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Nest thermostats with 2-wire installations

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Alan (California Radiant) Forbes
Alan (California Radiant) Forbes Member Posts: 4,109
edited June 2023 in Thermostats and Controls
I've had good luck installing Nest thermostats on jobs that have only two wires going to the thermostats by running a resistor from the "C" terminal to the "W" on Taco zone valve controls. As I understand it, this will allow a trickle of current back to the thermostat for charging.

Taco supplies 1,000 ohm, .5 watt resistors with the ZVC line, but they recently recommended using 220 ohm, 5 watt resistors instead which I used on the last job. Both resistors work fine, but the smaller ohm resistor gets very hot. I'm inclined the use the 1,000 ohm resistor because of this heat problem, but imagine that it takes longer to charge the thermostat with the greater resistance. Any thoughts?
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab

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  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,176
    edited June 2023
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    I've had good luck installing Nest thermostats on jobs that have only two wires going to the thermostats by running a resistor from the "C" terminal to the "W" on Taco zone valve controls. As I understand it, this will allow a trickle of current back to the thermostat for charging.

    Taco supplies 1,000 ohm, .5 watt resistors with the ZVC line, but they recently recommended using 220 ohm, 5 watt resistors instead which I used on the last job. Both resistors work fine, but the smaller ohm resistor gets very hot. I'm inclined the use the 1,000 ohm resistor because of this heat problem, but imagine that it takes longer to charge the thermostat with the greater resistance. Any thoughts?

    That is one way to save on fuel. Placing the resistor on the thermostat C to W will warm up the thermostat and keep it from calling for heat.

    I'm not sure that is what you are looking for Alan! LOL >:)
    We could always ask @pecmsg his thoughts.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    Solid_Fuel_Man
  • hot_rod
    hot_rod Member Posts: 22,497
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    Adding 5W resistors? Are you going to run out of transformer Va, running stats, resistors and zone valves?
    It’s tough getting any info from Nest on the current the stat pulls in recharge mode. I think it has been a moving target across the various versions of that stat.

    It would be nice to know the current draw of all the components to add up and see how much transformer you need.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SteveSan
    SteveSan Member Posts: 239
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    @Alan (California Radiant) Forbe We reached out to Nest for their recommendations on the correct resistor to use with their products and suggested to use a 220ohm and up to 5watt resistor. Our 1000ohm @ 1/2 watt can be use with all other t-stats.
    Alan (California Radiant) Forbes
  • pecmsg
    pecmsg Member Posts: 4,967
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    I've had good luck installing Nest thermostats on jobs that have only two wires going to the thermostats by running a resistor from the "C" terminal to the "W" on Taco zone valve controls. As I understand it, this will allow a trickle of current back to the thermostat for charging.

    Taco supplies 1,000 ohm, .5 watt resistors with the ZVC line, but they recently recommended using 220 ohm, 5 watt resistors instead which I used on the last job. Both resistors work fine, but the smaller ohm resistor gets very hot. I'm inclined the use the 1,000 ohm resistor because of this heat problem, but imagine that it takes longer to charge the thermostat with the greater resistance. Any thoughts?

    That is one way to save on fuel. Placing the resistor on the thermostat C to W will warm up the thermostat and keep it from calling for heat.

    I'm not sure that is what you are looking for Alan! LOL >:)
    We could always ask @pecmsg his thoughts.
    Why it is always the NEST? o:)
    EdTheHeaterMan
  • 109A_5
    109A_5 Member Posts: 1,388
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    Hello @Alan (California Radiant) Forbes
    With a heat only system without a C wire connected or that power adapter thing, I think the NEST does not do much charging of its internal battery during a call for heat. The higher the resistance of the added resistor the lower the supplemental charging current is during periods of no call for heat.

    During design days and near design days (when you need the heat the most) when the call for heat duty cycle is much higher will the proper state of charge of the internal battery be maintained ? What happens as the internal battery ages and its capacity diminishes ?

    With the C wire connected or possibly that power adapter thing these issues may be minimized.

    IMO the 220 Ohm is just a Band-Aid for a bad situation to try to maintain some reliability.
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    Alan (California Radiant) ForbesHomerJSmithhot_rod
  • HomerJSmith
    HomerJSmith Member Posts: 2,511
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    ...running a resistor from the "C" terminal to the "W" on Taco zone valve controls.
    I don't understand this.
  • mattmia2
    mattmia2 Member Posts: 9,974
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    With a 220 ohm resistor you're providing about 100 ma and losing about 2.5w in the resistor if it were directly across the 24v supply. The nest probably isn't anywhere near a direct short so it doesn't pull anything like that when it is charging. When there is a heat call the thermostat connects the resistor across the transformer, at that point you will get about 2.5 w dissipated in the resistor.
  • 109A_5
    109A_5 Member Posts: 1,388
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    Hello @HomerJSmith,

    ...running a resistor from the "C" terminal to the "W" on Taco zone valve controls.
    I don't understand this.
    The NEST needs to receive power for its operation and charging of its internal battery. The R and C wire are obviously the best way. Usually a constant 24 VAC this way.

    Additionally the NEST can receive power parasitically with other wires like R and W when the heating system is idle. Adding the resistor supplements the current the NEST can parasitically draw from a R and W wire only type hookup. The added resistor is at the heating equipment not at the NEST.

    Since with a two wire R and W NEST hookup there is a parasitic current draw, (an old school thermostat just opens a switch) when not calling for heat the resistor helps keep the heating equipment in the off state too. Parasitic current draws can cause odd or unexpected equipment behavior.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    Alan (California Radiant) Forbes
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,846
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    There are plenty of band aids for the nest and that's all they are, they will fail need a common and a separate relay to be reliable. All these work arounds are junk. A lot of controls don't have the va needed.

    How many times have we read that on a unit's instruction .....the warning about not adding load to the mfg supplied transformer. They seldom put in a trans that is too big
    pecmsg
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    I've installed at least 50 "smart" and wi-fi thermostats. Every single installation has involved a C wire. If there are no extra conductors, a new 22/4 is pulled. I keep RIBU1C and 20va transformers in the van just for this purpose. 

    Anything less is a hack
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    pecmsgHomerJSmith
  • HomerJSmith
    HomerJSmith Member Posts: 2,511
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    I always use 18/3 or 4conductor.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,176
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    I've installed at least 50 "smart" and wi-fi thermostats. Every single installation has involved a C wire. If there are no extra conductors, a new 22/4 is pulled. I keep RIBU1C and 20va transformers in the van just for this purpose. 

    Anything less is a hack

    I have heard that anyone that uses anything thinner than 18 gauge solid wire for a thermostat control system is a Hack. But we all have different teachers.

    From the Pot calling kettle black department

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    GGrossSolid_Fuel_Man
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,846
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    24v will give you a lot more voltage drop...because there is less voltage...less emf, so wire size is more important.

    people tend to think that "its just 24 volts" or "its only low voltage" so they think wire size does not matter.
  • Jamie Hall
    Jamie Hall Member Posts: 23,668
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    Remember Ohm/s Law? Well... to deliver the same power at 24 volts as you might at 120 takes 5 times the current (power equals voltage times current). But -- the loss in the wire is proportional to the square of the current, not the current itself -- so at 24 volts you lose 25 times the power in the wire as you do at 120.

    Still want to use itty bitty wires?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,846
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    That's right @Jamie Hall you said it better than me stumbling over my words
  • Alan (California Radiant) Forbes
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    I’ve seen 18 gauge wire used as line voltage to a 007 pump. The wire wasn’t even warm. Imagine my surprise. 
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    JohnNY
  • pecmsg
    pecmsg Member Posts: 4,967
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    I’ve seen 18 gauge wire used as line voltage to a 007 pump. The wire wasn’t even warm. Imagine my surprise. 
    And what does a 007 draw? 1 amp!

    NOTE : not saying it’s allowed, safe or advisable!
  • mattmia2
    mattmia2 Member Posts: 9,974
    edited June 2023
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    pecmsg said:



    I’ve seen 18 gauge wire used as line voltage to a 007 pump. The wire wasn’t even warm. Imagine my surprise. 

    And what does a 007 draw? 1 amp!

    NOTE : not saying it’s allowed, safe or advisable!
    It may be allowed as long as it is 120v or more wire and in raceway. You would have to look at the tap rules.
  • HomerJSmith
    HomerJSmith Member Posts: 2,511
    edited June 2023
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    A wire's current carrying capacity is dependent on size and length (distance), generally. Heat also impact capacity, too.
  • pecmsg
    pecmsg Member Posts: 4,967
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    mattmia2 said:
    I’ve seen 18 gauge wire used as line voltage to a 007 pump. The wire wasn’t even warm. Imagine my surprise. 
    And what does a 007 draw? 1 amp!

    NOTE : not saying it’s allowed, safe or advisable!
    It may be allowed as long as it is 120v or more wire and in raceway. You would have to look at the tap rules.
    Last time I check wire size is determined by breaker size.

    except for condensing units!
  • Jamie Hall
    Jamie Hall Member Posts: 23,668
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    18 gauge has a maximum rating of 14 amps. unless it is a flexible lead for a disconnectable fixture, it must be fused for that -- or less -- regardless of the voltage.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattmia2
    mattmia2 Member Posts: 9,974
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    Feeder Tap Rules
    10-Foot Feeder Tap Rule [240.21(B)(1)]
    Feeder tap conductors can be run not over 10 ft without overcurrent protection at the point they receive their supply, but they must be installed in accordance with the following requirements: Figure 1
    (1) The ampacity of the tap conductor is:
    1 Not less than the computed load in accordance with Article 220, and
    2 Not less than the rating of the device supplied by the tap conductors or the overcurrent protective device at the termination of the tap conductors.
    (2) The tap conductors do not extend beyond the equipment they supply.
    (3) The tap conductors are installed in a raceway if they leave the enclosure.
    (4) The tap conductors have an ampacity of no less than 10 percent of the ampacity of the overcurrent protection device from which the conductors are tapped.
    hot_rod
  • Jamie Hall
    Jamie Hall Member Posts: 23,668
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    One thing to sort of remember -- most heating equipment thermostats are hooked up to 24 volt circuits powered by low power (perhaps 2 amp max, maybe 3) transformers. This is sort of a safety... sort of... as overloading the transformer will let the smoke out of it, long before the wire gets too hot. Not quite sure whether that's a good thing...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,176
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    @mattmia2 This sounds like you are just tap dancing around the issue here!

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    And the .03 amps it takes to run a 24volt relay coil. I've never had voltage drop issues with that small wire. We are not talking about the old Taco heat motor heads here. 

    I always use a RIB as a set or dry contacts unless there is a switching relay already present. 

    Funny story, a few years ago I was called to a trouble with a ski tow at a local skiing facility. The 50HP motor at the bottom of the hill wouldn't run. It was 480V 3ø. Run of a soft start and a simple control system. There were 12 towers going up the hill about 200' apart for a total of abouy 2500 feet top to the peak of the hill. There is a 24volt DC positive wite which goes through 2 safties on each pole and several Emergency stops at the top of the hill. So a total of at least 5500 feet of wite. This was 18 gauge phone cable. After fixing some poor connections I had 16 volts coming back! 

    Think about that. I eneded up actually using a RIBU1C as it didn't care what the voltage input was as long as it was above 12VDC. 

    Last summer I put this in with an individual LED indicator for each tower. Some volunteers ran a 14 pair phone cable and we used a pair for each tower. I'm pretty sure it was 22 gauge or smaller. 



    Voltage drop is only dependent on the load. And most low voltage wiring is only running signals and very small relay coils. 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Jamie Hall
    Jamie Hall Member Posts: 23,668
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    Neat and clever!

    Thing is, though, you, @Solid_Fuel_Man , are clearly aware of how much current you are playing with -- in this case, very little -- and details like voltage drops and how much voltage is necessary to push enough current through a relay coil to make it operate (and coils operate on current, not voltage, by the way -- though they are rated in volts... longish story). Not everyone is. And solid state devices like soild state relays) are trickier -- and they operate on voltage, rather than current, unlike coils... just to add to the confusion (which, to go off on another tangent, is why the bleeder resistor trick for wi-fi thermostats works with some types of control devices, and not with others... another longish story).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Solid_Fuel_Man
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,846
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    @Solid_Fuel_Man

    Voltage drop isn't just dependent on load. Wire size and voltage have everything to do with it as well as load.

    the problem wit the Taco pump that @Alan (California Radiant) Forbes mentioned is that if it is bell wire thats not rated 600v which most all building wire is.

    anything smaller than #14 can only be used for line voltage under certain circumstances, Fixture wires, alarms and signal wires and some controls where the power is limited.