"High" stat temperature on Thorn Apollo 30/50B?

[Ric2013]

Hi,

Am trying to understand my mother's Apollo boiler. It has an intermittent fault.

In the last couple of years, it has had a new thermocouple, a new "low" stat, and a new pump over-run switch. The RGI was not throwing parts at it, and the replacement of these has solved some problems: I think we can fairly say these new components work. We're stuck with one remaining fault in that it will sometimes turn off the pilot soon after switching off the burner and then the pilot won't stay in (i.e. the overheat stat is preventing the pilot from being on: it is a thermocouple interrupter overheat stat design).

The Apollo 30/50B has a "high" stat and a "low" stat instead of an adjustable themostat. It also senses the need for pump over-run based off casing temperature, not off heat exchange temperature, so it can be a bit hit-and-miss as to whether you get over-run or not, so it is sometimes possible to have 80+°C flow temperature without over-run. On the plus side, fault-finding is, normally, simple.

The boiler is supposed to run at 82°C maximum, but it is currently rated at around 10kW output, and TRVs mean it can sometimes find itself running 2-3kW of rads only; I have measured 86°C+ flow temperatures in this condition if the boiler is run on the "high" stat. The "low" stat switches at around 70°C.

MY QUESTIONS:

At what temperature would you expect the overheat stat to 'trip'?

Given that the boiler output is excessive for the heating load at times, would you expect the flow temperature to reach 86°C+ before the "high" control thermostat cycles the burner, or would you expect the thermostat to maintain 82ish? I'm thinking these bimetallic themostats are probably quite slow at sensing anything?

It would be nice to know what is going on. Anyone have experience of these legendar(ily bad)y boilers?

 

[Deleted member 120897]

I think every boilers overheat stat has different temperatures that they trip at.

 

[PETE 123]

Scale in heat exchanger near overheat position so not cooling as other parts of h/ex. New oh stat would be a good idea.

 

[Ric2013]

Hmm. I think the question I should have been asking more clearly is whether the HIGH stat should be allowing 86 degrees.

I have observed the boiler further. It can fire for well over an hour if heating a cold house and cylinder and brings the system water up to temperature slowly. The OH stat trips only after this event which results in the flow has being above 82 for five minutes before the high stat switches the boiler off. The high stat is working, but at 86, not 82 as I would expect. There is over-run in this case, and then the boiler runs at about a 0.4 duty cycle (after the initial 'trip' of the overheat, it won't 'trip' again now the boiler is cycling on its own stat or on the room stat).

As I seem to remember the high stat used to keep flow to 83 (not far off the MI's spec of 82) and is now getting much higher (I've witnessed 89!), I think it's fair to say the high stat is not governing properly. And seeing as the overheat stat is allowing 89 for short periods, I think it's fair to say that is not over sensitive.

 

[Ric2013]

Scale in heat exchanger near overheat position so not cooling as other parts of h/ex. New oh stat would be a good idea.

I would agree, but only if the high stat were working correctly, which it does not appear to be. Since the boiler seems to be overheating, the OH stat would seem to be doing its job.

My next move is to monitor the boiler next time it's on a cold start, and manually cut out the burner at 82 (take over the high stat's job for a few minutes) and see if the OH stat still cuts out.