Spooky potterton puma

[David P]

having read all findable posts about pottertons I'm still no wiser about what ails mine.
its a potty puma 80 and seems to live happily enough until someone mentions it (or touches it) then it goes into apoplectic mode.
basicly it refuses to heat any rads. hw always fine xcept sometimes a too hot blurt at the start.
I have been trying to get some ch out of it for several hours this evening. and heres the spooky bit, the moment i gave up & started trawling the web for answers the damned thing started heating and now all rads are hot.

tommorrow it will refuse again - I know it - so perhaps one of you wise and generous people would like to help me get to the bottom of this no ch thing.

Details are: pressure fairly constant at 1.5 - water light on board works ok, pump works ok, overheat switches work fine, minimal air when bled at rads and pump (by the way pump bleeds by pushing the black nut to the side once it has been unscrewed) and what is more the ch/hot feed pipe gets hot - but only for about a distance of three feet from the combi casing (the exact spot seems to vary) where there is a sudden marked drop in temp as you move your hand along the pipe.
now i have blamed this on a blockage (the pipework looks decidedly cowboy installed - i guess 3 or 4 years) but it seems to shift up the pipe in either direction. so perhaps not. not sure if it is the diverter valve but the fact that what is in effect a small portion of the ch circuit gets hot (the first 3 feet of it) makes me think not.
tommorrow I hope to give it a sludge treatment - but I would be really interested to know what you reckon.

 

[weargas]

diverter valve.
normally combis have the diverter valve move to give hot water, in the puma it works the opposite way. it's not an electric valve but a wax module controled valve that is opened by heat. everything you describe is symptomatic of the diverter valve not working. were you gonna try and do it yourself??

 

[David P]

thanks for that.

I have found the problem and in a way you are right it is the diverter valve.

but it is not faulty - it is an undocumented feature for all pottertons that use the same diverter valve.

It seems the valve includes a cooling feature so that when hot water is drawn the demand pulls cold mains water through the diverter thus shutting it - this is so the combi can efficiently use all of its capacity to heat water for taps then return to ch when tap is closed (and diverter warms up again).

it seems that even the slightest leak in any hot tap (in my case 1 drip every minute or so) is enough to fool the combi into thinking hot tap water is being drawn - and so temporarily shut out the ch system - except of course a leak isnt temporary. result no ch.

all i had to do was tighten the hot tap that leaked a bit more and sure enough the ch began to work.

so the solution - new washers and regrind hot tap.