Worcester Bosch Greenstar 8000 System Boiler Issues

[John Carroll]

46 minutes ago, EinTopaz said:

I’ll give that a try. But I know already from testing that the 90% range rated test I tried yesterday took 5 mins longer

to get To target. Still overshot but only by 3.6c this time. But then weirdly never did make its way down to 60. It settled at 62.5 with a modulated output of 54%
 

I assume the higher modulated % was because it was a colder day and the pipes are in the subfloor with the ventilated air bricks etc. so assume it was just working a little harder to keep them at temp. Or above! 
 

EDT ^ this was all on pump setting 5, which was essentially locking it at 87% pump speed 

If you are happy enough with that overshoot then why not leave well enough alone, you probably may not have even noticed it if you hadn't had the other big problem.

 

But no way  IMO should the there be a offset of 2.5C between the target and flow temperature once steady state conditions are achieved, these controllers are/should be PID, where the "I", (integral) will achieve zero deviation between the target and flow temperatures especially under these benign conditions, you might see + or - 0.1/0.2C deviation now and then, the colder day/pipes etc has nothing to do with the control, the controller is just looking at the process variable which is the flow temperature in this case.

 

Edited January 1 by John Carroll

[EinTopaz]

9 hours ago, John Carroll said:

If you are happy enough with that overshoot then why not leave well enough alone, you probably may not have even noticed it if you hadn't had the other big problem.

 

But no way  IMO should the there be a offset of 2.5C between the target and flow temperature once steady state conditions are achieved, these controllers are/should be PID, where the "I", (integral) will achieve zero deviation between the target and flow temperatures especially under these benign conditions, you might see + or - 0.1/0.2C deviation now and then, the colder day/pipes etc has nothing to do with the control, the controller is just looking at the process variable which is the flow temperature in this case.

It’s a fair point, it’s not the most egregious thing now everything is getting warm enough. To be honest the overshooting in and of itself isn’t bugging me, I’m fine with everything getting a littler hotter then I’ve set… though what is bugging me is more the fact it’s staying at 100% of its rated output long after it’s reached target. It’s working way harder than I’ve asked it to and the bottom line is that’s costing me unnecessarily everytime I run it.

 

For every hour it’s running at 35kw it costs around £2. So that will add up a lot through the year if it’s not modulating down when it could/should.

 

 

as a slight aside. Do you know the best insulation method for pipework that runs in a ventilated, unheated subfloor. The pipes are already lagged but wondering what else I can do to keep them as optimally warm as possible. 

[John Carroll]

8 hours ago, EinTopaz said:

It’s a fair point, it’s not the most egregious thing now everything is getting warm enough. To be honest the overshooting in and of itself isn’t bugging me, I’m fine with everything getting a littler hotter then I’ve set… though what is bugging me is more the fact it’s staying at 100% of its rated output long after it’s reached target. It’s working way harder than I’ve asked it to and the bottom line is that’s costing me unnecessarily everytime I run it.

 

For every hour it’s running at 35kw it costs around £2. So that will add up a lot through the year if it’s not modulating down when it could/should.

 

 

as a slight aside. Do you know the best insulation method for pipework that runs in a ventilated, unheated subfloor. The pipes are already lagged but wondering what else I can do to keep them as optimally warm as possible. 

 

Yes, this flat out firing is very hard to explain thermodynamically.

For example, with a flowrate of your now 87%, 18.71LPM,  then the rads will output 16.33kW(45% boiler output)  under steady state conditions with flow/return/dT temperatures of 60C/47.57C/12.43C, assuming 20C room temperature, if the boiler firing is then increased to 36kW then the dT at that flowrate of 18.71LPM has to be, 36*860/60/18.71, 27.58C, so the flowtemp should/must then increase almost instantly to, 47.57+27.58, 75.15C, there will obviously be a delay in the sensors reading this correctly as they are surface mounted but surely 4 or 5 minutes should be well more than adequate time for this? also the boiler fires at 100% output for well beyond this period so a mystery as to why you are not seeing anything remotely approaching 75C., the sky's the limit then up to at least 100C, see below.

The rad output theoretically/thermodynamically can output 36 kW but would require flow/return temps of ~ 100C/73C, (even assuming a 25C room temp), but I just can't see this happening.

 

Don't know anything really re insulation, I just use the cheap as chips lengths.

Edited January 2 by John Carroll

[John Carroll]

Had a look at your data and did some calcs so things adding up much better there, you can see the measured flowtemps does lag the calculated ones but not hugely so you shouldn't be burning huge quantities of extra gas even if the flowtemp does exceed the target temp by 2.5c or so under steady state conditions, the boiler output will be outputting  ~ 1.0kW extra so say 15kWh over a 15 hour heating day or pro rata, at 6p/kWh?? = 90p/day, not huge?.

 

On for 5minutes
Flow 30.6
return 15.6   dT 15.00C
Burner 62% 22.32kW
Pump 93%   20.00LPM dT 16.00C Flowtemp 31.6C

On 15 minutes
Flow 50.9
return 29.0    dT 21.90C
Burner 100% 36.00kW
Pump   100%   21.50LPM dT 24.00C Flowtemp 53.00C

 

On 25 minutes (target temp reached)
Flow 60.1
return 39.0    dT 21.10C
Burner 100% 36.00kW
Pump   100%   21.50LPM dT 24.00C Flowtemp 63.00C

 

On 30 minutes
Flow 63.2
return 42.8    dT 20.4C
Burner 95%  34.2.00kW
Pump   100%   21.50LPM dT 22.80C Flowtemp 65.6C

 

On 35 minutes
Flow 64.8
return 46.4    dT 18.4C
Burner 85%  23.40kW
Pump   100%   21.50LPM dT 20.4C Flowtemp 66.8C

 

On 45 minutes
Flow 64.8
return 50.2    dT 14.6C
Burner 65%  23.40kW
Pump   100%   21.50LPM dT 15.60C Flowtemp 65.8C

 

 

On 55 minutes
Flow 61.9
return 50.7    dT 11.2C
Burner 49%  17.64kW
Pump   100%   21.50LPM dT 11.76C Flowtemp 62.46C

 

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    dT 12.1C
Burner 49%  17.64kW
Pump   100%   21.50LPM dT 11.76C Flowtemp 59.56C

 

[marshian]

16 hours ago, EinTopaz said:

It’s a fair point, it’s not the most egregious thing now everything is getting warm enough. To be honest the overshooting in and of itself isn’t bugging me, I’m fine with everything getting a littler hotter then I’ve set… though what is bugging me is more the fact it’s staying at 100% of its rated output long after it’s reached target. It’s working way harder than I’ve asked it to and the bottom line is that’s costing me unnecessarily everytime I run it.

 

For every hour it’s running at 35kw it costs around £2. So that will add up a lot through the year if it’s not modulating down when it could/should.

 

From my non technical viewpoint and I'm going to phase it as such

 

For the burner to be running at 100% then the water flowing thro the boiler must be taking that heat input - which means the return temp water is cool enough to enable the boiler to "have at it" 

 

Only when the heat capacity of the return flow is less than the full power of the boiler will the boiler modulate down (OK it may rise above target set point before it does that but that will be in the programming of the boiler MBU)

 

Basically you have a circuit of water (Pipes and rads) and it's taking a fair old time before the return water starts to warm up enough to cause the boiler to modulate down.

[JohnMo]

4 hours ago, marshian said:

From my non technical viewpoint and I'm going to phase it

 

20 hours ago, EinTopaz said:

though what is bugging me is more the fact it’s staying at 100% of its rated output long after it’s reached target. It’s working way harder than I’ve asked it to and the bottom line is that’s costing me unnecessarily everytime I run it.

You have what you have, you seem to want run an on off system and have loads of modulation immediately. It isn't going to happen, the boiler has plenty of work to do playing catch up. Then you switch it off again.

 

20 hours ago, EinTopaz said:

Do you know the best insulation method for pipework that runs in a ventilated, unheated subfloor.

Thick stuff - 25mm wall thickness, cover in aluminium tape to stop mice eating it.

[marshian]

2 hours ago, JohnMo said:

You have what you have, you seem to want run an on off system and have loads of modulation immediately. It isn't going to happen, the boiler has plenty of work to do playing catch up. Then you switch it off again.

Perfect summary

[John Carroll]

In non technical terms I would say its behaviour is bordering on bizarre.

It takes 85 minutes to fall from 64.8c to its targettemp of 60C, despite a very steady ~ 17kW demand.                                                                                                                                               

 

As per Tests, all temps are Flow, (target temp is 60C)

zero hour 60.1C (after 25 minute warmup from 15C)

+5mins    63.2C

+10mins   64.8C

+20mins   64.8C

+30mins   61.9C

+95mins   60.0C

 

 

Edited January 3 by John Carroll

[EinTopaz]

On 03/01/2026 at 00:15, John Carroll said:

In non technical terms I would say its behaviour is bordering on bizarre.

It takes 85 minutes to fall from 64.8c to its targettemp of 60C, despite a very steady ~ 17kW demand.                                                                                                                                               

 

As per Tests, all temps are Flow, (target temp is 60C)

zero hour 60.1C (after 25 minute warmup from 15C)

+5mins    63.2C

+10mins   64.8C

+20mins   64.8C

+30mins   61.9C

+95mins   60.0C

 

 

Just to update this… It won’t even drop to 60 anymore. For the past 4/5 times it’s been on it settles at 62.5c now despite that being 2.5c over target. The only thing I’ve changed is I’ve rated the boiler down to 95% as this didn’t seem to have a huge affect on how long things took to get hot. 

[John Carroll]

7 minutes ago, EinTopaz said:

Just to update this… It won’t even drop to 60 anymore. For the past 4/5 times it’s been on it settles at 62.5c now despite that being 2.5c over target. The only thing I’ve changed is I’ve rated the boiler down to 95% as this didn’t seem to have a huge affect on how long things took to get hot. 

 

Just to see if this 2.5C over target applies to all target values, reduce the target temp to 57.5C, see what happens.

[EinTopaz]

On 02/01/2026 at 21:28, JohnMo said:

Thick stuff - 25mm wall thickness, cover in aluminium tape to stop mice eating it.

It has some of the 25mm stuff on already but struggled to get it that thick in many places as the pipes are tied directly to the underside of the floor joists. So a good portion of it is only 13mm thickness too in tighter spaces 
 

I was thinking of getting a plumber round to unpin them and run them along the concrete slab floor instead, well not directly on the floor I’d put 100mm PIR board on the floor and run them along that. Then I could go as thick as I like with the insulation. Or put even  more insulation ontop of it too then. Or box all the pipes in in using more PIR. 
 

I also saw there’s a pipe insulation called phelonic which has a much better insulation rating than the foam stuff. Though it’s about £6 per meter. Wondering how far is too far. But it is -4 here today and that subfloor is very well ventilated. Probably best to treat them as external pipes, or as good as. 

Edited January 5 by EinTopaz

[John Carroll]

On 05/01/2026 at 10:18, EinTopaz said:

Just to update this… It won’t even drop to 60 anymore. For the past 4/5 times it’s been on it settles at 62.5c now despite that being 2.5c over target. The only thing I’ve changed is I’ve rated the boiler down to 95% as this didn’t seem to have a huge affect on how long things took to get hot. 

 

Have you contacted WB re this issue?.

[EinTopaz]

On 07/01/2026 at 10:52, John Carroll said:

 

Have you contacted WB re this issue?.

I have, they're coming back out on the 16th Jan. I've got a 100% repro case waiting for them. And have confirmed; no matter what I set the target to, it's exceeding it by either 5c and cutting off, or very nearly exceeding by 5c, then settling at around 2-3c above target.  

And can also show it continually climbing at 100% output, 10s of minutes after target is reached. So in summary, not settling at target, overly slow to modulate post achieving target, far exceeding target and tripping. 

 

[EinTopaz]

Picking up on a slightly earlier topic in this thread that I want to revisit. As well as the boiler in question here that's having the problems... im using a separate boiler upstairs for CH on the top floors, I may've mentioned already, but it is a 40kw CDI conventional CH only. (it's that large because it used to do the entire property before we extended etc). That powers 10  x type 22 rads/ circa 18kw, and is set to setting 4 (8max) on the potentiometer on the front, this means it gets to 65c. All the rads on this circuit get warm within 5mins, and hot within 10minutes. It's 22mm piping for the first 3meters, then drops to 15mm, then down to 10mm down the wall cavities to each of the rads for the last 2meters or so, so id assume way more resistance in this system than the boiler downstairs (the one i'm having all the problems with in this thread).  Also worth mentioning The pump on this is external and a grundfos 25-55.

 

 

On the boiler downstairs, the one i'm constantly having problems with. I have wondered now that the HEX is fixed and the temps are atleast displaying correctly, if I wanted to speed up the heating time to get it closer to the boiler upstairs, what can I do to improve it? I've looked inside this system boiler now and can see it has a 15-70 pump in there. Which does make me wonder, does that essentially make it a stronger pump with more head than the one I have upstairs? and if its 15-70, why is setting 6 (400mbar) show as pump output 100% on the boiler LCD display?  Does that essentially mean its got way more head than it's able to use? i.e it's capable of 700mbar, but can only run 400mbar max? I find the manual quite confusing here and it'd be great to understand this so I can work out the viability of adding a second external pump to this setup, or running an LLH and an external pump too. The goal would be hopefully significantly shorter heat up times. 

Images attached of the pump and the manual again, for ref.

 

 

Edited 11 hours ago by EinTopaz

[John Carroll]

If you look at the pump curve above, you will see "H/M" (M is meters) right at the top and it states right at the bottom that H is the Residual head.

Residual head is the pump head available after the boiler HEX, you can see the  pump head just starts falling, 1200LPM (21.7LPM) @ 7.0M head which indicates to me that the boiler HEX has a pressure loss of  7.0-4.0, 3.0M at a flowrate of 21.7LPM, you are getting very close to this flowrate, 21.5LPM at full pump speed. The residual (remaining) net head of 4.0M  is lost through the CH system.

If you now install a LLH where the primary pump just circulates through the boiler HEX and a (new) secondary pump circulated through the CH system, then, all the secondary pump head is used to increase the flow through the CH system which will increase the return temp/reduce the dT and get a increase in rad output and a faster warm up. If you install a "6M" secondary pump then the flowrate will increase from 21.5LPM to, 21.5*sqroot(6/4), 26.3LPM, not a lot as the rads dT will only fall from ~ 12C to 10C which only gives ~ 3% in rad output, say a increase of 0.6kW from 18.0 to 18.58. The easiest way to get a faster warm up is to increase the flowtemp to say 75C but would need reducing again when roomtemps reached to maintain reasonable boiler efficiencies.

 

When the engineer next attends ask what the pressure loss through the boiler HEX is, dP & flowrate.

 

Edited 3 hours ago by John Carroll