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,