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 Tuesday at 15:23 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 Tuesday at 19:15 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 3 hours ago 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!