boiler problem

[cotain]

Hi,
I have a Grant combi 90 v3 boiler which heats up the hot water ok but will not restart the burner to keep water temperature up. I have done all the work on my boiler to date but i,m not sure how to carry out tests.
Can some one please help?
Cotain.

 

[dcawkwell]

Does the burner fire when you turn the tap?
Could be thermostat has gone and it is tripping out on overheat?

 

[Boilerman2]

Above the plate H.E. is a Giannoni Diverter valve, on the front is a rectangular black plastic box with an electrical flex running to it a VERY common fault on the Grant is for the plastic collar that secures it to the front of the brass valve to become brittle meaning the pin on the diverter cannot make the electrical switches inside the box, a new microswitch assembly will be needed if this has happened

Another problem may be if the Riello burner motor runs constantly but the burner does not actually fire up, in this instance look at the Photocell on the burner 99% chance it is fooked.]
So there it is - 2 things for you to check

 

[cotain]

Hi,
Many thanks dcawkwell and boilerman2 for your help. The burner does not fire when I turn on the tap. I checked the microswitch and it seems ok and you can hear it work when you turn on and off the tap. the electric continunity is good with switch closed. I checked the photocell and cleaned it, it was black with soot. After the boiler has heated the water and I use some, if I try to restart the boiler the motor runs for just a few seconds and cuts out and I have to let it all cool down before it will start up again. If its the thermostat how do I check it.
Look forward to hearing from you.
Cotain.

 

[dcawkwell]

Boiler should fire as soon as you turn the tap on.
So trace from the flow switch. Look at the wiring diagram most likely
a thermostat problem. Overheat stat possibly.
With the boiler cold check the resistance across thermostat and
overheat stat. Then once boiler heated up do same again.

 

[cotain]

Hi,
Thanks for your reply dcawkwell. I fitted a new thermostat but the problem still remains. Any more suggestions ?.
Regards,
Cotain

 

[cozzmic]

Needs a service and checked with a FGA. (photocell sooty)

 

[dcawkwell]

Hi,
Thanks for your reply dcawkwell. I fitted a new thermostat but the problem still remains. Any more suggestions ?.
Regards,
Cotain

Need to check them before blindly replacing the parts.
You'll need a multimeter to diagnose the fault. Follow the circuit
diagram for your boiler it should be in the manual for the boiler.

 

[Boilerman2]

I still think the fault lies with the Diverter valve/Microswitch, possibly a faulty HW diaphragm???

 

[cozzmic]

Does nobody think that the sooty photocell is not a clue also the motor runs for a few seconds before going off. Sounds like burner problem too.

 

[cotain]

Hi,
I have checked the electrics with a multimeter that's why I put in a new stat.
I thought it was showing a problem but I was wrong. I have also checked the diverter valve. the diaphragm seems in good condition,I cleaned it, no holes or perishing of the rubber, but how can I be 100% sure. When the boiler did heat the water it did'ent seem to be as hot as normal. When I switch on now it only fires for about 1 minute and cut out so I'm not getting hot water at all. I have given the boiler a service and the firing chamber was quite clean but I know the photocell was'ent checked for a few years so that might account for the soot. Is there anything I can check in the burner?
Regards
Cotain.

 

[dcawkwell]

Hi,
I have checked the electrics with a multimeter that's why I put in a new stat.
I thought it was showing a problem but I was wrong. I have also checked the diverter valve. the diaphragm seems in good condition,I cleaned it, no holes or perishing of the rubber, but how can I be 100% sure. When the boiler did heat the water it did'ent seem to be as hot as normal. When I switch on now it only fires for about 1 minute and cut out so I'm not getting hot water at all. I have given the boiler a service and the firing chamber was quite clean but I know the photocell was'ent checked for a few years so that might account for the soot. Is there anything I can check in the burner?
Regards
Cotain.

The only way you will find the problem is with a multimeter.
No need to mess about with diverter valve diaphragm easy to tell
if this is working or not without. You need to follow the 240v.
If you turn on the tap you need to check there is 240v all the way from the flow switch to the burner. You need to follow the wiring diagram for the boiler.

 

[cotain]

Hi,
I taught a miracle happened today after switching on the boiler and checking the voltage through the burner flow switch and down to the burner. I turned on the tap had piping hot water and the boiler fired immediately after turning on the tap, I tried it a few times and had success. I left it for a while than used some water but it failed to fire, there was no voltage across the burner flow switch and no 240v reaching the burner. Could it be a faulty flow switch and why would it suddenly work and stop again ? Thanks again for your continued help
Cotain.

 

[dcawkwell]

Hi,
I taught a miracle happened today after switching on the boiler and checking the voltage through the burner flow switch and down to the burner. I turned on the tap had piping hot water and the boiler fired immediately after turning on the tap, I tried it a few times and had success. I left it for a while than used some water but it failed to fire, there was no voltage across the burner flow switch and no 240v reaching the burner. Could it be a faulty flow switch and why would it suddenly work and stop again ? Thanks again for your continued help
Cotain.

If there is 240v at one side of the flow switch but when the tap is running nothing at the other side. Fault is the flow switch. A common problem.
Take it to pieces it could just be sticking with limescale.

 

[cotain]

Hi,
At last a happy ending to my boiler problem, Tracing the 240v with my multimeter I found that I was getting voltage from the microswitch up to one side of the burner flow switch but when the tap went on there was no voltage on the other side. I replaced the printed circuit panel which houses the boiler heating thermostat, burner flow switch etc. I could'ent get a burner flow switch in a hurry and my wife's patients were growing thin. Every thing is now working as it should and I have piping hot water again.
Many thanks for all your help, its a great comfort to know of your expertise out there and and your unselfish willingness to share it.
Kindest regards
Cotain