Overheating Potterton Prima 80F

[crogthomas]

I have a overheating problem with a Potterton Prima 80F boiler. It’s operating a heating and hot water system with a hot water storage tank and a three-way valve to control the flow via a timer switch. There is a tank thermostat but no room thermostat. I’ve just bought the house and have no previous knowledge of how the boiler was operating before.
What is happening is that the boiler is tripping the overheat thermostat, so that I have to manually reset it once it has cooled down. The trouble is it doesn’t do this all the time, only occasionally in warm weather, but much more frequently in cold weather (around freezing) it seems. It’s worse with the boiler thermostat set to MAX and much less frequent with it set lower. I have the boiler thermostat set to ‘3’ normally, but it still does occur at lower settings.
I measured the temperature of the nearest (non TRV) radiator with a non-contact thermometer. With the boiler set to ‘3’ the radiator heats up to around 90 degC before the boiler thermostat switches the boiler off (if it doesn’t overheat first), the radiator then cools down to about 60 degC before the boiler switches back on again. This tells me three things:
1) The pump overrun thermostat is working correctly (pump is running all the time over 80degC)
2) Assuming the boiler is going to be a few degrees hotter than the radiator, the overheat stat appears to be working correctly. Which stat gets there first seems to be pot luck.
3) The boiler is overheating as a result to the boiler thermostat not functioning correctly. Or is it?

The boiler thermostat does work, I can rotate it and hear the contacts clicking inside at the approximate point relating to the current water temperature. It seems to be ‘lazy’, i.e. not keeping up with the changes in temperature. I expect that it is supposed to have some sort of lag or damping, but is this 60 to 90 deg C (at the radiator) band normal?

I have tried cleaning and reapplying some heat transfer grease to all of the thermostat bulbs since the original stuff was dried out, but this does not seem to have made any difference. All of the radiators are bled and heat up evenly, nothing has been replaced recently and changing the pump speed makes no discernable difference.

So the question is, if you are still awake; is this lagging behaviour normal for the boiler thermostat? I would expect it to maintain the radiator temperature within a much tighter range than 30 degrees. Should I be looking elsewhere or shall I just get the stat replaced?

All of the gas engineers I have contacted so far have suggested either replacing the boiler or replacing parts one at a time until it is fixed. I’m not happy with either of those suggestions. I’d rather diagnose the problem with the help of your collective wisdom, then get them to replace the part.

Thanks in advance for any advice.

Craig

 

[twgas]

try changing the,stat sound nakkered to me

 

[crogthomas]

So that isn’t normal behaviour then? I don’t want to replace it if it is functioning perfectly normally and the problem lies elsewhere...

 

[twgas]

you have already said temp gets up to 90c on setting 3, on max should shut off about 82c-stat almost certainly not working

 

[Agile]

I
All of the gas engineers I have contacted so far have suggested either replacing the boiler or replacing parts one at a time until it is fixed. I’m not happy with either of those suggestions. I’d rather diagnose the problem with the help of your collective wisdom, then get them to replace the part.

Thanks in advance for any advice.

Craig

I am a little dubious of what basis you have been contacting boiler engineers. I suspect its for free advice?

Somehow I get this feeling that you are going to fit the new thermostat yourself!!!

Your boiler is so simple the only part which is relevant is the thermostat. That should turn off the boiler when the flow reaches 82°C so if your temperature measurement is correct then set it to 2 or 1 and see what happens then.

It sounds as if the hysteresis is too great and that the stat should be replaced but its the flow temperature of the boiler thats relevant and not the rads. It should only be about 3-5°C lower when the boiler comes on again.

Tony

 

[tryitandsee]

Just reading your script. I have the 60 primaf and looking through my service instructions note that the flow and return connections to the 80F should be 28mm. Just thinking out aloud, are your flow and return pipes 28mm and not 22mm (like mine). This would not allow sufficient water flow through boiler to absorb heat output of burners. Just a thought.

 

[bengasman]

The solution is extremely simple: next time you contact a heating engineer, ask him to come and repair the boiler.
This is a very busy time for us, and when we get a call from someone who is obviously only asking a 17th opinion, you give a general answer and carry on working for the customer that is actually willing to pay you.

 

[crogthomas]

The inlet and outlets are both 28mm, so that’s all okay. I wouldn’t imagine that would cause the water in the radiators to become too hot though.

Thanks Agile for confirming my thought that the thermostat has too much lag in its operation.

And to answer the replies about contacting engineers, I HAVE asked three to come out and repair my boiler. One suggested that due to its age it wouldn’t be worth fixing and that he would fit a new condensing boiler. I’m not interested in that solution right now. Two said they would come straight out and replace the overheat stat and see what happened. I wasn’t convinced that the overheat stat was the cause and when I asked them whether they could diagnose the problem before replacing the parts, the answer was no. Willing to take a chance I asked one of them to come out and do the work anyway. He didn’t turn up. I didn’t bother asking the second chap to come.
All three were advertised as being CORGI registered.

Perhaps as heating engineers yourselves you do not realise how difficult it is to find a decent engineer?
I am also aware that I am not qualified to replace the parts myself, this is why I will be getting someone to do it for me, they can perform a general service at the same time.

I do get annoyed that someone who is a member of a professional body that is supposed to offer some sort of quality guarantee would be unable to diagnose and repair a boiler. Presumably there is more money in fitting complete systems or multiple callouts, replacing parts one at a time until it is fixed. Or am I being too cynical?

 

[Agile]

Perhaps as heating engineers yourselves you do not realise how difficult it is to find a decent engineer?

I am also aware that I am not qualified to replace the parts myself, this is why I will be getting someone to do it for me, they can perform a general service at the same time.

I do get annoyed that someone who is a member of a professional body that is supposed to offer some sort of quality guarantee would be unable to diagnose and repair a boiler.

Presumably there is more money in fitting complete systems or multiple callouts, replacing parts one at a time until it is fixed. Or am I being too cynical?

Unfortunately your assessment of the heating industry is very accurate.

There are very few boiler engineers who have any significant diagnostic skills or even much understanding of the finer workings of boilers or the electronics which control the later ones.

Yours is comparatively simple and most would know how to deal with it so its probably just bad luck that you have not found anyone very competent.

Fitting a new boiler for around £2000 is a much more profitable job and most engineers who cannot do repairs follow that option. Even some who can do repairs will try that on too,!

At least you have heat and hot water over Christmas unlike some of those in social housing where the contractors are already off for the week.

Tony

 

[tryitandsee]

If you can use your system with controls which allow the boiler to operate when both the hot water and central heating circuits are closed, the system must be fitted with a by-pass circuit capable of dissipating heat. Otherwise the boiler will activate the cut out.


Perhaps your system has not got enough by-pass circuit to dissipate the heat?
If you are happy that it has, then your boiler thermostat (which when working normally controls the water flow temperature at between 55 to 82 degrees C) is perhaps out of calibration and needs changing. You have already measured the heat at some point and say it reaches 90 degrees C.

Some thing you could easily do yourself, at a material cost of £10, worth doing anyway, just to illiminate it.






http://www.hccfspares.co.uk/Appliance_Type3.aspx?Manufacture_ID=6&Appliance_Type=GAS+BOILERS

 

[crogthomas]

I thought I'd add a follow-up to this post, for the benefit of anyone else with a similar problem.
I eventually got someone to come out after the holidays, who appeared to know what they were talking about. They replaced the boiler thermostat and the boiler has been working perfectly ever since. The temperatures now fluctuate by less than 10degC.

Thanks for all advice.

Looking at the old thermostat its now obvious what was wrong. Where the small capliliary copper tube entered the thermostat bulb there was a crack all the way around the soldered joint. Presumably whatever the gas or fluid used inside had leaked out. Quite how it was working at all is what confuses me.

 

[twgas]

only took 4 weeks for you to believe me then!!!!!!!!!