Brittany Combi SE80 No Hot Water PLEASE HELP!!!!

Hi I wonder if anyone can help me? My family and I have had no hot water for three weeks now. Although I'm not a plumber, I am a marine engineer so am not a complete incompetent.
I have a fault on my SE80. The CH works fine but I get no hot water at all. It's not the PCB because I've replaced it. I've also replaced the diaphram which looked in fairly good condition but it was only a fiver so I replaced it anyway. I also replaced the flow control valve which was in a poor state. I also stripped down the whole black plastic valve housing and cleaned it all. I saw no obvious problems other than the flow control valve.
Symptoms:
No fault codes.
No leaks.
1) With CH turned off.
When the hot tap is opened the diaphram depresses the plunger which engages the microswitch. The PCB display lights then flash in the sequence which tells you that it has engaged DHW mode, however the flame does not ignite.
2) With CH turned on.
When the hot tap is opened the diaphragm depresses the plunger which engages the microswitch. The flames which were blazing away (because CH is on) then cut out (because DHW is overriding the CH).

So it can't be a fault with the gas/solenoid side of things because CH is working fine. It's not a PCB fault and I don't think it's a diverter or secondary heat exchanger issue because the flame is not igniting at all. It's not the microswitch because the valve depresses and engages it which successfully switches the boiler to DHW mode (apart from no flame ignition!)
Could it be the thermistor? If so which one as my manual tells me that DHW uses both themistors. Or the the pressure switch? I don't want to keep chucking money at the thing. And I really badly need to bath myself and the kids!

Have you checked the system flow switch is operating, with a meter?

Tony

Agile said:

Have you checked the system flow switch is operating, with a meter?

Tony

Click to expand...

agile i don`t think on this model the flow switch is wired in as changed one on britony after i broke it changeing the hydrolic block

when you say the system flow switch do you mean the microswitch which is activated by the plunger (which depresses when a hot water tap is opened), then yes I have checked it with a meter. There is another switch which is a pressure switch mounted on the side of one of the pipes. I'm not sure how to check this with a meter as I don't know whether it should be open or closed.

its peice plastic what look like a control knob on left hand side hydrolic block just replace their cheep but the only way i know to test it

but i am sure some one will put me right

It's definately not the microswitch that is faulty. The Boiler definately does go into DHW mode (as proven by the sequence of LED's on the panel) and also proven by the fact that when the CH is running, the activation of the switch turns the flames off. And I'm getting continuity on the switch. I suspect the thermistor but I want to know if there is a way of testing it rather than going out and spending £25 on a new one?

What you call pressure switch, check that it does the same thing on DHW as it does on CH.

You test sensors by measuring the resistance with the boiler cold.

Tony