If you do not see any steam coming from the vent cap at the end of each warmup cycle (Do you? Are you sure it's not there? Sounds like you've got a lot of exposed surface if you're hearing boiling when you refill it), then I'd have to think you've got a plumbing leak somewhere. (Can't be in a hot water heater sideleg, or you'd be losing water pressure and overfilling the furnace, due to the pressure differential.) Since you're not seeing anything in the house (Are you? Have you checked thoroughly?), it would have to be either within your furnace, or in your underground line. If within the furnace, I'd suspect a leak into the firebox, where it's being vaporized and sent up the chimney, because with the volumes you're describing, you should see water/ice draining onto the ground otherwise. An in-ground leak would be harder to locate - others may have suggestions there. For a start, if you have snow on the ground, I'd look for any spot where the snow is disappearing, or quite thin (similar to the snow over most septic tanks, where heat loss from domestic water and decomposition keeps the ground thawed all winter, if the tank is working properly).
With my system mostly set to run at 185 degrees, I've lost about 10 litres(just under 3 US gallons) in 3 years of operation, and most of that happened when I had an air leak at the primary solenoid (hanging up), resulting in tank boiling after warmups).
To recap, water loss possibilities:
- boiling it off (most likely)
- losing it in house (easy to check, not likely without evidence)
- losing it in furnace due to crack or pinhole (hard to detect, unless you cool down and look for dribbles in the firebox, or heat exchanger)
- losing it in ground (hard to check, painful to fix)
If you do have a leak somewhere, I have to ask - what's your water PH? Have you tested regularly? Or is this a corrosion result?
Other possibilities may exist, depending on your system particulars.
Blair
(apologies, most of this is obvious, but it's good to cover all the bases).