DIY Doctor UK DIY Forums

hi i have a puma 80 boiler everything is working fine except when i turn the central heating on
the boiler fires up on full burn but stays like that for 20 mins then the pilot shuts off it will relight straight away but if i put the central heating on it will do the same again.
i think its the same with the hot water but i have not run the water long enough to test it

any ideas

im desperate

thanks

The overheat stat will cut the pilot off, does radiators or water seem too hot ? if you turn the thermostat down does the burner cut out ?


htg

hi thanks for quick response
the radiators seem to heat up slowly however yesterday i cleaned out the pilot etc changed the pcb and still the same prob.
the burner only stays on for a few minutes now then the boiler clicks and the pilot goes out.
on the hot water side the tempreture is normal and it seem to burn longer without cutting out.

my first thought was the thermocouple but i would have thought this would affect the hw and ch equally.
so are you thinking the ch overheat stat? if so do you know the cost of repair.

thanks

That sounds like like wax capsule/diverter problem.

Turn on the central heating and hold the DHW pipe, this should not get hot. If it does - turn the boiler off and run the hot tap, the cold water will make the wax capsule shrink. Turn off the hot tap - turn on boiler, the heating should then work.

Basically, if the DHW pipework gets hot then the diverter isn't switching across for the heating, with the heat not being able to go anywhere (all taps are closed) the boiler will overheat and cut out.


htg

hi tested the pipe but it does stay cold so i think process of elimination is in order. when the boiler switches of there is a strange sound like steam escaping from an old whistle kettle but without the whistle.
could it be the fan is not keeping the system cool enough?
boiler did run for about 40 mins though today, it came on high droped down after 15 secs then came on for 20 mins droped down again once again it fired up for 10 mins then the boiler cut out.

i did relight it straight away but it only worked for a few minutes
think it may have been to hot

cheers.

No, the fan is not for cooling - just for the safe removal of the products of combustion.

The heat isn't getting away from the boiler, if not diverter it could be the pump (quite common for the impeller to come away) or qa blocked heat-exchanger ????

htg

here's what has been done so far
bleed radiators, cleaned pilot, checked air cap top of boiler is loose,
undid black pump spindle pulled forward slightly and small amount of black water came out. is this normal?

when ch is on turned hot tap on for a minute then of, ch stays on longer

Q if pump faulty would it still make a noise

how do i diagnose heat exchange fault and/or diverter fault

and finally where are you based if you dont mind telling me.

thanks

if the pump is faulty it can still make a noise.

sounds like it's overheating to me. either dodgy boiler stat or blocked/scaled heat exchanger. i've had to do a few heat exchangers on these in my time, they can run the tap for a whole bath then the pilot will go off or heat the whole house before the pilot goes out and every time it was the heat exchanger.

What's the pilot like ? should be a strong blue flame.
When the fan is running (heating or water on) does the flame lift away from the thermocouple ?
What does the thermocouple look like ? any soot or white deposits, does it look old/past it.

If the Hot water works when you open a tap, and the central heating works when turned on - then the wax diverter will probably be ok - rule that out.

If the radiators get hot, the boiler is not banging etc then i'd assume the heat-ex is ok - for now. Get the pilot working properly, maybe change the thermocouple (cheapest part to replace).

Does the pump get mad hot ? it will obviously get hot, but sometimes they get too hot to touch, when the happens the impeller will turn for a short while, once it's too hot it will slow andf the boiler will overheat. Easiest way to check, is to remove the pump head (drain down, undo 2/4 bolts securing pump head and visually check impeller.

I'm in the Newcastle area.


htg

ok for some reason the boiler has started working, good central heating and hot water, although the shower did run slightly cold for a few seconds then was hot again.
as i dont believe in miracles i know there is still something wrong.
when the boiler is in idle there is a faint banging noise poss the flame?
also when i look through the window the main flame is burning fine but just to the left i think the thermocouple but not sure is glowing orange when full burner is on.
and the last thing is the pressure gage, i filled it to 1 bar last night and now this morning it has droped to just above the red. this is with the c/h off. so i am guessing there is a leak somewhere along the system
any ideas the easiest way to find a leak.

cheers.