How to control secondary pump from Evohome

[mtmcgavock]

I have to say 38kw for a heat only boiler is ridiculously big, out of interest why on earth did you have one so large?

[bruce_miranda]

I was stupid and listened to my plumber...he just said it's easier to wind a boiler down than realise it's too small. If there were models between 28Kw and 38Kw we would have had a bit more discussion. Also at the time it was my first new boiler and I just went on blind faith.

A few other plumbers since then have told me that it wasn't a bad choice as a 438 has a bigger and better heat exchanger so works better than a 428, even with the 438 wound down to 28Kw.

[fergie]

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!

[dty]

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.

[bruce_miranda]

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

[dty]

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.

[bruce_miranda]

Interesting bit of kit. I finally learnt what D.27 and D.28 are used for. I wonder if the internal and external pump can have different over run times. I doubt it but would be good to know.

[dty]

I doubt it. There's nothing in the parameters about separate overruns. In fact, d.27 and d.28 are the only things in the parameters that reference this board.

[mtmcgavock]

Surely this means your pump will run whether the boiler is pumping whether that be a HW or CH demand. So why not just wire the pump into the S/L on the boiler and then it wouldn't over run?

[dty]

Because I'm using OpenTherm and don't have a switched live. I mean... the wire is still there, but there's nothing connected to the other end of it. If I could bind a boiler BDR, that would work fine. But I can't... because OpenTherm.