Potterton Puma 80 Combi

Any suggestions as to the cause of the following fault.
Hot water is working OK.
Central heating switches on for 10-20 seconds, hot water emerges form heat exchanger into diverter valve and would appear to the enter the CH return side of the CH system (yes the valves are open). Water would just appear to be pumped around the combi `end` of the system. Water gets so hot, gas valve cuts out.
Diverter valve (full) has been replaced, problem persists. CH is also full of crap and would appear to need a good flush, this has been attempted (possibly not completely flushed) but problem still persists.
Any experts with this combi ??
Glenn
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glenn wrote:

These boilers are a great boon to heating engineers and a bigger one to the manufacturer who only sells the complete diverter valve (best part of 100).
If you had not had the diverter valve replaced I would say without hesitation that the wax pellet acutator on the front end of the diverter valve has gone tired and is not shifting the valve to CH mode.
However the symptoms you describe are absolutely the same.
1) A dripping HW tap can keep the actuator cool enough to prevent CH coming on especially when the pellet is nearing the end of its shortish life.
2) Was it possible that the actuator was not replaced? Or the exchange unit was not new?
3) Is it possible that all the radiators have TRVs and the temperture is still too warm to let any circulation through the circuit?
4) The diverter valve can't move due to much debris (it would have to be really bad for this). HTH
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glenn wrote:

The sludge could cause the problem - in which case you will need to get everything cleared out in due course.
If there was so much air in the system to cause this problem then it would also cause HW problems - chekc the auto air bleed unit is working and its dust cap is open.
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Is there any reliable way to test the air bleed unit ?
Glenn
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It usually has a shrader valve on the top. While the system is off press this down. If air comes out then it is malfunctioning. It should automatically vent air out of then system.
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> Is there any reliable way to test the air bleed unit ?

Glenn
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The better auto air vents can be changed without a drain down. If you change the vent get one of these.
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IMM wrote:

By no means all types of auto air vents have schrader valves. Also replacing parts on any gas appliance with anything other than an manufacturer's original spare part or upgrade kit is contrary to the regs. Ergo it's illegal, however innocent or well meaning your 'improvement' might be.
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You are obviously hard of thinking/reading. I "usually".

Asute observation our unqualified excet for a CORGI certificate cowboy.

In your dreams cowboy.

Keep cleaning the boilers, that is all the likes of you are fit for. BTW, is the fish smell strong in Grimsby today?
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geoff

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Maxie, you need therapy. Have you got a thing about fish?
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writes SNIP

Thats why he's in my killfile Geoff! If you hadn't left his quotes in I wouldn't have seen his post but hey if it satisfies his little ego he can bay at the moon all he likes. You know my quals and so do I, my previous employers, my Institution, and CEI so his comments are lost on deaf ears. (Perhaps he might like to learn a bit of geography as well)
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writes

Your qualifications are not in CH/gas. You have wormed your way into a CORGI ticket. That is clear by your plain ignorance of the business. You are the clear...a little knowledge is dangerous. No wonder people are screaming to bring back the apprentice system.
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