Temperature and Pressure Relief Valves (TPR)

Section P2803.1 of the International Residential Code for One and Two Family

Dwellings requires the installation of a combination Temperature/Pressure Relief Valve

(commonly referred to as a TPR valve) or a separate temperature relief valve and a

pressure relief valve on every water heater.

The purpose of the TPR valve is to provide an important safety mechanism that allows

for the immediate discharge of excess temperature and pressure that builds up inside

the tank as the water is heated. If not allowed to properly discharge, this excess

temperature and pressure will eventually cause the tank to explode.

Consequently, Section P2803.3 dictates that a TPR valve must discharge once the

pressure inside the tank reaches 150 psi (pounds per square inch).

Section P2803.4 specifies that the TPR valve must be installed within the top 6 inches of

the water heater tank in order to enable the valve to discharge at a temperature of no

greater than 210°F. As a result, any discharge piping must be rated to withstand at least

210°F to prevent melting and blockage of the TPR valve when discharged.

The presence of a functioning Temperature/Pressure Relief (TPR) Valve on a water

heater is extremely important. However, there are other factors to consider in ensuring

the protection of property and people who may be standing near the water heater when

the valve actually discharges. Namely, where does the scalding water or steam go once

the valve is allowed to discharge?

1) Improperly-Sized Discharge Pipe

Section P2803.6.1 requires that the TPR valve is fitted with a discharge pipe that

provides the same size opening as the valve outlet itself. This is a common defect that is 

strictly prohibitedbecause smaller discharge pipes restrict the flow of the pressurized water

released bythe valve. This creates a dangerous build-up of pressure at the point of discharge that

can cause the tank to explode.

2) Missing Discharge Pipe

The discharge pipe required in Section P2803.6.1 was never installed at all. This

normally occurs during replacement or initial installation of the water heater.This creates a dangerous

condition whereby the TPR valve will spray scalding steam or scalding hot water onto anyone (or

anything) that happens to be near the water heater when discharge occurs. When you

consider that the TPR valve discharges water at 210° F., serious injury or property

damage will result.

3) Improper Termination of the Discharge Pipe

Ideally, the TPR valve discharge pipe will be vented to the outside of the building where

the scalding water or steam can be discharged without posing a threat to people or

property. However, in many installations direct piping to the outside is not possible or is

deemed to be too costly to provide. Accordingly, Section P2803.6.1 does allow for the

discharge pipe to terminate inside a receptacle or drain pan as long as an air gap is

provided between the end of the discharge pipe and the top rim of the drain pan itself

without creating a hazard or potential cause of damage.

The Temperature/Pressure Relief (TPR) Valve serves an extremely important function

ensuring the safe and proper operation of the water heater. As a professional home

inspector, I encourage home owners to take the time and effort to identify and correct

them.

Consequently, the conditions presented in this article should be corrected immediately

by a qualified and licensed contractor.