Nest Tstat Problems

[stickboy1375]

Just use relays to keep it simple and the transformers isolated from each other...

 

[stickboy1375]

The Nest is compatible with dual transformers, So i'm not sure why you are having issues, unless you wired something incorrectly, but you should have a 2 conductor from the TACO valve to the NEST t-stat, which would land on RH and W1, Then you would run a 4 conductor from the zone panel to the nest as RC, C, Y1, G....

This should work fine....

 

[Sierrasparky]

The Nest is compatible with dual transformers, So i'm not sure why you are having issues, unless you wired something incorrectly, but you should have a 2 conductor from the TACO valve to the NEST t-stat, which would land on RH and W1, Then you would run a 4 conductor from the zone panel to the nest as RC, C, Y1, G....

This should work fine....

That will work fine if the RH and W1 are isolated contacts.

 

[stickboy1375]

That will work fine if the RH and W1 are isolated contacts.

The nest will know if you are using two transformers or one... if using one, the NEST will automatically jumper RH RC together...

 

[stickboy1375]

This is how is should be wired...

If using TACO control panels, just run TT from the control board to the RH and W1 on the nest.... not sure if you were using zone valves or circs.....

 

[Mgraw]

That is what I originally thought, but then I reconsidered: If a thermostat has separate terminals for Rc and Rh, and has its own power (say batteries), then why can't it control two separate systems with separate control transformers? When it calls for cooling it will connect Rc to the cooling wire (Y?), and when it calls for heating, it will connect Rh to the heating wire (W?).

In the case of the Nest, it wants power from the control system, so it has a C common terminal, but the link I provided tells you which common to use when you have two separate control transformers: use the common associated with Rc. So now aren't we back to the above situation?

Cheers,
Wayne

Nest T-stats "auto sense" two power supplies. Rh - W is just a switch, it is either open or closed depending on what the t-stat calls for. The heater C is not needed at the t-stat.

 

[stickboy1375]

Nest T-stats "auto sense" two power supplies. Rh - W is just a switch, it is either open or closed depending on what the t-stat calls for. The heater C is not needed at the t-stat.

All stats use the RC common, this is a given... and I concur with your above statement.

 

[mkgrady]

The Nest is compatible with dual transformers, So i'm not sure why you are having issues, unless you wired something incorrectly, but you should have a 2 conductor from the TACO valve to the NEST t-stat, which would land on RH and W1, Then you would run a 4 conductor from the zone panel to the nest as RC, C, Y1, G....

This should work fine....

I agree and that is exactly what I did, yet the heat comes on the when it should not where the nest is controlling both heat and ac. The only thing a little bit unusual is that two of the ac zones are from one air handler which are connected through an ac zone control panel

 

[stickboy1375]

I agree and that is exactly what I did, yet the heat comes on the when it should not where the nest is controlling both heat and ac. The only thing a little bit unusual is that two of the ac zones are from one air handler which are connected through an ac zone control panel

As long as the zone panel is not a master / slave type zone panel, it should work just fine, I don't see why it wouldn't work, but if the zone panel is using O/B to tell the other t-stats to dictate heat or cool, then you will have issues.

 

[stickboy1375]

If you are still having issues though, just install relays to isolate the heat from the cooling systems.

 

[mkgrady]

If you are still having issues though, just install relays to isolate the heat from the cooling systems.

Thanks and thanks to others for input. I gues I'll try the relay idea. I'll wire it like the heat and ac are one system. Meaning the nest will not get an rh wire. When heat is being called the nest will send 24 volts down the w wire and pull in a relay. This relay will cause the rh and w to close at the taco panel.

 

[stickboy1375]

Thanks and thanks to others for input. I gues I'll try the relay idea. I'll wire it like the heat and ac are one system. Meaning the nest will not get an rh wire. When heat is being called the nest will send 24 volts down the w wire and pull in a relay. This relay will cause the rh and w to close at the taco panel.

Yep... but in all honesty, its weird that it isn't working the other way, but whatever the nest T-stats sometimes are too smart...

 

[mkgrady]

Yep... but in all honesty, its weird that it isn't working the other way, but whatever the nest T-stats sometimes are too smart...

The relay idea worked! I am so grateful to all who helped. It really did not make sense that the nests could not handle the task. It made me think I was doing something wrong with the wiring. Using the relays proved to me that there is just some incompatibility between the heat and ac controls being used.

 

[stickboy1375]

The relay idea worked! I am so grateful to all who helped. It really did not make sense that the nests could not handle the task. It made me think I was doing something wrong with the wiring. Using the relays proved to me that there is just some incompatibility between the heat and ac controls being used.

I do a lot of wifi tstats, they all need common wires, a lot of heating systems simply just don't supply a common with a basic TT end switch, so I have to install a transformer and RIB relay to get the job done, hopefully manufactures see this issue and adjust accordingly...