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

[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.
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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.