baxi instant 105e no central heating

[waspdog]

hi all. the hot water on my boiler is working fine, but the central heating will not start.there are no fault lights up.the timer and room stat are ok, ive measured 240v switching on terms 1 and 2, and tried linking 1 and 2 out but nothing happens.any ideas?.im an electrician so i can cope with a wee bit of technical stuff ( i hope).

 

[baxpoti]

Could be

Stuck in DHW or preheat mode (green tap light on or flashing)

Has it got an onboard timer or external?

Is selector switch set to heating and hot water?

Keep link across 1 and 2 and remove all other wiring from 1 and 2.

230V on 1 and 2?

 

[bengasman]

Being a sparky, the quickest way for you to eliminate control failures, is to check the M.I. and bridge the timer and the roomstat connections.

 

[waspdog]

thanks for a fast reply.in response, the boiler has external timer and room stat.selector switch is set to water and heating.green tap light is not on (except when preheating or when water is demanded).ive removed outgoing wires from 1+2 as suggested and linked them out, no joy.230v going from selector switch through link (1+2) then to pin1 on A4 plug.im suspecting the pcb but i dont know if anything else could be a problem.any more ideas ?cheers.

 

[waspdog]

i forgot to ask, what is the MI?.thanks

 

[bengasman]

Manufacturer's Instructions.
The thank you button is the right hand one
I don't have the manual for this one at hand I think, (what is the gc number?) so I am unsure if the timer and thermostat are 2 different connections. Should be easy enough to work out by looking either at the wiring or at the diagram in the book.
I always keep the pre-heat off, and especially when fault finding, you should eliminate anything that could be of influence.
Combi's override the ch when there is a demand for dhw, and it is possible that a "sticky" pre-heat stops the ch from being activated. Don't hold your breath about this, it is more a matter of eliminating things one by one.

 

[waspdog]

cheers beng, i tried knocking the preheat off, no change.by linking 1+2 out the external controls are out of the equation.i think this is the gc no- 47 075 09

 

[bengasman]

gcn is important to make sure you get the right manual and correct part numbers. It should be somewhere on the boiler, possibly behind a flap, under the boiler, or behind the cover.
I seem to remember that some baxi combi's are designed to be in dhw mode in rest position, but not entirely sure. I would need to have the manual and possibly be in front of the boiler to get any further. Some of the guys posting here will no doubt know the entire boiler by heart; they might be able to get closer without the manual.

 

[twgas]

do you get the ch demand light on,

 

[baxpoti]

do you get the ch demand light on,

Does the pump run?


Remove the microswitch with red wires and operate it by carefully inserting a small screwdriver, or similar. What happens?

 

[waspdog]

no ch demand light.pump does not run (no voltage to it).tried the microswitch with red wires, nothing happens. ive also took a few measurements if they help. overheat stat- 0.4 ohm, ch water sense- 11.43k ohm, hotwater sense-60k ohm when cold.once again any ideas much appreciated.thanks

 

[twgas]

I'd be double checking the power at the link, and the selector switch.

 

[baxpoti]

I'd be putting a new PCB in...................but then, I don't have to pay for them.

 

[waspdog]

thanks for the superb response chaps.central heating has just decided to start working (after unplugging a few connectors to take measurements ?).ill leave it alone while its working and hopefully thats the end of it.least i wont have to buy a new pcb.well not just yet anyway.(hope im not being premature here).many thanks for the advice

 

[waspdog]

still working !