Worcester Bosch Greenstar 8000 System Boiler Issues

[John Carroll]

1 hour ago, SimonD said:

 

I think it's worth testing pump setting 0 to see what happens. Who knows, it may not work at all but if it were me, I'd like to see variation in flow rate proportional to burner and DT and then see what happens through the system. 

 

It was, on this very boiler, but in another Site/Thread, "Bypass Valve" in DIYnot.com, the pump speed fell to 30% which by my calcs meant a flowrate of 6.3LPM at a head of only 0.35M, shared through 11 Type22 & 4 Triple rads.

This was before the boiler HEX was changed, so yes, would be interesting to see a full set of data if tried again.

Edited February 25 by John Carroll

[EinTopaz]

On 25/02/2026 at 20:55, SimonD said:

I wonder what happens when these things are installed with weather compensation, or maybe nobody ever does and they build these things for relay controls only for the UK market, just like other manufacturers like to reduce specs for us?

 

Hey all sorry was away for a bit on holiday... lovely not having to stress about heating when you're in a warm country eh. 

I ran some more tests and logged the data. I may have mentioned already but I have a mix of type 22 and triple column rads in this system too. Those triple column rads always take a lot longer to warm up because of the amount of water content in them, its maybe 70L just in those as they're huge. So i wanted to do an A/B test with all rads on Vs all rads on bar the triple column ones. To see if it would affect the ramp up time by much. My thinking was, if the heat up time is very similar then the issue is likely that the pipes in the subfloor are losing lots of heat (as they're maybe 50-60meters of piping down there in total). 

Also taking your advice from the forum I ran both of the tests in pump setting 0 (proportional to burner mode), in which the range min was set to 60% and the max set to 100% Test A was all rads on, Test B was all rads on Bar the triple column ones. Note: i've set the target to 68c now rather than 60 just to see if it helped speed the ramp up too.

 

 

 

On test B it did heat up quicker, but also tripped the burner out at 27minute mark....I imagine because without the triple column rads TRV's on, it was just too much heat for the system.

 

 

I've got a few other things i want to try in my next set of runs.... but before I speak to them, what do you think of the above? On Test A i'm still a little baffled that the boiler is reacting so slowly to modulating down. 

 

Edited Sunday at 13:29 by EinTopaz

[SimonD]

22 hours ago, EinTopaz said:

I've got a few other things i want to try in my next set of runs.... but before I speak to them, what do you think of the above? On Test A i'm still a little baffled that the boiler is reacting so slowly to modulating down. 

 

I'm not that surprised. Sometimes it's better to have a system that is not too reactive and reactivity can significantly reduce efficiency. I think the figures in test A are okay. But I also caution against focusing too much on just the boiler temperatures and consider radiator temperatures and room temperature changes in relation to room volume. That way you'll properly know if the whole system you've got is in the right ball park. What are your expectations of what your heating system should be doing - specifically in regard to heating the space?

 

I'm still of the belief that to get this thing running properly, you need to add some decent modulating controls, like weather compensation - but that, I know, is not what you want.

 

 

[EinTopaz]

3 hours ago, SimonD said:

 

I'm not that surprised. Sometimes it's better to have a system that is not too reactive and reactivity can significantly reduce efficiency. I think the figures in test A are okay. But I also caution against focusing too much on just the boiler temperatures and consider radiator temperatures and room temperature changes in relation to room volume. That way you'll properly know if the whole system you've got is in the right ball park. What are your expectations of what your heating system should be doing - specifically in regard to heating the space?

 

I'm still of the belief that to get this thing running properly, you need to add some decent modulating controls, like weather compensation - but that, I know, is not what you want.

 

 

That's fair re: boilers not being too reactive, but it does seem awful slow to react this one. Like it carries on way past the target at nearly full strength output. I'll do a longer test next time to show you but it really does take a while for it to come back down to target.  That doesn't seem efficient to me and means it does sometimes turn the burner off if it exceeds the target too much. None of that seems like its doing what it should to me. Maybe 10minutes to react and load its self down would be fine, but in my case its way longer.

My expectations and frustrations are two fold with this boiler, they were three until the hex got fixed, now just the two. They are - I want the rads to get hot sooner, and, I want the thing to run efficiently and modulate down correctly. (the overshooting stuff above bothers me regarding efficiency). If it fixes both the frustrations; I am prepared to put the boiler in the bin if I need to and go for a larger Vaillant conventional type. This one is 35kw and doing 28kw-ish of rads at T50

Perfomance wise, i want it run more like the other boiler I have at this property. I'll put the data log of that one below. Its 41kw CDI conventional (with grundfos 25-55 pump) and powers around 22kw of rads. i've no issues with that boiler. It warms up fast, Runs exactly as I want it to. im sure it is overpowered for that system but again, no issues with overshooting or anything like that. Just works (tm). I wish the greenstar 8000 life in question here ran more like that.

 

[JohnMo]

1 hour ago, EinTopaz said:

That doesn't seem efficient to me

Sorry confused - the way you want to operate isn't efficient anyway. So are you fussed about efficiency? You seem willing to spend thousands to get a poor efficiency system anyway. I'm not even convinced you need you need two boilers, but hay ho.

 

I keep looking at this thread and I shouldn't.

[EinTopaz]

1 hour ago, JohnMo said:

Sorry confused - the way you want to operate isn't efficient anyway. So are you fussed about efficiency? You seem willing to spend thousands to get a poor efficiency system anyway. I'm not even convinced you need you need two boilers, but hay ho.

 

I keep looking at this thread and I shouldn't.

 

I'll spend thousands for a thing to function correctly and me not have to fanny around with it, yes. In simple terms it takes too long to heat up my system and its not running cheaply enough to outweigh the time i have to wait for it to heat up. Id rather go for a commercial boiler at this point.

it was on one system at one point but its been extended many times now. I cant be bothered linking it all back together now, rather have 2 boilers. I just need my downstairs one to be less shit. Incase I hadn't mentioned 🙂 

[John Carroll]

On 08/03/2026 at 13:24, EinTopaz said:

 

Hey all sorry was away for a bit on holiday... lovely not having to stress about heating when you're in a warm country eh. 

I ran some more tests and logged the data. I may have mentioned already but I have a mix of type 22 and triple column rads in this system too. Those triple column rads always take a lot longer to warm up because of the amount of water content in them, its maybe 70L just in those as they're huge. So i wanted to do an A/B test with all rads on Vs all rads on bar the triple column ones. To see if it would affect the ramp up time by much. My thinking was, if the heat up time is very similar then the issue is likely that the pipes in the subfloor are losing lots of heat (as they're maybe 50-60meters of piping down there in total). 

Also taking your advice from the forum I ran both of the tests in pump setting 0 (proportional to burner mode), in which the range min was set to 60% and the max set to 100% Test A was all rads on, Test B was all rads on Bar the triple column ones. Note: i've set the target to 68c now rather than 60 just to see if it helped speed the ramp up too.

 

 

 

On test B it did heat up quicker, but also tripped the burner out at 27minute mark....I imagine because without the triple column rads TRV's on, it was just too much heat for the system.

 

 

I've got a few other things i want to try in my next set of runs.... but before I speak to them, what do you think of the above? On Test A i'm still a little baffled that the boiler is reacting so slowly to modulating down. 

 

The boiler's behaviour in reaching 100% output seems quite reasonable IMO, 5 minutes to reach 75% and a further 5 minutes to reach 100%, my daughters Vokera Vision 20S is pretty similar except, as it states, in the MIs.... it takes another 10 minutes to increase the output from 75% to 100%., it probably doesn't take quite 5 minutes to reach 75% but ~ 2 or 3, not a whole not different. But it certainly behaves much differently once the targettemp is reached, it immediately ramps down to maintain the targettemp very accurately, if one zone is switched off, the flowtemp might rise by a max of 2C and then returns to exactly its targettemp. This would be my bone of contention with WB. It shouldn't matter where the "flow" temperature is being measured, the controller is looking at this temperature and should modulate the boiler output to maintain the targettemp.

 

What is the offset now between the Boiler displayed flowtemperature and the targettemp? and how does your temperature sensor, (if installed),  compare with the boiler displayed one?.

 

If you change the targettemp up or down by say 3C, is the boiler sluggish in responding and is there overshoot and undershoot?.

 

I seem to remember that before the HEX replacement that the targettemp and boler displayed flotemp were withim a degree or so even though way higher than the actual flowtemp?.

 

Edited 5 hours ago by John Carroll