Hi all,

Been browsing this site for while but this is my first post (and apologies as it is quite long!). Great forum with loads of useful information.

My query relates to my set up. I have an Ideal Vogue System 32 boiler in a traditional S plan set up for heating and hot water cylinder.

The Evohome controls the heating 2 way valve via a BDR91 relay box. The hot water 2 way valve remains controlled by the original timer unit.

I have HR92's fitted to approximately half of the rads in my house which is quite large (approx 260 square metres) with the other half on TRVs (landing, hall, etc) and a couple (bathrooms) with no TRV at all. Some rooms with TRVs are affectively are basically on frost protection settings as they are not currently used.

The system works pretty well but some of the posts on here have got me thinking about helping my boiler run in condensing mode more of the time.

I currently have the output flow set at 65C to ensure it can sufficiently heat the hot water.

This means that when just running the heating I think the boiler will spend a fair amount of time with a return temperature in the mid 50s and not condensing so well.

I'm still playing around with balancing rads to get reasonable temperature drops for the return but but am now also thinking about adding an outside temperature sensor or the opentherm bridge so the boiler can modulate the flow temperature automatically.

I would estimate that a CH flow of approx 65 is only really needed when it is approaching 0C outside and the house would comfortably warm up with lower CH temperatures as external temperature increases.

My initial thinking was therefore to add the outside temperature sensor to my boiler as it is all set up to take one and they are only 25. The boiler would then modulate the CH flow temperature accordingly with me picking whichever curve is most appropriate.

The only potential issue here is that the Evohome system will not be aware of this and therefore can't take into account the fact that the CH flow temperature is being changed. However this affect might be self cancelling i.e. on a cold day the boiler will give high temp CH flow and evohome will control from it's perspective as it does now. On a warmer day the boiler will lower the CH flow temp which evohome will not be aware of but at the same time, because it is warmer outside, the heat loss from the rooms will be lower and therefore the rooms might warm at the same rate regardless of the fact that the flow temperature has reduced..............

I've tried researching the opentherm option (as my boiler has the input for this too) but I'm struggling to understand exactly what the evohome system will do differently if I replace the BDR91 with an opentherm unit. I understand the principle of opentherm and that it should allow the opentherm controller to control boiler burner modulation for exampl but some of the feedback around the web seems mixed.

At the minute I'm leaning towards the outside temperature sensor option but any advice on either option would be much appreciated.