Worcester Bosch Greenstar 8000 System Boiler Issues

[John Carroll]

2 minutes ago, John Carroll said:

Double Post

 

Edited Friday at 14:58 by John Carroll

[MrPotts]

With the radiators getting to 52c isn’t that hot enough? My radiators normally get up to around 50c (with an 65c flow temp set but never reached) and are very hot to touch so I wouldn’t want them any hotter.

 

 

[JohnMo]

So, boiler will have a target flow temperature with overshoot hysterisis. But, it also has a control set point for dT. The circulation pump and boiler modulation will adjust to maintain dT then add more heat as dT closes down. I suspect the heating system return temperature is holding everything back, but this must be at or close to target otherwise the boiler output would not modulate down.

 

So boiler is doing what it should it has a decent run time, it's modulating down, it possibly massive for the duty. Other than a desire to have radiators you burn yourself on, I don't see a real issue.

[John Carroll]

1 hour ago, JohnMo said:

So, boiler will have a target flow temperature with overshoot hysterisis. But, it also has a control set point for dT. The circulation pump and boiler modulation will adjust to maintain dT then add more heat as dT closes down. I suspect the heating system return temperature is holding everything back, but this must be at or close to target otherwise the boiler output would not modulate down.

 

So boiler is doing what it should it has a decent run time, it's modulating down, it possibly massive for the duty. Other than a desire to have radiators you burn yourself on, I don't see a real issue.

The way I interpret how any boiler controls its output is that it bases it on the target flow temperature ONLY, the flow temperature is obviously influenced by the return temperature which in turn is determined (for a steady heating demand) by the flowrate and the room temperature.

You can see below the effect that the flowrate has, the design flowrate of a 36kW boiler is 25.8LPM/1547LPH which I doubt very much that this system is running at. We do know that the steady state output is 14.4kW (40% boiler output) so the numbers below show exactly the flowrates required at different flow temperatures to achieve this 14.4kw (40% output)

54C/46C/dT8C. 25.8LPM/1547LPH

57C/43C/dT14C. 14.67LPM/889LPH

60C/40C dT20C. 10.29LPM/617LPH

65C/35C/dT35C.  6.87LPM/412LPH

Its very unlikely IMO that the flowrate is > than 25.8LPM so 54C would seem to be the very minimum flowtemperature required to achieve 40% boiler output, its alos very unlikely that the flowate is as low as 6.9LPM so a 65C flowtemperature is unlikely at this end.

I would "guess" that the flowrate is ~ 14.67LPM/889LPH so the steady state flowtemperature is ~ 57C, return,43C, dT14C.

 

Edited Friday at 16:13 by John Carroll

[JohnMo]

34 minutes ago, John Carroll said:

The way I interpret how any boiler controls its output is that it bases it on the target flow temperature ONLY

But it has a modulating circulation pump which will change output based on dT, so not sure that can be fully true.

 

36 minutes ago, John Carroll said:

40% boiler output) so the numbers below show exactly the flowrates required at different flow temperatures to achieve this 14.4kw

If 14.4kW isn't enough the return temp will always be low. So if the boiler has been range rated to 40% it will constantly chase it's tail.

 

So @EinTopaz what is the boiler range rated too?

[SimonD]

2 hours ago, EinTopaz said:

The flow pipe is 20'c cooler than the boiler is saying.

 

1 hour ago, John Carroll said:

Since WB know that a huge dT exists as measured by their own instruments then they just can't wash their hands of the problem?.

 

Any engineer worth their salt would open up the boiler and measure the actual temperature of the heat exchanger particularly where the flow exits the chamber and even better next to the sensors the boiler is using, not just the flow and returns outside the boiler. I don't think they measured a huge DT? but suggested increasing the set temp to increase the flow temp. Now they're by accounts coming back with a new pcb, which seems sensible.

 

There a just too many unknowns right now.

 

What controls is the boiler wired up to? Are they WB modulating ones or third party?

 

 

 

 

[John Carroll]

3 hours ago, SimonD said:

 

 

Any engineer worth their salt would open up the boiler and measure the actual temperature of the heat exchanger particularly where the flow exits the chamber and even better next to the sensors the boiler is using, not just the flow and returns outside the boiler. I don't think they measured a huge DT? but suggested increasing the set temp to increase the flow temp. Now they're by accounts coming back with a new pcb, which seems sensible.

 

There a just too many unknowns right now.

 

What controls is the boiler wired up to? Are they WB modulating ones or third party?

 

 

 

 

WB certainly confirmed that the external flow sensor was reading the same as their one and presumably the return, but not sure about this one. 

 

I might have missed it but what did or do  the external sensors read after a extended boiler run?. if both known, then very easy to calculate the flowrate as the constant boiler output will also be known, 40%??.

[John Carroll]

4 hours ago, JohnMo said:

But it has a modulating circulation pump which will change output based on dT, so not sure that can be fully true.

This boiler doesn't measure the dT because it doesn't have a return temperature sensor, also the pump is (customer) clamped to virtually constant speed.

[EinTopaz]

On 19/12/2025 at 14:54, John Carroll said:

What's your next move?. Since WB know that a huge dT exists as measured by their own instruments then they just can't wash their hands of the problem?.

I've WB coming back on Monday, with a new  control board. Will show them the issues. I would've liked to go down the sub floor myself between now and then and have a look at where the single 28mm flow and returns each split into dual 22mm ones. If those splitters have valves on em, im wondering if they've been left partially closed.

I've mentioned already... but last WB guy, as I stated above very much did try to wash his hands of it and said its normal to have a difference between boiler target temp and flow pipe temp. I challenged that. He didn't really have a good come back. And just said we should raise our boiler target temp to 80. Which has brought the new kettling symptom in now. This is why I've asked for a more senior gentleman to come, this time. 

 

[EinTopaz]

On 19/12/2025 at 15:11, JohnMo said:

Other than a desire to have radiators you burn yourself on, I don't see a real issue.

I see an issue, it gets to 70'c on the boiler within 10mins. then modulates down, long before the radiators get hot. 

[EinTopaz]

On 19/12/2025 at 20:21, John Carroll said:

I might have missed it but what did or do  the external sensors read after a extended boiler run?. if both known, then very easy to calculate the flowrate as the constant boiler output will also be known, 40%??.

If the boiler is set to 68'c target temp, it settles at around 70'c after an hour. At this time, the flow pipe will max out around 54'c and the return 45'c, I appreciate that delta between flow/return is too small for a modern boiler. it does maintain 20'c delta for the most part but really closes down after the boilers been on for a while.

[EinTopaz]

On 19/12/2025 at 21:18, John Carroll said:

This boiler doesn't measure the dT because it doesn't have a return temperature sensor, also the pump is (customer) clamped to virtually constant speed.

Ish, it's set to min 80% max 100% on the pump. Though tbh this didn't make a huge amount of difference compared to running it at constant pressure.

[EinTopaz]

On 19/12/2025 at 17:01, SimonD said:

 

 

Any engineer worth their salt would open up the boiler and measure the actual temperature of the heat exchanger particularly where the flow exits the chamber and even better next to the sensors the boiler is using, not just the flow and returns outside the boiler. I don't think they measured a huge DT? but suggested increasing the set temp to increase the flow temp. Now they're by accounts coming back with a new pcb, which seems sensible.

 

There a just too many unknowns right now.

 

What controls is the boiler wired up to? Are they WB modulating ones or third party?

 

 

 

 

They did acknowledge a huge DT between boiler and flow pipe. But tried to tell me that it's normal. Which sounded like BS. Also tried to say it was 100% likely a system issue. Which sounded like I was being palmed off. 

The boiler is wired up to Hive, and for all these tests ive been running it on boost mode. I dont think that makes any diff to running it normally tbh, but just for clarity.

[EinTopaz]

On 19/12/2025 at 16:47, JohnMo said:

So @EinTopaz what is the boiler range rated too?

It's max isn't rated down at all currently. It's something im going to give a try to tomorrow to see if that enhances heat up times and stops the kettling. Do you think range rating to 70% would be a good starting point?

I think i've mentioned already but when I add up the KW of the rads we have here, its there abouts 27kw. (11x large double panel) + (4 x large vertical triple column)  

 

 

[EinTopaz]

@SimonD replying to an earlier suggestion of yours of running the boiler at a much lower target temp an seeing if the delta between the boiler and flow pipe is consistent vs running it at higher. Im going to try that tomorrow. But something I forgot to mention is... when the boiler has been off for a while, all 3 displays are pretty close. (boiler,flow,return). As in the photo below. But when the CH is demanded, the boiler one rockets up to a delta of 20-25 between that and flow within a couple mins, then maintains that delta thereafter pretty much.

 

[John Carroll]

53 minutes ago, EinTopaz said:

@SimonD replying to an earlier suggestion of yours of running the boiler at a much lower target temp an seeing if the delta between the boiler and flow pipe is consistent vs running it at higher. Im going to try that tomorrow. But something I forgot to mention is... when the boiler has been off for a while, all 3 displays are pretty close. (boiler,flow,return). As in the photo below. But when the CH is demanded, the boiler one rockets up to a delta of 20-25 between that and flow within a couple mins, then maintains that delta thereafter pretty much.

 

 

1 hour ago, EinTopaz said:

If the boiler is set to 68'c target temp, it settles at around 70'c after an hour. At this time, the flow pipe will max out around 54'c and the return 45'c, I appreciate that delta between flow/return is too small for a modern boiler. it does maintain 20'c delta for the most part but really closes down after the boilers been on for a while.

 

One very important bit of data...what was/is the boiler output % with the above 54C/45C??

My calcs show 38%, 13.6kW (based on the above) and a very healthy flowrate of 21.6LPM/1299LPH.

 

54C/45C/dT9C. 21.6LPM/1299LPH. (See below)

 

On Monday you might ask this senior engineer to monitor the flowtemperature with a independent sensor attached as close to the actual sensor as possible and compare them

 

 

 

Edited 23 hours ago by John Carroll

[marshian]

11 hours ago, EinTopaz said:

@SimonD replying to an earlier suggestion of yours of running the boiler at a much lower target temp an seeing if the delta between the boiler and flow pipe is consistent vs running it at higher. Im going to try that tomorrow. But something I forgot to mention is... when the boiler has been off for a while, all 3 displays are pretty close. (boiler,flow,return). As in the photo below. But when the CH is demanded, the boiler one rockets up to a delta of 20-25 between that and flow within a couple mins, then maintains that delta thereafter pretty much.

 

 

That to me is normal - after a period of time the HEX, Flow and Return are all going to equalise temp wise until the boiler fires up again when the HEX and flow pipe are going to rise rapidly. The return pipe will only start to rise when it's recieving warmer return water from the heating circuit if the emitters can't transfer all of the heat into the rooms

[John Carroll]

1 hour ago, marshian said:

 

That to me is normal - after a period of time the HEX, Flow and Return are all going to equalise temp wise until the boiler fires up again when the HEX and flow pipe are going to rise rapidly. The return pipe will only start to rise when it's recieving warmer return water from the heating circuit if the emitters can't transfer all of the heat into the rooms

 

Of course they will equalise, the $64,000 question is why is there is such a difference in temperature between the alleged flow target temperature and a measurement on the flow pipe a foot or two away from it, hopefully, all will be revealed tomorrow.

Edited 12 hours ago by John Carroll

[MrPotts]

18 hours ago, EinTopaz said:

If the boiler is set to 68'c target temp, it settles at around 70'c after an hour. At this time, the flow pipe will max out around 54'c and the return 45'c, I appreciate that delta between flow/return is too small for a modern boiler. it does maintain 20'c delta for the most part but really closes down after the boilers been on for a while.

My boiler delta between flow and return is also around 10c after the boiler has been on for a few hours and it doesn’t cause any issues. I have a WB Greenstar system boiler though.

[John Carroll]

Rad dT is a moveable feast if the flowrate remains constant like it does on quite a lot of boilers. In the above (subject boiler) a 54C flow temprature results in a boiler output of 13.6kW with dT of 9C, if that flow temperature was increased to say 70C either manually or by outside temperature compensation, the boiler output will increase to 22kW but the dT will also increase, to 14.6C, for the same flowrate of 21.7LPM.

 

 

 

[EinTopaz]

18 hours ago, John Carroll said:

One very important bit of data...what was/is the boiler output % with the above 54C/45C??

By that time, it will have modulated down to 40% output and stayed at around that.

[EinTopaz]

6 hours ago, John Carroll said:

 

Of course they will equalise, the $64,000 question is why is there is such a difference in temperature between the alleged flow target temperature and a measurement on the flow pipe a foot or two away from it, hopefully, all will be revealed tomorrow.

Exactly this, hopefully the Senior engineer doesn't try to say that this is normal. a couple degrees, sure, but not 20,

[EinTopaz]

1 hour ago, MrPotts said:

My boiler delta between flow and return is also around 10c after the boiler has been on for a few hours and it doesn’t cause any issues. I have a WB Greenstar system boiler though.

Yep, this is a WB Greenstar system too. Do you know if the theres a large delta between your boiler target temp and flow pipe by any chance? I had wondered if this is just an issue all these Greenstar System boilers have, and WB are refusing to acknowledge. You're the only other person i've spoke to online who has one. 

[MrPotts]

2 hours ago, EinTopaz said:

Yep, this is a WB Greenstar system too. Do you know if the theres a large delta between your boiler target temp and flow pipe by any chance? I had wondered if this is just an issue all these Greenstar System boilers have, and WB are refusing to acknowledge. You're the only other person i've spoke to online who has one. 

Mine is an older Greenstar 24i which doesn’t have a digital display just the old fashioned dials. FWIW mine is set by the dial to 65c but rarely gets above 60c at the flow pipe, not great accuracy but better then you are experiencing.

[John Carroll]

2 hours ago, EinTopaz said:

Yep, this is a WB Greenstar system too. Do you know if the theres a large delta between your boiler target temp and flow pipe by any chance? I had wondered if this is just an issue all these Greenstar System boilers have, and WB are refusing to acknowledge. You're the only other person i've spoke to online who has one. 

 

Just thinking thermodynamically.............

 

We know that your measurements of 54C/45C are correct, I would also be quietly confident that my calulated flowrate of 21.7LPM (based on your data) is correct), if we accept that the actual flow temperature leaving the HEX IS say 70C then there is only one thermodynamic explanation for your measured flowtemp of 54C and that is that there is massive bypassing going on internally for one reason or another where some of return water at 45C is mixing with the water at 70C leaving the HEX. 

By calculation, this means that 21.7LPM return water at 45C is entering the boiler, 13.9LPM (at 45C) is bypassing the HEX, the remaining 7.8LPM at 45C is entering the HEX and leaving at 70C to mix with the bypassing 13.9LPM at 45C to give 21.7LPM at 54C exiting the boiler.

(13.9*45)+(7.8*70)=(21.7*54).

Far fetched?? I wonder.