Hi all.

Boiler - an old Baxi 105e.  

Many new parts fitted in the past couple of months, including the complete diverter valve, prv, auto air-vent, hot water plate exchanger.  The PCB was replaced with a 'good used' one, though i do realise how dodgy that can be in terms of fault-finding.

There are no leaks and when it is working it produces good heat around all rads, and very hot water with no faulty heating of the rads when on the 'Water Only' summer setting, a problem i had before the diverter was changed.  (The old diverter was so choked with black crap that nothing moved inside it!)

Current problem :  first attempt of the day at firing up the central heating, it will not fire up and though the pump keeps running the 60-degree fault light comes on, which is the 'Fault on Pump or Low Pressure'.  The pressure gauge shows 1.5 bar.  I then turn on the hot tap at kitchen sink three meters from the boiler for a minute, and the central heating problem disappears.  Also, the CH will continue to work on demand for the rest of the day.  There doesn't seem to be anything wrong with the pump in so far as it is quiet and does the job of pushing heat to all the house rads.

Where on the boiler is the sensor which causes a 'Low pressure' error to be created - is it created by a failure of the CH micro-switch to be fully activated - as for example by the activating pin in the diverter not lifting fully when that diaphragm lifts ?  My only guess as to the cause of that would be a small amount of air in the diverter or nearby, given that running the hot water for only one minute or less clears the problem.

In for a penny etc... the only other gremlin at the moment, is a weird problem with pressurising the boiler through the filling-loop - don't know how to describe it quite, but it's as if the boiler is resisting the input of new water - the normal wooshing noise of water entering the system is completely absent.  I've checked the water pressure at the filling loop and it is 2.5 bar after passing through a magnetic cleaner in-line.  Obviously i don't need the 2.5 bar for the working pressure of the boiler but i've never seen a boiler which kind of resists the filling-loop input like this.  Can't see why or how, but i thought i should include this weird feature in case in some way i can't see, this is connected to the main problem above.

Thanks for any ideas.

[ Ps:  forgot to mention - a new expansion vessel was fitted last year just under the boiler on the ch return pipe, as the one buried at the back of the boiler is full of water - split rubber.  It works fine, but i never checked the pressure that it came pre-charged with until just now - it was at 2 Bar, definitely on the high side as the unit has a label with 1.5 as the recommended pre-charge.  I let out some of the air and hey presto my boiler pressure gauge came back to normal, dropped to 0.5 Bar, and i was able to use the filling-loop in the normal way to raise it to 1.25.  So was the 2 Bar in the expansion vessel pushing against the 2.25 Bar in the filling-loop or is that too simple? ]

Thanks for those ideas.  The magnetic filter that was in the system when i moved in a year ago was fitted the wrong way round with the arrow pointing back to the mains inlet/stopcock under sink !  So i don't think it had ever done much of a job.  New one fitted right way round.

I have it working A1 right now (fingers crossed) and the final fix seems stupidly simple.  It has taught me about the gamble of using kits of parts made god-knows-where by god-knows-who.  I've bought a lot of parts via ebay etc and had to return quite a few for poor quality or ask for missing bits to be sent.  With this renovation i had to have a full kit of all the bits for the diverter valve.  I have dismantled it a couple of times so i'm really familiar with it, so i was confident about it when i re-fitted it re-built.  But as my post says, it did not work A1 although it would fire up after 2 or 3 attempts.  Anyway, i re-opened the CH diaphragm chamber as that's where the pin was lifting by about 75% of what it should do.  Could not see anything dramatically wrong but when i got the new diaphragm out and compared it to the old one, it felt a lot thicker and stiffer.  I've tried to show in a couple of pics, but it's the way it feels that really matters - i put another new (but thinner) diaphragm in, and the boiler immediately fired up first time with the pin showing full travel up to the microswitch.  I've tested it 3 days in a row and both CH and hot water are great.  

No wonder pros refuse to re-build diverters and insist on a proper Baxi replacement !

In the pics the problem diaphragm is the thicker one (on the left) with the obvious ridge all around - when pressed in the middle it has very little 'give' so it was restricting the upward movement of the pin towards the microswitch.  That's my theory anyway !

Thanks again.

[Now to find out how to upload pics... ]

Before you chuck too much at the old boiler consider buying an Intergas, they don't have any of the problem parts.

Em Spend more on the Old Boiler than the Wife