Boiler Operating Pressures


Old 12-02-08, 11:12 AM
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Boiler Operating Pressures

I have a cast iron boiler with 4 zone valves. The limit switch is set at 160F(Low)-180F(High). The gauge reads around 11-12 psi(Low) and cuts off at 19 psi(High). Had a new expansion tank installed but was wondering if there should be consistant pressure on the gauge or is it quite normal that when the boiler is firing the pressure is going up and dropping when it it hits the LL limit switch? The PSV is set for 25 PSI but also curious if 11-12Psi could be a warning for low water level? Any advice as I am trying to figure this out. This is on a 4-plex.
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Old 12-02-08, 01:37 PM
Join Date: Oct 2008
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The system pressure will normally increase several psi as the boiler heats up. 12-15 psi when the boiler is cold is typical for a 2-story house with the boiler in the basement.

The cold pressure is established by the setting on the automatic feedwater pressure regulator that should be connected to the city water supply. The hot pressure increase depends on the size of the expansion tank in relation to the total volume of the system.

Low water level is not indicated by your numbers. Low level is pretty difficult to come by with a hot-water system that is properly filled, etc.
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