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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 6022
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Boiler water pressure is 12 psi cold and expansion tank is

Customer Question

Boiler water pressure is 12 psi cold and expansion tank is 12 psi. When hot, boiler pressure goes to 21 psi. The pipes are very noisey, sounds like water boiling. The boiler is 23 years old, but other than the pipe noise, works well.
Submitted: 6 years ago.
Category: HVAC
Expert:  Douglas replied 6 years ago.
Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment.

The expansion tank may not be large enough but it sounds like there is still air in the system if you are hearing noise or the velocity is too high in the pipes. Fresh water is not always a good thing since it contains alot of oxygen. Do you have an air vent to let some of the air out of the system (not the expansion tank).
Customer: replied 6 years ago.

The expansion tank was replaced with exact replacement. Yes, the automatic vent is new. Also added a manual vent at the time and no air bleeds out. Previously, without the auto-fill/regulator, there was air in the lines which is why I added the valve. The boiler does have some minor perculation noise, which seems to be amplified through the pipes.

Expert:  Douglas replied 6 years ago.
can I ask what the boiler temperature is when it's hot? Is it over 200f?
Customer: replied 6 years ago.

It's set at 195 deg.

Expert:  Douglas replied 6 years ago.
Do you have radiators or baseboard fin tubes?
Customer: replied 6 years ago.
Baseboard fin tubes.
Expert:  Douglas replied 6 years ago.
do you have zone valves?
Customer: replied 6 years ago.
4 zones, mounted by the boiler along with flow valves
Expert:  Douglas replied 6 years ago.
does the noise occur when all zones are calling or only when one or two are?
Customer: replied 6 years ago.
All or one. Question though. When the boiler is hot and pressure is 21 lbs., wouldn't the expansion tank be useless seeing that it is at 15 lbs.
Expert:  Douglas replied 6 years ago.
The boiler pressure increases, its a function of the capacity of the expansion tank volume to absorb this. Was the expansion tank precharged? (pump it to 12 psig before attaching it to the boiler.
Customer: replied 6 years ago.
Expert:  Douglas replied 6 years ago.
ok... you may want to verify it. I would turn the boiler off, drain pressure off until you are below 5 psig. Use a tire pump to pump the expansion tank to 12 psig and re pressureize boiler and see if it changes anything.
Customer: replied 6 years ago.
OK. Does it matter if hot or cold? I'll have to wait till tomorrow, since I'm at work now. Will this answer still be available tomorrow?
Expert:  Douglas replied 6 years ago.
yes, i should be here...

Just get the pressure below 5 on the boiler (dont go to zero) it doesnt mater if the water is still hot. Just dont let the boiler run while you are doing this.
Customer: replied 6 years ago.
Thanks, Greg.
Expert:  Douglas replied 6 years ago.
No problem, let me know