Baxi Solo 3 PFL boiler keeps overheating!

[Billy Boy]

Hi Chaps,

Just wondering if any one can help me identify a problem I have with my boiler?

My Baxi Solo 3 boiler is approx 6 years old.
I had a heating engineer replace the PCB 3 years ago when it kept doing the same overheating as described below.

The boiler works fine and gives hot water and heats the rads fine but keeps cutting out and the overheating light comes on.
When it has cooled down (say 10 mins) you can reset and all is fine again for approx 30 mins, when it overheats again.
The overheating sends very hot water up into the small tank in the loft and then out of the overflow pipe.
The pump appears to be working fine ie I can feel and hear it turning at all times (when boiler is working)

I wonder if the overheat thermostat could be at fault or if I should buy a new £120 and fit it myself!

Any thoughts from you guys would be appreciated.

 

[Slugbabydotcom]

The PCB provides pump over-run so that the water circulates after the boiler has switched off. I bet this is what it isn't doing. The residual heat in the boiler overheats the static water that the over-run was supposed to clear. This then overheats and boils up resulting in your 'loft geyser'

Now I'm not telling you to cheat, but imagine if the pump were wired temporarily to a permanent source then it would prove what your fault is and keep you running until you get another PCB. When the eco-squad gets hold of me I'm dead meat

If I am wrong about that there will be a temperature sensor in your boiler that is not shutting it off at the right temperature. I can think of another possible right now but lets cross that bridge if we get to it

If your overheat stat wasn't working it wouldn't cut out and have to be reset

 

[Eco-Squad]

Now I'm not telling you to cheat, but imagine if the pump were wired temporarily to a permanent source then it would prove what your fault is and keep you running until you get another PCB. When the eco-squad gets hold of me I'm dead meat

YOU ARE DEAD MEAT M8

 

[stem]

Basically, a boiler will overheat if either the boiler is faulty, for example, the boiler thermostat / PCB that doesn't switch off the gas valve and so the burner keeps going until the increase in temperature makes the overheat trip, or there is insufficent flow of water through the boiler it to take away the heat. This flow has to continue for a while even when the burner has extinguished, this could be caused by.

No pump overun as suggested by slugbabydot com

Insufficient flow of water through the system so the heat is not dissipated.
(all radiators off, or shut down by thermostatic radiator valves, By-pass valve closed or incorrectly set)

Blocked /sludged pipes

Have you changed anything recently? for example fitted themostatic radiator valves to any radiators that didn't have them before, or turned off a radiator that is fitted with lockshield valves. If you have a bypass valve has the setting been moved?

 

[Billy Boy]

Hi Chaps,

Well you are spot on.
I have just got the boiler up and running nice and hot and then switched it off and the pump is not running on as you so quickly pointed out.
Wait 15 seconds and hey presto the very hot water is warming up the overflow tank in the loft nicely!

So, in your valuable opinion is the PCB most likely to be shagged?

If so I shall purchase another and fit it.

I am a little surprized to have had to buy a PCB every three years, is this a recognized weakspot on these Baxi solo 3 boilers and if so, is there anything I can do to make life easier on the PCB ie, will turning down the water temp on the tank or on the boiler help?

I really appreciate your help with this slubbaby and stem

 

[twgas]

when you say you turned the boiler off, what switch/control did you actually turn off?

 

[Billy Boy]

Hi Mick,

What I mean by I turned it off is;
I switched the control panel in the airing cupboard so that it turned off the hot water and central heating
I did not adjust/touch the actual boiler

Is that ok?

 

[scatmanjohn]

Is the pump actually wired up via the boiler?

Left hand side of the boiler terminal block should be wired up to the pump.

 

[Slugbabydotcom]

Part number 231711. They do go quite often.
Bit of a fiddly job so take your time with it. The white blocks to the left can be unplugged so there's no need to undo all the wires from them.

Unless you have a thermal store if you run your boiler on max, I equate that to driving your car with the accelerator pedal on the floor. Turning it back to about 2/3 or 3/4 will make your boiler run a little cooler and may reduce the likelihood of another burn-out. Try it and post back in three or four years time

I have known a dying pump to take this part out so consider renewing it if it's old or noisy and also consider what stem says about flushing your system as if it has sludge in it this reduces the life expectancy your pump.

The manufacturers state that no additional compartment ventilation is required. Thinking about that it may be a good idea as a sealed compartment will contain a fire.
Now don't hit the panic button cos I said that. Yes they do burn out and I have seen a few badly melted pcb boxes but I've never known it to cause a full blown fire.
I would nevertheless be inclined to stick a smoke alarm above it if I owned one and it would be a good idea to not have any flammable clutter around it.
Your pcb will probably just have a faulty relay but when they really go you can't miss the fish like smell of burnt pcb. It gets up your nose quicker than a line of coke.

 

[stem]

I am a little surprized to have had to buy a PCB every three years

Actually I also have the same boiler as you, mine's 7 years old and I'm on my 3rd PCB (original plus two replacements) The first developed an intermittent fault meaning it took about 6 attempts to fire up sometimes, and the second was a failure of the pump overrun. Mine failed in the other direction to yours. The pump ran 24/7.

 

[Billy Boy]

Hi Guys - me again,

Thanks for your valuable help, but I still have problems!

I have purchased and fitted a new PCB took my time ensured I placed everything as I should, all looked good and started up fine ect.

After I had the water and Rads warmed up and when the boiler switched off, it sent the water into the loft again and tripped the overheat again, it wouldn't reset for 30 mins - I guess it just got too hot! The new PCB has ensured that the pump is working on overrun.
One of the pipes coming out of the top of the Boiler really is very hot, I can hardly touch it, is that normal?

I have just run the boiler for hot water only and got it nice and hot then switched the boiler off and it did not overheat.
I have just now run the Rads without hot water and got them hot and switched boiler off and hey presto I am throwing hot water into loft and got the boiler to overheat again.
So I would assume that there is there a part blockage in the rads / pipework somewhere? I should say that the rads do work ok, one I cannot get any warmth into, 2 are mildly warm and the others I would say are fine.

I believe that my pump is sound, it does run quietly, it is 5 years old now.

So your valuable thoughts appreciated and thank you Slug Baby for your fire warnings.

Thanks Billy Boy

 

[twgas]

do you have boiler stat turned up very high, try turning it down a bit, can you actually chect the temp of flow pipe with a thermometer should be no more than about 82c

 

[Whiter]

Do all of your rads have T.R.V.'s or is there a system by-pass, it sounds like there is no where for the water to circulate when the heating circuit shuts down. As mentioned by Stem.

 

[uselessFCUK]

Billy Boy

Have you shut-off any rads?

Has any work been done recently?

Regards

UFCUK

 

[Dizzyellis]

Hi, I too have a solo 3 that keeps overheating/tripping out .
Now some times I can just reset it but other times it needs a bang in the side then it fires up.
I'm suspecting the gas valve is sticking . Dose this sound about right???