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# Question about boiler static fill pressure

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Member Posts: 38
Is the max pressure on a boiler rating plate determined by the pressure relief valve packaged with the boiler? I have to pump up 4 floors, about 50 feet. Using H x .433+5, I get a fill pressure of close to 27 psi. Can I set my fill valve to 27 psi and install a larger pressure relief valve (36 or 40lb)? Or do I risk damaging the block or components if the boiler pressure exceeds the number on the rating plate, 30 psi?

• Member Posts: 864
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First and foremost, the pump does not "pump up four floors," instead the static pressure fills the pipes with water and the circulator pump moves the water through the pipes. In other words, the water stays inside the pipes all summer when the pump is off.

In simple terms, for every pound of pressure over zero the water will rise 2.3 feet. So for a typical four story building (with boiler in basement) the highest pipe would be about forty feet above the boiler. 40 feet divided by 2.3 equals about 17.4 pounds of pressure, let's add a few pounds for a slight cushion, so I'd go with 22 psi.

If the boiler is rated at 30 psi, it could be dangerous to install anything more than a 30 psi relief. If the boiler is rated at 40 or 50 psi then it would be acceptable to replace the relief valve with either one.

If the expansion tank is large enough and working properly, you should be able to sneak by with that 30 psi relief valve. If not you might be forced to use a larger expansion tank or add a second tank.

Make sense?
• Member Posts: 38
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Thanks Scott. The height to the top of the highest radiator is 48’. That gives me a fill pressure of 25.784. I called the manufacturer. They said I can install up to a 50lb pressure relief valve.
• Member Posts: 23,652
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48 feet is right up there. True, that is about 26 psi -- but if the manufacturer of the boiler is content with a 50 psi relief valve, and the boiler is labelled to handle the pressure, I'd add about five pounds... and shoot for 30 psi at the expansion tank and the inlet to the circulating pump.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 22,479
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Also be sure the expansion tank is sized properly to that fill pressure.
use the commercial tank size at the Amtrol site

Tank pre-charge needs to be set also.

Building height above tank times .5 is a easy number to calculate with.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 15,840
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@BigErl

When you replace the relief valve make sure that the BTU rating of the relief valve is large enough. It should meet or exceed the rating on the boiler name plate

Also, any boiler with a relief valve over 30 psi is supposed to have a ASME rated expansion tank.
• Member Posts: 38
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Thanks for all the replies. I have a follow up question about setting the expansion tank precharge. There are 2 b&g 60 tanks. Should I set both tanks to the system fill pressure or split the difference between the 2?
• Member Posts: 7,607
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Both should be at the fill pressure.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 884
edited October 2019
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In my state (Colorado, according to my state boiler inspector), if we exceed 30 lb on the relief valve we have to install ASME rated expansion tanks, and they are VERY pricey, I would check with your AHJ to make sure.

Last one I did, the ASME tank was 4 TIMES the cost of the non-ASME tank.