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transformer voltage(s)

ronron Posts: 119Member
what are the voltages used for the [low voltage] transformers used in HVAC that wire to thermostats and taco zone valves or other equipment? Are they all "24VAC" or are there also 26, 28, 30, 40... volt ones?

I am NOT asking about the VA (volt-amp) rating.

In probing various places with a meter, I am reading 26 to 30 volts I'm guessing induced voltage?
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Comments

  • STEVEusaPASTEVEusaPA Posts: 2,979Member
    They’re listed at 24, but can always test a little higher if good. Lower, bad
    steve
  • hot_rodhot_rod Posts: 11,608Member
    edited August 12
    24VAC is most common in the HVAC world. It is not unusual to see them read 30V. Depends on the load and input voltage also.

    You can purchase more accurate models but they can be $$
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,853Member
    @ron,

    24 volts is the standard "nominal" voltage. Normally it is considered ok to supply voltage to a piece of equipment that is 10% higher or 10% lower than the equipment nameplate.

    Transformers operate on the ratio of #number of turns of wire in the primary circuit to the number of turns of wire in the secondary circuit.

    So a transformer rated 120 volt primary and 24 volt secondary would have a wire ratio of 120 x 24. Feed it with 120 volts you should get 24 volts out of it. The 24 volts may read slightly higher, say 26 volts with no load on the transformer.

    If the transformer is rated 120 x 24 and the primary voltage is only 115 volts (about 4.5% low) then the secondary would be about 4.5% low or 22.9 volts.

    Overvoltage would do the same thing. A 120 x 24 fed with 125 volts would be about 25 volts on the secondary.
  • pecmsgpecmsg Posts: 797Member
    As the incoming fluctuates so does the output.
    110 Volts + 15 % = 127
    24 Volts + 15% = 28
  • ChrisJChrisJ Posts: 9,809Member
    I would expect the secondary to also vary with load.
    A transformer unloaded should read higher than one near it's maximum capacity.

    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • Jamie HallJamie Hall Posts: 10,548Member
    ChrisJ said:

    I would expect the secondary to also vary with load.
    A transformer unloaded should read higher than one near it's maximum capacity.

    To which I would add that generally they are supposed to be designed to deliver rated voltage at rated load, with the rated nominal voltage input. Small, cheap ones may droop a surprising amount between no load and rated load.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • ronron Posts: 119Member
    edited August 12
    i guess my question is... for "low voltage hvac transformers" they are all spec'd @ 24vac for the secondary, but any xfr < ~40vac secondary would likely operate any thermostat and heater zone valve just fine?

    i see at supply house the filter has 24vac, 26.5 v.o.c, 27v.o.c. and 27.5 v.o.c. voc = voltage open contact. everything in hvac is low voltage (~24vac) secondary transformers? and doorbells?
  • ronron Posts: 119Member
    what VA rating would be appropriate to run 2 taco 570 (gold) heater zone valves?
  • Jamie HallJamie Hall Posts: 10,548Member
    I see from your comment that you have a slight confusion between the voltage output rating (Volts AC) and the available power rating (Volt-Amperes) of these gadgets. Unhappily, they both are sometimes abbreviated as "VAC" which is no help; the Volt-Ampere -- power -- rating should be abbreviated VA.

    To directly answer one of your questions -- almost all HVAC controls are intended for a nominal 24 volts AC, unless they are specifically marked for "line" voltage -- that is a nominal 120 volts AC in North America. You can determine the power rating you need by determining the current draw of the device -- such as a zone valve -- and multiplying it by the rated voltage. But beware: some devices, such as some zone valves, are intended to operate on line voltage supplied through a relay from the low voltage control.
    Jamie



    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.



    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • hot_rodhot_rod Posts: 11,608Member
    ron said:

    what VA rating would be appropriate to run 2 taco 570 (gold) heater zone valves?

    Volts X Amps

    If the valve has a .9 amp rating X 24V = 21.6VA each
    So technically, by the numbers 2 valves on a 40VA transformer.

    Considered an intermittent load, you will usually see wiring diagrams showing 3 actuators wired to a 40VA transformer.

    If all 3 happened to call at the same time from a cold start condition, you would have 21.6 x 3 = 64.8
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • SteveSanSteveSan Posts: 21Member
    @Ron what VA rating would be appropriate to run 2 taco 570 (gold) heater zone valves?

    Taco says you can run 3 heat motor zone valves ( 570 series ) or 12 zone sentry's zone valves per 40va transformers.
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