LPG Boiler cutting in/out too frequently when heating water only

[John.g]

I was hoping that the balance valve was throttled too much, I have never throttled mine as IMO it gives a faster cylinder warm up, I also have the DHW timed to come in for 1.5 hrs in the morning before requirement for CH, in the evening its programmed to be open all the time (via cylinder stat) from 330 until system shutdown at midnight, it doesn't seem to have any noticeable effect on the CH but of course the house is fully warm then.
I think there is yet another regulation or other now that balancing valves are not to be fitted??.

 

[Chuck]

It takes 5.5 turns to wind down fully, so wound down fully and then opened just one full turn. That should slow the flow sufficiently for a test later.

If you've been operating with it only open one turn, that's probably your issue. Open the valve fully and close it half a turn (to stop it seizing).

These valves are virtually redundant these days. They were needed in the past when the hot water was not on it's own independent zone and the heat exchangers in tanks had a lower capacity than do these days.

 

[rhino666]

If you've been operating with it only open one turn, that's probably your issue. Open the valve fully and close it half a turn (to stop it seizing).

These valves are virtually redundant these days. They were needed in the past when the hot water was not on it's own independent zone and the heat exchangers in tanks had a lower capacity than do these days.

No Chuck, the balancing valve was always previously operating fully open. I thought this issue may have been too fast a flow through the small tank where contrasting powerful flow required for CH on a 250m2 house. Have reset valve fully opening and then wound back in half a turn as advised. Just to test the hot water issue have turned Grundfos back from highest speed setting to lowest speed to see if this makes any difference; will report back later.

 

[John.g]

IMO if circulation problems on HW only more likely to be too low, not likely with the balancing valve fully open but a air lock in the coil can cause havoc, there should really be a air vent as close to the coil entry to the cylinder as possible otherwise one would have to slacken/break the connection and pull the pipe back very slightly to check/release any air, however if the coil is circulating and heating the cylinder with both HW & CH on then unlikely as lukewarm water is normally a sign of air.

 

[rhino666]

IMO if circulation problems on HW only more likely to be too low, not likely with the balancing valve fully open but a air lock in the coil can cause havoc, there should really be a air vent as close to the coil entry to the cylinder as possible otherwise one would have to slacken/break the connection and pull the pipe back very slightly to check/release any air, however if the coil is circulating and heating the cylinder with both HW & CH on then unlikely as lukewarm water is normally a sign of air.

No difference tonight; in fact possibly worse.
Pump runs very quietly and water always piping hot but there are signs that there could be an air lock or at least a failure to automatically bleed effectively.
Please see photo of bleed stack that runs off feed pipe from boiler to tank. Only gets warm at the base and almost cold at the top. Took another couple of photos - it's a Flamco Flexvent automatic bleed valve. Any way to check operation/manually bleed without physically unbolting, thanks?

 

[John.g]

Can you try running with HW+CH and see if boiler is stable and if the HW tank heats up?
Also ensure boiler pressure gauge 1.5 bar.

Vent: not familiar, others may advise.

 

[rhino666]

Can you try running with HW+CH and see if boiler is stable and if the HW tank heats up?
Also ensure boiler pressure gauge 1.5 bar.

Vent: not familiar, others may advise.

The pressure guage on my system is remote from the boiler, fitted under the expansion vessel. I have always kept it at 1.2 bar cold.
Undid auto bleed valve and seems to have water underneath it. My idea that there may be an airlock in the stack appears wrong. My idea that water heat should rise up the stack appears incorrect; perhaps no potential for circulation in the stack.
Boiler with cental heating and hot water runs exactly as expected; constant burn with temperature gradually rising. I turned the CH off and the Hot water issue returned. Running out of ideas with this now; perhaps I just have to put up with the 15 mins or so of cycling when Hot Water heating only engaged.
Appreciate the feedback on this thread; just a shame I haven't found the solution to this.
Getting back to the fundamental problem. It appears that the boiler is firing up and running on full power on the Hot water circuit, heating up the limited circulation volume too quickly for application. Perhaps the problem is simply that the boiler is not regulating itself correctly? Would have expected the replacement main PCB to remedy that but it was a refurbished board so perhaps the same issue as the original.

 

[John.g]

A few thoughts, all gas boilers fire up at relatively high rates, (I think) up to ~ 70% of output but should then modulate down very rapidly, they generally fire up at the SP-5C and will cut out at SP+5C so if the demand is < 5.5kw (your boiler minimum) the boiler will cycle on/off but not like shown in your post. One of the reasons I asked you to drain down your cylinder until cold is that the coil requirement should ensure that the boiler will run continuously for 20 minutes if everything is "normal" and then start cycling.
Another test (with CH only on) would be to shut off all rads except 1 on say your downstairs circuit, turn up the room stat and see will the boiler cycle normally on this, if the boiler acts up similarly as originally, then IMO it may point to some modulation or other boiler problem.
[automerge]1607271436[/automerge]
Just looking at your readings in post #11 I notice that the boiler fires up again ~ every 30 secs or so after cut out, Afaik all gas boilers have a anti rapid cycling time of around 3 mins and also most have a pump overrun to cool down the heat exchanger, this also lowers the return temperature so when the boiler fires up it has modulated down before it reaches its cut out temperature, one way or the other that doesn't look like normal operation to me, someone familiar with Glowworm boilers might like to comment.