I have a MAIN 30 HE boiler and it loses pressure when the heating is on, seems to be fine and no loss of pressure if the heating is not on or only use hot water. Would this indicate a leak somewhere, iv'e tried to read up on it but unsure of the answer still. Anyone able to advise where to look, on the pipe outside for any drips? Iv'e checked the radiators and can see no sign or any water/leak. It will take two days of heating for it to drop 0.5 bar. Really appreciate any help on this one :)
Your expansion tank may be the problem.
An expansion tank or expansion vessel is a small tank used to protect closed (not open to atmospheric pressure) water heating systems and domestic hot water systems from excessive pressure. The tank is partially filled with air, whose compressibility cushions shock caused by water hammer and absorbs excess water pressure caused by thermal expansion.
If it's busted you will have pressure fluctuations in the heating circuit.
I would look inside the unit. Sometimes leaks in heat exchangers close up in the off cycle. Also, is there automatic fill on this or is it separated from cold water supply? Does the system contain glycol?
Is there a sight glass looking into the burner compartment? If so get a picture through there if you can. Also if you can isolate the heating loops from the boiler one at a time, would help to eliminate a leak in the field. If no-heat would be catastrophic, I would definitely have someone out to take it apart and look for evidence of weeping on the inside. If no other leaks can be found, though pinholes can be hard to detect. some technicians carry ultrasonic leak detectors, might be worth asking around about that.