Boiler not firing sometimes for DHW when cold!

5 Oct 2004
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United Kingdom
Hi all

I've a Ravenheat HE85T combi which recently has had new (dry type) CH sensor, diverter valve and microswitches (twice now).

I noticed a fault of intermittently boiler not firing for DHW when cold (when CH is in OFF mode). The boiler fails to sense the demand for DHW and therefore does not fire. I can hear the cold water running through the boiler (like a hissss sound) but the boiler does not fire up at all.

I replaced the microswitch with a new one but the fault is still there. Its very intermittent and generally comes when the boiler is ice cold...

I've also checked that the pin from diverter valve is coming out and pressing the buttons on the microswitch. More than 50% of times it fires (when CH is OFF).. There are 2 pink wires going from microswitch into the front instrument panel. Do I need to open that and look for a loose connection type of fault or could it be the PCB?

The CH is working fine and the DHW works just fine when the CH is ON!

Any ideas where to look?

Thanks in advance...
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Hi all

I've also checked that the pin from diverter valve is coming out and pressing the buttons on the microswitch. More than 50% of times it fires (when CH is OFF)..

Thanks in advance...

But have you seen if the pin comes out properly ont he 50% of occasions when the boiler does not fire up?

95% of these faults are that the pin does not come out enough!

With the pin out it should be very difficult to push it back with a naked finger.

Yes the pin does come out good enough.. I've even manually placed something in its way so it presses even harder on the microswitch (when the boiler is not firing), but even then the boiler does not fire..

If the boiler is repeatedly not firing, then even if I try to switch on the microswitch myself with a screwdriver, it won't fire unless I put the CH on...
You say you have changed the CH sensor.

But have you measured the resistance of the DHW sensor ??? Or just changed it ???

I think they should be 25 k cold.

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DHW sensor has not been changed so far, its the original 2 year old one..

Could you please elaborate a bit on how to check the resistance with a multimeter? Do I've to take it off?
Another symptom that I forgot to mention is that sometimes when the boiler is OFF (CH OFF, DHW OFF), on its own the boiler can be heard clicking and we can hear the boiler coming on for a few seconds..

I am sure the pump starts as well in this case as I can hear water running through rads but all this lasts barely a few seconds to max 1 minute.. Then the boiler stops completely..
This sounds like the problem I have with my Ravenheat Whitestar.

Do you get waves of cold water through your hot?

I had noticed that on warmer days my problem is very slight as compared to cold mornings.
I checked the continuity between microswitch and PCB. It seems connecting properly (resistance going from 1 to 0 ohms when boiler comes on).
Also connected to the pins of DHW sensor but could not measure resistance between the 2 pins as it stayed 1 (open circuit)..

Any other places to look? Am I looking at right places or now?

Is everyone too busy or no one has a clue?
Many of us professionals are very busy at the moment.

If you want to repair your boiler then you will have to learn how to measure the resistance of a component. Evening classes???

Or just replace it???

Or fit a 15 K resistor costing 3p in place of it to see the effect??

Tony Glazier

Point taken.. I want to learn how to fix this.. I've done this in the past, I am just note sure where to measure in the boiler with multimeter..

Ever since I opened the instrument panel, the boiler has been firing OK but I am not convinced as its an intermittent problem.. When the boiler was firing, resistance was changing at the ends all the time which meant that it was OK... I think I need to do this probing again when the boiler fails to fire and see if values change... am i right?

Could you please inform which resistor I need to change? So far, I've understood that DHW sensor is my next call as it has not been replaced yet.. I will order that now.. Should I get the wet screw type or dry clip on one?

Slugbaby advised in some other thread to get the clip on type as they are more reliable?
Anything else I should be looking for?

Many thanks once again for taking your precious time to help the needy!
Just get the wet one for DHW. They hardly ever go but do go occasionally.

Ever since I opened the instrument panel, the boiler has been firing OK

Tends to point to a loose wire or connection sometimes a faulty wire and occasionally a pcb.
Intermittent faults are the worst to sort out. When it aint broke you can't fix it! Sometimes its just best to live with the fault until it becomes frequent enough to work with.

Follow the wires and make sure the current makes the full trip from pcb to switch and back.

IIRC Resistance of thermistor is 7k @ 36.9ºC [10 mins under armpit temp!] and 9 at room temp 21ºC ish.

Manufacturer states its operating range is from 500ohm to 20k.

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