Vaillant Ambisense, how does it modulate, help please!

2 Nov 2020
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United Kingdom
I have a Vaillant ecofit 425 heat only boiler on a Y plan system. Controls are all Vaillant, a VRC 700f, VRC 920 gateway, 16 radiators with 15 Ambisense TRVs, the one remaining radiator TRV free as a bypass.

So far so good. I'm broadly happy with the system though the Vaillant app is unreliable and quite frustrating at times. In my somewhat obsessive desire to minimise gas usage and too much time spent making my house well insulated I've got to considering how the boiler is modulated by the Ambisense TRVs. Before the Ambisense bits were fitted I had Drayton TRV4s throughout, had a heating curve of 2.2 (which, after experimentation seems to be best for my house), and worked OK save that i was forever fiddling with individual TRVs.

I would like to understand how smart TRVs and, in particular Vaillant Ambisense TRVs work with heating curves/modulation given that there will be a number of different target temperatures throughout the house. Does the system average them in some way? Are they load modulated, if that's the right phrase?

I'm new to this forum and, clearly no expert, but I would be grateful if an expert could help me. I tried calling Vaillant but they were really no help at all.
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i'm not sure how the vailant system works, however i'm sure it is similar to evohome with the opentherm bridge.

each room stat learns its environment and how long it takes to reach target temperature, it then modulates the boiler and itself to avoid temperature overshoots.

obviously if rooms are at different temperatures then the boiler is controlled by the highest heat demand, but then this is negated by the trv opening and closing
The heat curve will select the boilers target temperature based on the outside temperature and the curve selected, the lower you can have the curve the better.
The system then temporarily alters the heat curve depending on the difference between the current room temperature and the desired temperature, so when there is a large difference between what heat you want and whats in the room the flow temp will be higher and gradually reduce as the current and desired temperature come together at which point it will be flowing the temp based on heat curve.

With ambisense it will select the valve set to the highest temperature to base its flow temp off of, an average of all the valves would leave some rooms not reaching temperature so it will always go with the highest.

So in the case ever room is actually at 15 degrees, and they are all set to request 16, a low flow temp would be selected, should you turn one up to 20, then a higher flow temp will be selected regardless of a majority of the house being almost at desired temp.

In reality its not very noticable, the rads being roughly sized correctly should mean that most of the rooms would heat fairly evenly and as you say most over shoot temps will be caught by the valves closing.

My system currently sits on a heat curve of 1.6 I think, and all but one room heat to within 0.5 degrees of what I want (the one cold room has a designer rad in it which looks nice but not really built to throw out heat)

Im curios though, you say its on a Y plan? what wiring center are you using? as the Ambisense is only spesified for a VR70/71 wiring center with the VRC700 or SensoComfrot control. And none of them support Y plans
Thank you Scottish Gasman, that is very helpful and just what I needed. I have a VR66 with the monozone option selected on it. It seems to work fine and is listed somewhere in the Vaillant controls brochure as being compatible with the VRC700 (just checked, page 27). It seems to work fine with the ambisense TRVs and they seem do what they are supposed to.

Re your heating curve observation, I need to start reducing it and see how I get on. The current 2.2 heat curve was set largely before I started on my lets insulate, fill in loads of holes I've drilled in the house over the last 30 years etc.etc. project. Its a 1980s built 5 bedroom detached house and is quite prevalent to winds though I live just north of London, not in Scotland!

Thanks again.
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Good stuff.

Yea, the lower heat curve will get the boiler condensing more, its very dependant on the insulation levels and the size of the radiators as to how low you can go before your not matching the heat lost from the property, but a bit of trial and error and you'll get there. 2.5 is the standard setting for an older UK heating system, and will still save some gas even on that as the full radiator temperature is only needed when its very cold outside, so the rest of the heating season you can heat the house using lower temperatures.

My house is a 1900 semidetached sandstone bungalow, but I've intentionally oversized the radiators as much as I can without it looking ridiculous, and adding insulation where I can loose a few inches of space, planning on doing the same re: filling all the gaps and holes thats appeared over the years so will see what the final result it when we get there.

The app is without question slow and frustrating, but they are planning on moving to a different app (which looks much nicer) and I belive newer servers as well which will hopefully speed up the responsivness of the overall expirience, it was due the last 1/4 this year but not 100% it will be done by then with covid and all the rest of the upheaval, but you should get an email when their close to doing it to inform you about what to do

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