Obvious boiler fault?

[inigopete]

We've just moved into a new (to us) flat. It has a Potterton Puma 80 combi boiler that seems to have been installed in 1999.

Up until yesterday, heating and hot water worked more or less OK, but there were a couple of problems: if hot water was run continuously for more than about 2 minutes, about 10-15 seconds after the hot water was turned off there would be a "clunk" from the boiler and the pilot light would go out. Also sometimes the pressure dial on the underside of the boiler would drop into the red zone near zero or to nothing at all; I'd have to take the wall panel off below the boiler, turn the fill loop tap on until the dial went back up and then turn the tap off again.

I called out an engineer today who came and looked at the boiler, said it seemed to be in pretty good nick. He replaced the thermocouple, cleaned the pilot burner and dusted down something large with fins in what looked like the main burner chamber. He said the boiler didn't sound scaled ("kettle-y").

This evening the heating didn't come on and the flat got cold. I found the pilot light was out, re-lit it following the procedures on the inside of the boiler door and the heating came on again. However after around a minute of the boiler firing, there was the familiar "clunk" and the gas shuts down, although a motor still sounds like it's running (don't know if it's a fan or a motor).

This happens every time I try and re-light the boiler. If I turn the heating off, re-light the boiler and run hot water, the boiler stays lit to provide hot water as long as necessary (so I have at least had a hot shower!) but shortly after turning the hot water off, clunk. If no hot water is run and the boiler is re-lit, the pilot stays alight until the heating is turned on. Then, again after about a minute of running, clunk.

Can anyone offer any advice? I'm not planning on tinkering with the boiler myself, but I phoned the engineer and he was puzzled, couldn't come up with a potential solution and said he'd have to look into the costs of replacing a few things: a couple of thermistors and a wax valve of some sort.

As it might take a few days for him to order parts, and then it'll cost a fair bit to come out and replace them as well, am I in for a cold few days and an unhappy credit card, or does anyone have any suggestions?

Thanks,

: P

 

[inigopete]

Blimey, I've just had a bit of a read through several other forum threads that inevitably involved Potterton Pumas.

Is it basically a Rubbish Boiler? Am I better off getting it replaced before I get through more valves, boards and suchlike? I don't much fancy spending a couple of grand, but if it's going to save a lot of money in the long term...


: P

 

[Gasguru]

The pressure gauge should read about 1 Bar with the rads cold and ideally no more than 2 bar when hot. Did the low pressure light come on when the pressure dropped? If not the pressure sensor is blocked or it has been disabled.

Does the burner continue to fire after a hot tap is closed? This is a common fault casued by a sticking flow switch assembly and will result in overheating and pilot outage.

The pilot burners gradually corrode/fatique...this results in a smaller pilot flame despite adjustment on the gas valve. The smaller flame can be pulled off the thermocouple by the fan.

Was the thermocouple set at the correct height?

After the overheat thermostat has tripped a few times I replace it since the contacts can be poor and the temperature set point seams to vary.

When you re-light the pilot how long do you need to hold in the gas valve. A good thermocouple should opnly need 4 or 5 seconds.

Did he check the burner modulates correctly, is the boiler range rated for heating - most are just left set to maximum.

 

[inigopete]

Thanks for taking the time to reply, Gasguru!

The pressure gauge should read about 1 Bar with the rads cold and ideally no more than 2 bar when hot. Did the low pressure light come on when the pressure dropped? If not the pressure sensor is blocked or it has been disabled.

The pressure gauge reads about 1/2 (on the edge of or well into the red zone) when rads cold and about 2 1/2 when hot. The low pressure light hasn't come on when the pressure has dropped to zero, although I _have_ seen it come on once while we've been in the property. (so I know it works sometimes, I just don't know what it takes to turn it on)

Does the burner continue to fire after a hot tap is closed? This is a common fault casued by a sticking flow switch assembly and will result in overheating and pilot outage.

I don't think so - it fires for maybe a couple of seconds longer (at most) but then shuts down.

The pilot burners gradually corrode/fatique...this results in a smaller pilot flame despite adjustment on the gas valve. The smaller flame can be pulled off the thermocouple by the fan.

Wouldn't this mean that it happened every time the boiler started up (I mean, for both CH and HW) - so far, it seems to be only the CH that makes it shut down after a minute, HW keeps running as long as you need it to, but then shuts the boiler down with that "clunk" about 15 seconds after you turn the HW tap off.

Was the thermocouple set at the correct height?

I'm not sure. I can see it through the pilot viewing window, it looks to be about 15-20mm above the pilot burner. When it's lit, the end is near the tip of the flame and glows red hot - is that right?

After the overheat thermostat has tripped a few times I replace it since the contacts can be poor and the temperature set point seams to vary.

That sounds like it may be the fault. Am I right in thinking that that would shut it all down with a clunk if it thought the boiler was "too hot", e.g. after a long spell of creating HW or, for some reason, at the start of the CH demand?

When you re-light the pilot how long do you need to hold in the gas valve. A good thermocouple should opnly need 4 or 5 seconds.

The instructions inside the boiler door say 20 seconds, but usually from cold it only needs about 10-15. However, I've noticed when trying to re-light it after a "clunk" that it will light in the usual time after HW has set it off, but that sometimes it still won't light after 30-40 seconds holding down if the CH has set it off. That motor keeps running (pump? fan?) and I have to switch everything off and leave the boiler for 5 minutes or so before attempting to re-light the pilot.

Did he check the burner modulates correctly, is the boiler range rated for heating - most are just left set to maximum.

I don't know what "modulates" means in this case, sorry. The dial on the front panel for CH is set to about 2 o'clock, a fair way off maximum.

Hope this helps!


: P

 

[ChrisR]

I've never yet had to scrap a puma, they're easy to mend for any boiler bod who understands them a bit. Seems the last guy didn't.

Most likely is the dhw flow switch sticking on. Look inside the boiler (two screws under to remove front) and you'll see this red light when it thinks the tap's on. Make sure it goes out the instant you turn the tap off.

 

[inigopete]

Brilliant, thanks ChrisR! Will check it out.

Is it the sort of repair I could attempt myself? I'm fairly mechanically adept (I run a bicycle workshop)


: P

 

[Agile]

have yopu checked that the pump is running?

Even if it is not then it will still give DHW but no CH.

Easy to check, see FAQ topic on this site at top of list.

Look on pcb for red light to go out immediately the DHW demand ceases.

Could be a sticking/intermittent pump. Must be completely free when turned by hand.

Tony