CH Boiler - Identification, and replacement of Flow Thermistor

[Mike Dodd]

I have a "Main 12 HE" boiler, which I think is a Baxi/Potterton? (certainly looks alike - but there's some confusion on the web as to whether "Main" is actually a manufacturer in it's own right)

In any case, looks identical to that given here:-

's a problem with this in that the boiler will heat for a couple of minutes, then shut-down with a solid override LED illuminated. Following the flowchart this suggests a faulty PCB, or faulty Flow Thermistor. I've checked the flow thermister vs the casting thermistor (which should be within 25% of the resistance of each other) and *frequently* find the flow thermistor open-circuit (I have seen it not, and close to the casting thermistor, leading me to think I have an intermittent fault).

Anyway, as look would have it, to diagnose further requires some form of gymnastics normally associated with 1/2 bottle of vodka and a "Twister" game mat - as the boiler is installed in a cupboard in the corner, overlooking the sink.

Easier, would be to remove the thermistor and test on a bench.

Now, I'm not clued up as to how these boilers work - is the thermistor "wet" (i.e. if I remove it, does the hot water flow from the HW pipe that it's screwed into), or if dry, can I remove without draining the system?

Are these fairly standard items? Are the casting and flow thermistors one and the same type of device (in which case, considering natural ageing, is it worth replacing both?)

I did try getting a spanner on one, to slowly remove, but havn't the right size to hand, and it seemed to require gorilla strength (I was worried about putting too much force on with an ill-fitting spanner) - is this normal? (the installation manual suggested hand-tight then 1/2 turn).

Any info, as always, gratefully received.

Mike

[Roger Mills]

Main certainly used to be a manufacturer in its own right - my late mother used to have a Main cooker dating back to the 1950's.

However, it looks like it's now part of Poxi-Batterton. On the Main site

) it says "Mainn (sic) is a trading division of Baxi Heating UK Ltd." - and the address they give in Warwick had used to be a Potterton office.

[geoff]

In message , Roger Mills writes

Yes its Baxi and a common pcb fault

[Mike Dodd]

Thanks Geoff and Roger.

I've now sourced replacement thermistors, which I'll try out first (I do think the flow thermistor is u/s).

If this isn't the case, then - Geoff, do you carry this PCB (part no.

5110550) ?

[geoff]

Sorry, its a processor problem and there's not much I can do about it

[Mike Dodd]

Old thread, now, but for the benefit of anyone trawling the NG for a similar problem : Flow thermistor replaced, problem solved. Interpart part number 5113044, "Potterton Sensor". Deep 13mm socket is a bonus for removal. Thermistor is dry-fit (i.e. can remove without draining system), just smear a dab of heatsink compound on the nipple of the replacement thermistor before refitting. Hand tighten, then nip up 1/4 turn.

[Mike Dodd]

Oh, and the part was £6 from eBay

[Doctor Drivel]

In the 1960s they had a TV ad' with the tune, "Cook modern with Main".

[geoff]

In message , Doctor Drivel writes

Was that when you got your O level in domestic science ?

[Doctor Drivel]

Fantastic Maxie. 12 ways of cooking spam. Fabulous indeed.