Boiler thermistor broken?

[Robert Irons]

Hi all. Apologies for the long post.

I have been having some central heating issues recently....started after I had to drain down the system after a leak, refilled and ever since has been problematic. However I have always had problems with this system and have needed to run the pump on full speed to get 3/4 of the rads warm (and then never particularly warm)

Anyway, in my travails I have:

1) reinstated old separate feed and vent pipe on the advice of a local plumber (had been installed with separate feed and vent but someone in the past had capped off the vent and combined them). I was told this would make it easier for the system to clear any air locks...made sense to me as given things got worse after a drain and refill air locks seemed likely culprit
2) backfilled the system twice via the draincock, drained and refilled. Flushed out several of he more problematic radiators. I have automatic vents fitted on both the CH and hot water circuits after the 3 way valve (its a Y plan system). Have bled everything multiple times.
3) Added CH system cleaner and ran it through groups of radiators for several days at a time
4) Removed old pump as anticipated it would be gunged up. It was not, but replaced anyway with a grundfos UPS3.


At this point I had hoped that maybe old pump was just a bit knackered but putting in new pump doesnt seem to have made any difference ( as i was warned by some on this forum it may not!)

everything fires up straight away, 3 way valve seems to be working correctly. However boiler comes on for only a few minutes a time, whether its the hot water cylinder thats being supplied or whether its the rads. It comes on for longer when it goes to the cylinder vs the rads (usually about 3 minutes to the cylinder, 2 minutes to the rads). It then switches off for a few minutes and then comes back on again in a similar pattern

I had assumed this was because the boiler was reaching its max temp ( which I think should be about 80 degrees right?) too quickly because of poor flow round the system due to sludge.
However Ive now checked the temperature of the feed and return pipes when it comes on. When the boiler comes on the feed pipe is at about 30 degrees. It seems to shut off consistently at about 52 degrees without fail. I have not seen it get any hotter than that.

I initially thought maybe it was the honeywell programmer not calling for heat consistently, so timed it as surely if it was the programmer there would be a time dependent pattern to the on/ off - but this does not seem to be the case as if I turn the boiler temp down it goes off for longer etc..

Basically Ive come to the conclusion that the boiler is shutting itself off due to an error reading the temperature that the feed pipe is getting to.......and the reason my CH has been so crap is that my pump spends alot of time circulating cool water around the system when the boiler is off......

Any thoughts?

 

[BlueLoo]

That pump...

Is it fitted correctly?
Is it pointing the right way?
Do you have it on the right setting?

Check that first. Do the obvious stuff. Valves open, etc.

Have a read of some recent threads in this sub. Much of what you have is commonly discussed.

 

[Robert Irons]

Thanks
Yes it is fitted correctly
I have it set on proportional pressure curve 2 which according to manual fits my system as closely as possible (relatively long distribution pipes, TRVs).

Irrespective of what the pump is doing surely the boiler should be heating the feed side beyond 50 degrees?

 

[DP]

Get professional help
You will be up a gum tree listening to some YouTube watching pundits.

You have several issues with the system that a helpful heating engineer might be able to advise you on or fix these.

 

[Robert Irons]

Thanks - Afraid I have lost a bit of faith in the professionals. Last one that came round (he was there fiddling for around half an hour), advised me to basically do what ive done. And has made no difference....

 

[CBW]

What boiler is it and what is the boiler thermostat set to?

 

[Robert Irons]

Worcester bosch Greenstar r15i
Have had the boiler thermostat on max or jus below

Interestingly it used to cut out with the blue flashing light sometimes when it was left on max for too long - Id always assumed this was because it overheated.
Hasnt done that for the last few weeks when on max but again has done so today and the boiler (or at least the exiting feed pipe...) certainly was not 'over hot'....

 

[DiyNutJob]

Short cycling can be a symptom of the boiler thermostat being set too low or defective. On my boiler at least, insufficient flow would result in kettling.

 

[DP]

Thanks - Afraid I have lost a bit of faith in the professionals. Last one that came round (he was there fiddling for around half an hour), advised me to basically do what ive done. And has made no difference....

Crack on then old chap.

 

[DiyNutJob]

Thermistors do fail. I have one right now on my bosch dryer. The thermistor can be diagnosed with with resistance measurement. It has a certain value at room temperature according to service information. Lot's of how to's are on youtube. But, the specific measurement range is usually proprietary.

 

[oldbuffer]

1. When you say the pump is the right way round, are you sure you mean the arrow cast into the body is pointing the same way as the same arrow on the old one was pointing? The fact that the new may be in the same orientation (e.g. electrical box on top) as the old, doesn't necessarily mean they are pumping the same way. If pumping the wrong way you will need to:
1a. Electrically isolate and remove the pump.
1b. Undo the four cap headed bolts securing the pump to the body and gently separate the two parts.
1c. Rotate the pump 180 degrees and tighten cap headed bolts, making sure the seal isn't damaged.
1d. Replace to pipework with the arrow pointing in the right direction.
1e. Re-connect.
2. Personally I'd try the pump in fixed mode on speed 3. The worst that can happen is that you get pumping over into the F&E tank. If so try lower speeds.
3. Beyond that, I'd get someone Gas Safe registered (GSR) to change the main sensor. Its a dry job that shouldn't take more than 1/2 hour. You aren't supposed to remove the boiler cover yourself. Relatively cheap potential fix.

 

[Robert Irons]

Thanks for previous advice. I can confirm that the pump is definitely installed in the correct orientation.

Bit of an update if I may. Up until about about a week ago the system was unchanged vs whats described above.

One evening both my wife and I heard a slightly odd clunk come from somewhere and since then the boiler has more or less STOPPED cutting out/ rapid cycling.

Yesterday I re-balanced the radiators and all rads are now a pretty even temperature, aside from one black sheep which I dont really care about.

The pump can now circulate to all the rads fairly easily on speed I or II (currently using I).

When the heating is on, the boiler now seems to stay on all the time, only very occasionally turning the burner off.

However what Im now observing is that the flow pipe directly where it comes out of the boiler never gets above about 52 degrees when the heating is on, and above about 61 degrees when the cylinder is being heated. This is with the boiler temp set to maximum. The return is consistently about 13-14 degrees lower than the flow. When the water is on, if the feed pipe temp gets to about 62 degrees then sometimes the return is about 30 degrees lower and the boiler cuts out, which I assume is to protect the heat exchanger as the temp differential is too high. As the water cylinder heats the return pipe temp rises and it doesnt cut out (as I would expect).


So I am now back to thinking that while I may have sorted the flow round the system to a fairly decent degree, the boiler itself is not heating the water to the correct temperature. As I understand it you would like the return temp to be about 50-55 degrees, with the feed being about 70-75 degrees. I assume that my boiler hopefully just needs a service and possibly a sensor change?!

 

[DP]

Not making much progress I see
When I say seek professional help I mean a person who is adept at fault finding
What you have done so far is no different from what a heating engineer would have done who really is a part changer with no diagnostic skills.

 

[Robert Irons]

Thanks DP
I actually think I have made some progress in the sense that my heating now comes on consistently and stays on, albeit at a lower temperature than is needed.
I have also demonstrated to myself that a powerflush probably isnt the solution here, which is something Id been wondering about for several years.

I accept that I have now reached the limit of my competence/ abilities and if the fault lies with the boiler then clearly I will get a professional in to look at this.

The question is how to find someone who isn't 'just a part changer' isn't it though?

So far Ive had one engineer say to change the pump and reinstate the vent pipe (which didnt really make any difference), and most recently have had a second telling me that the whole system should be converted to a pressurised one with a new, larger capacity system boiler....

This latter person is the same person who installed the boiler before we bought the house. He at least did have the honesty to say that he thought the whole system is a 'bit crap' - and apparently told the previous owners that in his opinion the only way to overcome the poor design of the system was to convert it to a pressurised then but as they were in the process of selling they were only interested in doing it on the cheap. However what makes me a bit suspicious of them is that they are also recommending putting in a 24 - 28kw boiler, which from what Ive read really ought to be more than we would need. So again makes me feel like they might just be a 'parts changer' - as in make if bigger, newer, and it will inevitably be better. I am inclined to hear them out though as they did have the honesty to say they felt they had been forced to do a second rate job 4 years ago (which tallies with what I know of the previous owners).

DP you don't seem to hold DIY'ers in very high regard, but the way I see it whatever you say Ive saved myself at least £400, possibly £1200, but doing it myself, and not paying someone to do work I could do myself (the pump/ vent) or work I thought I needed but it turns out probably dont (and had been suggested - ie the powerflush).

Anyway - seeing as you clearly like expressing opinions (!) - any opinion on the argument that a somewhat poorly designed vented system can be made to work better by conversion to a pressurised one? I cant find many resources online suggesting that this is genuinely that beneficial. In fact I've read quite a lot of stuff arguing that vented is less problematic overall......
Logic would lead me to conclude that poorish flow in a congested system would be better at a higher pressure I guess....

 

[HERTS P&D]

Thanks - Afraid I have lost a bit of faith in the professionals.....

So why are you on here then? or do you just want help from Diyers and not Professionals?

Andy