Radiator / furnace problem

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Hi all,
I posted this over in alt.hvac a couple of weeks ago and didn't get much help. Just a bunch of smart ass answers so I thought I'd try it here.
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 by the boiler). I open the fill valve, purge the air and close it all back up at 20psi or so at 160 degrees. All is well for another couple of days until the pressure is 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 and the pressure keeps dropping. It's an endless cycle. What could cause the pressure to keep dropping? I'm out of ideas.
Thanks in advance,
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You're not the first person to have that experience. :-)

www.heating-help.com is a *much* friendlier and more useful forum than alt.hvac.
The hot water

All sounds good so far.

You have a leak somewhere.

Proof that you have a leak. Air can't get in if water can't get out.

That pressure must be going somewhere. You have a leak. You just haven't found it yet.

That's good...

.. but that's bad. That's where your leak is, I'm afraid.

Do you have any way of shutting off the water flow to the room on the slab? The hot water system in my house has multiple heating zones, with thermostatically controlled zone valves to control water flow into each zone. If yours is set up similarly, closing the zone valve to that room, while operating the system to heat the rest of the house, would help indicate where the problem is. However, even without doing that test, I think it's a pretty good bet that's where your leak is: in the slab.
--
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Doug Miller (alphageek at milmac dot com)
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On Jun 8, 9:40 am, snipped-for-privacy@milmac.com (Doug Miller) wrote:

No but I could drain the system and install valves (if I can get to the pipe). That will break the loop, right? I'd have to add pipe across to complete the circuit?
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No way for me to answer that without seeing how your existing pipes are laid out. As long as there is at least one loop *somewhere* that the circulator can pump water through, it doesn't necessarily have to bypass that room.
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Doug Miller (alphageek at milmac dot com)
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On Jun 8, 9:40 am, snipped-for-privacy@milmac.com (Doug Miller) wrote:

Not necessarily
If the expansion tank doesn't have a bladder to keep the air from contacting the water you can develop an air problem. http://www.bellgossett.com/Press/BG-proper.asp
I agree a leak is the most likely cause but it could be other things. Rich Trethewey explained a way the layout of a system can cause this problem on Ask TOH but I can't find it online.
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No, that's not his problem. He says he has to keep purging air from the system. That's unambiguous: he has a leak. Air can't get in, unless water's getting out.
Further, if the problem were simply air dissolving into the system from a non-bladder-type expansion tank, eventually that tank would become waterlogged. Everything the OP has written indicates that has not happened.
He has a leak.
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Doug Miller (alphageek at milmac dot com)
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leave it at 10 psi cold, you only need more if top floor radiators are cold, I have a 3 story boiler is 12lb cold.
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Is this a troll post? Your hot water is heated by this unit, you keep the "automatic" fill valve closed, and you can't figure why the pressure is dropping. You need to stop taking those hot showers, man , this is SUMMER. Crank up the heat and enjoy an ice-cold shower!
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On second thought, this has to be a separate loop, so excuse me- still, auto fill valve should be open. But yeah, must be leak some place.
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Oops. I responded to your first post before I read this. I bet it's in the slab. I just had the wood floor put in last year. CRAP. The water must be going down under the slab because I don't see water anywhere. Do you suppose it's washing the dirt out from under the concrete?
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Yep.
Unfortunately.
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Doug Miller (alphageek at milmac dot com)
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On Jun 8, 11:15 am, snipped-for-privacy@milmac.com (Doug Miller) wrote:

I wonder if my homeowners insurance will cover it. Then they can raise my rates or drop me.
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That would be a question for your insurance agent. My guess is that the insurance would pay to repair damage caused by the leak, but not for the cost of repairing the leak itself.
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Doug Miller (alphageek at milmac dot com)
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No, the auto fill valve should *not* be open. Multiple reasons:
1) Doing so continuously exposes the pressure regulator on the fill side to the full pressure of the water supply, and can shorten its life.
2) With the auto fill valve open, hidden leaks may go undetected for a long time, as the lost water is continously replaced.
3) Any failure (such as a ruptured pipe or valve) will result in continous flow of water until the problem is detected and the fill valve closed.
The OP is operating his system correctly, by opening the fill valve only when he knows he needs to fill the system.
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Doug Miller (alphageek at milmac dot com)
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On Fri, 08 Jun 2007 15:05:47 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

I"m sure you're right, based on all your reasosn, but why do they still call it an AUTO fill valve? Isn't that likely to confuse lots of people into leaving it open, with just the bad results you list?
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The B & G fill valve that I see on most installations is an auto fill valve with a lever for fast fill, and a screw to adjust the fill pressure. I would agree that the "fast-fill" lever shouldn't be left open, but would disagree that the water supply leading to the fill valve should be left off. Why even have it in place if it is only used to fill the boiler manually when the pressure is low. It would be nearly the same amount of effort to open a valve while watching the gauge on the boiler when you needed to fill it.
Of course, if I posted this at alt.hvac, they would be laughing their collective asses off, whether I was right or wrong.
JK
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No, this is not a troll post. I don't understand your response unless you are thinking that the same water that circulates through the rads is the same hot water that comes out of the tap. That is not the case. The boiler has a coil of copper pipe running through it that heats the potable water within. Two entirely separate circuits.
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When the system cools down, the water that was expanded from heating now shrinks and the system will suck in air through the valve on the expansion tank. Try leaving the feed valve open and it will solve your problem.

Expansion and contraction
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That is not correct. Air purge valves are one-way. They let air out, but not in.

Nope. He has a leak.
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Doug Miller (alphageek at milmac dot com)
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Maybe he does, but I don't and if I close the valve I have the same problem. It has been open for the past 25 years and if I had a leak, I'd guess I'd have found it by now.
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