hot water boiler pressure

mferrer

In most residential hw boilers, the safety relief valve (SRV) is set to 30 psi. On commercial HW boilers that may have a much higher static head (ex: 10 story building-100 ft or so tall), what pressure ratings max out at 160 psi?

mferrer

In my last post, I meant to ask:
On HW commercial boilers, is the maximum rated pressure 160 psi?
and, what is the highest safety relief valve setting?

Leonard

Column of water 100ft tall only makes static pressure of 44.6 psi
160 psi would support a static column of water 359 feet high ~ 36 stories.

Zman

It sounds like you are asking about boilers that heat DHW directly. Those commonly are shipped with 150# T&P valves.

Leonard

In my city I've seen city water pressure be 95 psi down by the river and 42 up on a hill. City water is gravity feed system.

JohnNY

I think what he's saying is that in some boilers, like the commercial-grade Lochinvar XL, the heat exchanger is rated for 160 psi and he wants to know what pressure relief valve is best installed with that boiler.
(side note: "SRV" is not a thing)
The answer is the relief valve should be selected after the pressure requirements of the system are determined.
Knowing the height of the system and the pressure tolerance of the system components are what will get you to that value. Then an expansion tank, pressure reducing valve (PRV) and relief valve can be selected.

mferrer

Thank you JohnNY for the info and clarification!
I sometimes confuse safety relief valves associated with steam systems, with pressure relief valves.
Regarding pressure ratings, I have read in texts about low press hw boilers being rated below 160 psi. And most discussions about residential hw boilers mention relief valves to be set at 30 psi.
Your post clarifies that the design has to take into account the static pressure and the pressure ratings of the selected components before determining the pressure set of the relief valve.
Thank you again!

EBEBRATT-Ed

Some of the newer boilers are rated at 125 psi or higher.
Weather enforced in your area or not any expansion tank used on a system with a relief valve over 30psi must be an ASME rated expansion tank.

Many times we have been able to avoid using the more expensive ASME tanks by installing a 30 psi relief valve on a boiler where the operating pressure allows

When doing this make sure the btu capacity of the new relief valve is adequate.

Also when using a lower pressure valve the pipe size may be larger that the boiler tapping you have available