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Thread: How to control secondary pump from Evohome

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  1. #1
    Automated Home Ninja
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    Default How to control secondary pump from Evohome

    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?

  2. #2
    Automated Home Legend
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    Why not fire the secondary pump using the system boiler's pump output itself?

  3. #3
    Automated Home Ninja
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    Because when the boiler pump is on overrun the secondary side will be deadheaded. I suspect the answer is to do exactly what you're suggesting and add an ABV on the secondary side.

  4. #4
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    I thought you said you had no CH valve on the secondary side. Do all your radiators have TRVs? In any case a ABV is the way to go to avoid the pump hitting a dead end.

  5. #5
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    Correct. No CH valve. But I'm using a BDR bound as a CH valve controller to fire the secondary pump together with the orange wire from the DHW valve. All rads have TRVs as per Honeywell recommendation.

  6. #6
    Automated Home Legend paulockenden's Avatar
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    Do you actually need the low loss header and two pump setup with a modern modulating boiler? Or is it all a bit too complicated to change?

    I know it says "low loss" on the tin, but it's always struck me as a bit inefficient having hot water racing round the primary all the time, and just dipping in via the secondaries when you need it. Why not just heat the water as and when required? Just seems like extra pipes to fill up with hot water before it gets anywhere near the radiators!

    I guess it makes sense in a commercial setting if there are loads of (separately pumped) circuits running off a single boiler. But even for a large domestic it seems a bit OTT.

    (Or maybe I'm just jealous of your mansion!)

  7. #7
    Automated Home Sr Member
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    Quote Originally Posted by bruce_miranda View Post
    How I wish I had not listened to my plumber and installed the system boiler that I wanted, rather than the heat only that he recommended. Ofcourse he did manage to convince me at the time, that keeping the boiler to the bare minimum was best and allowed other parts to be changed with ease. And since then I have changed the pump to a quiet Alpha2, would not have been able to do that with a system boiler.
    My Atag installer was the opposite- sold me on the system boiler by explaining it meant the 10y warranty would also cover the internal pump etc. Also solved the problem of the loud external pump in the airing cupboard by getting it removed.

    Like you I was between sizes - 24kw too small and 32kw possibly too big, but I'm planning to extend the house further and Scottish winters can be pretty cold!

  8. #8
    Automated Home Ninja
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    Meanwhile, back in the land of secondary pumps, the decision has been taken to drive the pump off the boiler. Vaillant say I need a VR40 (2-in-7 multi-function module) for this purpose. To solve the over-run issue, I just need to leave a rad open to act as a bypass until my heating guy can fit a proper ABV.

  9. #9
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    So are you planning to run the secondary pump off the VR40? Or just in parallel with the primary pump.

  10. #10
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    Electrically, the first thing. Logically, the second thing! :-)

    There's nowhere on the PCB to run the secondary pump from. I could bodge it by wiring the secondary to the same connector as the primary, but I don't know if the board is capable of driving that load. Or I could bodge it by wiring the secondary pump directly to the primary pump, but again I don't know if the board can drive that, plus the existing wire would need to cope with the load. I could use a traditional external relay, but I'd have to cut into the wires for the primary pump. All of these things are bodges. Also warranty.

    So, VR40. It has two relays on board each of which can be configured in one of several modes (more than 7, I think), one of which is "external pump". So the secondary pump will come on with the primary, which is what I wanted. And I really should have an ABV on the secondary side, so fixing that solves the overrun problem.

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