bottom of the water heater tank, unless there are popping/knocking noises.

If you suspect that your water heater is overheating, do not attempt to troubleshoot the problem without turning off power to the water heater first. The heating element maybe shorted and therefore passing current through the tank.

Dash to your main house electrical panel and look for the breaker labeled “Water Heater” or with the initials “WH” on it. Flip it to the off position.

After turning off the power to the water heater, it is time to troubleshoot it. For this job you will need a multimeter, a Philips and a flathead screwdriver.

Now here is how to fix a water heater when the water is suddenly too hot:

1. Test the Heating Element

An electric water heater will most likely have 2 heating elements and thermostats. The upper and lower heating elements are concealed using a cover plate in the 2 access panels.

  • Use a screwdriver to remove the upper access panel cover plate.
  • With the plate out, you will see a small piece of insulation. Remove it as well.
  • Pull out the plastic cover. There will be a piece of plastic covering the thermostat and heating element by snapping on them. Pull it out too.

You can now see the thermostat, reset button, temperature adjustment dial and heating element.

The thermostat is the piece with many screws and wires connected to it while the heating element is at the bottom with only 2 screws and 2 wires connected to it.

A water heater reset button is the little button on the thermostat which in most cases is usually red in color. It is also known as a emergency cut off switch or high temperature limit switch.

The function of the reset button is to turn off power to the water heater if the thermostat fails to turn off the heating element. It does so by tripping when the water temperature reaches and exceeds 1800 F.

Usually, when your water becomes suddenly very hot, it is followed by lukewarm water and later no hot water. This happens when the reset button trips.

To reset this button you will only need to press it till you hear it click.

If however the reset button is faulty as well, the thermostat will remain stuck in the on position. You may notice a leak in your water heater temperature and pressure relief valve unless it is blocked and no longer working.

A broken heating element may make contact with metals inside the water heater creating a short circuit. Current will therefore flow from the element to the metal, causing it to continuously heat the water.  

Here is how to test a water heater element using a multimeter:

  • Using a screwdriver, loosen the 2 screws on the water heater and pull out the wires.
  • Set your multimeter dial to the least ohms of resistance setting.
  • If you have an analogue multimeter, pinch the 2 probes together and move the needle to zero. With a digital multimeter, move the dial to the tone settings if you have that option.
  • Put one probe on one of the water heater terminals (screw) and the other on the water heater tank body.
  • Check the reading on your multimeter.

If there is an ohms reading on your multimeter, there is electrical continuity between the heating element and the tank. The heating element will need to be replaced.

After checking the upper heating element, remove the lower access panel plate and test the lower element as well.

I have written a detailed post on how to replace a water heater element. Read it here.

2. Test the Thermostat

If the heating element is not the problem, you most likely have a bad thermostat. When the thermostat is stuck in the on position, it will keep current flowing to the heating element even after the water is sufficiently heated.

Whenever I test the heating element and it looks ok, I usually replace both thermostats. You can as well buy a complete kit where you will replace both the heating elements and the thermostats.

Replacing the heating element also gives you the opportunity to flush out sediment from the water heater.

While at it, check the set water temperature on the thermostat temperature adjustment dial. If the temperature is set too high, use a flathead screwdriver to adjust it to 1200 F.

I have written a guide on how to test and replace an electric water heater thermostat. Read it here.

3. Flush out Sediment from the Tank

If you live in an area with hard water, dissolved minerals and especially calcium will settle at the bottom of the water heater forming sediment. This sediment will not only reduce your water heater’s volume, but it will also impact its heating efficiency.

If you are using a gas water heater which is heats water from the bottom, it will take longer for the heat to be transferred to the water through the layers of sediment. Before your water heater senses the water temperature has exceeded the set level, the water will already be scalding hot.

In electric water heaters, heating elements will also be coated with minerals deposits, forcing them to overheat in order to sufficiently heat the water. You may also here popping noises form the water heater as well.

A good way to fix this problem is flushing out sediments from the water heater. You just need to turn off the power or gas supply to the water heater then shut off the cold water valve.

After that, connect a garden hose on the drain valve bib and drain out the tank. While refilling the tank, have one hot water faucet open to prevent air building up in the tank.

I have written a complete guide on how to flush out sediments from a water heater. Read it here.

If you have not flushed your water heater in years, doing it now will make almost no difference. You can however attempt to vacuum the minerals through the water heater element opening as shown in this video.

4. Check the TPR Valve

The water heater temperature and pressure relief valve also known as the T&P or TPR valve is the valve at the top of the tank with a vertical discharge tube connected to it.

The function of a TPR valve is to discharge little amounts of water from the tank when the conditions are too high, to prevent it from bursting. TPR valves will usually open and discharge water at 150 PSI (pounds per square inch) or 2100 F.

To check if your TPR valve is working properly, place a bucket under the discharge tube and lift off the lever on the valve. If you see some water being discharged on the bucket, the valve is good.

On the hand, if no water comes out of the discharge tube then the valve is blocked. A blocked temperature and pressure relief valve is quite risky since the tank may explode and will therefore need a replacement.

I have also written a guide on how to replace a TPR valve. Read it here.

5. Call a Plumber

If you have tried troubleshooting you water heater with no success or you are not confident or able to do it, pick up the phone and call a professional plumber to solve the problem for you.

A plumber will have seen such a scenario plenty of times and will therefore know exactly where to look and what to do.

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