Greenstar 24ri - large temperature drop before re-fire

[desktop987456]

nearest tap was ~61C and even at 6:00pm this evening was still sufficient water at ~55C.

Interesting. You could teach me a few tricks as I am in the process of putting a HW cylinder.
When you at 6p.m water was at 55C; after how much time did you get the reading at 6p.m?
Depending on your usage, how much are you spending on gas in a day if it is reheated twice; once in the morning and at night? It would use more gas when you fire it first time in the morning but would use less during the day as water replenshises in the cylinder.
Does the boiler re ignite once the temperature falls below the cylinder stat?
Thanks

 

[Johntheo5]

You are not far away from getting the boiler to reheat the cylinder in one go, the burner is tripping at 73C which means the boiler SP is, 73-5, 68C, its refiring at 57C ish so, SP-11C, might be worth just increasing the the boiler SP to say 70/72C because its running at minimum output after ~ min 26, after which the cylinder coil is not able to absorb the boiler minimum output, that few extra degrees might allow it to run until the cylinder stat is satisfied.

When the HW (or CH) demand is satisfied then the zone valve(s) will close with the circ pump overrunning for another 3 minutes, is there a automatic bypass valve, ABV, installed somewhere?.
EDIT, You have, its in the garage, post# 12.

 

[Johntheo5]

Here's a 2015 extract from tests taken on my twin coil cylinder with the top 100L heated by a oil fired coil.

 

[Gwyndy]

When you at 6p.m water was at 55C; after how much time did you get the reading at 6p.m?

In the last plot, water was heated for a one hour cycle from around 09:00 to 10:00 - (much later than usual so that I could measure return temps) - so still adequate hot water after 8 hours. Our hot water usage is fairly low - there are only two of us so showers and a couple of washing up sessions.

Depending on your usage, how much are you spending on gas in a day if it is reheated twice; once in the morning and at night? It would use more gas when you fire it first time in the morning but would use less during the day as water replenshises in the cylinder.

Normally we only heat the water once per day - see above. Average gas usage for hot water heating only is ~6 kWh / day. On my current tariff that costs about 34p / day excluding the fixed daily charge. Interesting to note that with the old boiler the gas usage for hot water heating was double the current figure and overall for CH and HW my gas consumption is down by ~40% (excluding any consideratons of weather differences).
Yes, you would use more gas if you heat twice per day but I don't know how much - will depend on how much hotwater you have used between heating periods and the temperature of the water in the cylinder.

Does the boiler re ignite once the temperature falls below the cylinder stat?

Yes, if left on and the cylinder temperature falls the boiler will re-ignite.

 

[Gwyndy]

You are not far away from getting the boiler to reheat the cylinder in one go, the burner is tripping at 73C which means the boiler SP is, 73-5, 68C, its refiring at 57C ish so, SP-11C, might be worth just increasing the the boiler SP to say 70/72C because its running at minimum output after ~ min 26, after which the cylinder coil is not able to absorb the boiler minimum output, that few extra degrees might allow it to run until the cylinder stat is satisfied.

@Johntheo5 Thank you for that - you have prompted a 'lightbulb moment'! Looking back at my last temperature plot the flow temp is fairly flat and steady at ~67-69C for probably about 20 minutes after the initial heat up and then has a little 7 minute 'burst' up to 73C when it then shuts off. Is this the 'set point + 5C' in the WB data sheet that you posted? Maybe! I had assumed that the set point was 73C as that is the temp at which the boiler stops firing. If you are right, and I don't doubt that you are, then a tweak on the boiler stat may well do the trick. I'll give it a try and see what happens.
Watch this space!

 

[Gwyndy]

Here's a 2015 extract from tests taken on my twin coil cylinder with the top 100L heated by a oil fired coil.

That's very interesting. How did you determine the coil heat input?

 

[Johntheo5]

@Johntheo5 Thank you for that - you have prompted a 'lightbulb moment'! Looking back at my last temperature plot the flow temp is fairly flat and steady at ~67-69C for probably about 20 minutes after the initial heat up and then has a little 7 minute 'burst' up to 73C when it then shuts off. Is this the 'set point + 5C' in the WB data sheet that you posted? Maybe! I had assumed that the set point was 73C as that is the temp at which the boiler stops firing. If you are right, and I don't doubt that you are, then a tweak on the boiler stat may well do the trick. I'll give it a try and see what happens.
Watch this space!

Yes, thats quite correct, the 73C is the SP+5C, if you increase the SP to say 72C, then the burner will remain firing until the flow temperature is, 72+5, 77C which overall should extract a little more boiler/coil eneregy

That's very interesting. How did you determine the coil heat input?

The boiler coil heats exactly 100L and I have a number of very accurate PT1000 sensors on the cylinder, one probe is a little bit up from the coil bottom, the other is a good bit above the coil top, so I averaged these every 5 minutes and subtract them from the previous 5 min reading, for example the average dT (difference) between min25 & min30 was 3.45C so the coil energy extracted was, 3.45*100/860, 0.401kwh (over 5 mins) which gives a coil output of 0.401*60/5, 4.81kw, and so on.
I control the cylinder (60C) with my solar controller which drives the cylinder coil motorized valve and is using the bottom cylinder probe for reference.

 

[Gwyndy]

Yes, thats quite correct, the 73C is the SP+5C, if you increase the SP to say 72C, then the burner will remain firing until the flow temperature is, 72+5, 77C which overall should extract a little more boiler/coil eneregy

But hopefully the cylinder heat up will be accomplished before the +5C point.
Boiler stat turned up a touch - I'll see what happens tomorrow. (I use a paper ring that I have printed with divisions at every 7.5° so that I can roughly track the settings that I use. The 24ri has a range of approx 35-82C - min to max - so each division on my scale equates to about 1.6C. That, of course, assumes that the thermostat is linear in operation which it probably isn't, but it's a guide.)

 

[Gwyndy]

The boiler coil heats exactly 100L and I have a number of very accurate PT1000 sensors on the cylinder .....

That is very clever.

 

[Gwyndy]

Boiler stat setting was increased last night. Hot water heated this morning - unfortunately I was still in the land of nod so I don't know if the cylinder stat operated but looking at the temperature plot I would guess that it probably did. It looks as though I have increased the flow temp by 4-5C.

Interesting that once again I see that after the initial heat up the temperature is fairly flat (~72-74C) for about 20 minutes and then ramps up to a peak of 77.8C after a further 10 minutes. Boiler then shuts off (set point +5C?).

On the attached plot I have superimposed the data from 2 days ago, just for interest.

Anyway, I do have plenty of hot water but I do have some boiler behaviour that seems to be explained by having an indirect cylinder designed for gravity feed HW heating. I will still talk to WB to establish the parameters within which the boiler shuts off / re-fires and I'll discuss possibly fitting a flow restriction in the HW circuit with the installer although I do wonder if this would make the system any more efficient or whether it would just be an 'interesting exercise'.

 

[Johntheo5]

It does look as if the burner tripped at SP+5C.
I don't think throttling the flow will help to get the cylinder to reach its SP before the the burner trips as the mean coil temperature will be reduced meaning the coil output will be even less, however you can do this test without any pipe mods, just throttle the pump discharge valve to say 1/2 open (with HW only on) or whatever and see what effect this has. You might also feel the ABV downstream pipe, this should be cool/cold especially with the HW coil circulation at probably ~ 15LPM and should then only open on pump overrun when the HW motorized valve closes, also look at the ABV's indexed setting.

 

[Gwyndy]

It does look as if the burner tripped at SP+5C.

It looks like it.
I have e-mailed WB asking them to define the parameters for boiler re-ignition. I have sent them the flow / return plot from a couple of days ago so that they can see what is happening.
I'll set the water heating for later tomorrow when I'm up and about and I'll check the ABV.

... just throttle the pump discharge valve to say 1/2 open (with HW only on) ..

Certainly a thought but I think that I'll monitor the performance for a few days and see what happens.

 

[Harry Bloomfield]

I have sent them the flow / return plot from a couple of days ago so that they can see what is happening.

It's fairly obvious from the plot, but an indication of when the boiler was firing, on the plot, might have been handy.

 

[Gwyndy]

It's fairly obvious from the plot, but an indication of when the boiler was firing, on the plot, might have been handy.

I thought that it was obvious - up to first peak, off, up to second peak, off again!

 

[Johntheo5]

Yes, still a bit puzzling though.
the 4th Oct shows the burner tripped at 73C and refired after 16mins at SP-11C (68-57).
yet, todays shows it apparently tripped at 77C ish but did not refire in the remaining 22mins to timeout after 60mins despite the temp having fallen to SP-14C, (72-58).