Sizing a Relief Valve

Alan (California Radiant) Forbes

I do very little commercial work, but this one has come my way. A pharmaceutical lab wants me to replace this leaking Amtrol water heater which has a standard relief valve with a max at 105,000 BTU. The 1.5 million BTU rooftop boiler heats this water heater plus fan coils throughout the building. What is code for installing the correct relief valve in your area?

Larry Weingarten

Hi @Alan (California Radiant) Forbes , I’d figure out what the maximum BTU input for the tank is, and size the relief valve based on that, plus a bit-O-fudge

Yours, Larry

EdTheHeaterMan

Agree with @Larry Weingarten The relief valve can not be based on the entire system. The entire system capacity cannot possibly get to the indirect. The only amount that can get to the water heater is what the supply and return piping can handle. Even if you push it, a one inch feed and return can not possible put more than 100,000 BTU. Since most 3/4" T&P valves are rated for 105,000 or higher, you should be within the specifications of the code.

If the water heater can accept 1-1/4 feed and return, you may want to up your game to a WATTS 3/4" 40XL-5 T&P Relief Valve (150 psi) that can handle twice as much capacity.

Alan (California Radiant) Forbes

I've settled in to my code book which says,

"The hourly BTU discharge capacity of the rated steam relief capacity of the device shall be not less than the input rating of the water heater".

And I think that goes along with what you guys have said - Thanks!

I was worried that I'd have to find a relief valve rated at 1.5 million BTU.