changing PSI

simon hauger

I recently moved and this house has a 150,000 BTU gas hot water boiler. In my last house, the system pressure stayed around 15PSI. This system pressure varies from 5 PSI when the boiler has been off for awhile, to 25PSI when the boiler has been on for awhile. I have bled all the radiators and have checked them since. Non of radiators have air in them.
Is the changing PSI normal??

oil-2-4-6-gas

.

sounds like a cold start boiler--but thats a big differential --is the expansion tank large enough ? thats what i'd look at -though the 5 psi on start sounds low--1 story ranch house? -when a boiler starts cold all of the water in the system is getting heated---instead of the warm start boiler where just the water in the heat emmitters ---sounds like someone dropped the cold pressure to not blow off the relief valve essentially getting around having an undersized expansion tank or one that is losing its charge bladder type,or old style tank losing its air ---------any signs of leakage from the relief valve in the past ?

Kyle Kubs

expansion tank

No, that kind of pressure fluctuation is not normal and approaching dangerous. Check that the expansion tank is not waterlogged - There are two common types that you see - The old ones were usually set up between or just under the floor joists. They had space for air that was in contact with the water and the air charge would be lost as it was absorbed into the water. The newer ones are typically piped in right at the boiler and have a rubber bladder inside to prevent air loss due to absorbtion - The easy way to check is when the boiler is hot, feel the tank. Only the end near the boiler piping should be hot - If the far end is just as hot, that is cause it is filled with water which cannot be commpressed as the water in the system expands causing the high pressure - also you can open the schrader (like a car tire valve) valve and see if water comes out - Oil 2-4-6-gas is right 5psi to start is pretty low - they typically start at 12 - and if the expansion tank is not waterlogged, it may be undersized... Good luck

Kyle Kubs

simon hauger

THANKS FOR THE HELP!!
The expansion tank is the old style - it is right below the floor joists. The house is two story so I was told that the pressure in the system should be around 15psi. The relief valve has discharged recently because I found a puddle of water under it. I believe it opens at 30psi. Today my pipes were banging for the first time - they sound like they have air in them. I am assuming that since I bled the radiators about a month ago, that the air must have come in via the pressure valve release or the expansion tank.
Thanks again for the help. I really appreciate it.