Worcester Bosch Greenstar 8000 System Boiler Issues

[John Carroll]

Had a look at installing a 6M Wilo Yonos Pico as a booster pump. It should, IMO, provide a modest boost in flow from 20.9LPM to 25LPM or slightly higher, might not seem a lot. but if your problem is getting a higher flow to a few cooler rads then it may be beneficial because the residual pump head now available to the rads should rise from 3.98M (398mb) to 5.7M (570mb), a 1.72M (43%) increase.

[EinTopaz]

18 hours ago, John Carroll said:

Dynamic Head, above should read ~ 570mb, not 510mb.

Thanks, so looks like I’d be best going with an LLH and add an adjustable external  pump too then maybe something like a 25/65? And basically monitor the temps on both sides and adjust accordingly. I’ve read on the LLH if it’s unbalanced it can either short cycle the boiler or pull colder water through to the system. 
 

That seems like the only way to proceed or chuck out the boiler and go for a conventional type instead.

[John Carroll]

2 hours ago, EinTopaz said:

Thanks, so looks like I’d be best going with an LLH and add an adjustable external  pump too then maybe something like a 25/65? And basically monitor the temps on both sides and adjust accordingly. I’ve read on the LLH if it’s unbalanced it can either short cycle the boiler or pull colder water through to the system. 
 

That seems like the only way to proceed or chuck out the boiler and go for a conventional type instead.

There's no way a 6.0M or even the "normal" 8M pump will give you 30LPM even with a LLH based on our tests.

The Hex loss was 1.54M @ 20.89LPM, this = 3.5M @ 30LPM, The rads loss was 3.9M @ 20.89LPM which will require 8.04M @ 30LPM. Any of the conventional 8M pumps will only give a head of 5.5M ish at 30LPM but the 8M UPS2 which is a giant of a pump at 140W will certainly supply 30LPM and far more at a 8M head, its actually a ~ 10M pump but the "fixed" "CC" speed curves are almost fully CP (constant pressure)  curves, no other pump that I'm familiar with will match that performance.

If you do think of going for that pump then it might be worth just installing it as a booster pump  at minimal installation costs as the boiler circ pump will still have a residual head of ~ 0.66M at 30LPM, enough to avoid cavitation in the UPS2 which requires a minimum inlet head of 0.5M at 75C. The UPS2 other "CC" curves of 7M & 6M can also be useful if the need arises, the LLH can be installed if the above doesn't work out, it's your choice.

I will post a few screenshots later of a LLH based on one of your tests to show the effect of balanced & unbalanced flowrates.

 

 

 

Edited February 17 by John Carroll

[EinTopaz]

2 hours ago, John Carroll said:

There's no way a 6.0M or even the "normal" 8M pump will give you 30LPM even with a LLH based on our tests.

The Hex loss was 1.54M @ 20.89LPM, this = 3.5M @ 30LPM, The rads loss was 3.9M @ 20.89LPM which will require 8.04M @ 30LPM. Any of the conventional 8M pumps will only give a head of 5.5M ish at 30LPM but the 8M UPS2 which is a giant of a pump at 140W will certainly supply 30LPM and far more at a 8M head, its actually a ~ 10M pump but the "fixed" "CC" speed curves are almost fully CP (constant pressure)  curves, no other pump that I'm familiar with will match that performance.

If you do think of going for that pump then it might be worth just installing it as a booster pump  at minimal installation costs as the boiler circ pump will still have a residual head of ~ 0.66M at 30LPM, enough to avoid cavitation in the UPS2 which requires a minimum inlet head of 0.5M at 75C. The UPS2 other "CC" curves of 7M & 6M can also be useful if the need arises, the LLH can be installed if the above doesn't work out, it's your choice.

I will post a few screenshots later of a LLH based on one of your tests to show the effect of balanced & unbalanced flowrates.

 

 

 

Thanks John, I’m not the most technical so will need that simplified down a touch for me. How do I know what LPM will be enough for my system if I want it to heat up quick? If it’s 27kw of rads. And a mix of 28mm / 22mm / 15mm pipe (mostly 22mm) 

[John Carroll]

Increasing flow rate will have a very small effect on rad output, temperature increase, if possible, is the name of the game, again as said previously, a small increase in flowrate requires a big increase in head which can be very beneficial in balancing rads.

 

You can see, quite clearly below from your set of readings that a flowrate of 20.8LPM with a flowtemp of 59.9C gives a rad(s) output of 16.49kW, increasing the flowrate to 30LPM will only give a 0.96kW increase to 17.45kW, not a lot. IF the flowrate remains at 20.8LPM but the flowtemp is increased to 75C then the rad output increases by a substantial 8.1kW to 24.6kW, if the flowrate is now increased to 30LPM, (flowtemp 75C) then the rad output only increases by a further 1.53kW to 26.13kW.

 

Also, attached, some LLH numbers, for interest.

 

LLH Calcs Extract ET Rev0.xlsx

Edited February 17 by John Carroll

[John Carroll]

Here are the screenshots from your LLH with both balanced and unbalanced primary/secondary flowrates.

 

 

 

 

 

 

 

 

 

 

 

 

 

[John Carroll]

 

Meant to post these as well a few days ago.

 

If you do choose a 8M UPS2 Pump then you can see from the attached that it should satisfy a flowrate of up to 30LPM based on your present flowrate of 20.89LPM at a Rad head of 3.98M.

 

[SimonD]

I have to jump in an ask why would a system with the installed pipework as described ever need an secondary pump of 8meters head, let alone a LLH. I find it almost inconceivable that the index circuit on this system needs an extra few meters of head above the 4 it's already got, unless the installer had accidentally welded shut the pipes somewhere. I've worked on Worcester 8000 system boilers that are fine under more demanding system conditions.

 

I would suggest doing a factory reset of the boiler (maybe that was done when the engineer came?) to get everything back to default. Then run the system taking regular temperature measurements through the system. Pinpointing which rads are problematic and then taking a methodical approach to understand the whole system behaviour.

 

But I'm also wondering what exactly is the problem you're attempting to fix? If it's this:

 

On 17/02/2026 at 17:22, EinTopaz said:

How do I know what LPM will be enough for my system if I want it to heat up quick?

 

Then we need more specific information about what exactly you define as quick and from what beginning state to what desired state.

 

 

 

 

[John Carroll]

1 hour ago, SimonD said:

I have to jump in an ask why would a system with the installed pipework as described ever need an secondary pump of 8meters head, let alone a LLH. I find it almost inconceivable that the index circuit on this system needs an extra few meters of head above the 4 it's already got, unless the installer had accidentally welded shut the pipes somewhere. I've worked on Worcester 8000 system boilers that are fine under more demanding system conditions.

 

I would suggest doing a factory reset of the boiler (maybe that was done when the engineer came?) to get everything back to default. Then run the system taking regular temperature measurements through the system. Pinpointing which rads are problematic and then taking a methodical approach to understand the whole system behaviour.

 

But I'm also wondering what exactly is the problem you're attempting to fix? If it's this:

 

 

Then we need more specific information about what exactly you define as quick and from what beginning state to what desired state.

 

 

 

 

 

@SimonD, in your experience of Worcester 8000 system boilers, have you come across this strange? behaviour on both this boiler and others, where, after a HEX change, the indicated flow temperature doesn't match the Target temperature even under steady demand conditions and  with the boiler demand well above its minimum output, can you suggest any reason for this behaviour?. and which WB says is normal.

Edited Saturday at 11:08 by John Carroll

[SimonD]

9 minutes ago, John Carroll said:

 

@SimonD, in your experience of Worcester 8000 system boilers, have you come across this strange? behaviour on both this boiler and others, where, after a HEX change, the indicated flow temperature doesn't match the Target temperature even under steady demand conditions and  with the boiler demand well above its minimum output, can you suggest any reason for this behaviour?. and which WB says is normal.

 

I have to say, John, that I try to avoid WB boilers like the plague. I think their designs are awful, I hate working on them and I would definitely never install one (although I have installed one, once!). I think the 8000s are worse than ever, but lots of installers seem to like them for whatever reason.

 

I've never been called to investigate this specific problem, only that radiators aren't getting hot and this is almost always to do with a system balance issue and not the boiler itself. When I do system diagnostics involving temperature measurement I always ignore the internal boiler temperature given by the sensors and instead use my own clamps to as close to the boiler as possible. This is because I very often find discrepancies between displayed flow/return temps from internal sensors and those measured immediately outside the boiler, it's not just with Worcester. The only times I take the internal sensor readings more seriously, is if I see the boiler cycling when my readings suggest it shouldn't and 5C difference is just not unusual in these circumstances.

 

This is why my approach here would be to ignore the boiler and investigate how the wider system behaves over time. A recent example is a hall where I installed a boiler a few years back. I designed a low temp system and explained all this to the caretaker and for several years the system ran beautifully. This year I received calls to say the heating wasn't working and the hall wasn't getting warm. I went and looked at the boiler and found out that the new caretaker was expecting the system to heat the hall up from freezing cold to between 18-20C within 40minutes so they'd even cranked up the boiler output to 75C! I explained to them that they should drop the flow temp back down and be a bit more patient as their bills had dropped noticeably since I installed the system. So in short, sometimes it's about education!

[EinTopaz]

On 21/02/2026 at 09:39, SimonD said:

I have to jump in an ask why would a system with the installed pipework as described ever need an secondary pump of 8meters head, let alone a LLH. I find it almost inconceivable that the index circuit on this system needs an extra few meters of head above the 4 it's already got, unless the installer had accidentally welded shut the pipes somewhere. I've worked on Worcester 8000 system boilers that are fine under more demanding system conditions.

 

I would suggest doing a factory reset of the boiler (maybe that was done when the engineer came?) to get everything back to default. Then run the system taking regular temperature measurements through the system. Pinpointing which rads are problematic and then taking a methodical approach to understand the whole system behaviour.

 

But I'm also wondering what exactly is the problem you're attempting to fix? If it's this:

 

 

Then we need more specific information about what exactly you define as quick and from what beginning state to what desired state.

 

 

 

 

Now that the HEX is sorted and its dispaying the temperatures correct. I want the system to heat up alot faster.

The LLH was an option I was considering but doesnt have to be that solution.

I have a CDI upstairs, conventional, it does the radiators in this property for the top 2 floors. 22KW rads total @t50 or so. The rads on that system are hot within 10 mins. Which is what I want my downstairs boiler to be like, the downstairs boiler is the greenstar 8000 that we're discussing here. That one takes 30mins for the flow to reach 60 target. The upstairs one takes 10-12mins to get to 60 target. 

I mean, there isn't a huge amount of difference in the power of these 2 boilers. So im trying to do what I can to get it so the downstairs boiler can heat up faster. Hence second pump, hence LLH. Do you have any other suggestions I can look at? besides swapping the boiler out entirely.

 

[SimonD]

10 hours ago, EinTopaz said:

Now that the HEX is sorted and its dispaying the temperatures correct. I want the system to heat up alot faster.

The LLH was an option I was considering but doesnt have to be that solution.

I have a CDI upstairs, conventional, it does the radiators in this property for the top 2 floors. 22KW rads total @t50 or so. The rads on that system are hot within 10 mins. Which is what I want my downstairs boiler to be like, the downstairs boiler is the greenstar 8000 that we're discussing here. That one takes 30mins for the flow to reach 60 target. The upstairs one takes 10-12mins to get to 60 target. 

I mean, there isn't a huge amount of difference in the power of these 2 boilers. So im trying to do what I can to get it so the downstairs boiler can heat up faster. Hence second pump, hence LLH. Do you have any other suggestions I can look at? besides swapping the boiler out entirely.

 

 

A low loss header is not going to be the solution for getting your system to heat up faster. What is responsible for getting heat into your house from your boiler is mean water temperature, driven by the boiler. You've got more than enough output available from the boiler to do this. Now, with 8000 I have heard feedback that it takes a while until it ramps up to max output when the heating is initially turned on. First, test this by doing some gas rates. When you turn on your system from cold - go to the meter and take a reading at the same time as starting your stopwatch. At 2 minutes, take another reading. Then do this say 10 minutes later, and again after 10 minutes. By all means post up the before and after figures so we can do the calcs. If you want immediate feedback, you can do the first gas rate and then put the boiler into chimney mode and max. then do another gate rate and calculate the difference. So again, to do a gas rate:

 

Have boiler running

Take meter reading and start stopwatch at the same time

At 2 minutes, take meter reading again and post up results.

 

What we want to find out is if the boiler is programmed to run at a lower output for a period before ramping up to maximum output and how long this takes under normal running conditions. Ideally we want to do this with all the settings at default, before you've amended anything like pump setting etc.

 

As an approach to heating there is the idea that not ramping the boiler up to max output immediately provides energy savings as well as reducing overall wear and tear on the boiler itself. ATAG, for example, has for years had an approach where it starts on minimum output and slowly ramps up output over a long period of time. Others will modulate too rather than put everything in at once. The conclusion we may end up with is that you have to put the heating on earlier to get it to temp for when you want it and change your behaviour and expectations.

 

If it does turn out that it takes a while to reach its max output and that this is just a feature of the boiler, it's not really something you can put on the installer because it's unlikely they knew about this when specifying the boiler as it's unlikely WB made this explicit. The gas boiler industry and market have unfortunately been complicit in the de-skilling of the heating engineer industry for the last few decades, as, of course, they know best.....

[JohnMo]

As mentioned above a slow ramp is good, it's energy efficient. Fast ramp in flow temp requires a lot of energy.

 

The way to operate a boiler is exactly the opposite to what you want to achieve. Reduce flow temperature let everything run longer, minimise boiler wear on boiler and longer steady state running. The house will benefit from a more stable temperature.

 

Our Atag you can adjust how quickly it accelerated to max temperature, even doing DHW I had it dialed right back. It would take 20 minutes to get to 60 degrees, by that time the cylinder was at 52 degs and the boiler returned to heating the house. Slow accelerate spends the max time in high efficiency maximum condensation mode. Fast acceleration spends the least time.

 

Think you are now in the making issues for yourself mode.

 

10 hours ago, EinTopaz said:

Do you have any other suggestions I can look at? besides swapping the boiler out entirely

Yes make a cup of coffee or tea and read the paper, or take the dog for a walk. Stop making issues out of normal behaviour. 

 

LLH - no

Additional pumps - no

Get on with life - yes.

[EinTopaz]

5 hours ago, SimonD said:

 

A low loss header is not going to be the solution for getting your system to heat up faster. What is responsible for getting heat into your house from your boiler is mean water temperature, driven by the boiler. You've got more than enough output available from the boiler to do this. Now, with 8000 I have heard feedback that it takes a while until it ramps up to max output when the heating is initially turned on. First, test this by doing some gas rates. When you turn on your system from cold - go to the meter and take a reading at the same time as starting your stopwatch. At 2 minutes, take another reading. Then do this say 10 minutes later, and again after 10 minutes. By all means post up the before and after figures so we can do the calcs. If you want immediate feedback, you can do the first gas rate and then put the boiler into chimney mode and max. then do another gate rate and calculate the difference. So again, to do a gas rate:

 

Have boiler running

Take meter reading and start stopwatch at the same time

At 2 minutes, take meter reading again and post up results.

 

 

I will, but im already aware it doesn't start at 100% burner output when turned on from cold. it starts at maybe 22%, is around 75% by the 5minute mark and 100% by 8-10mins, i'd need to double check but thats roughly correct. 

When I draw a comparison to my upstairs CDI boiler. This is where I get frustrated...I want the  8000 life boiler downstairs to behave more like that. I'm happy to forfeit some economy in trade for a quicker heat up. I logged the the temps and times on the upstairs boiler before.  it does ramp much quicker. Let me put them below. I wish my boiler downstairs heated up as quick as it did.

WB CDI boiler (upstairs)
0 minutes
Flow 15
Return 15

5 minutes
Flow 46.5
Return 19.2

10 minutes
Flow 54.8
Return 29.2

 

15 minutes
Flow 55.2
Return 39.2

 

20 minutes
Flow 55.2
Return 46.0

After that the temps don't change on the upstairs boiler. All rads are plenty hot enough by the 10min mark. Obviously I can't see the output % on that boiler's LCD with it being more primitive. But I presume its 100% from the start, or atleast has a much more aggressive ramp. Which I like.

With regards to the downstairs boiler ....Next time i do the logging for that one, i'll paste that here too. I will have mentioned before but there are 4 x triple column vertical rads on that downstairs system with alot of water in them. And the pipework for the downstairs boiler is all in a ventilated sub-floor, theyre lagged but only with the cheap foam stuff. So I reckon all of these things are contributing to the longer warm up time. 

That said if I turn off half the rads downstairs on the greenstar life system, I should expect to see the target of 60c hit much sooner right? if not then would it then be fair to say i need an LLH?

Edited 10 hours ago by EinTopaz

[EinTopaz]

5 hours ago, JohnMo said:

As mentioned above a slow ramp is good, it's energy efficient. Fast ramp in flow temp requires a lot of energy.

 

The way to operate a boiler is exactly the opposite to what you want to achieve. Reduce flow temperature let everything run longer, minimise boiler wear on boiler and longer steady state running. The house will benefit from a more stable temperature.

 

Our Atag you can adjust how quickly it accelerated to max temperature, even doing DHW I had it dialed right back. It would take 20 minutes to get to 60 degrees, by that time the cylinder was at 52 degs and the boiler returned to heating the house. Slow accelerate spends the max time in high efficiency maximum condensation mode. Fast acceleration spends the least time.

 

Think you are now in the making issues for yourself mode.

 

Yes make a cup of coffee or tea and read the paper, or take the dog for a walk. Stop making issues out of normal behaviour. 

 

LLH - no

Additional pumps - no

Get on with life - yes.

Hey, it's just personal preference. I've replied to a separate comment showing the difference between my boiler upstairs and this one. Thats the behaviour and characteristics i want. Fast heat up. And prepared to amend what I need to do to achieve that. its important to me and this project. 

[JohnMo]

1 hour ago, EinTopaz said:

Thats the behaviour and characteristics i want

To get that, a LLH header or additional pumps will not change anything. The boiler ramp rate (degrees per min) will be just the same, it's hard programmed into the boiler controller.

 

The only way to get your boilers to work exactly the same, is to install exactly the same boilers, or simply delete one (old conversation).

[John Carroll]

This is some of your data after the HEX was renewed.

The Target Flowtemp was 60C.

The bottom line of each set of data are my calcs using the pump flowrate  and  your dT (as measured with the external sensors), it gives a  calculated flowtemp not too far away from your measured temp so will mean that the boiler output based on modulation is quite reasonably accurate but maybe no harm to do that gas meter rate.

 

 

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

[EinTopaz]

3 hours ago, JohnMo said:

To get that, a LLH header or additional pumps will not change anything. The boiler ramp rate (degrees per min) will be just the same, it's hard programmed into the boiler controller.

 

The only way to get your boilers to work exactly the same, is to install exactly the same boilers, or simply delete one (old conversation).

Will a LLH stop the boiler overshooting and tripping out atleast? As it is still doing that and it does concern me despite what Bosch say about it being by design.

Also, in the test I did last night, I turned off the 4 triple column rads at the TRV to see if that'd drastically speed up the flow temp ramp. As they do have a huge capacity of water, something like 80L for the four of them. So figured if those weren't needing to be heated i'd see a much quicker journey to target temp, but I did not.  Wondering if the system (pipework) in and of itself is just too long and cold for this boiler and its internal pump.

 

 

[JohnMo]

36 minutes ago, EinTopaz said:

Will a LLH stop the boiler overshooting and tripping out atleast?

No.

 

Overshoot is shut off hysterisis, it's tripping out because it cannot shift the heat fast enough, or in other words the house doesn't need the amount of kW being put out by the boiler. Once the boiler hits a predetermined number if degrees over target flow temperature it will trip.

 

A low loss header, just provides hydraulic seperation between boiler and heating system. If not designed correctly adds distortion to system, so boiler flow temperature is higher and radiators see a lower flow temperature.  Well designed it isn't going to fix your issue. It's not a magic bullet and generally I would avoid.

 

The thing your seeing is indicative of a boiler that is oversized, for the heat demand and cannot modulate down far enough.

[John Carroll]

 

A LLH will not cure a BOILER problem which IMO this boiler clearly has.

 

It reached Targettemp after 25 minutes yet it took another 20 minutes for its output to fall from 36kW (100%) to 23.4kW (65%) but reached a flow temp of 64.8C during this period.

It then took another 50 minutes to fall from 23.4kW (65%) to 17.64kW (49%) and finally return to its Targettemp of 60C (59.9C)

I would be very embarrassed to look someone in the eye and tell them this is normal operation.

 

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

Edited 3 hours ago by John Carroll

[SimonD]

30 minutes ago, John Carroll said:

It reached Targettemp after 25 minutes yet it took another 20 minutes for its output to fall from 36kW (100%) to 23.4kW (65%) but reached a flow temp of 64.8C during this period.

It then took another 50 minutes to fall from 23.4kW (65%) to 17.64kW (49%) and finally return to its Targettemp of 60C (59.9C)

I would be very embarrassed to look someone in the eye and tell them this is normal operation.

 

On all these, it shows pump at 100%. A correctly modulating boiler should modulate the pump too. As flow temp rises and dt falls, I'd expect to see additional modulation, including from the pump as the gas valve throttles back. Has this boiler been set at constant pump speed? If so, this might be fighting the boiler?

[John Carroll]

Like most Boilers, there are several hydraulic options, one of which is pump modulation based on boiler output, this option would not be the one I would recommend for a system with long pipe runs as the pump head then at low boiler modulation might not produce enough head to circulate sufficiently through the furthest away rads, resulting in boiler cycling, one of the options is a constant pressure Residual Head of 4M (400mb) which, because of the Boiler Hex loss and the rads/system loss will/can  result in the pump running at 100% to maintain this required residual head. None of the selected options should cause the above problems.