potterton puma ch burns for 20 secs

Hi. Need help. I have a potterton puma 80 (I know I should of bought something decent). I have HW but my CH cuts in for 20-30 secs then cuts off. Give it a few minutes and it does the same (relays keep clicking on the board). The flow gets hot but the return stays cold. The presssure is set at 1.5 bar and has been bled (many times). I have checked the temp sensor resistance and it is correct (about 7K at room temp). I also get a noise from the right hand side (up where the pipe enters the combustion chamber) when the CH kicks in (it sounds a bit like expanding air noises). If I put the CH temp sensor cables across the HW temp sensor the CH works fine till he CH overheat stat locks me out (which I assumed it would do). I have swapped the sensors around and still have the same problem. My next step is to possibly replace the modulator control board, does anybody think this is where the fault lies or does anyone have any other ideas?. I have turned off the cold water to the boiler to eliminate HW demand or an iffy diverter but this hasn't helped either. On the odd occasion when I do get it going if I turn on the HW then it knackers it again. I am assuming that the logic on the modulator board is shafted and gets confused?

Thanks in advance. Pete

or the diverter valve is stuck in hot water

How can I check that? Cheers

1st make sure you dont have a dripping tap


the diverter has a wax capsule fitted warm it up

if you need a manual try http://www.partsarena.com/baxi/

Cheers Kev.

Seems to have done the trick for the moment. I held a lighter under it for a few seconds and it flew into life. I should've come here earlier.

Another couple of quickies. how much for the part do you think? and I assume I need to take the pressure off the system to replace it (and turn off the cold fill )

Think I'll do all the hot water tap washers at the same time. God this house will be the death of me

Pete

Hi again Kev,

Thought I'd pop back and say thanks again. The diverter was £75 inc VAT and the man in the shop was extremely helpful (someone else wanted £110, thievin gits). The job wasn't that hard to do either

Anyway thanks and have a good New Year, I'm off back to do more refurbing/decorating now.

Pete