UFH from Vaillant combi boiler via VR 61 mixer controller

6 Sep 2014
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United Kingdom
I am planning to add underfloor heating to a 25 m² dining room. (11 No 2400x600 mm Wavin Hep2O Modular Wood plywood panels to replace existing raised timber floor.)

We have an existing Valliant ecoTEC plus 831 combination boiler with VRC 430f weather compensator, installed in late 2011, currently driving 10 radiators.

Vaillant offer the VR 61 mixer module for adding a second heating circuit.

I'd be interested in any experience you have made with this combination.

In particular, I have a number of questions:

a) Do I really need a low-loss header?

Vaillant's twozone pack manual contains a number of possible hydraulic drawings. Vaillant technical support pointed me in that file at the hydraulic diagram on page 10, and told me to ignore the cylinder (as I have a combi boiler).

That diagram (and similar others in Vaillant literature) shows a "low-loss header" and two external pumps, one for each heating circuit. Is that actually needed for a single-room domestic UFH system, and if so why?

I was instead hoping to just add a single pump and motorized mixer valve to tap and mix down heat from the existing radiator circuit, as shown in


I don't mind if the UFH circuit gets no heat while no water is pumped through the radiators, i.e. the two circuits do not have to be independent.

Is that sensible?

b) Can I use the VR 61 without room thermostat VR 81?

I want two heating circuits: HC1 for the existing radiators and HC2 for the UFH. I want to keep one of the two existing radiators in the room that is to be fitted with UHF, therefore I do not really want two thermostat zones, because the two zones would overlap in the room that has both UFH and a top-up radiator. Instead of using any room thermostat for feedback, I prefer to control the flow temperature in both heating circuits solely based on the outside temperature (i.e., open-loop weather compensator). This is supported by the VRC 340f weather compensator via the (rather obscurely named) setting "Switch-on room temperature: none", as explained in the Glossary of


This setting works already fine so far with our existing single heating circuit.

Is there any reason why I can't operate both heating circuits just from the out-side temperature via two heating curves?

Do I even have to connect a VR 81 thermostat to the eBUS in the configuration?

c) What is a suitable motorized mixer valve?

The VR 61 can drive a motorized mixer valve with 230 V motor-open and motor-close connection. Where can I buy such a device in the UK?

Several manufacturers offer pump/mixer/manifold kits for UFH projects. Do you know any that contains a motorized mixer valve that can be controlled by the Vaillant VR 61 mixer module?

Thanks for any insights!

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