Boiler Pressure

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billw
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Post by billw » Thu. Jan. 29, 2009 7:05 pm

Last summer there were a few lively conversations about putting pumps on the returns vs the feed. I'm not a plumber so I didn't understand a lot of the conversations. I put my pumps on the returns mostly because that was the way my old system was connected and it worked fine.

I expected that my boiler pressure would go up when the zones kicked on. I have noticed the opposite effect. At rest with the water temp at 180 my pressure is 22 lb. When my zones kick on the pressure goes down, not up like I expected. When both my first and second floor zones kick on the boiler pressure drops to 14 lb. when the boiler temp is 180. I've seen my boiler temp drop to 140 when both zones were calling for heat and the pressure was at 12 lb.

Did I misunderstand those conversations or is something else going on? There isn't a problem with the boiler. I just like knowing as much as I can about my system.

 
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coaledsweat
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Post by coaledsweat » Thu. Jan. 29, 2009 8:51 pm

Your boiler and system have a fixed volume of water in them. When you circulate the water and it loses heat to your house, its total volume shrinks as it cools and reduces the pressure. The pump stops, the water heats up and you gain back the pressure that was lost and the process repeats. Two zones means greater heat loss and thus a greater pressure loss.
Sound right?

 
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billw
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Post by billw » Thu. Jan. 29, 2009 8:57 pm

That makes sense, water shrinks as it cools until it freezes, then it expands. Thanks. I was expecting the pressure in the boiler to go up when the zones kicked on because I was pushing water into the boiler. I based that thought on all of the pumping away/pumping into stuff last summer.

 
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coaledsweat
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Post by coaledsweat » Thu. Jan. 29, 2009 9:32 pm

billw wrote:I was expecting the pressure in the boiler to go up when the zones kicked on because I was pushing water into the boiler.
The system is sealed so there is only a very small pressure differential in front of and behind the pump when it runs, maybe 2# at most. If the system were open and not under pressure, vented at the top say, it would behave as you thought earlier. The discharge side of the pump would be under pressure and the return would be under suction.
It goes back to Bernoulli's Law.


 
braindead
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Post by braindead » Thu. Jan. 29, 2009 11:16 pm

Seems like a big pressure change. Shouldn't the expansion tank compensate for that?

 
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Freddy
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Post by Freddy » Fri. Jan. 30, 2009 6:59 am

All info so far pretty much covers it. That is quite a large swing in pressure though. I might add some water so it ranges from 14 to 24 or 15 to 29. For a two story house 10 is absolute minimum. What do you have for an expansion tank? If bladder type, OK, but if old type with no bladder, maybe it's time to drain it.

 
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billw
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Post by billw » Fri. Jan. 30, 2009 7:31 am

I have two extrol 30's on the system. The one on the coal boiler is new the one on the oil boiler is about 14 years old. The pressure regulator is set at 12lb when the system is cold. The feed water is always on so any make up water should be automatic. It's a two story house with the heating system in the basement. I guess the high point for the radiators is about 20 feet above the boiler. Like I said I was expecting the pressure to increase when the zone pumps came on instead of decreasing because I'm pumping into the boiler but I guess I misunderstood those pumping away conversations from last summer.

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