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 2 hours ago 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)