Boiler Behaviour- Is this normal?

[hyeung288]

Hi,

Looking for some advice on our boiler system.
Our have a Potterton Promax SL boiler combined with an unvented santon premium plus hot water system. I have a dual zone system so separate thermostats for up and down stairs and a separate hot water controller/scheduler.

The system is working fine in terms of requests for CH and HW.
However I’ve recently noticed some strange behaviour in that our boiler (in garage) was firing constantly approx every 5mins or so; it ignites and pilot light comes on, pump comes on and a few seconds later the boiler goes off whilst the pump runs for a few minutes and then goes off. This repeats every approx 5mins for about 1.5hr roughly speak starting at midnight thru to 1:30am. Now during then the CH and HW are all off, so no heat on radiators and the 2 zones valves are confirmed to be off (no led lights on the zone actuators)

As the boiler is in the garage, there’s a frosts stat attached (it’s tamper free) and there’s a there some thermostat attached to pipe work above the boiler (Honeywell L641B1004 set to below min setting).

I think the temp in the garage was around 7C at the time....
But question is why is the boiler firing so frequently? Is this normal behaviour?
If it was triggered by the frost stat, surely it would stay on until it reached temp and then switch the boiler off and not fire the boiler for 1–10secs, turn off and repeat every approx 5mins for about 1.5hrs?
Is there something wrong here? Or just normal operation. Would appreciate if someone could advise and explain what’s going on? I’m by no means technical in this areas as you can gather?

Thanks
hyeung

 

[Ric2013]

The frost stats could explain it...

Typically, a properly installed frost stat system will have a pipe stat detecting that the pipe is cold, and a room stat for the vulnerable area where the pipes are (typically set at 5°Cish).

So, if the area is cold AND the pipe is cold (after your heating has switched off for the day?), the boiler fires until the pipe is warm and then switches off. In this weather, it is likely that the area will always be cold, you state 7°C, so that's probably the first requirement met. If the pipe stat is close to the boiler, it could then be satisfied with heat quite quickly. I wonder whether the pipe stat (they tend to be quite crudely made) has wandered off setting or out of calibration and is actually set above the maximum so it only stops switching on the boiler when the pipe is actually hot.

That said, 1-10 seconds seems to be too quick for even a pipe stat to react, unless the contacts are slightly buggered and so it is arcing and switching on and off at random? Have you any way of testing that pipe stat?

Or the boiler is about to break down, but I'll leave development of that theory to the RGIs.

 

[hyeung288]

Hi,

Attached is a view of the setup in the garage with the pipe stat attached to the return pipe.
I noticed that the pipe stat dial is set below the min value according to the dot. Assuming that just means it’s set at 0C?
I’m not sure what the frost stat is at as it’s got tamper free cover over it.

So if I understand your reply on how things work is that the boiler fires if both the criteria of the frost stat and pipe stat are both met?
Ie
IF
[frost stat < configured frost stat value (possibly 5C default)] AND
[pipe stat < configured pipe stat valve]
THEN
[fire the boiler and sequently the pump]
ELSE
[turn off the boiler]

Is that what you mean?

Firstly is there an issue with setting on my pipe stat for the return pipe? What should both the frost stat and pipe stat be configured to normally?

I’ve been monitoring this behaviour today and through the day I have not noticed it? Will check to see what happens around the same time tonight (midnight)?

Are we potentially saying that I could have a faulty stat on either or both the pipe stat and frost stat?

I wasn’t sure if this behaviour was associated to any other protection mechanisms built in to the boiler which would cause this sorta behaviour ie stuff about boiler overruns and fan overruns etc? Once again I’m not technical in this area just from a standard user I’m just concernd there something not quite right. As continual firing of boiler which only lasts 1-10sec and subsequently running of the pump for a few minutes followed by a repeated cycle of all this every 5mins across 1.5hrs isn’t right?

 

[Ric2013]

That is exactly what I meant, (nice work on with the BASIC programming language ) In short, there is a (usually) 240V signal wired through the two stats in series, so both sets of contacts need to close for the signal to get to the boiler to get it and the pump to switch on.

During the day, I wouldn't expect you to see the problem occur. Presumably the heating is running most of the time anyway and so the pipe will be at least warm enough to prevent the pipe stat contacts from closing?

I am thinking faulty stat - what do others think?

 

[hyeung288]

Thanks for that, I now understand concept of frost and pipe stat protection!

Need to ask, is my pipe stats setting sensible as per picture?

However, I’m still unsure as to which stat you reckon is likely at fault? Is it mainly the pipe stat or the frost stat? Or both?

Now, according to the picture of my pipe stat, it looks to be dialled below the min temp. So presume the value set is 0C. In your inital post, you mention the possibility of the pipe stat being out of calibration so rather than the set 0C it may in fact be a lot higher value. However just trying to make some sense on this, that probably won’t explain why the boiler continually fires on and off within about 10 secs? And repeats on what seemed like a fixed interval (5mins). I would have expected it to fire and run longer for the pipe to get up to temp before then turning off?

Likewise if the pipe stat was faulty such that it was randomly going on and off....wouldn’t I expect to see the same behaviour across the day?
The heating isn’t always running, so there is a cool down period. But a tricky one gauge...

But I guess still a potential area for investigation.
Is there any other avenues or possibilities that would result in these symptoms?

The hot water and heating schedules work as expected. So things turn off and on per defined schedules and nothing out of the ordinary. Other than what I’ve obsevered when the system is off.

Thanks,
Hyeung

 

[waddycall]

It would be worth prizing the little round cover off the front of the pipe stat as then you can see where the arrow on the spindle is pointing. Not sure that mark on the cover is actually relevant. Might be worth twiddling the pipe stat setting a little and see what effect it has.

 

[hyeung288]

I thought the mark on the pipe stat is an indication of the set temp on that stat ie below 0C?
With regards to cover, I’m not sure that it can be prized off...

Should this not normally be set to a higher temp?

 

[Ric2013]

I have heard 15-20°C, and I would imagine 20°C would be a perfectly reasonable setting for that pipe stat.

If you twiddle the knob, can you hear a click? That will be the contacts opening and closing and will indicate approximately what temperature the stat thinks the pipe is currently at.

I would expect it would take a minute or so for the stat to react to a temperature change, not 10 seconds, and no, I wouldn't expect the frost stats to give a perfectly regular 10 second/5 minute on/off duty cycle either.

 

[waddycall]

The white disc is a cover. They’re adjusted with a screwdriver. You need to prize the cover off to adjust it. The white disc is meant to make it tamper proof.

 

[hyeung288]

I have heard 15-20°C, and I would imagine 20°C would be a perfectly reasonable setting for that pipe stat.

If you twiddle the knob, can you hear a click? That will be the contacts opening and closing and will indicate approximately what temperature the stat thinks the pipe is currently at.

I would expect it would take a minute or so for the stat to react to a temperature change, not 10 seconds, and no, I wouldn't expect the frost stats to give a perfectly regular 10 second/5 minute on/off duty cycle either.

Hmmm so suggests my symptoms noted is caused by something else then?
Any ideas what else this could be?
I’m sure it’s not the CH as the zone valves are turned off (no led light on actuators) and corresponding room stats are off.
Is it an internal feature or protection triggered by the boiler perhaps? Any ideas?
Btw, since then I have not had any repeat instance of this behaviour?

Regarding pipe stat cover, will have a look again...