Potterton Netaheat profile - luke warm water

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15 Oct 2006
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Location
Glasgow
Country
United Kingdom
The boiler seems to fire up normally and run okay for about 10 to 15 mins. Then the burner seems to partially close down and so produces luke warm water to the hot water storage tank and the radiators. It was only through luke warm water in the taps that we realised something was wrong, since the central heating is not on yet for the winter. It also sounds as if the system is working normally until you look through the window at the flame.

I tried setting the boiler control thermostat to max temporarily- but it made no difference.

A new circulation pump was fitted a couple of years ago and seems to be working fine.
The two-port zone valve and actuator also seem to be operating properly.
The fan runs normally and the overheat thermostat hasn't triggered.
The room and hot water tank sensors also seem to be okay.

The system is about 15 years old and has had the fan replaced a number of years ago. Numerous valve actuators have also been fitted over this period of time.

It all seems to be pointing towards the boiler - but I don't want to be told to buy a new one unnecessarily. Watchdog type programmes make me fearful of being ripped off either for unneccessary parts or excessive labour charges.

Thanks in advance for any suggestions of possible causes.
 
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Hi, sounds like it could be a cylinder stat or boiler stat problem. Try turning the cylinder stat up and see if the boiler fires. Failing that it could be a knackered boiler stat or sludged up heat exchanger.
 
Thanks. I've just timed it and it took 10 minutes to go from normal to "shut down" to small flame. Fortunately I was looking in the window when it happened. It "shut down" quite abruptly [i.e. not a gradual change] and also heard a click inside the boiler. Does that suggest the boiler thermostat? I thought it would have shut the flame off completely. I've kind of eliminated the tank and room 'stats since the same problem occurs whether on HW alone or CH alone. Also experimented with turning these 'stats up and down until clicked and the boiler "fired up" at the appropriate times.

The overheat thermostat doesn't trigger - so does that suggest the control thermostat? Sludged heat exchanger - mmmmm - a possibility - there have been quite long periods without anti-corrosion protection.
 
Your boiler needs urgent attention. It should not do a "small flame". It could be producing carbon monoxide, sooting up, overheating, leaking unburnt gas when not alight, etc
 
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Thanks for the advice and suggestions. On his first visit the engineer reckoned it was either the solenoid, PCB or [sticking] gas valve. He was however puzzled by the fact it fired up and ran for 10-15 mins first. The "small" flame I referred to was actually the pilot flame - so the main burner was shutting down completely. He tried tapping the gas valve with his screwdriver handle to see it that freed it - no difference. He agreed to try the parts in reverse order of cost - and yippee - the solenoid worked! He showed me touching it with a screwdriver when activated - to show it was energised. He said that hadn't happened with the old one. I'm delighted - he didn't even comment on the age of my boiler never mind try to sell me a new one. I'm still puzzled re the predictabilty with which it worked for 10-15 minutes, I would have thought a coil would have worked or not. Maybe through age its resistance had increased [like heating elements] and 10-15 minutes was just long enough to heat it to a stage where its resistance had increased so much it didn't carry enough current to hold the main valve open. :D
 
It's common for solenoids to work cold but go open circuit when they get hot and expand. ;)
 

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