Does an Electric Tankless Water Heater Need a Pressure Relief Valve?


A pressure relief valve is common on water heaters. As more and more people start to use electric water heaters, they wonder if electric tankless water heaters need a pressure relief valve? So, I'll provide the answers.

Electric tankless water heaters do not need a pressure relief valve because they don't built-up pressure. Electric tankless water heaters don't store and heat water. Hence, pressure will not accumulate within and a pressure relief valve is not needed.

Some people don't really understand the function of a pressure relief valve on water heaters. So, getting to know them can give you a clearer picture.

How Electric Tankless Water Heater Works?

Electric tankless water heaters do not store hot water. They don't take in cold water and heat it up like storage water heaters do. Hence, they don't built-up pressure.

Basically, an electric tankless water heater has a heating coil that is housed in a copper tank. Cold water goes into the tank, passes through the heating coil and comes out as hot water.

Below is a diagram showing how cold water turns into hot water after passing through an electric tankless water heater:

Electric Tankless Water Heater Working Diagram

Although it is called a tankless water heater, an electric tankless water heater still has a tank for water heating. What's different is that hot water is not stored inside a tank as opposed to a storage water heater that holds hot water for later use.

Hence, tankless water heaters are also known as instantaneous water heaters and on-demand water heaters.

Since they don't store hot water, pressure won't be built inside them and therefore, a pressure relief valve is not needed.

To understand how storage water heaters built pressure by storing hot water, see my post  How Electric Storage Water Heaters Work?.

What is the Function of a Pressure Relief Valve?

Storage water heaters use a thermostat to regulate the temperature of the stored hot water. When the hot water reaches the temperature setpoint, the thermostat will stop the water heating process.

However, like any other device, the thermostat can fail. When a thermostat fails, it can't sense the correct temperature anymore. Thus, it poses a risk of overheating.

When a storage water heater is overheating, it can turn hot water into steam. As water keeps turning into steam, there is more room for new cold water to come inside the tank. Subsequently, more water will turn into steam.

Steam is compressible and hence, pressure can built-up of a storage water heater is overheating. 

Therefore, a pressure relief valve (PRV) is needed to relieve the pressure and prevent any catastrophic event from happening such as an explosion.

A typical water heater PRV has a pressure rating of 0.8 MPa (8 Bar). If the pressure within the water heater exceeds that, it'll start to drip water and relieve the pressure.

Sometimes, storage water heaters are equipped with a temperature and pressure relief valve (TPV). It works the same as a PRV except it has an additional checkpoint on water temperature.

What Type of Valve is Needed for an Electric Tankless Water Heater?

A typical electric tankless water doesn't need any extra valves except for a pair of isolation valves; one at the cold water inlet and the other one at the hot water outlet. The isolation valve can be a simple ball valve or a gate valve.

Is It Safe Not Having a Pressure Relief Valve on Electric Tankless Water Heaters?

Electric tankless water heaters don't build pressure within like storage water heaters do. Hence, a pressure relief valve is fundamentally not required. So, it is safe not having a pressure relief valve on electric tankless water heaters.

Why Some Tankless Water Heaters Have a Pressure Relief Valve?

Although tankless water heaters don't need a pressure relief valve, some people still find a pressure relief valve on their tankless water heaters. Why is that?

The answer is that the pressure relief valve is not actually a pressure relief valve.

Well, it is a misunderstanding. Some people use what they thought is a pressure relief valve to release the air within the tankless water heater but in fact, it is called an air release valve or an air vent valve, not a pressure relief valve.

A pressure relief valve is supposed to drip water or release steam automatically to prevent a water heater from over-pressuring.

Conclusion

Electric tankless water heaters do not need a pressure relief valve. They don't store hot water and thus, no pressure is built within. Hence, no pressure needs to be relieved and a pressure relief valve is not needed.


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Yu Chang Zhen

I enjoy working on air conditioners and ventilation fans because Malaysia is a very hot country.

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