The Efficient Home Forum

Morning,

I have a Y plan heating system run by a pair of BDR91s to control the v4073 valve, a cylinder stat, and a Vaillant ECOTEC 428 heat only boiler. I have HR92s on all my rads, except for a few towel rails as a bypass. All pretty standard according to the installation specs for Y plan on my 1980s system.

I've been looking into whether I can make use of the opentherm functionality without too much electrical rewiring, or any replumbing - but I can't seem to find any definitive answer. I was wondering if I could simply disconnect the switched live to my boiler, and replace this input with a VR33 Ebus converter and the opentherm bridge. Both BDR91s still stay in situ to control my 3 port valve. my expectation would be that when the evohome controller demands CH or DHW it energises the respective BDR91 as per before, as well as giving the appropriate signal to the opentherm bridge to fire the boiler (as opposed to just energising it via the switched live).

I know there are posts about DPHW, X plan and other changes to improve efficiency, but would this simple rewire give me opentherm functionality and the associated efficiency benefits? Or am I fundamentally misunderstanding how evohome goes about things with opentherm?! Thanks in advance!

Have a look on the Automated Home forums,under heating control,lots of info on there and some very helpful clued up people.

Thanks, i'll do that!

itswinter wrote:Morning,

I have a Y plan heating system run by a pair of BDR91s to control the v4073 valve, a cylinder stat, and a Vaillant ECOTEC 428 heat only boiler. I have HR92s on all my rads, except for a few towel rails as a bypass. All pretty standard according to the installation specs for Y plan on my 1980s system.

I've been looking into whether I can make use of the opentherm functionality without too much electrical rewiring, or any replumbing - but I can't seem to find any definitive answer. I was wondering if I could simply disconnect the switched live to my boiler, and replace this input with a VR33 Ebus converter and the opentherm bridge. Both BDR91s still stay in situ to control my 3 port valve. my expectation would be that when the evohome controller demands CH or DHW it energises the respective BDR91 as per before, as well as giving the appropriate signal to the opentherm bridge to fire the boiler (as opposed to just energising it via the switched live).

I know there are posts about DPHW, X plan and other changes to improve efficiency, but would this simple rewire give me opentherm functionality and the associated efficiency benefits? Or am I fundamentally misunderstanding how evohome goes about things with opentherm?! Thanks in advance!

The answer is yes to this (Opentherm Bridge + 2x BDR91 for HTG & DHW). The integration of the VR33 is the gap in my knowledge as I have never done one.

Having just done this exact conversion today, it is way simpler than I thought. I had a Vaillant boiler which needed the VR33, slotted it into the boiler, plugged into X32 port - my VR33 had X32 and X31 terminals that I could connect to, it came out the box wired to X32, and there was a corresponding X32 port on my boiler PCB so I linked them. I figure if my boiler had X31 (as some do), I'd have done X31 <-> X31 instead. Then disconnected my switched 240v live (from BDR91) and bridged the 24v connection (to reenable low voltage control of my boiler) then connected up the Opentherm terminals to the evohome bridge.

I capped my flow temp to 65C (D71) at the boiler (this was my pre-OT fixed flow rate anyway) as evohome requests up to 95C for DHW etc so I wanted to dial it down a little, but I checked an hour later and it was requesting a flow temp of 46C so looked to be working fine.