CH on permanently - despite settings...

[jimothy]

Hello all,

our Baxi 80 eco boiler has started firing up the CH whenever it's switched on, no matter what the timer, integral thermostat and room thermostat are set to, and even if the function knob is set to HW only.

Had a guy come to look at it, and he said "it's probably the frost stat" but the manual doesn't mention any frost stat, and the only (very brief) reference to frost protection says that it only operates if the boiler's set to CH+HW anyway. (pg12 section 7:4:2) www.acwilgar.co.uk/pdf/combi-80-eco.pdf

SO, this leaves me wondering - does my plumber know what he's on about, and if so, where's the frost stat?

Thanks in advance to anyone willing and able to help...

 

[Mbear]

Can't see one in the wiring diagram, is there an external Frost stat fitted?

Hot water Flow switch could be stuck on, this would cause the boiler to fire all the time

 

[jimothy]

No, the boiler's in the airing cupboard, so no need for any frost stat, and plumber definitely indicated something internal when he said about it. (In fact he motioned towards the position of the main pipestat and overheat stat shown on the diagram) could a fault with one of these cause the same problem, or has he just misidentified a component?

 

[oldplumber]

could be several things and they may be faulty programer, microswitch jammed on, room stat jammed on wont help, etc etc. need to investigate each possible cause in term to eliminate them as an issue.

 

[Mbear]

Put the selector switch to hw only, does it still do it in this mode?

If it doesn't then I'd be looking at timer or room stat as OP said

 

[jimothy]

Put the selector switch to hw only, does it still do it in this mode?

Yes, the only way to stop the heating coming on is to turn the boiler off entirely, but this means no HW either

 

[street elbow]

Hi Jimothy,

Hope the plumber did not charge you for this,

With a combi boiler (working correctly) you cannot have HW & CH on together, (if CH is on and you turn on a hot water tap the CH is turned off by the diverter valve so all heat goes to the hw)

So the problem is with the diverter valve,
and its a job for a GSR installer unless you are very good at DIY.

David

 

[aggis]

Sounds like dhw micro switch, Check gland behind m/s isn't leaking, may be shorting so constantly calling/on.Turn off power, disconnect wires to m/s if heating working ok, order m/s also gland service kit if req'd

 

[jimothy]

With a combi boiler (working correctly) you cannot have HW & CH on together, (if CH is on and you turn on a hot water tap the CH is turned off by the diverter valve so all heat goes to the hw)

Yes, sorry if what I wrote was confusing, I meant when the function knob is turned to CH+HW, rather than just HW. Diverter valve working fine, HW supersedes CH, but CH comes on whenever there's no HW demand.

Plumber accepts he was wrong to describe it as frost stat malfunction -claims he meant the primary thermistor, but I've checked this and it seems to be operating correctly (9.6 kilohms before turning on boiler, dropping to about 5 kilohms when hot - manual states R=11kilohms "when cold" and drops as T increases, so sounds about right)

 

[aggis]

Thermista sounds fine. norm 10 -13 k/ohms when cold.
Just to confirm, you're getting heating when it's suppose to be off.
If so your diverter valve will be at rest [in the heating position], but if the dhw micro switch is faulty and has continuety [closed] you'll get the boiler firing & heating rads.

 

[jimothy]

Thermista sounds fine. norm 10 -13 k/ohms when cold.
Just to confirm, you're getting heating when it's suppose to be off.
If so your diverter valve will be at rest [in the heating position], but if the dhw micro switch is faulty and has continuety [closed] you'll get the boiler firing & heating rads.

Absolutely right, Aggis.

Plumber came round again, and got talked through a series of checks by the tech help guys at Baxi on the phone. Turns out the microswitch wasn't clicking when the diverter valve disengaged, so exactly the situation you foresaw - well done and thanks for the help - don't happen to live anywhere near Cardiff do you? We need someone who knows their stuff around here...

Thanks also to everyione else who's tried to help with this - great forum you have going here

 

[RoryD]

either the pcb or clock (internal / external?)is sending a signal to start a sequence of operations. I would remove the clock / programmer wires to see if it stops, if not the it's a good chance the pcb is faulty

 

[Mbear]

either the pcb or clock (internal / external?)is sending a signal to start a sequence of operations. I would remove the clock / programmer wires to see if it stops, if not the it's a good chance the pcb is faulty

Voltage u mean, sorry it's a pet hate when ppl say signal instead of voltage

 

[RoryD]

okey dokey