Normal/abnormal Boiler pressure

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Old 01-05-11, 06:25 PM
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Normal/abnormal Boiler pressure

Hello,

I have a 7 zone residential hot water baseboard system. When the system is idle (all zones off) the pressure sits around 10 to 12 psi. When any one or more zones is activated, the pressure drops off to below 5 psi until the boiler temperature increases, then the pressure will stabilize at 12 psi.

I have done some troubleshooting and found that....if the boiler temp is at the low limt...(just before the furnace kicks in (160), then any zone that is turned on will cause the pressure to drop well below 5 psi (sometimes it looks like 2 psi) until such time the boiler temp rises...in fact you can see the pressure rise as the temperature increases.

If the zone is activated at the high temp cutoff of 180-200 then the pressure stays @ around 10 psi or so.

I do not hear any air in the system...the circulation pump is running normally and with my ear up to it I cannot hear any cavitation.

The expansion tank was changed out last year....but it does feel hot when the system is running.

thank you

Jim
 
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Old 01-05-11, 07:19 PM
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Cool the system down to 100 deg, or so, and raise the pressure to 12-14 psi. Something is wrong with your system or your gauge.
 
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Old 01-05-11, 07:48 PM
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Not necessarily Mike...

Jim, Is your circulator pump on the SUPPLY OUT of the boiler, and is the pump pumping TOWARD the connection for the expansion tank?
 
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Old 01-05-11, 08:47 PM
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Hello NJ tropper...yes...the circulator pump is on the supply out side of the boiler and is pumping toward the expansion tank then up to the zone valves.
 
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Old 01-05-11, 09:17 PM
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Then in that case, what you are seeing is normal and expected. If the system is heating OK, and there are no other issues... with air in the system, or with the relief valve opening, then I wouldn't worry about it at all.

If you wanted to 'fix' the 'problem' (and I use those terms loosely), you would have to do some repiping. Swapping the postions of the pump and the expansion tank would stop the pressure gauge from doing that.

Let me try to explain a little... getting late so I'll try to use as few words as possible... not easy for me!

When a pump runs, there is naturally a pressure difference across that pump.

The pressure in the system at the point that the expansion tank is connected is not affected by the pump running. It is known as the POINT OF NO PRESSURE CHANGE (PONPC).

If the pump is pumping TOWARD the expansion tank connection, the difference in pressure between the suction and discharge of the pump will be SUBTRACTED from the pressure on the SUCTION side. (because the discharge side is at the PONPC) This is why you are seeing the pressure on the boiler gauge (on the suction side of the pump) drop when the pump runs.

If the pump is pumping AWAY from the expansion tank, the differential pressure across the pump is ADDED to the pressure on the DISCHARGE side. (because the suction side is at the PONPC). If you placed a gauge at the discharge of the pump, you would see an INCREASE that would be EQUAL in magnitude to the decrease that you see in the boiler.

I know it's confusing... google the term (in quotes) "pumping away" (and ignore any hits for porn sites) for more information about this concept.
 
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Old 01-05-11, 09:33 PM
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Hello NJ trooper

Thank you for the explanation, it actually makes a lot of sense to me now.

Thanks again for taking the time.

-Jim
 
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Old 01-05-11, 09:35 PM
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Trooper I give you credit. Most plumbers dont know this and still pipe boilers with the circ how its packaged on the return... Yes 2 out of 10 plumbers I know move the circ to pump away... the others are clueless..

And your not a plumber.... Your one of them there smart ones....

My two cents

Mike NJN
 
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