Hello all,
I have Evohome installed on a large system (24 rads). The system has a DHW valve but no CH valve. It's not possible to add a CH valve because of the way the system is piped. Importantly, the system also has a low-loss header with pumps on the primary and secondary side.
Before today, I had a BDR91 controlling the DHW valve, and another one controlling the boiler and both pumps (the dinosaur-era boiler had no pump control). This was fine. If DHW was called for, the valve opened on DHW BDR, the boiler fired and the pumps ran on the boiler BDR, and if the boiler hit its thermostat (which it frequently did), it would shut itself down, but the boiler BDR would keep the pumps running to circulate the heat. This did mean I had no pump overrun, but since I knew I was going to be upgrading the boiler, I was OK with this.
Today, I upgraded the boiler to a Vaillant EcoFit Pure 625 system boiler with OpenTherm. The primary side pump was removed as the system boiler has its own. The boiler fires, the pump comes on and I have primary-side circulation on my LLH, and the boiler controls the pump overrun which circulates heat around the primary loop to help cooling. All good.
The problem comes with the secondary-side pump. We have connected it as follows: for the DHW side, we've taken the orange wire from the valve to the pump - this is fine. For the CH side, however, we have no simple way of controlling the pump. What we ended up doing was taking the (now redundant) boiler BDR, clearing the binding and re-binding it as a CH BDR, and wiring it to the switched live for the pump. Effectively, as though there is an instantly-opening CH valve whose orange wire is controlling the pump, but without actually having a CH valve.
This seems to work, but the CH valve BDR seems to still be doing TPI, even with OpenTherm. We'd rather hoped that it would manage load by leaving the (pretend) CH valve open, and varying the flow temperature, but it appears that it is also using TPI on the CH valve. The effect of this, of course, is that when there's low heat load the boiler can be on, and modulated down, but the secondary-side pump will be on a low duty cycle and not circulating water to the radiators.
If I were to control the secondary-side pump from the boiler (if that's even possible - I didn't look), then I would be dead-heading it when the boiler demand is removed but the pump is on overrun.
So... how to control the secondary pump?
I have Evohome installed on a large system (24 rads). The system has a DHW valve but no CH valve. It's not possible to add a CH valve because of the way the system is piped. Importantly, the system also has a low-loss header with pumps on the primary and secondary side.
Before today, I had a BDR91 controlling the DHW valve, and another one controlling the boiler and both pumps (the dinosaur-era boiler had no pump control). This was fine. If DHW was called for, the valve opened on DHW BDR, the boiler fired and the pumps ran on the boiler BDR, and if the boiler hit its thermostat (which it frequently did), it would shut itself down, but the boiler BDR would keep the pumps running to circulate the heat. This did mean I had no pump overrun, but since I knew I was going to be upgrading the boiler, I was OK with this.
Today, I upgraded the boiler to a Vaillant EcoFit Pure 625 system boiler with OpenTherm. The primary side pump was removed as the system boiler has its own. The boiler fires, the pump comes on and I have primary-side circulation on my LLH, and the boiler controls the pump overrun which circulates heat around the primary loop to help cooling. All good.
The problem comes with the secondary-side pump. We have connected it as follows: for the DHW side, we've taken the orange wire from the valve to the pump - this is fine. For the CH side, however, we have no simple way of controlling the pump. What we ended up doing was taking the (now redundant) boiler BDR, clearing the binding and re-binding it as a CH BDR, and wiring it to the switched live for the pump. Effectively, as though there is an instantly-opening CH valve whose orange wire is controlling the pump, but without actually having a CH valve.
This seems to work, but the CH valve BDR seems to still be doing TPI, even with OpenTherm. We'd rather hoped that it would manage load by leaving the (pretend) CH valve open, and varying the flow temperature, but it appears that it is also using TPI on the CH valve. The effect of this, of course, is that when there's low heat load the boiler can be on, and modulated down, but the secondary-side pump will be on a low duty cycle and not circulating water to the radiators.
If I were to control the secondary-side pump from the boiler (if that's even possible - I didn't look), then I would be dead-heading it when the boiler demand is removed but the pump is on overrun.
So... how to control the secondary pump?
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