bolier losing pressure but cant find any leaks

Hi there, Depending how your pipework is configured, I would start by draining down the system. I'd then install a isolation valve at the inlet and outlet of the water jacket close as possible to your boiler, they may already be in place. Id also install a pressure gauge somewhere close to the isolator valves, water jacket side, not boiler side. I'd then recharge the system as per normal. Bleed and make sure the system is operating as per usual. Id then re-open the mains inlet valve and take the pressure within the jacket to say 2.5bar. Id then close the two newly installed isolation valves and switch off the system. What this will achieve is that the water jacket (radiators & pipework) will now be physically isolated from the boiler. Monitor the pressure, if it falls then its water jacket side, if it doesn't its the boiler. Working on the assumption that the radiators and pipework are integral (have no leaks) then over to the boiler. If it was a PRV issue the problem would be self evident, the PRV discharge pipe would be blowing off externally. My guess, I would try and disconnecting the condensator discharge pipe, it usually discharges into the nearest waste pipework. Its usually plastic and often push fitted into the pipework with a rubber grommet seal. If you can disconnect it, operate the system and monitor the flow. Normally you may see a few drop at most, any more than that, and you've found the culprit. Long winded, but I hope this assists. Regards Neil

This could be a tricky one to work out I'm afraid. My initial thoughts would be that you have a leaking pipe somewhere so tracing these pipes and examining them is the obvious starting point. The catch here is that much of this pipework will be concealed within walls or underneath floorboards so you may find yourself looking for evidence of a leak rather than being able to get sight of the pipe. There may be a faulty component or a leak in the boiler itself but as you've had it recently serviced the engineer would've spotted something I'm sure. You seem to know what's what when looking to self diagnose and fix these sort of issues having bled the radiators and checked the expansion vessel. Equally if the PRV isn't discharging water then my thoughts go right back to the leak in the system pipework. It might be worth trying a leak sealing solution - Fernox F4 or similar is worth trying - it is put into the system and finds it's way into leaks and seals them and hence could quite possibly be a quick and easy fix. I'd try this first of all - its cheap and fairly easy. Hope this is useful. Matt.

Call a professional, please. Money well spent. The previous answer is good, lots of detail, but sometimes it's better to have time-served veterans to ease your pain.