Baxi 105e grief

[Spooked]

Hi all.
Had this problem a bit ago.

The burner is definately not regulating on the DHW and i don't think it is on the central heating either (not so obvious due to the time it takes to get up to temp).

When the combi shower is running the water temp lights go to max and then the burner cuts out until the water cools then it fires up again. This goes on throughout resulting in a kind of dance when having a shower, wash when cold, rinse when warm

I get NO warning lights.

I changed the heat exchanger a few months back and all seemed well.

Since then we have had to have a new meter put outside and new 22mm pipe work to the boiler as our supply was low.

Could this have changed the gas pressure at the boiler, could wrong gas pressure cause this and would this then need resetting?
The manual points to the DHW temp sensor and it says to clean it. On parts arena this sensor is down as a special so i take it they don't stock them as they only need cleaning rather than replacing?

It also mentions the pcb but obviously, due to cost, this would be the last thing to change?

Any help much appreciated.

 

[ChrisR]

The sensor isn't special but it's less likely to be the problem than a dirty secondary heat exchanger or maladjusted gas valve.
Which heat exchanger did you change previously, and why was that?

If the 2ndry hex is getting that dirty as quickly as is sounds, you have a seriously dirty system, which needs cleaning.

 

[Spooked]

Hi Chris.
Thanks for the reply.
I changed the 16 plate DHW heat exchanger as i was getting this sort of grief before and the manual suggested this may be one problem.
I have just run some fault finding tests on the boiler and as i was doing it i noticed that when i turn the control dial to DHW only, the CH light comes on and the pump runs even with no taps on??
So the CH is firing up even on DHW only settings?
Wow it gets worse.
Do you think it may be the pcb? I can't see how gas pressures being wrong could do this sort of thing, can you?
Cheers[/url]

 

[namsag]

If you system is full of muck the plate heat ex maybe clogged again
, Turn your hw stat to half way and put hot tap on then observe flame size.
Turn hot tap right down so water is getting hot and observe flame size it should have reduced in size(this confirms modulation side of gas valve is working ) and leaves it at out of sinc sensor or plate heatex.

Or if its only doing it on shower it may be faulty shower thermostat cartridge

 

[Spooked]

Hi namsag
No, it does it when the taps are running as well so not just the shower.
I turned the dial halfway, the flame does change when i turn the tap up and down.
With the dial turned halfway the shower can run without the flame turning out but is not as hot as it should be.
I tried turning the dial up slightly and this caused the temp lights to go to 80+ and the flame switch off.
So does this confirm that the modulation is working?
If so, what could be the reason for the overheating and the burner just turning off instead of modulating?
Also, why is the CH working when i turn the switch to DHW only?
Thanks for the help.

 

[namsag]

Modulation side seems all ok then i would go for plate again take it out and clean it. As when it is really hot it is not getting its heat away into HW which is why it overheats without modulating properly

If heating is coming on when just HW selected it is a faulty microswitch on divertor or pin is not retracting back into housing.
Edit
ignore that bit about sticking microswitch i read it as dhw light on not ch indicator light

 

[ChrisR]

SOunds like you have lots of dirt in your system.
If the 2ndry hex blocks up the boiler can't control its heat output normally so it overheats and has to turn off.
Dirt also stops diverters working properly, so rads get hot when HW alone should be on.
It also makes gland seals leak, if water gets into switches they can do strange things like look electrically on when they're mechanically off.

 

[Agile]

A DHW demand switch being faulty and bringing on the heating when its not selected is a very common fault and we usually carry a replacement at all times.

Its a little difficult as a DIY repair as the little switch comes with half of the wiring harness!

The lack of apparent modulation could be a faulty DHW sensor or even PCB but a diligent CORGI would first be checking the gas valve min and max settings.

Tony

 

[Spooked]

Hey all.
Job sorted (again).

While i have been away from the comp i was cleaning the heat exchanger and the DHW sensor and on my return were all these replies and it looks like you were right.

The heat exchanger was indeed full of black bits? Top section only, how do these things work?

So it looks like my system needs a clean, whats the best way of doing this?

Agile, i will get all settings tested and see if the DHW alone setting behaves now the system is working properly again.

Thanks again for all the replies and help, we can now have a shower without the dance, thought it was fun for a while