Puma Gas Valve lockout on CH only

[diyben]

Hi,

Had some good advice on here a while back. Can anyone help this time?

Overview:
Potterton Puma 80 PP version. It's had a new diverter valve, but continued to play up, (Lockout at random). I replaced the TC which seemed to fix the problem. Of course, it was warm weather and I didn't check the heating. The HW now functions perfectly, (since the new TC), however.......

When the Central heating is selected, the bolier fires up and then locks out within a minute every time. The Pilot can be reset after a few minutes.

(I'm thinking pump, or some feedback circuitry prob)???

Any help, tests or advice would be appreciated.

Many thanks

Ben

 

[GforceJames]

If the pilot light is extinguished chances are it is overheating, check all valves under boiler are open, and that there is proper circulation.

James

 

[baxpoti]

I'm thinking pump

I think your right!

 

[diyben]

Thanks. Is there any test you recommend to verify that it is the pump at fault? Are there any tests I can carry out to eliminate other components?

Many thanks

Ben

 

[ollski]

If the pilot light is extinguished chances are it is overheating, check all valves under boiler are open, and that there is proper circulation.

James

I'm thinking pump aswell but I'm also thinking don't touch the isolation valves whatever you do....they know when you look at them and burst into spontaneous water leaks. Not too tricky to replace I know except that the replacements have a different thread.

 

[possum_jenkins]

Take off the silver slotted cap of the front of the pump, cover the electrics because you will get a little bit of water out. The shaft should be spinning freely.

If not, and you are carefull and confident testing 240 V, check the terminals on the pump for 240. If there is voltage present. If there is your pumps duffed.

 

[diyben]

Many thanks....... but more help needed please.

I have checked that the pump is spinning, was worried that the impeller wasn't, so I removed the pump head. Impeller intact, no corrosion, pump spins freely by hand, plus I briefly ran the pump observing the impeller spining freely.

So, the CH still locks the gas valve out (Puma PP) after 30 secs or so.

The HW continues to work perfectly.

What other interlocks action the gas valve from the CH side?

Does the overheat stat, half way up on the RHS, relate to CH only? Could this be at fault? Is there a test I can carry out??

Again, any help would be greatly appreciated.

Ben

 

[Agile]

I assume the pump is spinning on CH ?

If so then it sounds as if the CH sensor is failed or out of calibration.

Can you measure the temperature the flow pipe in the boiler gets to when CH is selected after DHW has been used? It should turn the burner off at about 82°C

I suspect the temperature is being controlled and is excessive and operating the o/h stat and tripping the pilot light.

Its just possible the o/h stat is out of calibration but thats less likely. But the temp measurement above would identify that possibility.

Tony

 

[ChrisR]

WHen you put it on CH. how far dopes the heat get out of the boiler? Can you feel pipes to see?

I don't suppose you have anything to measure pipe temperatures with - 82 is as ot as rads should ever get and much too hot to leave your hand on.

I'd still suspect the pump. When you ran it off the boiler, it should have been hard to stop with one finger. Sometimes they go slow/weak.

The resistance of the CH thermistor should be about 10 k Ohms, cold, falling to 1-2k ish when v hot.

 

[diyben]

Thanks once again guys

pump is spinning, didn't try the finger test when I had it out.

I measured the cold resistance of the CH water temp sensor to be in the 10k region. Difficult to measure when hot, because lockout occurs very quickly, (10 to 30 secs) and pipe didn't seem that hot. I removed it and put it under the hot tap. Resistance dropped to about 2.9k

I ran the boiler again, on CH and the lockout occurs as the flow pipe is heating up. The pipe gets warm/hot, but not too hot to hold.

I ran the boiler with the CH water temp sensor connected, but out of its housing. The boiler ran slightly longer, but still cut out. This was long enough for some of the radiators to start to get warm.

Tried to measure the temp, I've only got an older style clip on thermometer, with a needle display. I think it may be too slow to react, as the temp only got to about 50 - 60 or so before lockout.

Serious head scratching this end???

 

[ChrisR]

Sounds like the overheat stat is opening too low then. That's the way they go.
Technically a corgi job to replace.

 

[diyben]

I assume the pump is spinning on CH ?

If so then it sounds as if the CH sensor is failed or out of calibration.

Thanks once again. I have repalced o/heat stat and CH temp sensor. The heating works, but the boiler seems incapable of clicking in and out to regulate the temp of the water in the boiler. The flame just stays on. The flame sometimes gets bigger and smaller, but still stays on. It doesn't click off while the water circulates and cools etc. The weird thing is, to test the new temp sensor, I removed it and plugged the hole with the old one. I connected the new one and left it dangling. Started the CH and gently heated the new temp sensor with a candle. Sure enough the burner went out. I blew on the sensor to cool it and the burner came on again. I repeated this several times. This is what I would expect the boiler to do as normal operation.

However, when the sensor is fitted, it doesn't do this. As if the water isn't getting hot enough. However, I can hear the water getting very hot and will, (after about 20 mins), lockout via the overheat.

So. Tony mentioned about the CH temp sensor being out of calibration?? Is there an adjustment elsewhere??

Once again, many thanks so far. Any more ideas greatly appreiciated.

Oh, one other thing. I think I'll replace the boiler before the winter this year. Any opinions on best combi's at the moment?


Thanks

Ben

 

[DP]

I would be looking at the diverter valve. This unit has to be in CH position to send water to rads. Looks like it is staying in HW mode during CH.

I would also be looking at the isolating valves on flow and return. Yes, I know, they leak but if the valve/s is/ are turned off, boiler will overheat and pop the pilot.

 

[twgas]

The flame just stays on. The flame sometimes gets bigger and smaller,

this is where it's modulating, leave it till all rads are piping hot and it should shut right off (you could turn some rads off to speed up the process)

 

[Agile]

Even if the diverter valve has not moved to the CH position the boiler will not overheat when its working properly.

It sounds as if the pump is not working properly or there is a blockage!

However, the pump should be running all the time on CH and DHW. Can you confirm that it is?

Can you measure the temperatures around the boiler?

Its odd that you seem to have found that it follows the correct protocol with a sensor externally mounted.

Tony