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
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