potty 80 CH failure question (number 9000)

[David P]

Have tried to search this but no luck so im going to ask in the hope of some kind soul giving time.

small two storey house
three rads downstairs
two rads upstairs
potterton puma 80 combi heater without any external controls/thermo's
combi is quite old
rads are cheap and nasty - one quite badly corroded at the bottom (bathroom)

HW works a treat
CH when called for by timer or constant will fire up and heat for a few mins then stop - the result of this short burn is to heat the rad feed pipe for about 3 feet (up to the T fitting to the upper & lower rads) this short stretch is too hot to touch - but beyond this the pipes are all cold.

last winter I had a similar prob that was caused by a dripping hot tap. I replaced all of my taps and that prob stopped. until now...

the water pressure is ok and constant in the system (although i do get a brief moment of warm circulation if i crank up the water pressure to the red line using the feed tap)
the rads are bled of air and the pump appears to be spinning.

Can anyone help by either -
miraculously giving the answer to the problem
or
suggesting possible causes and solutions
or
giving me a checklist to identify the cause

thanks in advance

 

[kevplumb]

wild guess the diverter valve is stucking or not opening enough for heating

instructions here

 

[ChrisR]

Seems likely.

 

[mmalone]

wild guess the diverter valve is stucking or not opening enough for heating

instructions here

guys came across 3 issues in 1 week with these diverter valves. whats the easiest way to free it if its sticking ? draindown,turn of thecold feed etc to see if it frees ?

the latest issue I had wouldnt free so I ordered the valve only to return today and be told its ok again.

basically should you just replace this when this issue arrises or is there suggested alternatives to try first or will this intermittingly continue if not fixed ?


oh nad has anyone replaced the full valve or is it normally just the wax capsule section that gets replaced.

 

[David P]

Well i went and bought a diverter valve, the whole assembly (just the wax capsule end wasn't significantly cheaper to cost the extra time it'd take to get one).
and guess what - it is like having a new boiler, it heats up quicker than ever before and is quieter.

for the record I only fitted the wax capsule end as the other bit has no working parts or critical clearences - on looking at the removed old part it seems that 'stuck' isnt really a good description, its more like the wax expansion capsule just stops responding to heat change. I would say that repair is unlikely.

One useful bit of info that this replacement seems to prove is this:

potterton pumas, when they get older begin to suffer from an anoying habit of being sensitive to any dripping hot taps. this is a symptom of the wax capsule of the diverter valve begining to give up the ghost.

when i first moved in the boiler would refuse to do CH if a hot tap dripped noticably, about a year later the boiler wouldn't do CH unless the taps were tightly shut (one still gave a drip every 5 mins or so) before the replacement the CH would only work if the hot taps were isolated.

upon replacing the wax capsule, the boiler will now tolerate a hot tap left dripping (hard taps and feeble relatives) at about 2 drips per second.

In fact there is hardly any noticable switch from the gas heating HW demand to CH demand - before the boiler would need a good five mins to think about firing up to heat CH water after an open tap DHW demand.

If your potterton is being slow to provide CH because of drips it means that your wax capsule is on its way out.
so if youve been scouring these boards for an answer to this problem my advice is replace the wax capsule - it is a simple job and the effect is really dramatic.