About T&P Relief Valves
Temperature and pressure safety relief valves are designed to open when either the temperature or pressure limits are reached/exceeded, discharging built-up pressure or scalding hot water to avoid unsafe operating conditions and replacing it with cold water from the supply line, effectively lowering both temperature and pressure.
The most important characteristics of a t&p valve are:
- Connection size - 3/4" is most common for all residential and some commercial uses, with 200K BTU’s or smaller water heater. Sizes 1" and up are generally intended for larger commercial and industrial uses.
- T&P relief setting - 150/210 (psi/°F) is the most overwhelmingly common configuration, with rare instances of 125/210. Systems which contain plastic piping may utilize an additional 100/180 valve to protect the pipe itself.
- Thermostat/probe length - can vary between 2" and 9", with 4" and 8" being the most common. The probe length must meet manufacturer’s requirements.
- Valve capacity (BTU/hr) - must be greater than the capacity of the water heater/vessel. A common source of confusion since equipment may contain both ASME/NB and CSA/ANSI ratings and the relief valve usually has either both or CSA rating only. The latter (CSA rating) is more restrictive and is usually a safer choice for t&p valve selection.
How does a Temperature & Pressure relief valve work?
The pressure part of the safety mechanism in the relief valve is the same as in a regular pressure relief valve - a spring holds a disk over the opening, which is opened when pressure exceeds the spring tension.
The temperature relief mechanism is operated by means of the thermostat/probe with a spring-loaded sliding piston inside a sealed chamber, which starts to push against the relief valve’s sealing disk when the pressure inside the chamber is increased due to rising temperature of the water in the tank.
Frequently Asked Questions
Q: Why are some t&p valves lead-free while others are not? Are the latter approved for potable water uses?
A: Non lead-free models are still allowed for potable water applications under Section 1417 of SDWA (Safe Drinking Water Act).
Q: Is it OK when a temperature & pressure relief valve discharges water occasionally, but does not leak otherwise?
A: No. Under normal operating conditions, T&P valve should remain closed and not leak or discharge. Occasional discharges are often a sign of an under-sized expansion tank, improperly sized t&p valve or exceedingly high water heater/vessel temperature setting, among other causes.
Q: Why does a t&p valve leak?
A: When there’s a constant dripping from the valve, it is usually caused by a fouled seat (i.e. the disc does not fully seal against the seat due to sediment/rust/dirt, etc.). It is important that such valves should be replaced and the underlying cause (if any) addressed, since leaking relief valves may seize over time and stop functioning completely, creating serious safety hazard.
Q: Do indirect water heaters require a T&P relief valve?
A: Yes, a separate relief valve is required on indirect tanks.
