System over pressuring

Old 11-22-09, 05:48 AM
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System over pressuring


I have a Slant Fin boiler and recently the pressure in the heating system is fluctuating over a wide range (5 to 25 psi). When the boiler is shut down the pressure is around 5 psi and when the boiler kicks in and the temperature is increased from 19 degrees centigrade to 21.5, the pressure mounts to 20 psi. Initially I was told that the pressure reducing valve is not working and that the pressure in the system when it is cold should be 15 psi. I increased the pressure in the system manually to 15 psi, however that resulted in the pressure releif valve opening to release the pressure which went over 30 psi when the boiler was operating. The pressure in the expansion tank is 7 psi (should be 15 psi) and the tank is cold from top to bottom when the system is operating, does that mean that the tank is waterlogged. The tank I have is branded Watts and has a diphragm that seperates the water form the air. I got two different technicians in to chek the system and non could give me a convincing reason for the problem I have. Can someone advise me what should the pressure be in the system when the system is shut down, should it be 0 or 15 psi?

Old 11-22-09, 11:51 AM
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What kind of techs are them? It's obviously water logged. Water does not compress. And heat raises the pressure. If you have one of those grill propane tank looking expansion tanks that end in 'trol, it is a bladder tank. New ones come preset to 12 psi. Bladder either got a leak, or the shrader valve let some air out over time.

You can drain back your system by opening the boiler drain valve to get to 0 psi. If the bladder tank also goes to 0, the tank has discharged. With a good tank, even if you drain down the system to 0 psi, the bladder tank should still say 12 psi.

Then try to add air and see what happens, with air compressor or bicycle hand pump. This will fix you up either permanently or for a while(if you have leak). If the bladder is shot, then water may even discharge from the shrader valve if you test push it, once you charge it. If you find out the shrader valve is not how it lost it's air -(you can test the valve by putting some spit on it and see if any bubbles form ), - then it has a bladder leak, and you need to replace it. Easy task.

I just did one and did it quickly where little water was lost from the piping above. Afterwards, you can bleed the radiators.

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