SMART Thermostats?


Recently, I had a call from a contractor, saying he was having issues with a unit making a lot of noise and the blower “bouncing off and on”.

The furnace had been in for a few years and, all of a sudden, developed this operating condition. We discussed the usual things to check, polarity, voltage, grounds, etc. I asked if anything new had been added or changed on the system? The only thing that had changed, as far as the contractor knew, was that the homeowner installed one of the new NEST Thermostats.  These are those fancy thermostats that give the customer access to it from their smart phones or the internet so they can “play” with the setting even when they are not home.

In the instructions that come with the new thermostat, it boasts No Problem Wiring To A  4 Wire System.  Well….. that is yes and no!

Depending on the control system in the equipment, it may work perfectly fine on the 4-wire hook-up. But, in some system it may need the C wire due to power stealing battery charging causing disruptions to the control programming.  What happens is, when the furnace is At Idle, it Steals MillIamps from the control board to keep it operating and charged.

Again, on some system, this may or may not be a problem.  Well, on the system our contractor was working on — it was a problem. When there was a call for Heat, and the thermostat was in a “charging mode”, it rattled the relays on the board, and the blower motor  bounced off and on and actually sounded like thunder through the duct work.

All I want you to be aware of is that the newest technology is a great thing and can make our lives easier but it can also create problems.  The solution here was simple, remove the “power stealing” aspect of the stat and “hard wire” it so it is powered and it worked just fine and so did the furnace and air conditioner it was intended to control.

(Thanks to John of Effective Air for providing the information for this post)

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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7 Responses to SMART Thermostats?

  1. Mo says:

    Does any one have experience with the nest t-stat and modulating furnaces?

    • Mo:
      My guess is it will cause the same problem with the Mod Furnace board as it did with the single stage board. If it is “hard wired”, it should not cause a problem. I have not had one yet with the Mod furnace.

    • Landon says:

      Yep, do yourself a favor and make sure you run that extra wire for C. We installed a York YP9C modulating with a/c and BOTH heat and cool would initialize when either one called at the Nest. Pain in the but and it says in their installation instructions that 99% of systems will work perfectly without the C wire. Whatever

  2. Bruce N says:

    Apparently the Nest promoters chose to mislead the public and contractors by ignoring what the HVAC industry learned 20 years ago!

  3. ThePilotman says:

    The “four wire” system the NEST thermostat instruction is talking about is NOT the 4 wire communicating (2 voltage / 2 data) format the York (or any other communicating type thermostat) is referring to. If one uses the current NEST v2 thermostat then you have the wire the thermostat system “the old way”. In speaking with fellow engineers at NEST, there are no plans (as far as I could get out of them) to mimic each different data format each respective furnace has. You can use a NEST, the biggest problem (if one wants to call this a problem) is that the NEST v2 cannot control the speed of the fan motor so when a circulating mode is selected the NEST v2 will turn the fan motor on or off. Not in variable speed mode as a respective brand communicating thermostat will.

    The NEST v2 is a GREAT thermostat and does save energy, (if the user knows how to program and “tweet” the operation) as it does things such as “automagically” reduce/increase temperature as the unit “learns” both the user occupation habits and manual adjustment habits.

    The average HVAC person doesn’t understand the software (and doesn’t need to) since the NEST v2 is far more complex than a “communicating” thermostat.

    I used the NEST v2 with an old 34 YO GE single speed furnace (the original one installed in my house when I had it built) and after 1 year of use, graphed (excel) both the gas and electrical usage and it saved about 20% in usage of energy (even in that “old dog” furnace) and the auto “away” features were fantastic.

    The modulating gas valve/variable speed fan combination in the new 98% eff furnace is nice but the variable speed motor appears to use an algorithm based on the position of the modulating gas valve (and temperature the thermostat calls) for efficiency.

    A two stage (blower motor) furnace works great with a NEST v2.

    If you use the NEST v2 with a modulating gas valve/variable speed motor combination, the furnace computer control will control the modulating gas valve/variable speed motor automatically. (Or that’s my take on it) IMHO this negates the extra efficiency one desires using s NEST v2.

    I heard NEST awhile ago released programming APIs and other info to encourage third party development of NEST apps. I hope there is a standard developed for a communicating thermostat protocol (standardization) and then maybe NEST (or other manufacturers) can develop a “SMART” thermostat to make energy use even more efficient.

    For more info, read the both the York installation manual for both the furnace and the thermostat. (I presume other HVAC manufacturers have the same information)

    Just my 2 cents.

  4. tony says:

    i have a York AFFINITY SERIES 8t CZH04812C ac unit and have ran the unit and gas furnace off a Honeywell pro 5000 t-stat and all was well. i bought a Nest (V3), home installed and wired it just like the Honeywell only i did not have to jumper the Rc off the R wire, Nest instructed me not to that the Rh and Rc are internally jumpier together. In testing the gas furnace worked fine but when testing the AC it would not come on, the fan would energize but the AC unit would not come on. i did get it to come on and it ran during the night but when cooling was called for after being off for several hours same problem, i called a HVAC company and the tech pulled off the cover to the AC unit and the control board was flashing red and yellow. he adjusted the pressure switch, the unit came on and again worked during the night but again after being off for several hours would not come on when cooling was called for. i reset the AC breaker to clear the fault but no go. i returned the Nest. but i have been thinking about the C wire and wondering if i just needed to pull a C wire. Can anyone help understand what might be going on here?

    • As my post states, some control boards require the C Common wire to function properly. York is one of those boards. Without the C, the thermostat is a “power stealing” stat and that can affect how the control board will work or not work. Since your air conditioner is set up for “communication”, you might look into either the York communicating stat or use the Source1 comm stat that is compatible with your unit

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