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Ideal Logic 35 Boiler Losing pressure *Solved*

Evening All
I used this forum over the last month to research my problem with the Logic 35 boiler losing pressure.

Background - My 4 year old boiler began losing pressure at around 0.3-0.5 bar every day about 4 weeks ago. Like any worried home owner i checked the following things:
1. PRV - tied a bag over to see if there was any water collected. None
2. Condenser pip was bone dry.
3. No obvious radiator leaks or towel rail leaks. (7 rads + 2 towel rails)
4. No Air lock in the system - I bled all the rads
5. No visible leaks in or around the boiler.
6. Isolated the boiler over night to see if the pressure dropped - no Luck.

I was using the filling loop daily to refill the pressure to 1.25bar and it became annoying. I had taken out homeserve cover last year so i called them out and the engineer said immediately there was a CH leak, (god knows how he did this without taking the boiler cover off). He then left and i was refunded my excess.

Two days later i called them again to say it was still leaking so they sent out another engineer. When the engineer came, Using my own research from this forum i asked him to check if the heat engine was leaking. So to my knowledge there is a heat exchanger and a heat engine, the exchanger is a smaller part at the bottom of the boiler and the engine the larger part making up most of the boiler. The condenser inspection cover was removed and a few drops of water were visible. The engineer said he'll speak to his manager to see if this was enough evidence to replace the Heat engine. 3 days later the engineer ordered the new part and spent around 3 hours fitting it.

3 days later the pressure seems to have held at around 1.2 bar. I haven't had to refill it and the heating system seems a lot quieter too. The engineer couldn't find any visible damage to the part he changed, so my theory is whilst the heating was on and under increased pressure it was leaking down the condenser trap and away through the drainage system.

I hope this helps someone with the same problem and felt it was important to share this as Homeserve weren't too helpful in identifying the problem, i kind of had to show them what to check. Apparently its a common fault and not a cheap one to fix if you have no cover with a company.

i'll continue to monitor the pressure over the next month and report back if the problem returns, but for now it seems to be solved.

Big thank you to all the people who posted indicating the typical areas to check.
It's a common problem I've replaced loads of these.
The homeserve chap said the same but i can understand from their point of view they don’t want to start with the most costly option, i was strongly against leak sealed as i didn’t feel there were any leaks on the CH side