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cparke 05-20-2008 10:17 PM

boiler / Over-pressurizing
It is the end of the heating season, so of course now is the time when a problem crops up in my heating boiler system.


I have an old gas boiler system with circulation pumps and an overhead expansion tank. The incoming water line is regulated by a relatively new pressure regulating valve apparently set to 10 (units). The system is over-pressurizing when heating up to 190 degrees and the relief valve is dripping out water as the system pressure exceeds 30 (units). When the system shuts off because we have warmer weather outside and cools down to room temperature, the pressure in the system must be dropping to below 10 (units) and therefore more water is added. Then, the system comes back on again because the temperatures outside dropped back down again, and the peak pressure is again exceeded and water comes out the relief valve again. I cannot see the top of the expansion tank to see if there is some kind of air pressuring valve or a pipe to take excess water automatically out of the system. However, the expansion tank does have a manual valve for me to hook up a garden hose to drain water out of the tank if I need to.

As a quick fix, I shut off the incoming water valve, drained the expansion tank to 5 units, then started up the system and manually drained the expansion tank more each time the pressure approached 30 units so that pressure would return to a safe 15 units and the system could continue heating up to 190 degrees. Pressure now seems to be oscillating between 10 units and 15 units on its own as the system is running through the heating cycles, incoming water remains turned off since I dont see the need to add any new water into the system at this time when there seems to be no way for water to get out.

My questions:

Is the pressure range of 5-30+ units of pressure for heating water from room temperature to 190 dregrees F seem proper? With the incoming water line turned off as this is happening, where is the excess water coming from?

How is water normally supposed to exit the system? As far as I can tell, there is no pipeline to automatically take water out of the system. Is there supposed to be a second pressure regulating valve for an exit line somewhere?

How is air supposed to get into a sealed pipe system like this? It seems the expansion tank must be allowing air in and out somehow, or else how could it ever get filled or emptied? Is the air in the tank supposed to be pressure regulated too, and could a fault there have something to do with my problem?

What is likely the faulty component in my system? Why is this just happening to me now (after more than 5 seasons of use) for the first time? Is there a way I can fix it?

Am I required to bleed and flush the system annually? (cause I know I haven't been doing that)

Thanks so much for any thoughts or other assistance as I try to figure this important home system out.

8 Ball 05-21-2008 05:54 AM

Most smaller hot water heating systems operate at 12psi idle, no pump running. When at full temp 180deg, the pressure should not exceed 15-20psi.

The water pressure regulator is factory preset to allow water to pass, or fill until 12 psi is reflected in your system.

An expansion tank is desgned to accept water as the water in your system heats up and expands.. If you have an overhead tank, the air trapped above the water acts like a cushion, and forces the water back into the system as it cools. If there is no air cushion, the heated water has nowhere to go, or no room for expansion, and the system will over pressurize.

Hot water systems should not need to have water added, or be flushed annualy. Oxygen in the water does nasty things when excessive.

You are apparently losing the air from the expansion tank due to a leak. If the tank is more than 20yrs old or has a sightglass, more than likely one of these two are the problem. If there is a pinhole in the tank replace it. If the sightglass is leaking, it can easily be repaired.

You may want to have the pressure relief tested also. It sounds like its doing its job, but it is the most critical safety device on your system and under NO circumstances should it be restricted or plugged.

If you have a bladder type expansion tank it probably needs to be replaced.

Hope this helps.

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