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

[rhino666]

I'm sorry to revisit my thread but the hot water issue continues. Main boiler PCB replaced and pump thoroughly checked; no change in this issue. Tonight I stayed with the boiler for 50+ minutes to document the hot water scenario.
Times are stop watch mins/secs............
0.00 15 degrees pump fired straight up and ran consistently and quietly throughout.
3.00 Steady climb to to around 60 degrees then all over the place, back to 30, hovered and then climbed quickly
3.45 cut out at 72 degrees
4.22 fired up at 62
5.20 cut out at 72
5.50 fired up at 55
6.04 cut out at 72
6.36 fired up at 55
7.00 cut out at 72
7.22 fired up at 53
8.20 cut out
8.51 fired up
9.12 cut out
9.43 fired up
10.01 cut out
10.32 fired up
10.55 cut out
11.24 fired up
12.21 cut out(settling)
16.00 54
18.00 52
22.00 50
25.00 49
32.20 48
39.30 47
49.00 46 fired up
51.15 72 cut out
Steady drop as before(stabilized)

To summarise the boiler and pump appear to be working perfectly. The problem is that the circulating water is initially heating up and cooling down too quickly, hence the nine fire ups and cut outs where there would ideally be just one. Boiler and hot water tank were installed in 2006. There is very little hard water deposit down here in South Devon but it seems to me that the issue may be a failure of the heat to effectively transfer from the coil system in the tank to the contents. I think this is causing the limited boiler heated circulatory water to have very unstable temperature until the hot water tank contents finally get up to temperature and stop a fast cooling of the circulating water. The Grundfos 16/60, 130 pump(pic earlier in thread) is set to speed three(of three). Quite a lot of radiators in the house so works well for central heating. Perhaps the hot water system with small 114 litre tank would work better if the pump was set at one or two? If anyone can think of anything else, input much appreciated.

 

[Chuck]

I'm sorry to revisit my thread but the hot water issue continues.

What model of hot water tank do you have?

 

[John.g]

I have a standard 0.62 M2 coil heating ~ 100 litres of a twin coil 150 litre cylinder with a oil fired boiler so I would think similar coil performance, my boiler runs at 70C.
I took a few readings years ago after draining the HW cylinder until full of cold water at 13C and heated it to 55C, took 65 minutes (20 kw boiler cycling on/off since it can't modulate)
Just to give you some idea of the power required/absorbed at some different cylinder temperatures.
7kw @ 13C
6kw @ 20C
4kw @ 40C
3kw @ 50C
2.5kw @ 55C

I would suggest draining off all the hot water until you have a cylinder full of cold water and fire up the boiler and see how it performs.
Presuming you have a modulating gas boiler down to maybe 6 kw then it should fire continuously for anything between ~ 2 and 21 minutes (see below) to reach 30C from cold at 10C, the heating demand is < 5 kw after this but shouldn't cycle excessively even up to 55C (2.5 kw).
If the boiler is of the modulating type then it should run at ~ 6.5 kw up to cylinder temp of 30C and as it requires 2.3 kwh to heat 100 litres from 10C to 30C then the boiler should run continuously for ~ 21 minutes but if not a modulating type should still run) for a minimum of 2 minutes before cycling begins.
Now I accept that it appears you have a different problem but carrying out the above
may just give you a clue as to the problem.
Was the boiler flow temperature probe changed?.

Just read that the boiler can modulate to ~ 5.5kw so should definitely run based on the above for 20/25 or so minutes initially with a cold cylinder until 30C.
[automerge]1607155622[/automerge]
You may have already checked but.....
Since you have a lot of radiators with a mid position valve which means that you can heat the CH & HW together its possible that a balancing wheel (gate) valve is fitted in the coil return, just check for this and shut it fully, then open it fully (3 to 4 full turns). If this makes a difference then you can fully shut it again and open it 3/4 to 1 full turn.
If or if not carrying out the above suggested cold test and if the boiler starts acting up switch on the CH+HW and assuming no boiler antics see if the HW cylinder is heating up as well as the rads, also feel the coil return, it should be fairly hot but way cooler than the flow.

 

[rhino666]

What model of hot water tank do you have?

Please see attached photos. Tank seems to have been installed at the same time as the Glowworm 30HXI boiler in 2006.
[automerge]1607171582[/automerge]

I have a standard 0.62 M2 coil heating ~ 100 litres of a twin coil 150 litre cylinder with a oil fired boiler so I would think similar coil performance, my boiler runs at 70C.
I took a few readings years ago after draining the HW cylinder until full of cold water at 13C and heated it to 55C, took 65 minutes (20 kw boiler cycling on/off since it can't modulate)
Just to give you some idea of the power required/absorbed at some different cylinder temperatures.
7kw @ 13C
6kw @ 20C
4kw @ 40C
3kw @ 50C
2.5kw @ 55C

I would suggest draining off all the hot water until you have a cylinder full of cold water and fire up the boiler and see how it performs.
Presuming you have a modulating gas boiler down to maybe 6 kw then it should fire continuously for anything between ~ 2 and 21 minutes (see below) to reach 30C from cold at 10C, the heating demand is < 5 kw after this but shouldn't cycle excessively even up to 55C (2.5 kw).
If the boiler is of the modulating type then it should run at ~ 6.5 kw up to cylinder temp of 30C and as it requires 2.3 kwh to heat 100 litres from 10C to 30C then the boiler should run continuously for ~ 21 minutes but if not a modulating type should still run) for a minimum of 2 minutes before cycling begins.
Now I accept that it appears you have a different problem but carrying out the above
may just give you a clue as to the problem.
Was the boiler flow temperature probe changed?.

Just read that the boiler can modulate to ~ 5.5kw so should definitely run based on the above for 20/25 or so minutes initially with a cold cylinder until 30C.
[automerge]1607155622[/automerge]
You may have already checked but...
Since you have a lot of radiators with a mid position valve which means that you can heat the CH & HW together its possible that a balancing wheel (gate) valve is fitted in the coil return, just check for this and shut it fully, then open it fully (3 to 4 full turns). If this makes a difference then you can fully shut it again and open it 3/4 to 1 full turn.
If or if not carrying out the above suggested cold test and if the boiler starts acting up switch on the CH+HW and assuming no boiler antics see if the HW cylinder is heating up as well as the rads, also feel the coil return, it should be fairly hot but way cooler than the flow.

John, I have an instinctive feeling you have nailed it if as I suspect this cycling is created by excessive flow speed upsetting heat permeation/diffusion. There is a balancing gate valve fitted; see picture and it is very possible that I upset it some years ago when tackling a series of leaks in the rotted copper pipe. Shut every valve off in sight and then wound fully open when issue with pipes finally resolved. 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. The temperature probe has not been changed in the nine years we have lived here but reasonably sure it is working correctly.

 

[Brambles]

Follow John’s advice, but also measure the dhw coil flow and return temps at the cylinder. On Dhw only they should be very close to the boiler flow and return temps.

If they are not look at the by pass valve arrangement - is it acting at the path of least resistance - or acting properly and you have a restriction in the DHW coil circuit - which could be at the valve

 

[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?