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Wiring for Nest

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Just finished adding two Nest thermostats to an old SR506-2 pump control. Taco says that the onboard transformer can handle up to two, but more Nest stats will come online later, so I’ve wired a larger transformer in series to handle the future load. 

My question is, once I’ve charged the “R”  terminal with the additional transformer on the first thermostat, do I have to do the same on all the others or have I now increased the energy available for all of them?


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

Comments

  • hot_rod
    hot_rod Member Posts: 22,327
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    Does the 506 have two 40va transformers? If so I think they are split 3 on one transformer 3 zones on the other.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    HomerJSmith
  • Alan (California Radiant) Forbes
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    There’s just one transformer on the pump controls. 
    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
  • Jamie Hall
    Jamie Hall Member Posts: 23,525
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    I hope you wired them in parallel, @Alan (California Radiant) Forbes ! But if they really are in parallel they will act as a single power source.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Alan (California Radiant) Forbes
  • HomerJSmith
    HomerJSmith Member Posts: 2,476
    edited November 2022
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    Ganging class 2 transformers, the transformers secondaries need to be wired in polarity (phase). The easy way to determine if the transformers secondaries are correctly wired together is to take one secondary of each transformer and tie them together. Take the other secondary of each transformer and strike them together quickly and they will spark. Reverse one secondary lead and connect them together again and strike the other untied leads which will spark. The connection that produces the smallest spark is the connection that is in the correct polarity.
  • hot_rod
    hot_rod Member Posts: 22,327
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    I'd use the @EdTheHeaterMan fuse trick when wiring transformers together
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,074
    edited November 2022
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    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
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    edited November 2022
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    Here's what I was taught...
    There is no hot or ground on the secondary side of a transformer. It is floating until one side is grounded.
    Ground one side of the secondary, measure L1 to R, should read about 96v.
    If it reads 144v, then you have the wrong side of the secondary grounded.



    There was an error rendering this rich post.

    hot_rodEdTheHeaterManHVACNUT
  • ratio
    ratio Member Posts: 3,674
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    Phasing single phase transformers is easy. Connect the common of both transformers together & measure the voltage between the two Rs. If it's zeroish (2-4 volts is common) they're in phase, if it's 50ish volts (2x nominal 24 VAC) they're out of phase. You shouldn't need to involve the primaries unless it's three phase. (Phasing transformers on 3 phase involves getting onto the right primary too. It does make a difference.)
    Alan (California Radiant) Forbes
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,074
    edited November 2022
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    I'd use the @EdTheHeaterMan fuse trick when wiring transformers together

    Yeah, I liked ETHM’s trick as well. I went the easy route with a circuit breaker on the transformer. 

    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
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,153
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    hot_rod said:

    I'd use the @EdTheHeaterMan fuse trick when wiring transformers together

    There is a minimum service charge for that idea. $2.50 for the fuse or $4.99 for the circuit breaker.

    Edward Young Retired

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

  • pecmsg
    pecmsg Member Posts: 4,909
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    I'd use the @EdTheHeaterMan fuse trick when wiring transformers together
    There is a minimum service charge for that idea. $2.50 for the fuse or $4.99 for the circuit breaker


    Operators are standing by. 
    place your order in the next 10 minutes and we’ll ship 2 for the price of one. Just pay the extra shipping and handling. 
    EdTheHeaterMan
  • hot_rod
    hot_rod Member Posts: 22,327
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    There will come a time when the Caleffi switching relays will come with a snap in 40Va, a second 40 can be snapped in. Both circuit breaker protected. Gone will be the small 15 Va transformer populated on the board.

    Plenty of power for stats and light arc welding projects :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    STEVEusaPAHomerJSmithHVACNUT
  • Alan (California Radiant) Forbes
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    So, when I buy a transformer with the primary wiring hot and neutral (black and white) and the secondary wiring marked "R" and "C", can't I assume that it's properly phased?

    And when I wire it in parallel to the transformer on the SR control, can't I assume that it is properly phased as well since the terminals are marked "R" and "C"?
    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
  • ratio
    ratio Member Posts: 3,674
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    IF the transformers are the same mfgr, it's probably a safe assumption that they're the same polarity so that wiring the primaries & secondaries in parallel will be uneventful. It's not a given that any old transformer will be in phase with any other. Transformers are inherently uncaring about things like polarity and even power input—a 120:24 transformer will happily (mostly, there is some risk of the insulation breaking down at the higher voltage) give you 600 volts out if you feed the 24 volt side with 120 volts.
    Alan (California Radiant) Forbes
  • Jamie Hall
    Jamie Hall Member Posts: 23,525
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    So, when I buy a transformer with the primary wiring hot and neutral (black and white) and the secondary wiring marked "R" and "C", can't I assume that it's properly phased?

    And when I wire it in parallel to the transformer on the SR control, can't I assume that it is properly phased as well since the terminals are marked "R" and "C"?

    No. Not only is electricity colour blind, it's illiterate. As someone once said -- "trust, but verify".
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,218
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    Also, be aware that Honeywell has 120/24 transformers out there now where the neutral is black and the hot is white.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,153
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    Also, be aware that Honeywell has 120/24 transformers out there now where the neutral is black and the hot is white.

    We are all trying to get along in this new politically correct world. Nice to know that Honeywell is allowing the roll reversal of the black and the white wires.

    Edward Young Retired

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

    Alan (California Radiant) Forbes