NEST Thermostat vs. Tekmar 256 Compatibility

sopcopotoptop

Any experience with the above combination? Thermostat calls for heat but won’t shut the boiler down once demand has been met. Unhook the thermostat wires and the boiler shuts down. Reconnect the thermostat wires and the boiler kicks back on with no demand call. Have read about potential issues with insufficient power to operate the thermostat correctly. Have used it for years without an outdoor air reset with no issues. Thanks much for your consideration.

Jamie Hall

Is the new thermostat provided with a C wire for constant power? Without that, who knows what it will do — though latching on is a new one… You really do need that C wire.

sopcopotoptop

The thermostat is not new. Been using it for at least 8 seasons or so. It’s the ODR that’s new to the system. As it happens the wiring to the thermostat does have a third wire available, although unused at the moment. I could connect it to the C connection on the NEST and then back to the boiler 24V C terminal? Sound correct?

neilc

sounds like the ODR is wired wrong or in parrallel to, and is overriding thermostat TT,

had the boiler run correctly since the ODR was added?

EdTheHeaterMan

Is this transformer in the diagram the one that comes with the boiler?

It is better if it is a separate transformer.

It will make a difference on your nest thermostat connection.

1. What boiler is the 256 connected to?
2. Did a Pro install the 256 or is it a DIY? …Nothing wrong with DIY, (Sometimes that is better)
3. Are there zone valves or zone circulators involved?

With this info I can offer you a proper wiring diagram.

EdTheHeaterMan

This is a tough concept to follow, (and I'll try to make it clear as mud) because it is counter-intuitive to assign the R on the transformer to the R on the Tekmar and to assign C on the transformer to R on the thermostat.  This must be because of the note at the bottom right of the diagram  As far as the Tekmer is concerned the R on the transformer is the source to power the control and the C on the transformer is the return path for the control's logic board. And that is how it is set up.   

In order to make the Terminal 9 and terminal 7 complete the circuit to the relay coil between 7 and 8 on the Tekmar the R to W on the thermostat must be the wires that are connected to the 7 and 8 on the Tekmer.  This can only be done if the C on the thermostat is connected to the R on the transformer.  And R on the thermostat is connected to C on the transformer.  As far as the Thermostat is concerned the power for the logic board inside the thermostat is starting out at C on the transformer thru R on the thermostat and the return path is C on the thermostat back to R on the transformer.  (Since this is alternating current, there is no positive and negative to be concerned with)

You must have a dedicated transformer on the Tekmar to Thermostat circuit because the transformer on the boiler may cause some interference with the phasing of the transformer to the boiler chassis, especially if the transformer is also phased with C on the low voltage side to N or L2 on the source side.