Old boiler engineers

[T i m]

Hi all,

Mum has an old wall mounted, low capacity, b/f conventional gas boiler and she's trying to keep it running till she revamps the kitchen.

It works fine (for what she needs anyway) but now and again the overtemp stat trips, even though the main stat is set quite low and I've never see it other than just 'ticking along'.

So, the (manually resetable) overtemp stat is of the remote type where the copper sensor feeds though the outer casing to the heat exchanger / case and into a tube sitting inbetween the main boiler tubes.

I managed to find and fit what I as told (and it looked like) a new old-stock replacement a while ago and whilst it seemed to help it hasn't cured the problem completely.

So, can someone confirm that it is where it is to be able to measure the temperature of the main boiler tubes (and cut the boiler out if above a preset threshold), if there is a 'typical' reason why they might trip and if it could be a false positive (tripping prematurely) and I can't get a direct replacement, can anyone suggest a safe alternative fitting and fitting location please?

Would strapping a std 'overtemp' switch to the flow as close as I can get to the heat exchanger be sufficient and if so what temperature switch would I be looking at please (I'm guessing under 100 DegC)?

I was thinking of something like this:

I would like to retain the manually resetable function as then I would know if summat was going wrong.

Cheers, T i m

[Tim+]

Typically they trip because the boiler is overheating. ;-)

That's usually going to be down to poor flow. Sludged pipes or failing pump probably.

Tim

[T i m]

Ok ... ;-)

Ok, well that not easy to measure is it?

This was an old gravity fed system and the pipes are comparatively

*massive* compared to a std system today. When the guy did the bathroom a while back and I helped him put it back together, most of the pipes were pretty clean (considering how long it had been running etc). After re-commissioning we chemically cleaned the entire system (left it running a week) and I even took the heat exchanger out and found it to be very clean.

The system was then flushed several times (till it ran pretty clean) and then was re-filled with suitable inhibitor.

Now, that's not to say it's not blocked somewhere *now* but judging by how fast the pipework gets hot once the boiler comes on I'd say it's not *that* blocked (if blocked at all).

It's yer classic Grunfoss 3 speed and sounds like it running ok (and again, you have a good flow round the system).

If you fire the boiler up from cold you can see the flames running nicely and it quite soon starts to modulate as the HW cct is quite short (and always open, no valves etc).

From memory the are two paths though the heat exchanger and I guess if one was blocked the boiler would appear to work but one 'path' could 'overheat'?

I'll see if I can get a temperature probe / IR thermometer on the top of the two paths and see what they each read.

The other thing I was wondering was if the tube that carries the overtemp stat probe had a hole in it somehow and the stat was being 'overheated'? I might be able to test that by blowing into it with a suitable pipe.

Cheers, T i m

[Tim+]

Not unknown for impellers to come adrift of the motor shaft or just get eroded away through corrosion/cavitation. But if pipes are heating up quickly downstream after firing up then probably okay.

Okay, I'm out of ideas. ;-)

Tim

[T i m]

Check.

Yeah, me too, apart from taking some measurements and seeing what I find ('you can manage what you measure' etc).

My background 'fear' is there is nothing wrong with anything and that the upper limit stat has just got weak?

It's just that it seems to be in a god place to protect the boiler from melting down in the event of a total water loss (where you may not get an overtemp reading further up the flow path)?

Subject to the tube that carries the overtemp stat not being compromised (and I think from memory it may / should be attached to one of the main exchanger tubes) and not being able to get a replacement, maybe I could use a thermistor [1] and a little PCB driving a relay make something more 'adjustable'?

Cheers, T i m

The thermistor and wiring run at 205 DegC on our 3D printer 'hot end' so the temperatures experienced in the boiler shouldn't be an issue.

[rick]

Suggest putting in a system cleaner run it for a week then flush .. may just be sludged up

[T i m]

Yeah, as mentioned elsewhere Rick we did that no so long ago when Mum had the bathroom re-vamped and considering how old the system was it was pretty clean.

This was one of the ground floor pipes where I fitted a drain c*ck.

There used to be one that came out thought the air brick under the kitchen door (it used to open straight onto the garden, now into the lean-to) but was 'lost' when it was converted from a gravity system with a floor mounted coal boiler to a wall mounted jobby (not by me).

This was from the days when they made stuff to last (this is a rad valve).

Huge bore pipes, all generally pretty clean and little sign of sludge or calcium build up (but not saying there was none).

That said, nothing to stop me flushing it again and as mentioned elsewhere, I might take some temperature measurements as the most narrow part is where the pipes spilt at the return in the heat exchanger and go though the heat exchanger itself. I should be able to measure the temperatures of the flow on both paths and see if they are particularly different.

Cheers, T i m

[Tim+]

Possible but given that it's a safety device probably not wise to assume it. ;-)

Is there any chance of localised overheating in the boiler due to an airlock in the top part of the boiler? Without sufficient water to carry away the heat an air pocket trapped at the top of the boiler could (theoretically) cause localised overheating.

Of course good design and pipe outlet positioning should eliminate this but without seeing your boiler I can only speculate.

Tim

[T i m]

Quite, and why I've not left it bypassed. ;-)

I don't *think* so. The design looks very much like any air would travel up and out by natural means, let alone because of the pump pressure / flow.

Understood. Unless an airlock was causing a lack of movement the side of the heat exchanger the overheat stat was fitted ... I'll have to have a better look first.

I'll take a pic the next time I'm round there.

Cheers, T i m

[Andrew Gabriel]

Presumably that was with a different boiler? I never heard of a low capacity boiler being suitable for a gravity system - it would overheat too fast.

or failed pump run-on timer?

[T i m]

Nope, same one.

With a pump?

It doesn't have one and has been running like this for over 25 years? However, the pipework was changed a while back and, coincidentally or otherwise, the 'issue' did start about the same time. That said, it has also been running for ~3 years after that (once it settles down).

So, for example, if the wind is in the wrong direction it can blow the pilot light out. It generally re-lights ok but now and again it doesn't. The last time it didn't (a whole back) it seemed that the overtemp stat was introducing just a bit of resistance into the flame failure detector cct as bypassing it allowed the pilot light to stay on ok. Replacing it seemed to fix the problem.

Changing the FF detector seemed to make a difference but later the same fault would return (and then be cleared after I tried a few things).

This last time I noticed the upper limit stat had dripped and resetting it allowed the boiler to run fine again (for a few days). I went round there today and it seems like the pilot has gone out again (hot water only warm) but I forgot to have a look at it (no issues as Mums away for a couple of days).

The last time I looked into it I think the flow and return temps were fine, everything seemed to be working fine (water flowing round the system in general), it's just that the upper limit stat seemed too 'sensitive'?

Cheers, T i m

[Cynic]

Have you actually measured the control stat response? They do get lazy and settings drift. The overheat stat could be entirely innocent in this saga

[T i m]

No and I'm not sure how I would?

In case it's relevant, Mum used to run the boiler at position 4 (out of 10) and that gave her the right temperature hot water and background central heating. After the most recent plumbing changes we found that it seemed to 'hang in there' better when set on 1. So, if you turn the boiler on from cold, the main burner flame comes up pretty well (but not in any way 'violent') runs nicely till the water temp starts to pick up and fairly quickly (minutes) modulates down slightly till it reaches the correct temperature (thermocouple in the return) and then shuts down. The pump runs all the time the timer is on (7-8, 5-6 or summat).

Understood, but without being there watching (or logging?) the process the two hours a day it's on, I'm not sure what I can do?

Something Andrew said re a pump overrun timer got me thinking (as it doesn't have one). A while back Mum had the system on 24/7 which would mean the pump would also be on 24/7. This would mean there was never a chance of any overtemp down to lack of water flow.

When I went round there yesterday (to do other things) I noticed the water wasn't hot and therefore the chances are the boiler overtemp stat may have tripped again. If manually resetting it works and the boiler runs ok again, I could temporarily re-wire the pump to be on

24/7 and leave just the boiler on the timer? If that works, can you get 'stand alone' overrun timers? [1]

Cheers, T i m

[1] I think you could years ago, or I made one with a delay timer relay on my Uncles very basic CH system.