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# transformer voltage(s)

Member Posts: 308
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?

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

• Member Posts: 5,982
They’re listed at 24, but can always test a little higher if good. Lower, bad
steve
• Member Posts: 18,268
edited August 2019
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
Living the hydronic dream
• Member Posts: 13,150
@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.
• Member Posts: 3,571
As the incoming fluctuates so does the output.
110 Volts + 15 % = 127
24 Volts + 15% = 28
• Member Posts: 13,905
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
• Member Posts: 20,500
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.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 308
edited August 2019
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?
• Member Posts: 308
what VA rating would be appropriate to run 2 taco 570 (gold) heater zone valves?
• Member Posts: 20,500
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.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 18,268
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
Living the hydronic dream
• Member Posts: 157
@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.