Multiple Low-Voltage Transformers on HVAC Systems

When to remove the thermostat Rc Rh jumper wire
Wiring diagram for two transformers, A/C & Heat
Which transformer powers the C-wire?
How many devices can one 24V transformer support?

Thermostat Jumper Wires vs Number of Power Transformers

When do we remove the R-Rc Jumper in a room thermostat? What about when we're using two or more separate 24VAC transformers, for example one for heating and one for cooling?

Bottom line: in a heating and cooling set-up that uses one single 24VAC power transformer to power thermostats and turn either heating or cooling on or off, leave the Rh Rc jumper in place.

If your system uses two separate 24VAC power transformers, one to power the heating system and a second to power a cooling system, remove the jumper and connect the cooling power transformer to Rc and the heating power transformer to Rh.

If we are considering a 2-wire thermostdat hook-up then we know it is able to control just one piece of equipment, either a single heater or a single air conditioner. In this case there will be just one power transformer powering the thermostat and the equipment that it is to turn on or off.

If we are discussing a 3-wire thermostat hook-up we're controlling both heating and cooling. In those installations there may be either one or two power transformers that operate the the thermostat and switch heating or cooling on when required.

1. Single 24VAC power transformer:

If your old thermostat had a with a jumper wire or clip between the thermostat Rh and Rc terminal and did not provide a simple or single R terminal, the jumper or clip assured that the single 24VAC power transformer was used to power both heating (Rh) and cooling (Rc).

2. Two 24VAC power transformers:

If your HVAC system uses two separate power transformers, one to control heat and a second to control cooling equipment, then you would remove the jumper or clip between Rh and Rc and instead connect the heating power transformer to Rh and the cooling power transformer to Rc.

Below this wiring schematic by Bob Scaringe at QwikProducts [Minor edits for clarity by us] explains in detail how two power transformers are wired in a typical HVAC system. The Rc to C jumper provides power to the thermostat itself.

The room thermostat acts as an on-off switch to operate HVAC equipment power on-off relays such as those shown in Bob's schematic of a two transformer basic circuit.

Watch out: as you might infer from the schematic above, if you simply connected both power transformer red wires together into an "R" terminal on a thermostat, IF both 24VAC transformers happen to be wired to different building power phases, you may see 48 volts instead of the desired 24Vac on the circuit - risking damge to the equipment! DO NOT connect the outputs of the two transformers together!

Notice that in this drawing that while there are two R-terminals, Rh and Rc, there is onlhy one C terminal in the thermostat.

That's all that's needed for the thermostat to act as a switch. By convention, you might run a single wire INTO the C-terminal, usually from the cooling system's 24VAC transformer - the right hand of the two transformers in Mr. Scaringe's diagram.

References

• Bob Scaringe, Ph.D., P.E., Thermostatic Wiring Principles [PDF] (2011) QwikProducts by Mainstream Engineering, Rockledge FL, Tel: 800-866-3550, Website: http://www.mainstream-engr.com/, retrieved 7/17/2011, original source http://www.epatest.com/store/resources/images/misc/how-a-thermostat-operates.pdf, [copy on file as how-a-thermostat-operates.pdf ]

How many devices can a standard 24VAC transformer support?

Our reader asked:

I have a 40 year old house that has been remodeled over the years with additional multiple heating zones, modernized thermostats with wireless computer access, intercom, etc.

My 24vac control transformer has started buzzing very loudly and I plan on replacing it.

With this new demand on the 24vac side should I consider increasing the existing transformer capacity of 40VA to something larger? Is there a way to determine the output capacity of the transformer? 2016/03/16 Peader

Reply:

Thanks for the question Peader.

For other readers, as used here, 24VAC or 24Vac usually means that we're referring to a 24-volt alternating current low voltage transformer such as those used to power room thermostas and switches that turn heating and cooling systems on or off.

In other texts, 'VA' stands for Volts x Amps. Volts multiplied by Amps is also the classic definition of a Watt.

Watch out: If you hook up more devices drawing more current than a transformer can provide, it'll burn up - fail.

Also to be clear for other readers, the OUTPUT VOLTAGE of a replacement transformer should not be changed - that is if you are running 24VAC devices you want 24VAC supplied by the transformer. You wouldn't change to a 12VAC transformer (for example).

Here is a typical 24VAC transformer product specification using Robertshaw data as an example:

Class 2 Transformer, 40 VA Rating, Input Voltage 120VAC, Output Voltage 24VAC, Multi Mounting, Protective Device Fuse, Primary Connection 8 In. Leads, Secondary Connection 8 In. Leads, 1 Phase, 50/60 Hz, Not Rated Temp. Rise, Open Enclosure Type, Enclosure Rating NEMA 1, Enclosure Finish Bare Steel, Height 2.75 In., Depth 3.25 In., Width 2.44 In., Standards UL 1585 File Number E110657, E92807, E99697, CSA File Number LR107325-1, NEMA

A 24VAC transformer rated for 40VA (volt-amps) means that it can support 40 / 24 = 1.6A or 1.6 amps of current draw (load). That is its output capacity and the answer to your question.

Add up the current requirements (amps) of the devices you want to support. E.g. one zone valve might want 0.3 Amps. so the most you'd put onto one 40VA transformer would be 4 such zone valves - though I've seen a few working installations at which five or even six zone valves on a single transformer.

The VA rating of the transformer should be greater than the load of all the devices connected to it.

So how did they get away with wiring so many zone valves onto one transformer? Luck. Usually not all of the zone valves are being turned "on" by their respective thermostats at the same moment.

During activation the zone valve motor runs for less than a minute.

But eventually, on a very cold night, and with bad luck, all five or six thermostats try to turn on all five or six zone valves at the same time. Maybe the homeowners came back to a cold house and ran amok turning up all of the thermostats in the course of a minute. Hiss. Buzz. Dead transformer.

Typically an older wiser, HVAC installer adds additional transformers rather than installing one giant one serving everything in the house.

That can also avoid having to snake long runs of low voltage wire - depending on where your various devices are. For example an installer hooking up 6 zone valves would install two 24VAC 40VA transformers and wire 3 zone valves to each.

I'd guess that your wireless computer access is talking to a wireless router that is plugged into a 120VAC wall receptacle so that ought not be a concern.

I've hooked up three NEST thermostats in a test building, all working just fine off of the original transformer that originally supported the old Honeywell round units.

See NEST LEARNING THERMOSTATS - the manufacturer probably took care to be sure that their new product would work in older homes without requiring homeowners to change out their thermostat transformer or add additional ones.

Usually an intercom system has its own separate transformer as does a traditional home doorbell and as does the receiver for a remote operated garage door operator.