Pressure relief valves (PRVs) are devices that are designed to open in case the water pressure inside of a water heater, boiler, or any other vessel that is designed to hold water under pressure.  Water heaters heat water and are supposed to maintain that temperature.  The process is simple.  When the water is not hot enough, the fire turns on.  When the water gets hot enough, the fire turns off.  But with every mechanical device, things fail.  The problem that the PRV is protecting is if the fire is on and the sensor/thermostat that says the water is hot enough malfunctions and the fire stays on.  Water at atmospheric pressure boils at 212 degrees.  When water boils, it turns into steam.  In other words it changes from a liquid to a gas.  Water changing state (from liquid to gas) creates a tremendous amount of pressure.  Way back when, they used to have steam locomotives and steamboats.  The boiling liquid was released to move the wheels of the locomotives and the paddles of the steamboats. It is possible to keep water in a liquid state above 212 degrees.  We have to keep it in an enclosed vessel such as a water heater.  Water heaters are designed by code to hold up to 150psi of pressure.  In order to get water to 150psi of pressure I would have to heat it up to 300 degrees.  When water turns to steam it wants to expand roughly 1000 times its size.  If a 40 gallon water heater should explode due to overheating, it will want to fill a space of 40,000 gallons.  The video of MythBusters below actually shows a water heater exploding from overheating.

There are three different codes that are followed in the Chicagoland area.

Depending on the location of the home, is which code is followed. To view which code is followed by community you can click here.

My home inspector friends are going to tell me (and you) that we don’t inspect to code.  This is true. We don’t.  But I do like to use codes to help base my opinion.  The following excerpts are from the International Plumbing Code.

504.4 Relief Valve

Storage water heaters operating above atmospheric pressure shall be provided with an approved, self-closing (levered) pressure relief valve and temperature relief valve or combination thereof. The relief valve shall conform to ANSI Z21.22. The relief valve shall not be used as a means of controlling thermal expansion.

This means that any sealed container of water that has any pressure on it whatsoever needs to have a pressure relief valve designed for the container and purpose at hand.  The only time it is not required would be in situations such as an open pot of water sitting on a stove.  When a container is open, it cannot build pressure of any kind and no PRV is required.

504.4.1 Installation

Such valves shall be installed in the shell of the water heater tank. Temperature relief valves shall be so located in the tank as to be actuated by the water in the top 6 inches (152 mm) of the tank served. For installations with separate storage tanks, the approved, self-closing (levered) pressure relief valve and temperature relief valve or combination thereof conforming to ANSI Z21.22 valves shall be installed on both the storage water heater and storage tank. There shall not be a check valve or shutoff valve between a relief valve and the heater or tank served.

I have seen where these PRVs have been installed away from the tank.  Along the piping.  This section of pretty specific and the PRV needs to be attached directly to the tank, or within 6″ of the tank.

504.5 Relief Valve Approval

Temperature and pressure relief valves, or combinations thereof, and energy cutoff devices shall bear the label of an approved agency and shall have a temperature setting of not more than 210°F (99°C) and a pressure setting not exceeding the tank or water heater manufacturer’s rated working pressure or 150 psi (1035 kPa), whichever is less. The relieving capacity of each pressure relief valve and each temperature relief valve shall equal or exceed the heat input to the water heater or storage tank.

This section of the code is specific to water heaters.  There are different types of systems that require different PRVs.

This valve is for a water heater. The PRV is set at 150 psi.

This valve is for a hydronic heating system. The PRV is set at 30 psi.

This valve is for a steam heating system. The PRV is set at 15 psi.

504.6 Requirements for Discharge Piping

The discharge piping serving a pressure relief valve, temperature relief valve or combination thereof shall:

1. Not be directly connected to the drainage system.

2. Discharge through an air gap located in the same room as the water heater.

3. Not be smaller than the diameter of the outlet of the valve served and shall discharge full size to the air gap.

4. Serve a single relief device and shall not connect to piping serving any other relief device or equipment.

5. Discharge to the floor, to the pan serving the water heater or storage tank, to a waste receptor or to the outdoors.

6. Discharge in a manner that does not cause personal injury or structural damage.

7. Discharge to a termination point that is readily observable by the building occupants.

8. Not be trapped.

9. Be installed so as to flow by gravity.

10. Terminate not more than 6 inches (152 mm) above and not less than two times the discharge pipe diameter above the floor or flood level rim of the waste receptor.

11. Not have a threaded connection at the end of such piping.

12. Not have valves or tee fittings.

13. Be constructed of those materials listed in Section 605.4 or materials tested, rated and approved for such use in accordance with ASME A112.4.1.

This last one is different for the Illinois Plumbing Code.  It states that the discharge pipe needs to be metallic.