Adding UFH zone without thermostat

[Tamlyn]

Is it madness to add a UFH zone to an existing combi boiler with radiators, without adding a thermostat in the UFH room?

I have a Vaillant Ecotec 832 Plus running radiators with the Vaillant vSmart thermostat modulating the boiler over eBus. This has been working for years and we've very happy with it. We're converting the attached garage and having underfloor heating fitted but the wiring is turning into a nightmare. Nobody seems to know how to do it.

I was wondering: can we just treat the UFH zone as a radiator? I'm aware that the heat response for UFH is very different to radiators, but equally the garage is insulated to 2024 standards whereas the rest of the house is at 1960s standards, so it's going to be much warmer anyway.

What I'm imagining is:
- we have the zone valves open all the time
- whenever the main house thermostat calls for heat, the radiators heat up and the UFH pump turns on
- the manifold blends the water to the correct temperature
- when the house thermostat tells the boiler to stop heating, the UFH pump runs on a bit until the water temperature drops below 30°C (the manifold handles this automatically)

Would this work? Has anyone done this?

 

[Madrab]

The main trouble is temps - Floor types dependent - UFH doesn't run much warmer than around 45deg - if you have standard rads then the flow will be at => 60Deg, that would usually be far too high for UFH.

You would need to use what's called a flow regulating valve (edit) (FHV - R or A), that lowers the flow water in the UFH circuit by blending the flow with cooler return water and sets the temp by either floor temp or air temp.

 

[Tamlyn]

Thanks, that's useful info. It looks like an FHV-A is just like a TRV on a traditional radiator: it restricts the flow when the air temperature reaches the desired level. An FHV-R does the same but based on the return water temperature. Neither actually blend the out and return water: that's what a manifold does and it has its own pump.

Unfortunately it's too late for us to fit an FHV but we do have a manifold which will take care of regulating the flow temperature.

So it looks like we may not need any wiring at all!

 

[Madrab]

Apologies - I was half way through the reply and was still to add that using an inline blending valve allows the input flow temps to be lowered to a suitable UFH temp. This then negates the need and expense or setup for an extra manifold, pump wiring etc as in a typical UFH layout.

 

[Tamlyn]

I set it up today as described in my initial post and it seems to work. The manifold pump switches on when the CH water heats up and switches off when it cools down again. I'll have to wait until winter to tell if it's sufficient to heat the new room.