I have a Saunier Duval Thelia Twin 28E combination boiler which was installed when the house was built seven years ago. Over the last 3/4 years it has caused us a lot of problems usually due to lack of pressure but for the last 8 months (since it was last repaired) it has been okay.
Now it is loosing pressure so quickly that it needs to be topped up every 3/4 hours. I have placed a bucket underneath and this is catching water leaking from an overflow pipe under the boiler. There is also an overflow pipe outside but this is dry. The boiler only seems to leak when I initially top up the pressure, it then settles down, but after 3/4 hours has lost so much pressure that it cuts out.
I have found that if I turn down the CH thermostat so that the boiler is off and ensure no hot water is being used, then I allow water in to top the pressure up, the pressure still falls slightly over a short period e.g. 15 minutes even though the boiler isn't being used. During this period no water is seen to be coming out of the overflow pipe under the boiler. Initially, this suggested to me that I had a leak in the CH system and so checked all the radiators and pipes in the house but couldn't see any evidence of this. The only pipes I can't check are those under the downstairs floorboards. I would expect to see damp patches in the ceiling if pipes between the floors were leaking. Besides, if there was a leak in the CH system then surely this would result in the highest radiators (in the loft) having "cold patches" where air had replaced the lost water. This is not the case, in fact, I have bled all radiators and none had any significant amount of air in them. Does this suggest that the CH system is NOT leaking and that the problem is definately with the boiler?
We have already had an engineer replace a part last week (after waiting for 2 weeks for the replacement part to arrive) but this hasn't made the slightest bit of difference. Sorry, but I don't know what part he changed!
Are there any more tests I can perform to try to confirm that this is definately a boiler problem and not caused by leaking radiators/pipes? The boiler is covered by an insurance policy, but the radiators and pipes aren't so I want to make sure that the engineer doesn't try to blame a leaking system for the fault rather than a faulty boiler on his next visit.
If it is the boiler, any idea what might be causing it?
Thanks.