Furnace pressure question


Hi all,
My house is heated with hot water baseboard radiators. The hot water is supplied by a boiler furnace with circulation pump. The furnace has an automatic fill valve that is kept in the off position unless the system needs water. It is them manually opened until proper pressure is achieved. The burner comes on at about 150 degrees and goes out at 160 degrees. I try to maintain about 18psi at 160 degrees system pressure. The safety blow valve is set to blow at 35psi. It has an air purge valve at the top of furnace just below the floor above it (about 10 feet from basement floor). The air purge valve is new and so is the expansion tank. The expansion tank pressure seems to match the boiler pressure.
Here's the problem. Every couple of days the system pressure has dropped to about 10 or 11 psi at 160 degrees. This effects the efficiency of both the radiator heat as well as the potable hot water (which is heated bythe boiler). I open the fill valve, purge the air and close it all back up at 18psi at 160 degrees. All is well for another couple of days until the pressure if back down and I repeat the process.
I don't see any leaks, no water dripping from ceilings or pooling anywhere that I can see. One room of the house is a concrete slab floor (converted carport). It has heating pipe within the concrete slab.
I keep adding water and purging. What could cause the pressure to keep dropping? I'm out of ideas.
Thanks in advance,
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coustanis wrote:

Quote: "It has heating pipe within the concrete slab."
I bet you have answered your own question.
After the heating season is over, maybe look for a way to isolate that loop, then pressure test it for leaks.
Jim
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That's the answer I was trying to avoid. I can't imagine replaceing pipe within concrete.
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coustanis wrote:

An automatic fill valve is designed to function all on its own and should never be turned off.
It is them manually opened until proper

Likely the pipe in the slab is leaking.

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What kind of idiot puts pipe in the slab. It used to be a carport so the pipe may have been added afterwards. Maybe in a small trench or something. I guess I have to take the floating floor up. It was just installed last year. Crap.
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coustanis wrote:
<SNIP>

Maybe go with a Plan "B": If the loop is leaking, consider doing a perimeter baseboard section instead of disturbing the floor.
Jim
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Are you talking about running the pipe along the wall tucked into the corner where floor meets baseboard? I was wondering if that was a viable option. Then I could cover it with something attractive running the length. Is that doable?
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It may be sucking in air. When the water heats up it expands. When it cools, it contracts. Instead of bringing in water, since the fill valve is shut off, it my be sucking in air, then purging it, then the pressure will be lower. Try just leaving the fill valve open.
Copper tubing in concrete has been know to corrode. That may or may not be the problem. Can that portion be isolated? It may be a smart thing to do to test the loop anyway. If the loop loses pressure but the rest of the system maintains, you know you have a leak there.
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The fill valve may be defective but not positive on that. It has a manual valve sweated into the pipe before it and that's the valve that is off. I can open it but I can't tell where the proper pressure setting is on the auto fill valve....although I can try it and keep an eye on it this weekend. I'll just start at it's lowest setting and work my way up. If it hits 35psi it should blow out the safety valve. Where would it suck in air from that it wouldn't also leak from at times? Would this give a low pressure reading on the system? I really feel that the quantity of water is low during these times because you can really hear the water in the pipes upstairs, really loud. Sounds like, well....water running through a pipe. When pressure is good it is totally silent. I would have to drain the system and solder in valves to isolate that portion. A pain but I'll do it if I have to.
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