Indeed, and what is very annoying is that even when the boiler output is well above its minimum output that the flow temperature can be 2 to 3 deg higher than the target flow temperature even under steady demand conditions, completely unacceptable, IMO, because the burner trips (shuts down) at target temp +5C, if a heating zone is shut down then its very likely that the flow temp will rise by a few degrees and will trip the burner, leading to recycling.Here is a thread with this same issue on another forum, looks like WB have an issue with these boilers….
Worcester Bosch Greenstar 8000 System Boiler Issues
Hi all, This one is turning into a bit of a saga... but, I've been having a hate / hate relationship with a boiler we had fitted a couple years back. Now it's getting colder again and we've started to use this boiler again, it still has issues. I'm hoping to get some help diagnosing what the prob...forum.buildhub.org.uk
Bypass valve
- Thread starter toneloc
- Start date
Thank you Johntheo5 and Mister Banks for your replies.
Firstly to Mister Banks, thank you for pointing me to the thread on Buildhub, which I see is not only on the same issue but was actually started on 15th December by the same person (GeddyMortgage) about the very same boiler and issue as this thread here on DIYnot.com has been discussing. One can see that much of the description and also the pipework diagram and photos are the same as on this thread. His last post here was on 16th December and he signs off his initial post on Buildhub as ‘Ged’. Thus he appears to have switched from this forum to Buildhub around 15th/16th December.
That said, I have read the whole of the Buildhub thread, which eventually reveals that the cause of all his problems (identical to mine) with actual flow temperature being well below that being sensed and displayed by the boiler was a scaled or clogged HEx. As soon as this was diagnosed and replaced by a senior WB engineer, the actual flow temperature increased to match the indicated one.
And on thinking this through further, I can see how a scaled/clogged Hex can explain all his and my observations, including the fact that the discrepancy is not constant but can vary between about 10C and 20C. In my own case, I would not be that surprised if my Hex were to be clogged, as existing pipework and some retained radiators were not cleaned before the new boiler was installed (despite the old system having been open vented and out of action for 17 years after the old boiler and pump were removed!). However, cleaner was run for a week in the completed new installation, which does have a strainer and a magnetic filter in the main boiler return. These have been cleaned at each annual service since.
So it looks as if I will have to get WB back again, with a view to having the HEx replaced. The previous engineer did say that would be the next thing to try.
Turning to Johntheo5’s reply regarding my ABV question, I understand that, with a modulating pump, we do not need an ABV during normal operation but, because all circuits (two heating + DHW) are controlled by motorised valves, we do need one for pump overrun when all three valves are closed at the end of a heat demand. There is no unvalved circuit to a towel radiator or similar. But the ABV should be closed during all normal operation (as is made clear in WB Technical Bulletin 0159 (03/2022), which also specifies the need for a pump overrun ABV).
I am quite sure that my ABV is passing flow during normal operation, as the 10m pipe loop it uses is hot throughout at virtually the same temperature as the concurrent heating flow. Presumably, this additional hot water being added to the return flow will have some negative effect on boiler control? So I will do as Johntheo5 suggests and increase the ABV pressure setting until the unwanted circulation stops; hopefully it will still open when needed for pump overrun.
Thank you both again for your assistance. I will post again when I have further news.
Firstly to Mister Banks, thank you for pointing me to the thread on Buildhub, which I see is not only on the same issue but was actually started on 15th December by the same person (GeddyMortgage) about the very same boiler and issue as this thread here on DIYnot.com has been discussing. One can see that much of the description and also the pipework diagram and photos are the same as on this thread. His last post here was on 16th December and he signs off his initial post on Buildhub as ‘Ged’. Thus he appears to have switched from this forum to Buildhub around 15th/16th December.
That said, I have read the whole of the Buildhub thread, which eventually reveals that the cause of all his problems (identical to mine) with actual flow temperature being well below that being sensed and displayed by the boiler was a scaled or clogged HEx. As soon as this was diagnosed and replaced by a senior WB engineer, the actual flow temperature increased to match the indicated one.
And on thinking this through further, I can see how a scaled/clogged Hex can explain all his and my observations, including the fact that the discrepancy is not constant but can vary between about 10C and 20C. In my own case, I would not be that surprised if my Hex were to be clogged, as existing pipework and some retained radiators were not cleaned before the new boiler was installed (despite the old system having been open vented and out of action for 17 years after the old boiler and pump were removed!). However, cleaner was run for a week in the completed new installation, which does have a strainer and a magnetic filter in the main boiler return. These have been cleaned at each annual service since.
So it looks as if I will have to get WB back again, with a view to having the HEx replaced. The previous engineer did say that would be the next thing to try.
Turning to Johntheo5’s reply regarding my ABV question, I understand that, with a modulating pump, we do not need an ABV during normal operation but, because all circuits (two heating + DHW) are controlled by motorised valves, we do need one for pump overrun when all three valves are closed at the end of a heat demand. There is no unvalved circuit to a towel radiator or similar. But the ABV should be closed during all normal operation (as is made clear in WB Technical Bulletin 0159 (03/2022), which also specifies the need for a pump overrun ABV).
I am quite sure that my ABV is passing flow during normal operation, as the 10m pipe loop it uses is hot throughout at virtually the same temperature as the concurrent heating flow. Presumably, this additional hot water being added to the return flow will have some negative effect on boiler control? So I will do as Johntheo5 suggests and increase the ABV pressure setting until the unwanted circulation stops; hopefully it will still open when needed for pump overrun.
Thank you both again for your assistance. I will post again when I have further news.
There's still a problem, maybe new, since the new HEX was installed, with the (a) dT between the target and flow temps even under steady firing conditions, but maybe sorted since engineer was scheduled to come back on the 16th.
Thank you Johntheo5. Yes, I followed all that on the Buildhub thread but am not so worried about the indicated flow temperature sometimes being a bit above the set flow temperature. My big concern is the actual flow temperature being unable to reach the set or indicated flow temperature by as much as 20C and the limitation that puts on the boiler's actual heat output. His problem like this did seem to be completely solved by the HEx replacement on 22nd December.
But I also have my problem with the ABV! With the boiler pump map set to constant pressure option 1, which equates to 150mb, I cannot stop the circulation through the Honeywell DU144 ABV, even when I wind it all the way to setting 0.6, which equates to a pressure of 600mb to 1000mb, depending on flow rate. Surely that should stop the flow? Seems like the valve must be faulty or stuck open. I was suspicious to start with, because it was passing flow at the installed setting of 0.3, which should not have been happening at pump setting 1 anyway.
What I do notice having taken the valve to setting 0.6, is that at any lower setting the knob just feels loose, as if it is not doing anything. Is that normal or does it also point to a faulty valve?
Thank you again.
But I also have my problem with the ABV! With the boiler pump map set to constant pressure option 1, which equates to 150mb, I cannot stop the circulation through the Honeywell DU144 ABV, even when I wind it all the way to setting 0.6, which equates to a pressure of 600mb to 1000mb, depending on flow rate. Surely that should stop the flow? Seems like the valve must be faulty or stuck open. I was suspicious to start with, because it was passing flow at the installed setting of 0.3, which should not have been happening at pump setting 1 anyway.
What I do notice having taken the valve to setting 0.6, is that at any lower setting the knob just feels loose, as if it is not doing anything. Is that normal or does it also point to a faulty valve?
Thank you again.
Looks very like a knackered ABV, easy enough to confirm.
Put the pump map to 1....150mb
After a few minutes read off the pump modulation in 1-C2 %
Then go to pump map 3...250mb
Again read off the pump modulation
Ensure ABV at index 6
Put the pump map to 1....150mb
After a few minutes read off the pump modulation in 1-C2 %
Then go to pump map 3...250mb
Again read off the pump modulation
Ensure ABV at index 6
OK Johntheo5 ……
ABV set to 0.6 (600mb+).
Boiler started by raising room stat for one heating circuit.
Pump at 150mb; wait 5mins; Pump modulation 46% and rising slowly.
Pump at 250mb; wait 5mins; Pump modulation 63%.
Pump back to 150mb; wait 5mins; Pump modulation 50%.
Pump back to 250mb; wait 5mins; Pump modulation 63%.
Even at 150mb pump pressure and ABV at 600mb+, the whole 10m ABV loop gets hot at almost the flow temperature from the boiler. And the ABV adjustment knob still now feels loose over its whole range from minimum to 600mb, having once set it to 600mb. Only when it is rotated to the 600mb setting does any resistance now start to be felt. But it is always passing hot water.
ABV set to 0.6 (600mb+).
Boiler started by raising room stat for one heating circuit.
Pump at 150mb; wait 5mins; Pump modulation 46% and rising slowly.
Pump at 250mb; wait 5mins; Pump modulation 63%.
Pump back to 150mb; wait 5mins; Pump modulation 50%.
Pump back to 250mb; wait 5mins; Pump modulation 63%.
Even at 150mb pump pressure and ABV at 600mb+, the whole 10m ABV loop gets hot at almost the flow temperature from the boiler. And the ABV adjustment knob still now feels loose over its whole range from minimum to 600mb, having once set it to 600mb. Only when it is rotated to the 600mb setting does any resistance now start to be felt. But it is always passing hot water.
Yes, Renew the ABV before doing anything else.
That appears to be a 7M (700mb) pump, Pump head is proportional to speed squared so pump head at 50% modulation = 700*0.5^2, 175mb and pump head at 63% modulation = 700*0.63^2, 278mb (2.78M), nowhere near index 6 on the ABV.
You might also, for interest, when the boiler & system have settled down take the following readings.
1-A5 Flow Temperature
1-A6 Target Flow Temperature
1-b2 Burner modulation
1-C2 Pump modulation
Pump map setting
That appears to be a 7M (700mb) pump, Pump head is proportional to speed squared so pump head at 50% modulation = 700*0.5^2, 175mb and pump head at 63% modulation = 700*0.63^2, 278mb (2.78M), nowhere near index 6 on the ABV.
You might also, for interest, when the boiler & system have settled down take the following readings.
1-A5 Flow Temperature
1-A6 Target Flow Temperature
1-b2 Burner modulation
1-C2 Pump modulation
Pump map setting
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