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Calvin Taylor
Furnace trouble
Calvin Taylor

I have a hot water heating system. The boiler constantly blows water out the overflow past the presure release valve. All valves have been replaced. This system is about 40 years old but has been completely rebuilt except for the boiler. Please help!

Re: Furnace trouble


Water blowing out the pressure release valve (PRV) is a common problem in HW heating systems.

You have a BOILER, btw, and not a furnace.

The PRV is a safety device built into the system to prevent the boiler from causing worse problems.

Water EXPANDS about 4% when it is heated to the 180 degrees usually seen in residential boilers.

This extra water needs a place to go, so there is an expansion tank 1/2 filled with air that acts as a spring, & absorbs the extra water until the system cools down.

Could you advise what type of expansion tank you have; one kind is a long green steel one that is propped up between the floor joists of the boiler room ceiling.

The other type of expansion tank is a smaller tank that looks like a 20 lb. propane tank they use for barbecue grills.

Could you also try to find the rated btu heat output of the boiler; this may be posted on the boiler.

If it's not there, could you provide the total square footage of the area the boiler is heating.

If you have the old style expansion tank it may well be "waterlogged", but is an easy matter to get the water out & get things back to normal.

There is another post on this forum in the "heating" section labeled "Hot water circulation problem"

Go to post #11 and read the listed sites concerning expansion tanks to get some background info.

Please post back.

Calvin Taylor
Re: Furnace trouble
Calvin Taylor

Sorry it has taken me so long to reply. I have the old green expansion tank. I emptyed it as suggested and now there seems to be no problem. Thanks so much for the info.

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