This is sort of connected to the S-Plan vs Y-Plan for Evohome thread which is ongoing, but this is separated for clarity.
I am replacing an old boiler with new Worcester Bosch 40 CDi (LPG not that it matters).
My current controls are Evohome with three BDR91 relays controlling CH pump, HW pump and ancient boiler, no valves.
The installers aren't really happy with this and would prefer conventional S plan with one pump and two MOVs controlled by CH and HW signals (they don't really want to know that these come from two BDR91s).
I am happier with valving CH and HW separately anyway so I have agreed. I have a problem with Evohome losing contact with some remote radiators and can re-deploy the spare BDR91 as a second CH trigger controlled by another Evohome controller sited so that it can see both the missing radiators and the relay, but that's another matter.
At the moment a bypass loop for overrun pumping is not necessary as Evohome takes care of that, having control of both boiler and pumps (correct me if I'm wrong).
As I understand it, reading the manual, the new boiler does not have a dedicated overrun output signal, and just runs the pump when it wants. So I would need an automatic bypass loop after the pump to stop it pumping against closed valves.
I don't like that and would prefer the overrun to pump via the DHW loop when the demand for both CH and HW drops. I have an aversion to dissipating heat in a bypass loop and would rather see it do some useful work.
I think the way to do this is via an extra changeover relay, which switches the boiler input (Common contact) between the output from the HW MOV (Normally Closed contact) and disconnected (Normally Open contact), controlled by the boiler pump signal. The HW BDR91 switches the HW MOV to either live (Normally Open) or the boiler pump signal.
The net effect of this (ignoring CH which operates normally) is that the boiler runs when there is HW demand, and the MOV is forced open when there is pump demand, regardless of HW demand.
Is this a reasonable approach or should I just let the installers have their bypass loop?
I am replacing an old boiler with new Worcester Bosch 40 CDi (LPG not that it matters).
My current controls are Evohome with three BDR91 relays controlling CH pump, HW pump and ancient boiler, no valves.
The installers aren't really happy with this and would prefer conventional S plan with one pump and two MOVs controlled by CH and HW signals (they don't really want to know that these come from two BDR91s).
I am happier with valving CH and HW separately anyway so I have agreed. I have a problem with Evohome losing contact with some remote radiators and can re-deploy the spare BDR91 as a second CH trigger controlled by another Evohome controller sited so that it can see both the missing radiators and the relay, but that's another matter.
At the moment a bypass loop for overrun pumping is not necessary as Evohome takes care of that, having control of both boiler and pumps (correct me if I'm wrong).
As I understand it, reading the manual, the new boiler does not have a dedicated overrun output signal, and just runs the pump when it wants. So I would need an automatic bypass loop after the pump to stop it pumping against closed valves.
I don't like that and would prefer the overrun to pump via the DHW loop when the demand for both CH and HW drops. I have an aversion to dissipating heat in a bypass loop and would rather see it do some useful work.
I think the way to do this is via an extra changeover relay, which switches the boiler input (Common contact) between the output from the HW MOV (Normally Closed contact) and disconnected (Normally Open contact), controlled by the boiler pump signal. The HW BDR91 switches the HW MOV to either live (Normally Open) or the boiler pump signal.
The net effect of this (ignoring CH which operates normally) is that the boiler runs when there is HW demand, and the MOV is forced open when there is pump demand, regardless of HW demand.
Is this a reasonable approach or should I just let the installers have their bypass loop?
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