How to control secondary pump from Evohome

[dty]

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?

[bruce_miranda]

Why not fire the secondary pump using the system boiler's pump output itself?

[dty]

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.

[bruce_miranda]

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.

[dty]

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.

[paulockenden]

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!)

[dty]

Yes, because the boiler only has 230 mbar (about 2.3m) of remaining feed head and that's not enough. My secondary pump displays its working head and it will often deliver over 4m under certain loads and configurations.

[bruce_miranda]

Why not run a single loop with two pumps? Also a system boiler will alter it's output if the heat is not getting away from it fast enough. A heat only boiler can't do that. 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.

[paulockenden]

Surely a system and heat only boiler will modulate down in exactly the same way? The only difference is the venting, isn't it? Or have I completely misunderstood?

[bruce_miranda]

That was my naive understanding too. But apparently a system boiler has the ability to vary it's heat output down dynamically if the pipework or pump is undersized. In my heat only boiler if I set the heat output too high my boiler will just lock out with a fault. I have had to wind the boiler output down manually to avoid it over heating. This was something a Vaillant engineer told me. I have a seriously over specced 38Kw boiler that has been set to 28Kw to avoid the issue. In a system boiler, the boiler can vary this value on it's own - apparently!