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Did you need to post that chuck
broadsword calling danny boy
Any pics of the water heater ?
Yes that's because it's an oil boiler
That's one you said there's two what's the other one
What's open when heating up mode is on the diverter valve egvtc ?
Think you have a problem with gravity circulation either by a port valve not totally shut or no
So there's zones on all 3 sets
If you can set up a thermo on the cylinder feeds
Flow and return
Where's / is there non returns on the return pipework?
Storage combi at min something like a Viessmann 111 or 222 best option is an unvented cylinder
You can try it as is and see how you get on and as you say if it's slow remove and either leave or cut down
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