Worcester Bosch Greenstar 8000 System Boiler Issues

[John Carroll]

For info only.

Came across some data recently from a WB Greenstar 36CDI Combi that had its PHEX renewed due to very poor DHW performance, the primary flowrate increased from 8.2LPM to 23.0LPM with DHW back to normal so seems a bit strange that (even though the primary HEX was renewed in the above case) that there was no increase in the flowrate, obviously the system&rads are in series with this so a major factor in the head loss, still, one might have expected some increase in the flowrate?, wonder was it just some form of scaling or perhaps nothing wrong with the HEX and reattaching (IF) the flowtemp sensor to the HEX made a proper connection, it would point to the former though, highly insulating scale of some form or other?.

Edited yesterday at 09:01 by John Carroll

[JohnMo]

8 minutes ago, John Carroll said:

23.0LPM

Are you sure - 23 is a lot of flow, nameplate says 13?

[John Carroll]

1 hour ago, JohnMo said:

Are you sure - 23 is a lot of flow, nameplate says 13?

That's the primary flow John, not the secondary, DHW flow, strangely enough, there didn't appear to have been any restriction to this in the old PHEX.

[EinTopaz]

2 hours ago, John Carroll said:

For info only.

Came across some data recently from a WB Greenstar 36CDI Combi that had its PHEX renewed due to very poor DHW performance, the primary flowrate increased from 8.2LPM to 23.0LPM with DHW back to normal so seems a bit strange that (even though the primary HEX was renewed in the above case) that there was no increase in the flowrate, obviously the system&rads are in series with this so a major factor in the head loss, still, one might have expected some increase in the flowrate?, wonder was it just some form of scaling or perhaps nothing wrong with the HEX and reattaching (IF) the flowtemp sensor to the HEX made a proper connection, it would point to the former though, highly insulating scale of some form or other?.

Yep could be. Though It is strange there is no increase there, as the flow pipe and rads were all measuring 10'c hotter than before the HEX was changed. So its definitely outputting hotter water than it was before. 

[EinTopaz]

18 hours ago, marshian said:

 

Depends if you are heating to a schedule or 24/7

 

If heating to a schedule - you really want the boiler to throw whatever kW is needed to bring the house up to temp and you might want the pump to be managed by the boiler.

 

If heating 24/7 - you are just asking the boiler to replace the heat lost (per hour) - in this case range rating (with a margin to allow for extreme temps) and pump speed could be completely different settings wise.

We're closer to the former of those two, but honestly we just boost it if and when rather than on a schedule. When the house feels chilly we boost it for a few hrs. 

[EinTopaz]

18 hours ago, John Carroll said:

 

I suppose so, you can play around with it.

 

It would be interesting to see what the pump modulation goes to and the other data if you remove the minimum clamp.

 

Did the engineer renew anything else apart from the HEX?, you mentioned that he might renew some control card or other??.

He never did no, they brought a new board with them. But he said that replacement is for the older version of my boiler that suffered from a different set of problems, mainly something to do with the boiler idling for long periods of time when there's a clear demand, mine never had that as it's the newer physical version of the same boiler. So he only changed the HEX.

I will do some logging now that the boiler is reading 1:1 with the flow pipe. I can try all the different pump settings which previously due to the issue I was having made it unclear which was the correct set of settings. I should easily be able to monitor and decide on that now though. 

[marshian]

14 minutes ago, EinTopaz said:

We're closer to the former of those two, but honestly we just boost it if and when rather than on a schedule. When the house feels chilly we boost it for a few hrs. 

Then you probably want to give the boiler the ability to run at whatever kW it can or potentially your warm up time will be extended.

 

We used to run a similar process with the old boiler - Summer to Autumn transition - boost for a couple of hours when you feel the house needs it.

 

Moving thro to fixed scheduled heating in the winter months increasing the scheduling period to ensure the house was at target temp when we got in from work

 

Mrs Alien is a stupidly light sleeper and mornings were an issue because if I set the schedule earlier in the morning it woke her so we had to compromise on heating early in the mornings.

 

Then back to boosting whenever required for the spring transition

 

But I found it very unsatisfactory, temps swings, rapid cool down because you aren't putting any warmth into the fabric of the building (so it can't give it back to slow down the temp drop) Needed to heat rooms to a much higher temp to feel the warmth. 

 

Moving to weather compensated flow temps enabled me to run lower target room temps - with a little bit of set back on the boiler overnight - house is just so much more comfortable all the time and the additional gas usage compared to scheduled heating isn't worth worrying about (for us it works out to £60 extra a year)

[EinTopaz]

Gave it another run last night. Still on pump setting 0 (relative to the burner setting) and locked in a range of 60-100% min/max for the pump too.

It got to target of 60'c within about 30mins. All rads hot by then and measuring around the same on the flow pipes for each of them. So all fine.

The not fine bit is it stayed at a max output on the burner for 30mins after reaching target temp. Then took another 30mins after that to very slowly modulate down to 60%.  So that feels like its not modulating down soon enough.  Or that the system really needs the boiler to be on 100% even to maintain the target temp of 60'c... which i'd be very surprised by.

So to test the latter, I tried it again but range rated it down to 75%, it took longer to get to target, but then frustratingly didn't drop from 75% once it got to target, it did exactly the same as it did when it was rated to 100%, just stayed on 'max' for half hour after hitting target, then dropped. 

So in conclusion, if it can raise the temp to target at 75%... why would it think it needs to stay at 100% to maintain the target? What exactly is telling the boiler to modulate down on the greenstar 8000 life? 

[marshian]

2 hours ago, EinTopaz said:

Gave it another run last night. Still on pump setting 0 (relative to the burner setting) and locked in a range of 60-100% min/max for the pump too.

It got to target of 60'c within about 30mins. All rads hot by then and measuring around the same on the flow pipes for each of them. So all fine.

The not fine bit is it stayed at a max output on the burner for 30mins after reaching target temp. Then took another 30mins after that to very slowly modulate down to 60%.  So that feels like its not modulating down soon enough.  Or that the system really needs the boiler to be on 100% even to maintain the target temp of 60'c... which i'd be very surprised by.

So to test the latter, I tried it again but range rated it down to 75%, it took longer to get to target, but then frustratingly didn't drop from 75% once it got to target, it did exactly the same as it did when it was rated to 100%, just stayed on 'max' for half hour after hitting target, then dropped. 

So in conclusion, if it can raise the temp to target at 75%... why would it think it needs to stay at 100% to maintain the target? What exactly is telling the boiler to modulate down on the greenstar 8000 life? 

That does appear to be rather odd behaviour

 

How big is the circuit volume?

[marshian]

Another thought

 

Read the gas meter at the start and then say every 20 mins noting the modulation claimed

 

Post the readings lets see what it's really consuming

[John Carroll]

As above, more or less, get the boiler to fire at 100% for say 4/5 minutes, during that time take  two gas meter readings exactly 3 minutes apart, should then get a fairly accurate feel for the actual boier output in kW. 

If the output is 36kW, then the flow/return dT should be  23.9C until the target flowtemp of 65.5C is reached, assuming a flowrate of 21.6LPM. Target temperature of 65.5C should be reached when the return temperature rises to, 65.5-23.9. 41.6C Maybe clamp the pump minimum output to 95% until all tests completed.

Post back.

 

 

[EinTopaz]

1 hour ago, John Carroll said:

As above, more or less, get the boiler to fire at 100% for say 4/5 minutes, during that time take  two gas meter readings exactly 3 minutes apart, should then get a fairly accurate feel for the actual boier output in kW. 

If the output is 36kW, then the flow/return dT should be  23.9C until the target flowtemp of 65.5C is reached, assuming a flowrate of 21.6LPM. Target temperature of 65.5C should be reached when the return temperature rises to, 65.5-23.9. 41.6C Maybe clamp the pump minimum output to 95% until all tests completed.

Post back.

 

 

Thanks John, does the boiler need to be at target temperature when I do these tests or does that not matter? 

[John Carroll]

It's more than likely that it won't or shouldn't be at 100% firing at target temp so best window is well before it reaches target temp as it should then be firing at 100%.

[marshian]

14 minutes ago, EinTopaz said:

Thanks John, does the boiler need to be at target temperature when I do these tests or does that not matter? 

 

Not answering for @John Carroll but I'd do readings from start up 

 

Logic is I'd want to establish that what the boiler is stating it's modulation state actually was.

 

My 16kW Viessmann modulates from 100% to 10.8% - 100% is 19kW 10.8% is 4.2 kW the % values between the two aren't very linear or put another way if 10.8% is 4.2 kW 100% should be 38 kW

[marshian]

Similar boiler on third heat exchanger

 

 

 

[EinTopaz]

Will give it a try tonight. It’s going to be hard to get an accurate reading from start up as it’s output % does change quite for the first 10 mins from start up. It goes higher and higher. I’ll wait for it to settle at a high % then do the 3 min tests.  

[EinTopaz]

Ok, gave it a whirl and logged the temps and burner/pump outputs at each interval too. TLDR it did run well but overshot the target by 5c and stayed there for a while whilst modulating down slowly. Some things worth noting before looking at the data below. I set the pump setting back to pump 6, the highest constant pressure setting, rather than setting 0 (which makes it relative to the burner). Now that the HEX is fixed, I did want to try that to see if it ran well and still got everything hot etc. Also worth noting the target temp on this run was set to 60'c

On for 5minutes
Flow 30.6
return 15.6
Burner 62%
Pump 93%

On 15 minutes
Flow 50.9
return 29.0
Burner 100%
Pump 100%
 

On 25 minutes (target temp reached)
Flow 60.1
return 39.1
Burner 100%
Pump 100%

 

On 30 minutes
Flow 63.2
return 42.8
Burner 95%
Pump 100%

 

On 35 minutes
Flow 64.8
return 46.4
Burner 85%
Pump 100%

 

On 45 minutes
Flow 64.8
return 50.2
Burner 65%
Pump 100%

 

On 55 minutes
Flow 61.9
return 50.7
Burner 49%
Pump 100%

 

big time gap now as I stopped measuring till it settled to actual target temp of 60'c

On 1h35mins
Flow 59.9
return 47.8
burner 49%
pump 100%


Lastly, I did two lots of measures on the meter, first set when the boiler was at 100% and second set when it had settled to 49%

100% meter readings (in m3 meter cubed)
0mins  - 7293.39
3 mins - 7293.56
6mins - 7293.74

49% meter readings ( in m3 meter cubed

0mins -7296.14
3mins -7296.22
6mins -7296.31

 

 

 

What do you guys think? To me im not sure why it's staying above target for so long before it starts modulating down. If the greenstar 8000 has no return sensor, i wonder what is making it think it needs to stay so hot at such high % long after target is achieved? 

[marshian]

1 hour ago, EinTopaz said:

Ok, gave it a whirl and logged the temps and burner/pump outputs at each interval too. TLDR it did run well but overshot the target by 5c and stayed there for a while whilst modulating down slowly. Some things worth noting before looking at the data below. I set the pump setting back to pump 6, the highest constant pressure setting, rather than setting 0 (which makes it relative to the burner). Now that the HEX is fixed, I did want to try that to see if it ran well and still got everything hot etc. Also worth noting the target temp on this run was set to 60'c

On for 5minutes
Flow 30.6
return 15.6
Burner 62%
Pump 93%

On 15 minutes
Flow 50.9
return 29.0
Burner 100%
Pump 100%
 

On 25 minutes (target temp reached)
Flow 60.1
return 39.1
Burner 100%
Pump 100%

 

On 30 minutes
Flow 63.2
return 42.8
Burner 95%
Pump 100%

 

On 35 minutes
Flow 64.8
return 46.4
Burner 85%
Pump 100%

 

On 45 minutes
Flow 64.8
return 50.2
Burner 65%
Pump 100%

 

On 55 minutes
Flow 61.9
return 50.7
Burner 49%
Pump 100%

 

big time gap now as I stopped measuring till it settled to actual target temp of 60'c

On 1h35mins
Flow 59.9
return 47.8
burner 49%
pump 100%


Lastly, I did two lots of measures on the meter, first set when the boiler was at 100% and second set when it had settled to 49%

100% meter readings (in m3 meter cubed)
0mins  - 7293.39
3 mins - 7293.56
6mins - 7293.74

49% meter readings ( in m3 meter cubed

0mins -7296.14
3mins -7296.22
6mins -7296.31

 

 

 

What do you guys think? To me im not sure why it's staying above target for so long before it starts modulating down. If the greenstar 8000 has no return sensor, i wonder what is making it think it needs to stay so hot at such high % long after target is achieved? 

 

I would think it's managing the modulation based on flow temp target

 

Anyway kWh usage below for the for the two sessions 

 

100% being the first one shows the boiler is really chucking everything it can at the circuit

 

49% being the second one shows it's modulation percentage is not far out

 

[marshian]

The other interesting thing is the DT at the boiler

 

On initial start it doesn't apply full power to hit target temp because the DT would be greater than 20 

 

Once the DT is at 20 Deg it applies full power

 

Once the DT starts shrinking it modulates down (it's not doing that by knowing the return temp it doing it based on the flow temp rising above target and then it modulates down 

 

At the end of the day you are intermittently heating - your circuit temp at the start is less than 15 Deg C (now the rads will normally be at the same temp as the room when the system has been off for a while so clearly your tolerance for cooler temps is greater than mine or rather Mrs Alien)

 

I do wonder if you would be better of heating low and slow rather than blasts of heat, cool down, blast of heat etc

 

If your flow temp was say 45 deg C and initial circuit temp was 15 the boiler wouldn't have to throttle back and wait for a DT of 20 before hitting the circuit with full power. The return temp would rise and the boiler would modulate down sooner and probably much more........

 

However with a flow temp of 45 you'd need to have rads that were big enough to emit the heat the rooms needed.

[John Carroll]

Overall, not too bad really, 25 minutes from a cold 15.6C to 60C Target/flowtemp in 25 minutes isn't outrageously slow IMO.

 

 

Edited 2 hours ago by John Carroll