Myson Apollo Overheat Stat versus Hi stat specification

[TonyJMB]

Hi,

First post - please be gentle..

I have a Myson Apollo 50/65B boiler. I can get the pilot light to stay on, but the burner will only light if I short the overheat stat out. This is even from a completely cold start, so I am guessing that the stat itself is faulty. So long as the short-link on the overheat stat is there the burner runs, but without it, the burner does not even fire up..

But.... I have a problem as I went to a gas spares place this morning (before I did any tests) , in anticipation of the O/H stat being faulty intending to buy another one. However, in my rush, I misread the part number on the spares list and got a pump overrun stat (I did not even look to check unfortunately).

So - my question is - can I use one of my spare Hi thermostats in place of the overheat stat (it is exactly the same shape) or will the trip temperature be different?

If it is (too) different, I am stuck with no heating (and 1 very angry wife) for the weekend.

Thanks in advance!

TonyJMB

 

[twgas]

no you cant use the Hi stat as an o/heat

I cant see how bridging out o/h stat fires boiler, if as you say pilot stays on.

my advice is to get a RGI as from your statements you are obviously not competent to work on a boiler.

 

[TonyJMB]

no you cant use the Hi stat as an o/heat

I cant see how bridging out o/h stat fires boiler, if as you say pilot stays on.

my advice is to get a RGI as from your statements you are obviously not competent to work on a boiler.

Well thanks very much for the nicely insulting response to my first post.

A simple yes/no would have been fine, with perhaps a statement on why not.

As I have managed to change just about every single key ancillary part on this boiler over the last five years without blowing up my house then my competence seems adequate so far. When I get into areas where the answer is not obvious, it seems sensible to ask those who know.

My diagnostic process was based on the installation instructions which came with the boiler and it has always worked up until now. My only mistake was quoting the wrong part number at the gas supplies place and not checking what I got.

And FYI -

1. the boiler is actually running now, without having changed the OH stat. I have no idea why, it is such a sensitive beast and always has been
2. the RGI who originally fitted the boiler, frequently had to rely on my diagnostics when fixing it in the intervening years. He is a really nice chap, and I would be happy to use him now, but he has moved and I no longer have have his contact info.
3. Other RGIs that I have contacted in recent months - with the temptation of installing a new boiler - don't even bother to turn up, or frequently don't even return telephone calls.

Boiler still running. No loud noises so far....

 

[gas4you]

If the overheat stat had actually gone, you wouldn't get the pilot to stay alight. It is in series with the thermocouple.

You fault must lie somewhere else, hence why it is now working.

 

[TonyJMB]

If the overheat stat had actually gone, you wouldn't get the pilot to stay alight. It is in series with the thermocouple.

You fault must lie somewhere else, hence why it is now working.

Agreed. I just wish it would be consistent. I know the thermocouple seems extremely sensitive to outside influences. I have checked the t/c voltage and on ignition last night it was about 13mV, which was less than it normally needs (>18mV typically) for the pilot to remain lit. This has got to be the most sensitive and unpredictable boiler made.

 

[gas4you]

Is the thermocouple spotlessly clean?

I find that the few I look after on my patch need the thermocoupling replacing every year to give reliable service.

I have also wrapped the overheat connector on the thermocouple with insulating tape, as the slightest contact with the case earths it out.

If the pilot is ok but main gas unreliable, I would be suspecting the main gas valve.

 

[giblets]

TBH I'd be looking at more like 30mV from the thermocouple.

Have to agree with Dave - gas valve.

 

[TonyJMB]

TBH I'd be looking at more like 30mV from the thermocouple.

Have to agree with Dave - gas valve.

Main gas valve was done last May, so I trust it does not need doing again so soon. On that point, it was surprising how much carbon build up (in the incoming line filter) was in evidence (the previous valve had probably been in for 3 or 4 years).

Is rapid carbon build up a symptom of anything else being amiss?

 

[TonyJMB]

TBH I'd be looking at more like 30mV from the thermocouple.

Have to agree with Dave - gas valve.

Never been able to get more than about 23 - 24mV

In the last 2 days, maximum has been around 19mV from memory, but it does seem to vary a lot, dropping down to maybe 12mV, though not quickly IYSWIM.