Gas boiler overheats on valve closure - vented system

[stardanny]

Both valves closed it still runs. If hot water cylinder demand met, then heating demand met, both valves close and pump continues to run on a cooling circulation I guess determined by the boiler. I'm pretty sure the pump is wired to the boiler.

Hmm, make sure the boiler does not fire when both valves are closed and the pump overrun for 10 mins to cool down.

Baxi solo 70pf is a cast iron heat exchanger so should not overheat when not firing. I have a baxi solo 2 50pf with same 2 2 port valves and bypass and it never happen to me.

Dan.

 

[penguinventures]

is the bypass the right way round

Good question! Just checked and yes it is.

 

[hazetimesfive]

do you have any idea what the flow temperature is... seems to me if the water returning to the boiler when the system is satisfied is hot enough to start boiling then something isn't quite right.

 

[penguinventures]

Both valves closed it still runs. If hot water cylinder demand met, then heating demand met, both valves close and pump continues to run on a cooling circulation I guess determined by the boiler. I'm pretty sure the pump is wired to the boiler.

Hmm, make sure the boiler does not fire when both valves are closed and the pump overrun for 10 mins to cool down.

Baxi solo 70pf is a cast iron heat exchanger so should not overheat when not firing. I have a baxi solo 2 50pf with same 2 2 port valves and bypass and it never happen to me.

Dan.

The boiler does definitely stop firing when both valves are closed and the pump def overruns for maybe 10 mins.

It's so weird. The valves can close. The firing stops. Nearly a minute can pass. Then you hear the boil. A click and the overheat light glows. Air bubbles and boiling continues (which I can kill if I operate the room thermostat and loose the boiling water into the rads).

 

[penguinventures]

do you have any idea what the flow temperature is... seems to me if the water returning to the boiler when the system is satisfied is hot enough to start boiling then something isn't quite right.

I can measure this tomorrow. Systems all off now and I'll get it in the ear if I fiddle about with it now. Seems like a fair enough test. I can measure the temp on the flow & return on the boiler and record temperatures as the boiler cycles and when it goes faulty.

 

[stardanny]

Has the boiler been serviced, gas safety checked? It will need to be check by a gassafe engineer to make sure it not letting by or pilot light if you have one correct set.

Dan.

 

[penguinventures]

Has the boiler been serviced, gas safety checked? It will need to be check by a gassafe engineer to make sure it not letting by or pilot light if you have one correct set.

Dan.

Dan

All work on the system has been by the same engineer and he is good gassafe engineer whom I trust to have looked after that aspect. The boiler's serviced & safety checked by him though not this season, and its always been his call to do whatever he likes in checking the boiler. Personally I think the boiler's working fine.

 

[hertsboilers]

Very interesting problem!

How far the auto bypass valve is located from the boiler? If the boiler is big in heat output and there is no enough pipe length from the boiler to the auto-bypass?

 

[gas112]

Couple of things it will be.
Does the boiler rumble at all when the valves close if so the boiler stat is getting to hot and has gone out of sinc and overheat trips.
No flow through auto bypass whip it out go back to gate valve see if fault disappears

 

[penguinventures]

Very interesting problem!

How far the auto bypass valve is located from the boiler? If the boiler is big in heat output and there is no enough pipe length from the boiler to the auto-bypass?

The auto bypass is adjacent to the pump in the airing cupboard which is 5 metres pipe length away. The boiler connections are 22mm - pipe changes to 28mm for the 5 metre run to the airing cupboard. Half of the run is exposed pipe high on the wall, other half is reasonably insulated (with a tumble dryer and general clutter) as it travels round a cupboard then into the airing cupboard.

Here's the airing cupboard

Airing cupboard

• penguinventures
• 15 Nov 2012
• 1

Sorry you have to tilt your head!

 

[penguinventures]

Couple of things it will be.
Does the boiler rumble at all when the valves close if so the boiler stat is getting to hot and has gone out of sinc and overheat trips.
No flow through auto bypass whip it out go back to gate valve see if fault disappears

I'll have to listen out for rumbles as you suggest. Not sure I've heard that.

As you'll see from the photo posted the old gate valve is still there and can be partially opened. I'll open it partially this weekend see what happens. I'll also measure temperature differential across the boiler during the overheats. As is often the case, no overheat today? Fault appears in cycles?

 

[ianmcd]

The gate valve isnt the old by pass (valve with red handle) the old bypass is the silver poss Brass hard to tell in pic ballofix valve running parallel with the auto by-pass, open that up fully and see if that stops your problems

 

[Agile]

First time I have seen an inline auto bypass.

Can I recap?

The motor valves close and the boiler stops firing ???

The pump continues to over run as controlled by the boiler ???

The latent heat in the boiler is not always dissipated by the pipework and the boiler overheat stat trips ???

To see whats happening you need to measure the boiler flow temperature during operation and just after the valves close as it over runs.

Tony

 

[penguinventures]

First time I have seen an inline auto bypass.

Can I recap?

The motor valves close and the boiler stops firing ???

The pump continues to over run as controlled by the boiler ???

The latent heat in the boiler is not always dissipated by the pipework and the boiler overheat stat trips ???

To see whats happening you need to measure the boiler flow temperature during operation and just after the valves close as it over runs.

Tony

Tony

I confim;
The motor valves close & the boiler stops firing.
The pump continues to over run as its controlled by the boiler.
Once the valves close that heat in the boiler is not always dissipated by the pipework and the overheat trips.

I've setup two meters monitoring the boiler's flow and return pipes as they exit/enter the boiler casing.

Max flow temp (before stat cuts firing) is 62 deg C (setting at 3.5). Firing resumes at 57deg C
Differential between flow and return whilst firing is 13 deg C (average monitored over 30 mins)

When valve shuts.
Flow goes from 62 deg C to 67 deg C in less than minute
Return goes from 49 deg C to 59 deg C same time frame
After two minutes flow is 63 deg C
Return is 58 deg C.

Differential went from 13 deg C to 5 deg C in two minutes
Overheat did not occur.

I need to do some further tests and will report temperatures during an overheat.

I did see flow go from 54 to 80 deg C with return from 36 to 39 deg in <1 minute. Two minutes later the flow was down to 57 with return at 42.

Then again I saw 88 & 42, 86 & 46, 84 & 47

I need to do the tests again keeping track of what the valves are doing.

 

[xr4x4]

That would suggest to me that the bypass isn't letting enough flow.

Did you try opening the old bypass as suggested earlier?