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. 

sopcopotoptop

neilc, yes, the boiler ran just fine with the NEST thermostat. There just seems to be some conflict between the NEST thermostat and the Tekmar ODR.

Ed, you helped me with a very fine diagram some months ago to connect the ODR to my boiler. The only mistake I made was using the ‘G’ terminal on the 24V transformer instead of the ‘C’ terminal for the common connection for the power to the ODR and white wire of the NEST thermostat.

And I am using the 24V transformer on the boiler and not a separate one. No separate zoning. One zone, whole house.

I’m beginning to think it’s the “logic” between the thermostat and the ODR that are conflicting. The ODR is attempting to maintain a constant 70 F degree temperature based on its algorithm while the NEST is running its own program based on its ability to adapt to meet the temperature needs. Maybe I just need to go back to a simple thermostat and set it at 70 and try to balance the ODR to maintain that temperature.

neilc

follow Ed's diagram and be sure the R, C, and W are correct between the pest and tekmar

EdTheHeaterMan

@sopcopotoptop. you will need to purchase a separate transformer to power the thermostat and the Tekmar This is the only way i can see to keep the Tekmar and the Nest from interfering with the boiler transformer and possible burning it up as a result of creating a dead short.

Pay careful attention to the R and C being reversed on the Nest and not on the Tekmar. This part should do just fine for you https://www.supplyhouse.com/Functional-Devices-TR20VA001-Transformer-20VA-120-to-24-Vac-Foot-and-Single-Threaded-Hub-Mount

sopcopotoptop

Thank you again Ed and neilc for the advice. I’ll purchase the separate transformer and connect accordingly.

Ed, to answer your earlier question my boiler is a 30 year old Weil-McClain, HE-6. It has a replacement Honeywell S8910U controller instead of the original Fenway controller. It just keeps chugging along.

Jamie Hall

Ah… a useful bit of apparent trivia. "I’m beginning to think it’s the “logic” between the thermostat and the ODR that are conflicting. The ODR is attempting to maintain a constant 70 F degree temperature based on its algorithm while the NEST is running its own program based on its ability to adapt to meet the temperature needs."

Whatever else may be wrong, that surely is. You can either have one — ODR — or the other — the Nest chasing the occupancy and trying to adapt to who's where when. Of the two, on a hydronic or steam system, the Nest algorithm is much the worst. It will try to adapt, but the system can't respond fast enough and things go completely bats.

For starters, get deep into the programming of the Nest and turn off every single adaptive or scheduling feature there is Defeat everything, including scheduled setbacks (those don't play nice with ODE).

I have nothing against the Nest — and similar — for forced air heat, which is what they are meant for. In my not so humble opinion, they are useless for anything else. If you insist on setbacks (as I say, not compatible with ODR), then get a simple programmable. Otherwise get a simple thermostat, set it, forget it, and let the ODR do its thing. 

sopcopotoptop

Update: Followed Ed’s wiring diagram and installed a separate transformer for the NEST and ODR. Now the thermostat called for heat and signals the ODR to start the boiler. Seems to be operating normally in that fashion now.

I also Jamie Hall’s advice and dumbed down the NEST to a simple programmable thermostat. Will negate the seat backs as well and let the ODR take care of that.
Thanks again for the great help and advice! Really want to dial the ODR in for comfort and stability once the real winter weather sets in.