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Member Posts: 38
Homeowner here so I apologize in advance for my ignorance on this

Three somewhat related questions:
1). When finalizing the pressure on a radiant system, should the cold system pressure match the expansion tank charge (12 psi)?
2) Does the working pressure of the system change the gpm that a circulator can produce? I.e. All other things equal does a circulator produce the same flow rate in a system running at 20psi vs say 12psi
3) Is there a typical recommend pressure for a one story house?

Thank you. I've learned so much on the various discussions on this forum.

• Member Posts: 22,143
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12 psi is fine for one or two story homes.

Ideally the static fill pressure is adjusted to provide 5 psi at the highest pipe or heat emitter in the building. .433 psi to lift water 1 foot up, round that to .5 and the math is easy to do in your head.

The fill pressure you use assures the system is filled to the top, the circulator does not lift the water up to the top.

So if the high point is 24' above the pressure gauge at the boiler, 12 psi covers the elevation and some positive pressure at the top. here is a graphic I use to get a visual.

It also shows a piping layout that will be tough to keep air free with a bunch of up and downs
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 38
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Thanks for your reply, I think what you are saying is that 1) 12 psi should work fine for a two story and 2) system psi has no bearing on pump speed

Thanks again,

Allen
• Member Posts: 22,143
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Experienced troubleshooters will sometimes increase fill pressure to get rid of a problematic air condition. Increasing pressure squeezes air pockets smaller, often just enough so they can flow back to the central purger.

But know that increasing fill pressure reduces the acceptance of the expansion tank. You might rise above the pressure relief valve setting, typically 30 psi, when you hit high operating temperatures with high fill pressure.

Boiler pressure relief valves may start to seep within 2 psi of relief pressure, so around 27- 28 psi a 30 lb relief valves will often start to dribble.

If the system heats and operates quietly at 12 psi, no advantage to a 20 psi operating pressure.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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What type of boiler do you have?
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
• Member Posts: 38
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It's a Lochinvar Noble
• Member Posts: 2,646
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You are all set at 12 psi.
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
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Thank you!
• Member Posts: 38
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As a follow up to this...

When the system is cool pressure is about 14 psi. It's been cold recently here and so the boiler setpoint is 140F. Operating pressure when hot is 20-21 psi.

Is It better to make adjustments to keep it closer to the idle pressure or does it not really make a difference?

I know that increased heat will increase pressure, but I would have thought the expansion tank would have kept pressure steady.

Thanks and happy New year!
• Member Posts: 1,048
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Your fine were the pressure is at.
D
• Member Posts: 2,646
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You are correct that the expansion tank should keep pressure fairly consistent. There are formulas for total system volume, and total temp swing of the system to size tanks correctly. Yours may be a bit on the small end for your system volume, but an operating pressure of 21 psi is perfectly acceptable. You still have another 9 psi as a margin of error per se.
Serving Northern Maine HVAC & Controls. I burn wood, it smells good!