Boiler Pressure Varies/Expansion Tank not reducing pressure

Old 10-21-05, 04:37 PM
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Boiler Pressure Varies/Expansion Tank not reducing pressure

I have a Weil McClain 468 oil fired boiler for a single zone single level hot water baseboard heat with a tankless water heater. The circulator pump is on the return side. On the outlet side the water first flows through a 1 inch check valve and then into an American air purge both of which are close to the boiler. A 12PSI bladder type expansion tank is attached to the bottom of the air purge. An automatic air vent is installled on top of the air purge. The water supply to the auto water feed has been closed off. The system has worked well for 8 years regulating the boiler pressure; however, now the pressure is no longer controlled. When the boiler has been off for a while the pressure drops to 5PSI and when the boiler is ON the pressure rises to 20PSI. (I drained some water to keep the pressure in this range) The pressure cycles like this as the boiler heats up and cools down. The expansion valve has never discharged any water. The expansion tank does not seem to be taking on water when the system heats up as the top of the tank does not get very hot. I've bypassed the check valve by loosening the nut on the top completely counter clockwise with no effect. I've replaced the expansion tank with no effect, and the old tank seems fine also with a full charge, no water, and the bladder is at the top under pressure. The heat is working and water is flowing through the system. Can there be something wrong with the air purge to which the expansion tank is attached? The boiler manual says a air vent should be installed on the boiler in the 3/4" NPT fitting next to the outlet pipe when using a bladder type expansion tank. There is no vent here. The air vent is located on top of the air purge device. Thanks in advance for any suggestions.
Old 10-21-05, 06:51 PM
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If it is working without problems, leave it alone. "If it ain't broke, don't fix it".
Old 10-21-05, 07:50 PM
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It sounds weird.

I'd move the circulator to the other side and install an air vent to the boiler as it says to also you need the water on to maintain water pressure. If the pressure begins to blow off the safety then you will need to either install a larger tank or double it up. Check your diaphgram tank pressure 12PSI when you have the boiler water pressure at zero. The regulator should be set to 12 also if you have a 3 story bldg, use 18 instead of 12 for both.
see this diagram only its for a Burnham boiler.........

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