BAXI Combi Instant 105e Central Heating Problems

Had the BAXI Combi Instant 105e Boiler about 3/4 years. Having pressure problems, when cold can be down to zero, when been on for a while can be well over the recommended 2.5. I have known for some time how to pressurise or depressurise, but quite often when central heating is being asked for, it won't come on. Even if pressure is OK. Plumber friend is baffled, have already replaced the thermostat and checked for leaks etc. Next thing to replace would be the PCB, quite expensive, maybe just count my losses and get a boiler that works consistently. Also, if the preheat hot water is on, Central Heating stays off. Any ideas what to do next?

PCB???? No no no no.

The expansion vessel is goosed... have it checked to see if the diaphragm has split or it just needs recharging.

This is a very very easy job to check

Nothing wrong with the PCB if the pressure is going up/down

Dave
IEng MIGEM

Thanks, l'll get the expansion vessel checked out.

You can check this yourself.....

At the back of the boiler on the top (as if looking down on it) there will be a plate held on by 4 screws..... undo these and you will see a large red drum - this is the expansion vessel... Take off the valve cover ( looks like a car tyre valve) and depress the centre of the valve.

If water squirts out, sorry the vessel is done for and needs replacing. If its just a pathetic 'psst' of air then you just need to have it recharging, there is I think an FAQ on here for how to do this, all you need is a foot pump.

Dave

IEng MIGEM

i wouldnt recommend replacing the expansion vessel yourself unless you know what you are doing. as your boiler is an 'instant' the expansion vessel also acts as a small thermal store for the pre-heated hot water. changing the vessel will involve removing the boiler from the wall (disconnecting the electrical supply, flue, gas and water pipes) and is best left to someone with experience (and a big box of replacement washers!)

My plumber tried to inflate the exp vessel, but water blew out. Rather than remove and replace, he attached a new Exp Vessel to the in-pipe and this seems to have fixed the pressure problem. However, the intermittent but regular occurrence of the central heating not coming on when heat is asked for is still happening. It now seems to be consistently caused by the Hot Water green light staying on for much longer than it should. Whenever we run the hot water, as expected this takes priority and the Central Heating green light goes out. Unfortunately, the Hot Water light stays on even when no hot water is being asked for. So the on and Hot Water green lights are the only ones on, the burner light is off and it can stay this way for ages! So this seems to be the reason, but what could be causing this, is it the PCB done for? Any ideas?

It will be heating up the ( now larger ) expansion/store however this should only take a few minutes at the most.

Perhaps you need a competent boiler engineer rather than a "plumber" ?

Tony