I have a gas boiler which feeds a pressurized heating system and pressurized hot water tank with no header tanks involved.
There is a pressure guage that displays pressure in the heating system. This drops in pressure gradually, and can be repressurized by small valves to let more water in.
I am giving it 5-10 seconds open every 2-3 days which my plumber estimates is 2-3 pints. I have no sign of leaks at all! Where is it going? No sign from pressure relief overflow, no sign from any pipes. I have only about 1 metre of pipe under downstairs floor boards so easy to check. All other pipes would flood carpet or tiles - no sign at all. When pressure drops I get banging from the pipes when run downstairs shower in the morning as pipes heat up (presumably just air in system replacing water lost).

Andrew D, October 2008

have you checked the expansion vessel?

paul the plumber, November 2008