Combi Boiler DHW only when CH on - help!

[haggettd]

Hi all, first post - hope you can help.

The Boiler:- SIME format 100C

The problem.
No DHW when you turn the hot tap on whilst the CH is off.
DHW available in abundance whilst the CH is on (Burner firing).

Observations.
Microswitch can be heard clicking when the hot water tap is opened and closed.

With the hot tap running, you can manually fire the boiler with the ignition knob and the burner will stay lit for 15-20 seconds then go out.

There is not the usual sound of gas flowing before ignition that you got when all was well.

Central heating works normally, firing and shutting off in line with the thermostats requirements.

When the DHW is coming out of the tap (CH on) there is an unusual metallic buzz/electrical fizzing sound which stops 5 seconds after the DHW tap is closed.

The bigger problem!
My local corgi man has had a look, is baffled and suggested a new Diverter valve which he proposes to fit this Friday.

My thoughts are that the diverter valve is doing its job as far as diverting the hot water to heat the DHW since I get hot water when the CH is on.

Does the diverter also trigger the flow of gas to the burner as well?

Any thoughts much appreciated😕

 

[RoryD]

Hi all, first post - hope you can help.

The Boiler:- SIME format 100C

The problem.
No DHW when you turn the hot tap on whilst the CH is off.
DHW available in abundance whilst the CH is on (Burner firing).

Observations.
Microswitch can be heard clicking when the hot water tap is opened and closed.

With the hot tap running, you can manually fire the boiler with the ignition knob and the burner will stay lit for 15-20 seconds then go out.

There is not the usual sound of gas flowing before ignition that you got when all was well.

Central heating works normally, firing and shutting off in line with the thermostats requirements.

When the DHW is coming out of the tap (CH on) there is an unusual metallic buzz/electrical fizzing sound which stops 5 seconds after the DHW tap is closed.

The bigger problem!
My local corgi man has had a look, is baffled and suggested a new Diverter valve which he proposes to fit this Friday.

My thoughts are that the diverter valve is doing its job as far as diverting the hot water to heat the DHW since I get hot water when the CH is on.

Does the diverter also trigger the flow of gas to the burner as well?

Any thoughts much appreciated😕

This probably is the diverter diaphragm mate. I think the default position is CH so no real diverter movement there. if the diaphragm is loose / worn it won't raise up correctly and so no sustained dhw. but enough to allow dhw to flow when heating on. There is a much cheaper option by using a service kit rather than a very expensive piece of brass.
service kit number 6281540

 

[haggettd]

Thanks Rory D.
Your suggestion makes some sense.
2 years after installation of the Boiler - the DHW became more and more reluctant to give hot water. You had to turn the tap on more and more to get it to fire up.
The diaphragm was replaced and all was well 'til now (4 years on).

This time the burner will not light no matter how much the tap is turned on but still you still get DHW with the CH on at low tap openings. Maybe that's why my plumber's a bit puzzled.

Could it be anything electrical?

 

[RoryD]

Thanks Rory D.
Your suggestion makes some sense.
2 years after installation of the Boiler - the DHW became more and more reluctant to give hot water. You had to turn the tap on more and more to get it to fire up.
The diaphragm was replaced and all was well 'til now (4 years on).

This time the burner will not light no matter how much the tap is turned on but still you still get DHW with the CH on at low tap openings. Maybe that's why my plumber's a bit puzzled.

Could it be anything electrical?

The cover has to come off and you have to observe the pin hitting the microswich, if it does then , with poweroff u can measue if there is continuity back at the board

 

[haggettd]

Thanks,
as mentioned, I can certainly hear the microswitch activating with the turning on and off of the hot water tap. (Luckily I have a basin within 2feet of the Boiler!)

Would this suggest that the diaphragm's ok since the switch is being activated?

Do you know whether the Microswitch activates a solenoid valve to let the gas through, or is it the movement of the diverter valve that does this mechanically.

Without seeing the logic circuits I don't know but I would expect the microswitch to control the gas valve and the ignition so that the microswitch fails safe.

Thanks for your input RoryD

 

[so-solar]

sounds very much like a diverter valve!

 

[haggettd]

Re: Update Combi Boiler DHW only when CH on

Plumber visited Friday.
The Diverter is working correctly - full deflection of plunger and is actuating the Microswitch as it should.

The loud buzzing noise is coming from the Gas valve. The gas valve has two valves in fact, the main one is for CH and when the DHW tap is turned on, a second solenoid actuates and opens a second valve to boost the burner output when hot water is required.

Now then. The Gas valve assembly was exchanged for another which the plumber had borrowed from a colleague.
The boiler behaved the same and the valve emitted the same loud buzzing noise.

Our thought was that the PCB may be faulty and is not supplying sufficient voltage to hold the solenoid open - hence the buzzing.

The puzzling thing is that the buzzing solenoid must be opening the valve since the DHW gets very hot. My plumber disconnected the buzzing solenoid while the tap was on and the hot water then ran lukewarm as expected. Re-connected ther solenoid and it ran hot again.

When the heating is off the DHW doesn't fire up the boiler - you can't hear the fan or pump working either.
If at this point you rotate the manual ignition knob to the ignition position then the fan and pump works and the burner ignites and you get hot water.

My plumber is a bit reluctant to buy and fit a PCB since they are £200 and non-returnable.

Could it be anything else???

Thanks

 

[RoryD]

Re: Update Combi Boiler DHW only when CH on

Mate rotating that knob will not have any effect on anything electrical - it's a mechanical thing.I think with buzzing and everything else that they r red herring. you will need a water flow switch!! part number 6281502 or gc number E92724

 

[so-solar]

i agree with the above! The first part of the hot water sequence is the flow switch detecting flow, which then sends the signal to start the dhw boiler sequence! It is very hard diagnosing over a pc but i think you will be far better trying a few quid flow switch which is returnable than a pcb which is not returnable and will probably not sort the problem

 

[haggettd]

Thanks chaps, yes, I'd much rather try the water flow switch first.
Where is this switch located? I presume it's not the microswitch which is actuated by the Diverter valve plunger.
The microswitch clicks on and off as it should - actuated by the Div valve piston - the only thing we didn't do was check continuity when the switch was actuated - I guess we need to do that first eh?