potterton promax 15he not cycling

[bikeman123]

My potterton promax 15he is not cycling - the burner remains on for hours until eventually it locksout, I guess because it overheats.

The bolier responds correctly to the room and water thermostats, and seem to respond to the various sensors in that it shuts off when they are disconnected.

Is this likely to be a faulty flow temperature thermister? It measures 10k ohms cold and drops to 250 ohns when hot which seems to be ok, disconnecting it does shut off the burner.

Any other suggestions?
Thanks

 

[D_Hailsham]

Are you saying that the boiler continues working after the room and/or cylinder stats have reached the set temperature?

Do you have a frost stat?

How many motorized valves?

 

[bikeman123]

No as per OP the boiler correctly turns off when room/cylinder stats reach temp.

The problem is that the boiler stays lit for the entire heating period - it does not cycle the burner on/off periodically to maintain a set flow temp. ie its wasting gas.

Yesterday the boiler locked out before set room temp reached because the flow temp got too high.

Main problem is the frightening gas bill expected.

 

[D_Hailsham]

If the thermistor is OK, then the fault is probably in the circuit board.

 

[bikeman123]

If the thermistor is OK, then the fault is probably in the circuit board.

too expensive to change a pcb on just a hunch - I need a competent ch engineer in buckinghamshire - not a part switcher - to correctly diagnose.

If anyone can help pm me.

 

[D_Hailsham]

too expensive to change a pcb on just a hunch - I need a competent ch engineer in buckinghamshire - not a part switcher - to correctly diagnose.

I wasn't suggesting that you went out and bought a PCB (which has to be changed by Gassafe registered engineer), only that this would seem to be the most likely cause. Any competent engineer will carry out a logical sequence of testing and fault finding steps, which cannot be done remotely over the internet.

 

[bikeman123]

Called out a gas safe engineer today and he changed the thermister at a cost of £50. No improvement. I cant afford to have someone change parts willy nilly at my expense. Unfotunately gas safe doesnt seem to equate to competent.
If it needs a pcb ill change it myself - its only a couple of cable connectors but I need to be confident that thats whats required first.
Any competent engineer in Bucks want to pm me?