baxi solo 3 not lighting, what is sequence of operation?

[Darren1234]

Hi, I have the above boiler and its doing nothing at the moment.
When the control unit switches on the CH/HW the 'Boiler on' light illuminates and that's as far as it goes.

The other indicators are Overheat, Fan on , Pilot on, Burner on.
These all stay off.

I've downloaded the pdf manual and it lists all the sensors etc involved in the boiler but it doesn't give any indication as to what is used and when.

Can anyone give a list of operations for me to help diagnose which electrical part to check first.

I read elsewhere that when the boiler comes on on, the Fan should spin first and then the pressure swicth monitors that that is up to speed before any ignition takes place.
When the boiler on light comes on there is no supply at the fan as though it is not being powered up to spin. is there another sensor/control that the cct board checks before supplying the fan? eg if the Thermal cutout sensor is broken this would stop the boiler trying before anythign else?

(The cct board fuse is ok)

Thanks for any help,
Darren.

 

[penfold46]

On a call for heat, is the pump running?

 

[Darren1234]

Yes. The pump is definately running. This is up in the airing cupboard and I've been up to verify that.

 

[Darren1234]

There are two sensors on the water pipes. According to the PDF manual they are :-

Thermostat sensor and Overheat Thermostat.

The fault finding chart suggests checking them with a meter but isn't clear as to the readings expected. I'm electrical/electronic trained and can't understand what it wants reading wise.

I'm guessing the overheat thermostat will be a open/closed switch , whereas the thermostat sensor will be a variable resistance dependant on temperature?

The overheat is reading ~0 Ohms. The thermostat ~600K Ohms

 

[onlyfitidealboilers]

if you have no continuity over the overheat stat then look no further

 

[Darren1234]

It reads 0 Ohms, ie short cct. Sorry if I confused the issue by putting ~0 , that means approximately 0.
I assume it would go open cct when it activates because of overheating?

Or are you saying it should be open cct, and then closes when activated? This isn't usually the logical way with electronics as it could fail unsafe.

 

[onlyfitidealboilers]

open when overheated. should be closed ie continuity if its a good stat.

OL is dead.

 

[Darren1234]

So the Overheat stat is ok, cause it currently reads 0 Ohms which is closed.

 

[nicknox]

Pcb fault

 

[Everest18]

is your meter testing your leads? if so there usually 2.0ohms or there abouts

what exactly does your meter read?

 

[Andygasman2010]

The overheat stat should be "continuity" and the sensor should be 1Mohm@25C (NTC)

Check the gas valve. The PCB checks resistance of the solenoids before it fires. Disconnect the solenoids and test with your meter. One should be 1.5kohms and the other should be 5Kohms.

 

[Darren1234]

If you're saying the thermostat should be 1MΩ at rt , does my 600KΩ suggest a problem or is that a 'thereabouts' kind of reading ?

The pilot valve coil reads 1.5KΩ, the gas valve coil reads 65KΩ.

Your comment about "the pcb checks the resistance of the solenoids before firing" brings me back again to wondering if there is a logical step sequence to explain what the PCB is doing/checking as it is used.

 

[45yearsagasman]

is your meter testing your leads? if so there usually 2.0ohms or there abouts

what exactly does your meter read?

This is the idiot who asked how electricity and fans work!!!!

 

[Andygasman2010]

If you're saying the thermostat should be 1MΩ at rt , does my 600KΩ suggest a problem or is that a 'thereabouts' kind of reading ?

The pilot valve coil reads 1.5KΩ, the gas valve coil reads 65KΩ.

Your comment about "the pcb checks the resistance of the solenoids before firing" brings me back again to wondering if there is a logical step sequence to explain what the PCB is doing/checking as it is used.

600kohms is of course 600,000 ohms

1Mohm is 1,000,000 ohms.

So as it's an NTC (higher the temp the lower the resistance) if it's lightly higher than 25C at the sensor it may read 600Kohms. I'd say that was ok.

The pilot solenoid sounds ok at 1.5kohms but the main solennoid is slightly higher than i'd expect. It should be 5000 ohms and you've got 6500 ohms. Maybe that's the fault but you can do no more to confirm that other than replace the solenoid.

The PCB checks the overheat stat and sensor along with the gas valve coils before doing anything. It may also check the APS is open circuit too but i'm not sure.

 

[transam]

dear oh dear you could'nt make it up really !

By the way Everest18 can you tell me how a multitester works I know it measures resistance & voltage but how does it do it ??