Combi boiler getting too hot (so cycling)

In article , Richard Walker writes

Did you work out whether it was modulating correctly?

I've just been re-running some tests, and can confirm that the gas valve modulates correctly when I drive it with a variable power supply. If I monitor the CH loop thermistor and adjust the modulation voltage accordingly (anywhere between 0 and 15V, steadying around

I re-connected the control PCB to the gas valve modulator and measured the voltage. Running the hot tap again showed the modulation voltage start on a nice high 15V, but never shifting. I even turned down the HW temperature potentiometer, and the modulation voltage didn't shift.

This kind of makes me think the PCB is refusing to modulate, so could be broken. But why would only that aspect of the PCB be broken, when everything else works fine?!

I did not try my (rather dodgy) manual modulation method with the CH to see if I could get it to run without cutting out, but I assume I could. In fact, I might give that a go now...

Many thanks, Richard.

That's a major step forward I think, you now know that the gas valve and the modulator are working fine. It certainly now points to the pcb or the sensors feeding it. How about now replacing any temp sensing thermistors with variable resisitors to see if you can manually control and modulate the boiler with them. If you can then it points to faulty sensors, if you can't then it's moving back to the pcb. As to one part of the pcb working while another doesn't, that doesn't surprise me at all, different functions in different sections of the pcb mean one function can work while another doesn't. I haven't re-read all of the old thread so don't know if you've already tried changing the pcb.

I have tried using spare thermistors just dangling in the air, so they don't realise things are getting hot. Unfortunately, the CH loop gets hotter and hotter, rather close to 95 degrees C (at which point, the overheat stat kicks in!).

I tested the PCB modulation with the CH, and found that the PCB does actually vary the voltage. It starts at 10V, and modulates down to almost nothing in the space of 10 minutes (once the rads are all hot). Unfortunately, even at minimum flame, the CH loop still gets too hot and the thermistor intervenes and cuts the flame.

I've also tried the manual modulation method with the CH, and it doesn't work! The CH loop gets too hot, even with the modulation set to 0V (minimum). So, my manual method is exactly the same as the PCB for CH (although for HW, the PCB always uses 15V).

No, I have not tried changing the PCB. I was kind of assuming they were an all-or-nothing affair. The CH modulation seems to work, so I find it rather hard to believe that only the DHW modulation function is broken! :)

I suppose that if the PCB is OK, then it does rather suggest that the CH loop is not able to circulate properly. Duff pump, or partial blockage, maybe?!

Many thanks for your comments.

Cheers, Richard.

Sorry matey, thought it was your DHW side that was screwed up, not your CH. The fact that the boiler is cycling so quickly suggests to me (as a non-expert) some flow issue. Is it possible the diverter is stuck on a little (or leaking) and the CH loop is being short circuited (even partially), that would result in no (or reduced) CH flow, no DHW demand so nowhere for the heat to go, hence the fast cycling. Don't know if your diverter has a diaphragm but it's a bit of a classic for splits & leaks, also a sticking diverter.

Sorry, I probably didn't explain it too well!

Tallies with my current thinking (pump or heat exchanger).

The diverter seems to be working OK - it doesn't pass to the full CH circuit when it is asked to link in the HW. I'll see if I can check it in CH mode, though.

I guess it could be rather sludged up which would screw up the flow?

Yes, in fact, I think I've got a diaphragm repair kit somewhere...

It looks like I need to remove all these bits and have a good look at them. :(

Cheers, Richard,

The boiler heats all the radiators in 10 minutes, so you say. It is most unlikely that there is a duff pump or partial blockage in the primary.

Is there a separate sensor for the DHW? If so and the sensor has been verified then it's likely the PCB. The modulator driving circuits are OK but perhaps the DHW sensing circuits are duff?

The only blockage there could be is in the secondary heat exchanger (but ISTR vaguely that you check that out). Even so I would expect the boiler to modulate rapidly from max to min before cutting out on DHW.

HTH

Yes. And on CH-only, the boiler keeps re-firing then dying for short periods (sometimes it will fire for as little as 10 seconds before deciding it's too hot, then wait for 30 seconds, then start again).

Yes, there is a wet thermistor on the DHW output pipe from the heat exchanger (boiler has a single-piece heat exchanger for DHW and CH).

I believe that the value from this sensor should determine the modulation level, but although the sensor is definately OK (was replaced as a precaution) the PCB doesn't bother to modulate down the gas.

Yeah, that makes sense for DHW.

However, when in CH mode, the boiler cycles (as I say above), even though there is an anti-cycle device which should prevent this for at least 2.5 minutes.

I see. So the CH side of the exchanger could be blocked?

That is exactly what happens on the CH side - it starts off with a high-ish flame, then as the CH loop warms up, it reduces down. Eventually, it gets to minimum modulation, yet the CH loop is still too hot (so the flame dies).

Perhaps I've got two problems here:

a) iffy PCB in DHW mode, possibly re. the DHW temperature sensing, as you say

b) poor flow on the CH loop (heat exchanger, pump or diverter?)

???

I think I can get hold of a new pump and diverter, so might try taking them out and checking for blockages. Depending on my findings, I could then be on the hunt for a PCB...

Anyone tried this sort of thing before? Any tips on removing these wet parts and flushing?

Cheers, Richard.

If there are TRVs and they have mostly closed then you could get that behaviour. I'm not familiar with this model there may be a bypass circuit. Or more simply the required heating load is 1/4 of the minimum power (typically 10kW) so 2.5kW to keep things warm.

I'm sticking by that. If the boiler heats the rads in 10 minutes a significant heat transfer has taken place.

Is is a tube-in-tube type that has to keep the primary circulator going in DHW mode; the heat transfer is then gas-metal-primary water-metal-DHW. Or is it more like a Ferroli with two direct exchangers in one unit?

The anti cycle device might only operate on the external control i.e. if the thermostat goes off and then on the boiler will impose a minimum 2.5 mins, but not on the internal stat?

That's exactly what it should do. After that it short cycles on the sensor between off and min. Remind me how few rads you have?

I doubt it. A) The rads get hot in 10 mins. The diverter valve simply short circuits the primary - in which case you almost certainly have a tube-in-tube type heat exchanger, which means the circulator must run in DHW mode.

What might be happening is that there is a problem getting the heat into the DHW. The primary is getting too hot so the gas has to be cut. This is being picked up by the CH sensor not the DHW sensor.

If you are in a hard water area I'd say that the DHW side of the exchanger is full of chalk.

I've lost track of the original symptoms IIRC they were:

CH OK. DHW cycles between warm and off. Boiler is cycling between min modulation and off in DHW. DHW unusable because of the fluctuations.

The boiler has a tube-in-tube one piece heat exchanger (I guess) The diverter valve simply short circuits the rads in DHW mode (I guess). The pump is OK (cause the CH works OK). The burner is OK The sensors are OK, both of them. The PCB is only a bit broken if at all.

I'd say if I cam across this and it was a hard water area that we are looking at chalk in the DHW exchanger. Just possibly crud on inside the primary heat exchanger - but the CH works well so can't be too much wrong.

In a softwater area you'd have to go for the PCB somewhat in hope over desparation.

If you are in a hard water area and don't have a descaling pump then you will have to find a a) bucket/bowl capable of immersing the entire heat exchanger in. b) remove the heat Xer c) obtain 5litre of screwfix patio cleaner say adding water to cover. d) put unit in bowl add acid leave for several hours turning occasionally to get new acid into the tubes. You could also spend a couple of hundred on a new unit and refurb the old one in readiness for the second run of this problem in a year or two.

Hi Ed,

Thank you very much for your detailed reply.

I should have said: I don't have any TRVs, although all but one rad has the return valve open only one turn (just to balance the system - otherwise, one of the rads does not heat at all!). For what it's worth, there is a room stat, and I could always short this out to eliminate it.

An external bypass should be fitted according to the book. In the pipework just below and boiler, the CH flow and return are linked with a gate valve which is open two turns. If I close this completely, the pump makes a dreadful racket when the CH is on!

Makes sense. Also, when I run the DHW, before the boiler cuts out, the water does get VERY hot and it comes out very fast!

The manual describes the exchanger as a 'double parallel' type which uses the 'countercurrent system' for DHW. From the picture in the manual, it looks like a tube-in-tube. And yes, the pump runs in DHW mode.

Maybe. The manual doesn't say much: only that the anti-cycle stops the CH re-firing for 2.5 minutes, and that the DHW operation is not affected.

I've got 7 in the house (small three-bed semi).

The cycles are VERY short, however. The flame stops, then within about 30 seconds, the boiler starts up and fires for maybe only 10 seconds. This is why I wonder if the CH loop is getting too hot.

True.

Yes, that's correct. I was also wondering if the diverter could be blocked.

Yes. I've just made another discovery: the DHW sensor doesn't appear to be powered-up by the control PCB! No wonder the boiler doesn't realise the water is getting hot and that it should modulate down!

The sensor does work (the resistances are good) but when the PCB is connected, it always shows 0V, whereas the CH sensor will be at approx

I suppose this explains why the CH does modulate and the DHW does not.

Thing is, the water is nice and soft here. :-/

Yes, apart from the excessive cycling when the CH has got warm (after being on for 10 minutes).

Yes. Although I may have just discovered why (DHW sensor not live).

If I can assume it's OK because all 7 rads warm up nicely in 10 minutes, then yes, it must be OK.

Yes.

Possibly. It is not powering (so therefore not able to read) the DHW sensor so cannot modulate on DHW. And the anti-cycle doesn't seem to work on CH mode.

Then perhaps this is where I am.

Yes, I can see me having to do that if I do end up cleaning the exchanger, and manage to break it! :)

Cheers, Richard.

I think you have found the smoking gun with there being no volts on the DHW sensor. Could it be a simple bad connection?

Otherwise the PCB really is the trouble.

Yes, this makes sense. And I'm also wondering if the CH anti-cycle function is broken.

I wondered this when I discovered the zero volts, so I bypassed the wiring loom and measured directly from the PCB - still zero volts. OK, so maybe there is a tiny dry joint, crack, single component failure somewhere on the PCB, but I'll never find it in a million years!

Sounds like it, doesn't it? I'll see what I can get hold of, and I'll post pack with my results. Fingers crossed!

Cheers, Richard.

I found a local engineer who carries plenty of Ravenheat spares (and will actually work on them, rather than complain about DIYers!). We tried fitting a new PCB. Result: exactly the same. I couldn't believe it!

Almost by fluke, he decided to change the wiring loom which runs from the PCB to the DHW potentiometer and DHW thermistor, as he had a spare one. Result: voltage present on DHW thermistor! Then we put my PCB back in. Result: the same, voltage still present on DHW thermistor! Hurrah!

The reason I didn't spot the problem in the wire is because I tested the loom from the sensor ends to the 'top' of the PCB plug. If I'd tested to the bottom of the PCB plug (or direct from the PCB) I would have seen that the PCB was fine, and the plug was shafted. D'oh!

Anyway, the boiler now seems to measure the DHW temperature and modulate the gas valve accordingly (I can hear the valve clicking slightly as it does this). The DHW is a lot more reliable.

However... it will still sometimes cut-out, wait 5-10 seconds, then re- fire when running the DHW. The CH does something similar once the rads are all up to temperature (I think the anti-cycle is too basic to counter this).

So... We're thinking that the CH loop is not flowing adequately (so the CH thermistor gets too hot). Blocked pump, diverter and/or heat exchanger? Anyone know of anything I can try before I strip these parts down (looks quite difficult, and a job for child-like hands!).

Cheers, Richard.

You now have a working boiler - the short cylcing on the CH is probably the way it's made.

To stop the cycling in DHW mode you either need to improved the flow rate on the DHW and/or increase the maximum DHW temp.

Perhaps, but the handbook says there is an anti-cycle function which should stop re-firing for 2.5 minutes. It re-fires well within that time. Mind you, perhaps it's just not very good!

I've been monitoring the temperature of the short CH loop (when in DHW mode). It always cuts when the CH thermistor resistance falls to

I have backed the DHW temperature potentiometer off to about half-way, and the DHW still comes out pretty hot. The modulation kicks-in sooner, and the CH loop doesn't get to 1.2k, so it doesn't cut out. I can run the hot tap for ages and ages. This is excellent!

I also noticed that the CH loop will overheat much sooner if I turn the pump speed down (it was on max).

In CH mode, the boiler dies when the CH loop reaches 1.2k (just like DHW mode) and it will re-fire at 2.3k. It only takes about 25 seconds for the loop to reach 1.2k again, and shut off!

My question, perhaps to all combi boiler owners: is this normal? Should the CH loop get too hot when it's running on max DHW temperature? And is cycling inevitable once the rads are hot?

Cheers, Richard.

Have you check whether the feature applies to the external control rather than the internal themrostat? Simple turn the WALL thermostat down for a few seconds and back up again and see how the system reacts.

Yes turn it lower if you provided that does not cause the burner to cycle.

The modulation kicks-in

And should be left there, otherwise you won't get the full DHW performance.

Better boilers can be instructed to not do full power for CH, they also have anti-cycle delays that work.

More or less for lower end kit.

Yes if there is scaling and/or poor DHW flow.

And is cycling inevitable once the rads are hot? Maybe for this model.

This may be a stock fault (or feature?) for this model. I know of one that does almost exactly what the OP is describing (for CH at least - seems to be OK on DHW).

I've just tried this, and the boiler does as soon as you reduce the stat, then re-fires as soon as I turn it up! If I reduce the CH potentiometer, the boiler simply cuts out sooner (at, say, 1.3k instead of 1.2k resistance on the CH thermistor). Not much of an anti- cycle, really!

The user manual claims that when the boiler reaches its set maximum temperature (whatever that is!), it will not operate on heating for 3 minutes. The installer guide says 2.5 minutes!

OK. I've had it at just over half-way and ran gallons of hot water, varying the flow every not and again. Not a single cut-out. If I put the tap onto full flow, the water doesn't get quite so hot, but I suppose that's because it's only a 24 kW combi.

Thing is, though... the user instructions say that the DHW potentiometer should vary the water temp between 35 and 65 degrees C, and that operating it on max is OK (the CH potentiometer should select between 40 and 85 degrees C).

Okey-dokey. It is a tad noisy on speed 3. Is this normal for Wylo pumps?

This one has a potentiometer on the PCB which can be used to adjust the burner gas pressure. The default is 'middle', and it can be tweaked higher or lower. Maybe I should play with this?

Ah well. You get what you pay for, I suppose!

The DHW flow can be very high (if the tap is fully open) so I think that side it OK. I do wonder about the CH side, though...

Cheers, Richard.

Hmm, interesting. Do you know if the DHW potentiometer also needs to be backed away from maximum?

I'm wondering if I can improve the CH cycling by fitting a more sophisticated room thermostat. Mine is a simple Honeywell analogue device. Don't some of the digital models offer an anti-cycle?

Cheers, Richard.