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 9 hours ago 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.