Viessmann 100-W Boiler, Wet UFH, 2 Port Valve and Opentherm

[Antec123]

We have a Honeywell T6R Thermostat Wired In Open Therm mode with a Viessmann 100-W 35KW Combi Boiler and working ok, the house has standard rads in each room.

 

We have just fitted wet under floor heating in our new kitchen and connected it to the boiler using a 2 port valve so the rest of the house doesnt heat up when the kitchen ufh is on, not an expert here but I assume this is standard practice.

 

The UFH stat they have installed is Heatmiser neoAir v3 Wireless Smart Thermostat.

 

We are now being  told by the boiler engineer that Opentherm will now not work due to the valve being 240v and Opentherm doesnt support it, so hes wired the boiler back to on/off switching.

 

is there any way (or workaround) to get Opentherm to work with central heating in my setup? 

 

Thanks

[JohnMo]

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[Nickfromwales]

16 hours ago, Antec123 said:

We are now being  told by the boiler engineer that Opentherm will now not work due to the valve being 240v and Opentherm doesnt support it, so hes wired the boiler back to on/off switching.

 

is there any way (or workaround) to get Opentherm to work with central heating in my setup? 

Yup.

 

If you have a signal, I assume low voltage DC, then you just feed that to a relay with a DC coil that switches a fused 230v feed, and Bob's your mothers brother.

 

You can do the same with zero volt contacts, you just have to generate the DC to send it, so it can become the return.

[JohnMo]

If running opentherm does the UFH even need a 2 port valve. Could the UFH pump just be wired direct to thermostat. UFH zone hot, UFH pump stops?

[SimonD]

On 05/07/2025 at 07:14, Antec123 said:

We have a Honeywell T6R Thermostat Wired In Open Therm mode with a Viessmann 100-W 35KW Combi Boiler and working ok, the house has standard rads in each room.

 

We have just fitted wet under floor heating in our new kitchen and connected it to the boiler using a 2 port valve so the rest of the house doesnt heat up when the kitchen ufh is on, not an expert here but I assume this is standard practice.

 

The UFH stat they have installed is Heatmiser neoAir v3 Wireless Smart Thermostat.

 

We are now being  told by the boiler engineer that Opentherm will now not work due to the valve being 240v and Opentherm doesnt support it, so hes wired the boiler back to on/off switching.

 

is there any way (or workaround) to get Opentherm to work with central heating in my setup? 

 

Thanks

 

Heatmiser sell an opentherm mini hub that will control 2 zones with 240v relay to zone valve. The opentherm is done by output from the mini hub to boiler. So you could buy another neoair to replace the t6r and then wire up the minihub.

 

The other alternative is to use EPH CP4 combination if you're not using a heatmiser wiring centre - with the cp4 you just have 2 stats with receivers with one receiver set up at the hub unit which is is wired to the boiler with opentherm.

 

Oh, and especially if you're running a low temperature heating system (i.e. 55C or below), make sure the installer has actually installed the UFH with hydraulic separation, otherwise you'll probably end up with balancing problems between the radiator system and ufh.

 

HTH

 

 

 

 

Edited yesterday at 10:26 by SimonD

[Nickfromwales]

3 hours ago, JohnMo said:

If running opentherm does the UFH even need a 2 port valve. Could the UFH pump just be wired direct to thermostat. UFH zone hot, UFH pump stops?

It’ll still allow bypass if not? 

[Nickfromwales]

21 minutes ago, SimonD said:

make sure the installer has actually installed the UFH with hydraulic separation, otherwise you'll probably end up with balancing problems between the radiator system and ufh.

👆👌

[Antec123]

2 hours ago, SimonD said:

 

Heatmiser sell an opentherm mini hub that will control 2 zones with 240v relay to zone valve. The opentherm is done by output from the mini hub to boiler. So you could buy another neoair to replace the t6r and then wire up the minihub.

 

The other alternative is to use EPH CP4 combination if you're not using a heatmiser wiring centre - with the cp4 you just have 2 stats with receivers with one receiver set up at the hub unit which is is wired to the boiler with opentherm.

 

Oh, and especially if you're running a low temperature heating system (i.e. 55C or below), make sure the installer has actually installed the UFH with hydraulic separation, otherwise you'll probably end up with balancing problems between the radiator system and ufh.

 

HTH

 

 

 

 

Thanks, so right now, the UFH is using Heatmiser neoAir Kit (with RF-Switch) - White v3.

 

So I need to buy a Heatmiser neoHub Wireless Combi Boiler OpenTherm WiFi Smart Control Kit

 

replace the RF-Switch with the neohub, then replace the t6r with the Heatmiser from above kit yes?

[SimonD]

39 minutes ago, Antec123 said:

Thanks, so right now, the UFH is using Heatmiser neoAir Kit (with RF-Switch) - White v3.

 

So I need to buy a Heatmiser neoHub Wireless Combi Boiler OpenTherm WiFi Smart Control Kit

 

replace the RF-Switch with the neohub, then replace the t6r with the Heatmiser from above kit yes?

 

Yes