Baxi 105e 'Fault on Pump or Low Pressure' error.

8 Jun 2018
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United Kingdom
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? ]
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the pin that comes up to activate the microswitch when pump comes on may actually be sticking up after a demand this activates this neon and stops boiler firing again so check this
Hi thanks for that. No, the pin drops down immediately. I can shorten the description of the issue a lot today - first switch on of the day, the pin comes up but the boiler fails to fire up (not the hot water, that works great); if i immediately switch off the power and switch the CH back on, it fires up ! It's as if the pressure at the diverter has dropped overnight, but is soon back up enough to push the pin properly after only one try at starting. But there are no visible leaks and the main pressure gauge is steady at 1.5 bar. A possibly dumb question - is it ok to leave the automatic air-vent on top of the pump half undone to allow venting of air, or should it be closed tight in normal use ? Cheers.
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.
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It seems a good model
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Could someone tell me please if this model was very good and what type of expected lifespan you could expect if you had it refurbished/repaired and or regularly serviced, thanks

In its day it was a decent boiler, but it was designed a long time ago so technically it is very out of date. As for reliability, i imagine all gas engineers would say that it will give good service IF it is properly installed - cleaned through and filled with inhibitor / and a magnetic filter fitted in-line on the cold main in...AND if it is properly serviced and repaired at regular intervals. But - i am not a pro, just a car mechanic who likes getting stuck in to anything mechanical. These days any boiler that develops a major fault or two tends to get taken off the wall and replaced especially if it is over 7 or 8 years old.
you fit magnetic filters on the return pipe not the cold feed.

Yes - thanks for that. I've been temporarily lazy and fitted the new in-line filter where the old one was cut into the cold water supply pipe - about two feet before it enters the cold inlet to the boiler. On the basis that it would be better than no filter at all, and better than it had been for years especially given that the previous bodger who lived here had the filter fitted the wrong way round. But i know that all it is doing is picking up any charged ions in the hard water coming into the hot water / plate heat exchanger part of the boiler, and not helping the CH part at all except to a small extent when using the filling-loop to re-fill from the mains after work on the system. Just a note on the general boge-up i inherited : i couldn't work out why it was always spewing hot water into the yard through the pressure relief valve, but didn't lose too much pressure on the gauge. I found a permanently fixed braided filling-loop fitted from the cold mains to the CH flow side that was always open, and was fitted with a flow regulator set to balance the loss of water by a constant inflow of stone cold water from the mains ! No wonder the gas bill was high and the house not exactly warm. I fairly quickly got up above the top of the boiler and tested the expansion vessel - the air side and valve full of water, so a split diaphragm for sure. So the permanently-connected filling-loop was a bodge instead of a new expansion vessel. One of my first jobs in the renovation of the old 105e was to fit an 8 litre external EV just below to boiler on the CH return pipe, which works fine without any further water loss through the prv to my back yard! Is that kind of bodge for a failed EV common ? I will next look to fit a magnetic filter on the return pipe the next time it's drained down for a new rad that's on its way. Thanks again.

Ah - afterthought : is the filter placed between the cold mains in, doing some good at least by taking out the calcium that's in all hard water areas, and thereby protecting the hot water plate heat exchanger ?
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