A year or so ago we had a Vaillant 428 installed on a basic S-plan system (one CH zone plus HW), along with new rads & cylinder. Currently system is pumped by Grundfos Alpha 15-50 set on position 3. Cylinder is a Condencyl HE which I believe is c. 22kW coil. System runs hot water priority.
Boiler is sized to allow very fast reheating of the (140l) cylinder with the aim to achieve a reheat time of 10-15mins. At the moment I cannot get close to this as with the boiler rated at anything over 20kW I am getting the "insufficient flow" lock-out error. Set sub 20kW it is modulating heavily presumably because of the return temp taking a long time to rise. So, "obviously" I need more flow through the circuit. So a 25-6 pump is on the cards.
My question is this; should I stick to a single pump to take care of both sides or put the Alpha on the CH side and use the bigger pump just for hot water? For CH the boiler is rated down to 15kW so really does not need the sort of flow the HW side requires.
If I do go with two pumps it looks like it would be easy enough to wire the pumps from the outputs of the VR65 by taking the zone-valve control signals to a relay that determines which pump is driven, with power to the pump taken from boiler so that overrun still functions. Presumably the diverter neesd to run on the CH side.
I realise that the two stage pump used in the Vaillant system boilers would be a good way to have avoided this issue, but for some reason the modulation on the heat only boilers goes significantly lower than the system boilers which is why we went with the 428.
Thanks
Graham
Boiler is sized to allow very fast reheating of the (140l) cylinder with the aim to achieve a reheat time of 10-15mins. At the moment I cannot get close to this as with the boiler rated at anything over 20kW I am getting the "insufficient flow" lock-out error. Set sub 20kW it is modulating heavily presumably because of the return temp taking a long time to rise. So, "obviously" I need more flow through the circuit. So a 25-6 pump is on the cards.
My question is this; should I stick to a single pump to take care of both sides or put the Alpha on the CH side and use the bigger pump just for hot water? For CH the boiler is rated down to 15kW so really does not need the sort of flow the HW side requires.
If I do go with two pumps it looks like it would be easy enough to wire the pumps from the outputs of the VR65 by taking the zone-valve control signals to a relay that determines which pump is driven, with power to the pump taken from boiler so that overrun still functions. Presumably the diverter neesd to run on the CH side.
I realise that the two stage pump used in the Vaillant system boilers would be a good way to have avoided this issue, but for some reason the modulation on the heat only boilers goes significantly lower than the system boilers which is why we went with the 428.
Thanks
Graham
