Furnace pressure problem

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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Are you serious, you add water and then pressure goes down, then you add more water and pressure goes down, then you add more water and the pressure STILL goes down...
And you DON'T know what's happening????
Here is you answer: Your water is contracting till it disappears... try filling the unit to 16 P.S.I. not 18, this will make the water more stable.
Also remember, if your car tire will not hold the proper air pressure, try 2 P.S.I. less.
You believe me, RIGHT? :-)
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replace the thermostat if that doesnt work. Its almost always the thermostat. Bubba

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You probably have your relief valve piped into a drain, and can't see the boiler relieving. Either that or you have a hell of a leak somewhere.
-Canadian Heat
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<snip>.
I wouldn't necessarily agree that reducing the system pressure causes the radiators, or the potable hot water system to be efficient. If anything, it may negatively affect them. In other words, it may have a negative effect on the efficiency. (Learn that there is a difference between those two words) especially when you are trying to esplain somthin.

You will..... Or the mosquitos and mice are drinking it fast enough.

They call that a snow melting system where I come from, but in Chicago, I guess that's what they use heated carports for.

It's also possible that you have a chemistry issue, causing electrolysis - no... it's got nothing to do with naked beavers. You could be electrolyzing the water. What happens is that the oxygen and hydrogen atoms split from the water molecules. The oxygen becomes part of the internals of your system. It makes a new substance called corrosion. The hydrogen that's left over... well, what do you think you're purging. Next time you purge - your heating system, not your system - hold a lit match in the escaping gas. (You know... how they tell you not to look for a gas leak with a match... But it does work.) If you get a ball of almost invisible fire that rises rapidly to the ceiling over the heating system, that would be a good indication of electrolysis. Depending on how long you've been going though this cycle of fill and purge - again, your heating system - you may need a complete replacement. Wait... you might get help from your insurance company. When you esplain the fire to the arson investigator, just tell them it's ok. The experts on the InterNet said so.
If it's not electrolysis, it might be something else.

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<snip>.
I wouldn't necessarily agree that reducing the system pressure causes the radiators, or the potable hot water system to be efficient. If anything, it may negatively affect them. In other words, it may have a negative effect on the efficiency. (Learn that there is a difference between those two words) especially when you are trying to esplain somthin.

You will..... Or the mosquitos and mice are drinking it fast enough.

They call that a snow melting system where I come from, but in Chicago, I guess that's what they use heated carports for.

It's also possible that you have a chemistry issue, causing electrolysis - no... it's got nothing to do with naked beavers. You could be electrolyzing the water. What happens is that the oxygen and hydrogen atoms split from the water molecules. The oxygen becomes part of the internals of your system. It makes a new substance called corrosion. The hydrogen that's left over... well, what do you think you're purging. Next time you purge - your heating system, not your system - hold a lit match in the escaping gas. (You know... how they tell you not to look for a gas leak with a match... But it does work.) If you get a ball of almost invisible fire that rises rapidly to the ceiling over the heating system, that would be a good indication of electrolysis. Depending on how long you've been going though this cycle of fill and purge - again, your heating system - you may need a complete replacement. Wait... you might get help from your insurance company. When you esplain the fire to the arson investigator, just tell them it's ok. The experts on the InterNet said so.
If it's not electrolysis, it might be something else.

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