Making Nest thermostat work with Uponor C5 UFH

[taroli]

Hi everyone,
I just installed Google Nest Thermostat 3 Gen, made for Europe together with Uponor C35 underfloor heating system (my version of the Nest comes with a heat link, which is a separate device installed near my underfloor control system. The thermostat talks to Heat Link via wireless or wire (in my case it's a wired connection, which gives power to the thermostat as well)).

A small explanation is needed: I have 3 floor zones to heat with the underfloor heating system. I replaced a thermostat of one of the zones by the Nest thermo, the rest 2 zones are still controlled by the native Uponor thermostats. The source of heating liquid is central district heating station, so in house, we just control the valves that let the heating liquid flow into zones

I wired the Heatlink in place of a regular thermostat. So, from the point of view of my underfloor heating system (which is Uponor C35) - the Heat Link is placed instead of its regular thermostat.

I connected the wires to the 1 and 2 connectors on the Heat Link ("Low voltage/dry contact combi boiler" setup as it says in the documentation). The reason that it was not 2 and 3 connector was that my Uponor C35 considered closed circuit to be "regular state" and open circuit to be "call for heat" state.

Everything seems to work fine for one exception: I see that my heating is sometimes ON, while it should be OFF.

For example, the current temperature in the room is 23 Celcius and the Thermostat is to set to heat up to 18 Celcius (which means it should NOT heat at all). And it works like that, but from time to time (say, 2-3 times a day) I notice that the floors are hot! and I see that the valves (that open the heating) are opened.

I can't figure out why it happens. It is either Nest thermostat for some reason gives the command to heat (when it shouldn't) OR it's some strange logic inside my Uponor C35 control system, which heats the floors even when Nest doesn't request it to do so. But such a situation doesn't happen on the zones with native Uponor thermostats.

Perhaps, Uponor control unit has a certain identification protocol of its native thermostats and it "feels" that not a native device is connected to the input.

Any ideas?

 

[ShaunCorbs]

Need to find out if the best is opening the circuit so easy way to do this is to test 1-2 on a multi meter

 

[taroli]

Need to find out if the best is opening the circuit so easy way to do this is to test 1-2 on a multi meter

Thanks! Good idea. Will check this out.

 

[taroli]

A small update. It appeared that the Nest relay works correctly. I hear the click when it turns states and multimeter confirms that.
But I noticed that after the Nest relay changes state, the Uponor relay (which is supposed to switch the valve ON/OFF) doesn't click.
I assumed that the Uponor control unit doesn't recognize Nest as a thermostat. So, I installed original Uponor thermo in parallel to Nest.
So far it seems to work. I will update the final observations to help those who might be looking into similar issues.

 

[Basher]

The reason that it was not 2 and 3 connector was that my Uponor C35 considered closed circuit to be "regular state" and open circuit to be "call for heat" state.

Are you absolutely sure "call for heat" is open contacts? Could you verify this on your added Uponor thermostat (with disconnected contacts) using your multimeter (ohms) to confirm?
The reason I ask is there is a bit in the manual about contacts closed for heat and open for cooling, but this relates to combined heating and cooling systems, to switch from heating to cooling, not thermostat connections. I couldn't see any specific reference to Uponor thermostats being 'reverse logic' to normal!

I may be wrong with the above, but I can't immediately see any other explanation for the symptoms.

 

[taroli]

Are you absolutely sure "call for heat" is open contacts? Could you verify this on your added Uponor thermostat (with disconnected contacts) using your multimeter (ohms) to confirm?
The reason I ask is there is a bit in the manual about contacts closed for heat and open for cooling, but this relates to combined heating and cooling systems, to switch from heating to cooling, not thermostat connections. I couldn't see any specific reference to Uponor thermostats being 'reverse logic' to normal!

I may be wrong with the above, but I can't immediately see any other explanation for the symptoms.

When the original thermostat is connected to the control unit (without Nest), the behavior is the following:

• if the temperature wheel is rotated fully clockwise (which means "call for heat" state), a led indicator is ON on the thermostat and the control unit gives 24 volts to the valves (checked with multimeter)
• I short-circuit the only two wires that connect the control unit and the thermostat - and led light goes OFF and the current on the valves goes to zero.

This makes me think it's "call for heat" when the two wires connecting the thermostat with the control unit are open, and it's "not heating" when they are closed.

So, finally I connected Nest in parallel to the original thermostat and put the original thermostat wheel to constant "call for heat" state (wheel turned fully closewise). This means that the whole circuit is open when both the devices (Nest and the original one) are in open state (call for heat in both of them). And it is closed (no heating) when at least one of them closes the circuit (Nest which in my case).

My control system (Uponor C35) is only an underfloor heating system, it is not intended for cooling, as far as I understand.

Anyway, having done the setup above, I am still observing if it behaves as intended.

 

[Basher]

When the original thermostat is connected to the control unit (without Nest), the behavior is the following:

• if the temperature wheel is rotated fully clockwise (which means "call for heat" state), a led indicator is ON on the thermostat and the control unit gives 24 volts to the valves (checked with multimeter)
• I short-circuit the only two wires that connect the control unit and the thermostat - and led light goes OFF and the current on the valves goes to zero.

This makes me think it's "call for heat" when the two wires connecting the thermostat with the control unit are open, and it's "not heating" when they are closed.

So, finally I connected Nest in parallel to the original thermostat and put the original thermostat wheel to constant "call for heat" state (wheel turned fully closewise). This means that the whole circuit is open when both the devices (Nest and the original one) are in open state (call for heat in both of them). And it is closed (no heating) when at least one of them closes the circuit (Nest which in my case).

My control system (Uponor C35) is only an underfloor heating system, it is not intended for cooling, as far as I understand.

Anyway, having done the setup above, I am still observing if it behaves as intended.

Thank you for replying, and by your checks you have confirmed that 'call for heat' is open contacts, and 'not heating' is closed contacts.
So your original assertion is correct, and my theory was wrong!

It will be interesting to hear what you observe. It is certainly strange to have those random 'call for heat' events you describe. I can't see what would trigger a random opening of the Heatlink relay circuit, unless there is a loose connection somewhere!

 

[Jose Madrid]

Thanks! Good idea. Will check this out.

Finally, did you manage to fix it?

 

[Nclayton]

I have exactly the same problem. I was trying to replace my Uponor T38 thermostat/timer attached to my C5 with a Drayton Wiser room thermostat attached to their HubR controller. This provides a 240V call for heat signal with which I hoped to control the C35. Initially I tried to connect the two using a relay connecting the 240V to one side and the two contacts on the C35 that my T38 had been connected to on the other. In other to either provide an open circuit or closed circuit but this displayed the erratic behaviour described at the start of this thread. I did a brief investigation to try and determine the impedance of the T38 in both its ‘call’ and ‘off’ states which was about 4k ohms for ‘call’ and 2k ohms for ‘off’. So I have now wired a couple of resistors across the relay so that it switches between these two loads and hey-presto the C35 works as expected.

 

[Nclayton]

I have exactly the same problem. I was trying to replace my Uponor T38 thermostat/timer attached to my C5 with a Drayton Wiser room thermostat attached to their HubR controller. This provides a 240V call for heat signal with which I hoped to control the C35. Initially I tried to connect the two using a relay connecting the 240V to one side and the two contacts on the C35 that my T38 had been connected to on the other. In other to either provide an open circuit or closed circuit but this displayed the erratic behaviour described at the start of this thread. I did a brief investigation to try and determine the impedance of the T38 in both its ‘call’ and ‘off’ states which was about 4k ohms for ‘call’ and 2k ohms for ‘off’. So I have now wired a couple of resistors across the relay so that it switches between these two loads and hey-presto the C35 works as expected.

Sorry. Just realised I got that the wrong way round! 4k = off, 2k = call