Potential overheating gas boiler

[dinners]

Hi,

I have an old Potterton Kingfisher 2 RS100. I think it may be overheating as I can sometimes hear what I think is kettling in the expansion tank. As it's an old big boiler there's no pump overrun or auto-bypass (as I understand it it is able to dissipate the heat safely itself after the valves close). I'm trying to measure the hot water outflow pipe with a contact thermometer to determine if it's a boiler stat issue, but I was hoping someone could help with how much you might expect the temperature to rise following the valves closing and the boiler shutting off.

Thanks in advance!

 

[Agile]

I do not know how you think you can hear kettling in an expansion tank.

Kettling is a sound made by water inside a metal heat exchanger when it turns to steam inside on the surface of the metal.

Generally those old cast iron heat exchangers are quite forgiving. Although a short pump over run would be far better for the boiler.

That is easy to do with any of several electronics solutions.

 

[dinners]

Thanks for your reply. Perhaps the wrong term, but it sounds like boiling coming from the attic - by your defintion soulds like it could apply to boiling water/steam in the vent pipe? Appreciate a pump over run would help, but it doesn't resolve the underlying issue (which I believe there is as it's not done this over it's long life span until the last couple of years).

Am just trying to do as much as possible to confirm the diagnosis of a faulty thermistor before forking out for the fix!

I should've mentioned this is an S-plan system for what it's worth.

 

[Agile]

When the valves close that normally removes the call for heat signal to the boiler.

This is achieved because the valves have a micro switch which is operated when the valve opens.

While the boiler is operating the temperature of the flow should not exceed 82 C and for economy it is best run a little lower than that.

If I remember correctly your boiler may have it's temperature controlled by a thermistor which is just a small thermistor pushed into a pocket. This pocket is meant to be full of heat transfer paste to ensure rapid response of the thermistor as the temperature increases.

But after a number of years with questionable service engineers I have often encountered some with very little heat transfer paste left in the pocket.

 

[Agile]

No, your boiler does not have the thermistor which described as that is on a later model.

Yours has an earlier type with a long thin bulb which is inserted into a drypocket and does not need any heat transfer paste.

But its setting should not allow the flow temperature to exceed 80 C.

It is a bit more economical to run it at more like 70 C

 

[DP]

But its setting should not allow the flow temperature to exceed 80 C.

About 82 degrees actually. Checked the MIs
Mf had the thermistor RS was Renco