Water reaches temp. Boiler shuts down.Pump keeps going?

Postby filip » Sun Mar 13, 2011 8:29 pm

My boiler fires up ok when the heating timer calls it into action. It burns and heats the tank up to temperature and then the boiler shuts down. the pump keeps going though. Every now and then the boiler kicks in to 'top up the heat'. Should the pump not switch off after a few seconds and only run when the boiler calls? The pump switches off only when the programmer reaches its 'off' time. :shock:

Otherwise all is well!!!!

Any suggestions gratefully recieved.
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Simply Build It

Postby Kenj » Mon Mar 14, 2011 6:58 pm

Hi filip,

Your pump should NOT run continuously.

It may continue to run, or startup for a couple of minutes after the boiler shuts down. This is to flush through very hot water in the modern small heat exchangers. Otherwise the heat contained in the metal could cause the water to boil.

If your pump only stops when the program finishes then there is a fault. The most likely cause would be the pump control pcb in your boiler.

Check that the boiler pipes cool down a little within a couple of minutes of the boiler shuting off. If they do then it will probably be the pump control pcb at fault. If not reply for more options.
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Postby htg engineer » Tue Mar 15, 2011 6:16 pm

The first question Kenj should have asked is what type of boiler ? he is assuming it is a combi.

If a gravity hot water system - when central heating is on the pump WILL run continuously.

htg engineer
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Postby filip » Tue Mar 15, 2011 8:58 pm

Thanks for both of your comments ,

The system is a fully pumped open system. It has a Baxi back boiler
( serviced). It has a new pump, new three way diverter valve, new pressure relief valve (set at 3.5), Its been cleaned,drained and flushed etc. It is set up correctly as far as I know

Water is selected as on ( tank set for 60 deg, boiler thermo is turned about 3/4s the way around)
Heating is selected as on (19 deg and not demanding cos room is warm enough)

The fault is intermittent...this evening it has operated correctly but later on it has misbehaved. Again tonight the tank called for more heat and the boiler kicked in after the pump had started up. The boiler then cut out but the pump kept going . and going... and going!
A red light shows on the front of the boiler so I turned the thermostat to zero. It changed to amber and I re set it and the boiler kicked in. the pump carried on after the boiler shut down again. If its the PCB can yo u tell me what that is and what ca nbe done / how to fix it to eliminate it please. If its not the PCB?

Thankyou Filip
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Joined: Sun Mar 13, 2011 8:17 pm

Postby Kenj » Wed Mar 16, 2011 7:46 pm

Not quite sure why you would have a pressure relief valve on an open vented system !

However, for the pump to intermittently keep running you either have a water flow problem or an failing electrical component.

That is why I advised you to check that the boiler pipes cool down a little within a couple of minutes of the boiler shuting off. If the pipes do not cool then you have a water flow problem. Possible causes: Pump, blockage, lack of water etc.

If the pipes do cool down then the boiler is not detecting this or is incorrectly responding to this cooling signal. This could be caused by a faulty Thermistor, Overheat Thermometer, or Control PCB.

If you folow the fault finding chart in your user manual, it should tell you what the red and amber lights means. It may well tell you how to test the overheat sensors.

If for some reason you do not have the manual you may be able to download it from here - Google "baxi.uk.com."
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Postby filip » Thu Mar 17, 2011 8:12 am

:D The answer to the problem was staring me in the face!!!

After removing the fire front I could see the lights for the boiler. After you mentioned the PCB I dug out the boilers paperwork and found there was a flow chart fault finder linked to the way the lights behaved. It indicated that the fault lay in a heat sensor or if not that the PCB!

The heat sensor had to be tested to check .... so it was a job for the professionals.... in came the fully qualified engineer and ran some tests as well as casting an eye on my DIY effforts and giving them the thumbs up. The sensor was faulty. A £20 part. the engineers call out fee and labour was rather a lot more .... but well worth it. I was impressed with how he went about his work. Methodical and logical. He is a member of check a trade so I will be making my ten out of ten feedback report.

Thanks all .... you got me in the right direction. :) :)
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