Because water does not expand like steam, the valve lifts gradually as pressure climbs past set. Residential hot water boilers almost always carry a 30 psi relief valve, and its Btu rating must equal or exceed the boiler output. The discharge line runs full size to near the floor or a drain with no valve or cap. A dripping relief valve usually points to a waterlogged expansion tank or a fill valve set too high.
Relief valve's dripping every time the boiler fires, check the expansion tank charge before you blame the valve.
Also heard as
- RV
- pressure relief valve
- boiler relief
Related terms
Safety valve
The spring-loaded valve on a steam boiler that pops fully open to vent steam before pressure exceeds the boiler's rating.
Pop-off
Shop slang for the safety valve on a steam boiler or the relief valve on a hot water boiler.
Expansion tank
A closed tank with an air cushion that absorbs the expansion of heating water as it warms up.
MAWP
Maximum allowable working pressure: the highest pressure a boiler is approved to operate at, stamped on its nameplate.