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My daughter has a Glowworm Fuelsaver 40bf mkii boiler. The

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Hi, my daughter has a...
Hi, my daughter has a Glowworm Fuelsaver 40bf mkii boiler. The pilot light goes out about 30-60 seconds after the heating system goes off, e.g. when the timer finishes and shuts off everything, There is a loud click and the pilot then goes out. The following parts have been replaced - boiler thermostat, thermocouple (twice) , overheat stat (tiwce), pump, motorised valve. The overheat stat has been bypassed for testing purposes only and without it the pilot stays on at the end of the cycle. When it was reconnected the problem comes back. Currently, she has the system set to come on twice for the hot water and continuous fr the heating, and she then just turns the thermostat up when she wants the heating on and the pilot stays on all of the time. The system has been flushed with a chemical cleaner.
The boiler clearly thinks it is overheating but I don't see why it only does this if the heating programs are completely off, once the overrun cycle has finished. I can't find a local plumber who really understands these systems and have all just suggested replacing the parts which have already been replaced by the engineer.
Any advise on what else to try would be much appreciated.
Submitted: 5 years ago.Category: UK Plumbing
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Answered in 27 minutes by:
2/1/2012
Plumber: Rick Ridley, UK Heating Engineer replied 5 years ago
Rick Ridley
Rick Ridley, UK Heating Engineer
Category: UK Plumbing
Satisfied Customers: 2,708
Experience: 30 Years on the tools, now retired, Ex CORGI
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Hi I am Rick, UK Plumber, and I specialize on Boiler problems, this is a common problem with overheat stats that are in line with the thermocouple, the thermocouple only develops a few mili volts and so a poor connection will lose it, the connections must be absolutely clean and tight that's all, Rick
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Customer reply replied 5 years ago
Hi, thanks for that but those connections have been checked by the plumber and are very tight. If they were loose would it not cut out at different times, not just at the end of the heating programme ?
Plumber: Rick Ridley, UK Heating Engineer replied 5 years ago
that is correct so it looks like you do have an overheat problem if you are sure the overheat stat is calibrated properly, What motor valves do you have? if two 2 ports have you a bypass? is it functioning? if three port is the pump running after the boiler shuts down and for how long, Rick
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Customer reply replied 5 years ago
The overheat stat is new, and has even been tried with another new one .. but I'm not sure what you mean by calibrated , are they not factory set ?
The system has a 3 port valve which is new and appears to work correctly (water comes out the right pipes etc. when just heating on or just water on). There is also a bypass with a gate valve that is open by half a turn.
The pump carries on for probably 1-2 minutes after the programme finishes and it is not long after the pump stops that there is a loud click and the pilot goes out. If it was really overheating, would that not cut out during normal operation as well ? Thanks for your suggestions so far, you seem to know more about this type of boiler than many local plumbers who seem to be more experienced in just replacing boilers nowadays.
Plumber: Rick Ridley, UK Heating Engineer replied 5 years ago
by calibrated I mean at the factory, it is not unknown to have a batch of stats calibrated incorrectly, to this end a temperature probe with readout would be useful to determine at what temperature the stat is tripping,with a three port valve no bypass is needed, as there is always a water path for the pump on overrun to deliver too, you can shut the gate valve down and get a better flow to the radiators, the over heat stat is there to do what it is doing, that is trip the boiler (requiring human intervention) before it will start again,and it is tripping because the residual heat is not being taken away from the boiler after shut down of the flame, the first thing to do is monitor the actual flow temperature, the trip temperature is normally around 95 Deg C (it will be on the stat) if it is tripping much lower than this then the stat is out of calibration and will need returning as faulty, Rick
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Customer reply replied 5 years ago
Thanks again and I understand what you are saying but I still don't understand why it only thinks it is overheating at the end of the programme. If it really was overheating, why does it not cut out during the heating cycle. Is there some sort of function to check if the boiler is cooling down at the end and if so which component actually does this, I would assume the thermostat ? The only bits which haven't been replaced are the gas valve and the small connecting block which the thermocouple and overheat stat plug into. Thanks
Plumber: Rick Ridley, UK Heating Engineer replied 5 years ago
It is a little puzzling, this is one of those where if was there to see, hear, smell and feel it, then a diagnosis would be easier, however if the OH stat is accurate? then for some reason the residual heat is not being removed after shut off of the burner, the only other point I can make is, does the burner actually shut down fully, the gas valve may not be shutting off completely, leaving a low flame, but I guess this must be very rare as I have never seen it, but the valve is a mechanical component and as such it can fail in this way, Rick
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Customer reply replied 5 years ago
I appreciate it's very hard to diagnose without seeing it. The flames do go out completely at the end so it's all a bit of a mystery really. I don't seem to be able to find any local plumbers in the Leicester area with your experience either who really understand it. The one we have used seemed OK but nothing he has done has fixed the problem and there's been quite a lot spent on replacement parts already. I know a new boiler will be required for them long term but they are getting married this year and had to borrow money for that hence why I've been looking on here to see if there are any other suggestions as it's had most parts replaced already. Do you think the connector which the thermocouple and overheat stat plug into (which then goes into the back of the gas valve) could be faulty ? I can't find the right ame for it to try and see if they are available to buy separately .
Plumber: Rick Ridley, UK Heating Engineer replied 5 years ago
it could be, it will depend on the method you used to bypass the overheat stat, if you simply screwed the thermocouple into the gas valve then you not only bypassed the stat but the connecting block also it will not be easy to make a temporary block so I suggest soldering a short length of wire to the inner and outer of the thermocouple and solder these to the overheat connectors direct, this will prove or disprove the connection, Rick
Rick Ridley
Rick Ridley, UK Heating Engineer
Category: UK Plumbing
Satisfied Customers: 2,708
Experience: 30 Years on the tools, now retired, Ex CORGI
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Customer reply replied 5 years ago
hi, over the weekend I borrowed an infrared thermometer and checked the temperature of the pipes going in and out of the top of the boiler just as the pilot clicked off and it was showing around 70 Deg C on the out pipe just above where the o/heat stat is (about 69 on the in pipe). So the main question I still have remaining is how does the overheat stat actually shut off the pilot. I assume when the pilot is lit the thermocouple sends a very small current to the gas valve and if the current isn't there the valve shuts off. So when the o/heat stat is added to the circuit ( you are correct about how it was tested , i.e. by screwing the thermocouple straight into the gas valve), does the current pass through the o/heat stat but if it thinks it is overheating will shut off the current , which then obviously shuts off the current to the gas valve ? If so, I'm not sure how soldering the wires would help as presumably the connections are in series, i.e. the o/heat stat needs to be the last connection into the back of the gas valve so I wouldn't have anything to screw into the valve. Sorry if I'm being a bit thick there :-)
Customer reply replied 5 years ago
hi, actually I've just had a thought, the problems probably started after the thermocouple was replaced with a universal one (as the pilot wouldn't even light) but I am wondering if the genuine glowworm thermocouple would kick out a higher voltage and therefore could be the root of all the issues ?
Plumber: Rick Ridley, UK Heating Engineer replied 5 years ago
the voltage from the thermocouple passes through the overheat stat and completes the circuit to the gas valve coil, if the overheat stat goes "open circuit" (high temperature, which you are not getting) then the voltage to the gas valve is interrupted and the coil de energises shutting the pilot off, yes I have had trouble in the past with "universal" thermocouple's, 20 mili volts is the required voltage, Rick
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