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What Causes Too Much Pressure in a Hot Water Heater


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If you notice a small puddle of water near your water heater, it's possible the T&P valve has released pressure from your tank. But what causes too much pressure in a hot water heater to happen in the first place? When water heats, it expands and increases the pressure within the tank.

This article will explain the reasons your water heater can reach a dangerous level of pressure, and the safety mechanisms in place to prevent it from causing damage, or even worse, becoming a bomb.

what causes high pressure

What Causes a Water Heater to Build Pressure?

When water is heated, it expands and takes up more space, hence building pressure within the tank. This is called thermal expansion.

Think about a bicycle inner tube. When air is pumped into the inner tube the pressure builds and the tube becomes harder. Eventually, there will be so much air within the inner tube, that the pressure will cause it to burst. 

The same thing can happen to your water heater. 

If you have a 50-gallon water heater filled with cold water, once it's heated (and with the help of thermal expansion) it'll become at least 52-gallons. The extra 2-gallons will cause additional pressure within the tank.

As the temperature rises, the pressure builds within the tank, and with no where for the water to go, the tank can reach unsafe levels.

Typically the water heater's safety mechanisms will kick in to prevent a problem, but should the systems fail, the tank can reach pressure levels that could destroy your home.

Here's a video of the aftermath of a water heater that burst because it was unable to release the pressure within the tank.

Watch the Video

Why is My Water Heater Building Up Too Much Pressure?

If your water heater is building up too much pressure there's likely three reasons. Either the temperature is set too high, there's a malfunction, or the water heater doesn't have an expansion tank. 

Temperature Setting

If you're a fan of really hot showers, it's important to remember that the hotter the temperature setting, the more pressure will build within the tank.

For health reasons your hot water heater should be set to at least 120°F or you run the risk of exposure to Legionnaire's disease, but any temperature setting above 130°F can result in scalding.

Fortunately, your water heater is equipped with safety devices from preventing it from exploding. But the unnecessary pressure will cause wear-and-tear on the heater and shorten it's service life, as well as lower it's efficiency.

In addition, if the pressure reaches a point that triggers the T&P valve, you may frequently find a puddle of water near your tank.

Water Heater Malfunction

There are a few malfunctions that could cause your water heater pressure to reach high levels.

The majority of water heaters have an upper and lower heating element that heats the water. Each heating element has a thermostat which communicates with the other.

The upper thermostat is the "boss" of the two and has a high limit switch that will activate should the temperature reach 180°F. Once the high limit switch is triggered the flow of electricity to the water heater will stop.

A reset button located on the thermostat will allow electricity to flow again. However, if your water heater trips the high limit switch again, you should contact a professional to troubleshoot the problem.

Four common reasons your water heater will trip and build too much pressure:

  • A short in the heating element
  • Faulty thermostat
  • Faulty wiring
  • Faulty reset button

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No Expansion Tank

As we've explained, thermal expansion causes the water to expand as it heats, which increases the pressure within the tank and plumbing system.

An expansion tank will give the excess water a place to go and relieve the pressure within the tank.

Expansion tanks are required in homes that have a closed plumbing system. Closed systems prevent the water that has entered your household plumbing system from flowing back into the municipal pipes.

Replace T&P valve

What Prevents a Water Heater from Building Too Much Pressure?

Water heaters have mechanisms to prevent an unsafe build up of pressure within the tank. Still there are times that these safety mechanisms may fail, or have been tampered with. In fact, they may not even be in place at all.

T&P Valve

The T&P valve, also known as the temperature and pressure relief valve is required on all tank-style water heaters.

It's located on either the side or top of your water heater, and by code has a pipe that runs down the side of the tank.

When the temperature within the tank reaches above 210°F or the pressure is above 150 psi, the T&P valve will open to reduce the pressure. Once the pressure is release, the valve will close.

If you notice a puddle of water around your tank, the pressure within your tank may have been high enough to open the T&P valve. 

It's a good idea to contact a professional to determine the root problem, especially if this is a common occurrence.

T&P valves need to be inspected and tested several times a year to ensure they are properly working. Sediment and limescale can cause the valve to malfunction. Flushing the valve may help, but it may also need to be replaced. 

This article will show you how to replace your T&P valve.

If your T&P valve is not properly working it could result in too much pressure within your tank.

Never cap off the T&P valve if its leaking. It's doing its job and relieving pressure from within the tank.

A consistently leaking T&P valve means your water heater needs attention. There's something that's causing your water heater to reach unsafe temperatures that activate the T&P valve.

High Limit Switch

As we mentioned earlier, the high limit switch will trigger when the water within the tank reaches 180°F. Once activated, the flow of electricity to the water heater will shut down. 

If you consistently need to hit the reset button on your water heater, there's a problem that needs to be addressed. you should contact a professional plumber to troubleshoot the situation.

Exploding Water Heaters

Although, water heaters don't explode often, it can happen.

Always keep your water heater well maintained and make sure your T&P valve is in good working order.

Watch the Video

Why You Need an Expansion Tank

Certainly an exploding water heater is the worst possible scenario.

But too much pressure in your hot water heater is cause for other concerns. Especially since it can create significant damage to your plumbing system, appliances, and the water heater itself.

Even in normal operations the pressure within the tank will build as the water is heated. This is because as the water expands there's no where for it to go, placing more wear on the water heater's components. In fact, they may even fail.

In addition, if the pressure reaches a point where the T&P valve triggers, you may experience water damage in the surrounding area of the water heater, such as walls and floors.

There will also be more wear on your hot water appliances as the incoming water pressure may be higher than they were designed to handle. And the added pressure on your plumbing system can cause micro leaks that can develop into a huge mess.

However, with the addition of an expansion tank the overflowing hot water will have some place to go. 

Expansion tanks help keep the tank's pressure at a constant level. When the pressure rises, the additional hot water can move into the expansion tank. You can learn more about expansion tanks here.

An expansion tank will only prevent your water heater from developing too much pressure during normal operations. If your water heater is malfunctioning, you need to call a professional to resolve the situation.

This quick video will give you an overview of how an expansion tank operates.

Watch the Video

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