faulty thermister?

[onlyfitidealboilers]

take no notice of the dial on the front just turn the blr off. then run the hot tap (the wtr will be cold) then put the multi mtr across the two pins on the thermister should read 10k or higher then turn the blr back on with the hot tap running as the temp rises the kohms on the mtr should drop. if it does then its fine normally..

 

[Agile]

I think he meant to tell you to take the boilers connection off the NTC before you test it.

 

[jackecole]

I might replace the thermistor with a pot to see if the PCB correctly makes the boiler modulate.

Agile, what's a pot? that's part of the PCB isn't it? Sorry am just trying to understand that part of this post!

 

[Agile]

A "pot" is a potentiometer.

Its a variable resistor but actually has three connections, each end on the resistance and the third is the variable wiper. The resistance therefore varies as the control is moved.

This enables a set value to be presented to the boiler representing the value of the thermistor at a partiucular temperature. It enables a engineer to see how the boiler responds at different temperatures.

Tomny

 

[bcoleman]

Hi guys,

First - a big thanks to all your help - I changed both thermisters at the weekend, the problem seems to have gone away and the wife is happy!!

A slight issue is that to change the CH one, I had to drain the boiler through the PRV and now I can't get it to stop dripping. It's only about a single drop every 15 secs or so but it's enough to drop the pressure a bit over the course of a day.

Will this sort itself out after a few days or is there something I can do to help it seal up again? Also, I've read a few posts about leaking PRV being a sympton of a busted expanision vessel - I wouldn;t have busted it just by draining the boiler would I?

Thanks all.
Ben

 

[Andygasman2010]

Hi guys,

First - a big thanks to all your help - I changed both thermisters at the weekend, the problem seems to have gone away and the wife is happy!!

A slight issue is that to change the CH one, I had to drain the boiler through the PRV and now I can't get it to stop dripping. It's only about a single drop every 15 secs or so but it's enough to drop the pressure a bit over the course of a day.

Will this sort itself out after a few days or is there something I can do to help it seal up again? Also, I've read a few posts about leaking PRV being a sympton of a busted expanision vessel - I wouldn;t have busted it just by draining the boiler would I?

Thanks all.
Ben

If the expansion vessel was faulty the pressure would go up to 3bar or so when the heating was fully hot. If it's not then you've probs just got some muck on the seat in the PRV. Just open it for a few seconds and let it spring back fast, maybe that'll shift it. If you can't sort it like that just strip it down and clean it manually. Or you could just change it...? they're pretty cheap

 

[DeltaT]

A "pot" is a potentiometer.

Its a variable resistor but actually has three connections, each end on the resistance and the third is the variable wiper. The resistance therefore varies as the control is moved.

This enables a set value to be presented to the boiler representing the value of the thermistor at a partiucular temperature. It enables a engineer to see how the boiler responds at different temperatures.

Tomny

Everyone that repairs boilers should take note of the very valuable information given above and understand this is why Telly Tony is held in such high regard on any forum. Pots & thermistors are such an important part of any modern control system, we all need a good understanding of electronics.