Main Combi 24 HE no DHW, Boiler fires up then turns off

[wheeliebinraider]

I have a Main Combi 24 HE. The central heating seems ok, but the hot water is playing up. When i run the hot tap the boiler fires up fine, all valves work as they should, but after about 4-5 seconds the boiler seems to reset, all the lights on the front go out (no fault lights flashing) then after about another 4-5 seconds the boiler fires up again. this just keeps repeating. I have temporarily disconnected one of the modulator wires as this seems to keep the flame on. Any ideas, is it the PCB? all sensors etc seem fine.

Cheers

CW

 

[Slugbabydotcom]

No guaranteed fixes here Just my first thoughts on the matter

all the lights on the front go out

Last time I saw that it was a pcb fault. Check all the plugs are in properly.

Could be a thermistor faulty. Dont rule out the CH thermistor even though its a DHW problem.

Could be a circulation fault perhaps a blocked plate heat exchanger, DV not moving as it should or pump tightening up

 

[gas4you]

I have temporarily disconnected one of the modulator wires as this seems to keep the flame on.

By doing this you are keeping the boiler at minimum burner setting.

NTC's can break down at a certain point, even though they test fine.

How old is the boiler? It sounds to me like the system side of the plate HE is blocked.

 

[wheeliebinraider]

The boiler is just under 2 years old.

mechanically the boiler is pretty spot on, plate heat exchanger is fine.

Think i will order new PCB on Tuesday, and just in case a CH thermistor.

If anyone has any other ideas i will give them a go!

 

[bengasman]

mechanically the boiler is pretty spot on, plate heat exchanger is fine.

Do you have a trick I don't know, or did you take it out?

 

[namsag]

edit.
read it as mini 24 instead of main, as slug says sensor is the usual but could be the board.

 

[Prof.plumb]

this is basically a baxi 105. two possibilities of hot water issue no 1 plate to plate blocked and overheating or dhw thermistor faulty. if it is the thermistor the central heating senor is the same as the dhw one. you can move wires from dhw one onto heating and vice versa if the boiler then displays fault on heating then it is safe to assume that the dhw sensor is knackered.

 

[namsag]

Professor plum basically its an 80eco and as such only has one sensor unlike the 105 which does have 2 so they will not be able to try what you are suggesting.

 

[wheeliebinraider]

i took out the plate heat exchanger, just drained the boiler and its only two screws!

there is only one temp sensor for both CH and DHW.

 

[gaspro1981]

so wot was the condition of the plate he

 

[baxpoti]

The PCB on this model is strong and rarely gives problems.

Before changing parts check the wiring.

Test for incoming Live, Earth and Neutral. Test L-N, L-E and N-E

Remove all connections on PCB and replace securely

If fault persists, in cost order, change.
Sensor
Harness
PCB

Last time I encountered a similar problem, the incoming Neutral was in the terminal block but not screwed down!

 

[wheeliebinraider]

plate heat exchanger is in good condition, water flows through both sides fine.

I have checked the incoming electrical connections and they are also fine.

A new PCB and Temp sensor are in the post so should be with me hopefully by the end of the week!

 

[wheeliebinraider]

PCB and HW sensor turned up today, fitted them and i now have hot water!

thanks for your help everyone.