What is an HVAC transformer? An HVAC transformer is the small step-down transformer that turns 120 or 240 volt line power into 24 volt control power for the thermostat, furnace board, gas valve and outdoor contactor. Goodman’s furnace unit is rated 40 VA, and Class 2 rules cap control transformers at 100 VA.
Key Takeaways
- Goodman’s furnace service manual specifies a step-down transformer, 120 volt primary to 24 volt secondary, rated 40 VA for heating and cooling models (Goodman RS6612022R1).
- A 40 VA transformer supplies about 1.67 A at 24 V. NEC Class 2 power sources are capped at a 100 VA nameplate (Sola/Hevi-Duty technical note).
- An ABB 2-pole definite purpose contactor with a 24 V coil pulls 30 VA at inrush but holds closed on 6.5 VA (ABB DP catalog).
- One Taco 571 heat motor zone valve is rated 0.9 A at 24 V, or 21.6 VA, so two of them exceed a 40 VA transformer on paper (Taco 571 spec).
- Resideo’s S9200U1000 universal furnace control accepts 19 to 30 Vac and guards the 24 V side with a 3.0 amp automotive fuse (Resideo 69-2075EF).
- Carrier’s 58MVP manual reads fault code 24, an open secondary fuse, as a short in low-voltage wiring (Carrier 58MVP).
What is an HVAC transformer?
An HVAC transformer is a step-down transformer that powers the 24 volt control circuit. Line voltage feeds the primary winding; the secondary delivers 24 VAC to R and C, which the thermostat, gas valve and contactor coil run from. Goodman describes its furnace unit as 120 volt primary to 24 volt secondary, 40 VA, with “ample capacity” for heating or cooling (Goodman RS6612022R1).
VA (volt-amperes) is the capacity rating; divide by 24 for secondary current, so 40 VA is 1.67 A. On a gas furnace the transformer feeds the integrated furnace control, which fuses the 24 V side and passes power to the thermostat terminals.
Control transformers are Class 2 power sources, which limits the energy reaching thin thermostat cable. Federal rules require a Class 2 source to be listed and marked, with its conductors kept out of power-wiring boxes unless a barrier separates them (29 CFR 1910.308(c)). The Class 2 nameplate ceiling is 100 VA under NEC Chapter 9 Table 11.
How much load can a 40VA HVAC transformer carry?
A 40VA transformer carries 40 VA of loads running at once, so add every 24 V device that can be energized together, at rated amps times 24. Single-stage heating and cooling fits easily; zone valves and humidifiers are what push it past the rating.
| 24 V load (maker document) | Rated draw | VA at 24 V |
|---|---|---|
| Resideo S9200U1000 furnace control, own draw | 0.5 A | 12 VA |
| ABB DP 2-pole contactor, 24 V coil, pulling in | 30 VA inrush | 30 VA |
| Same ABB contactor, held closed | 6.5 VA sealed | 6.5 VA |
| Aprilaire 600 bypass humidifier solenoid | 0.5 amp | 12 VA |
| Taco 571 heat motor zone valve | 0.9 A | 21.6 VA |
| Gas valve on the S9200U1000 (board’s ceiling) | 1.5 A or less | Check nameplate |
Worked example, cooling call: the S9200U1000 board (12 VA) plus a held-in contactor (6.5 VA) totals 18.5 VA, under half the rating. Pull-in is larger: 12 plus 30 is 42 VA for the instant the outdoor contactor closes. The board figure is from Resideo’s S9200U1000 instructions.
Worked example, heating with a humidifier: board 12 VA plus an Aprilaire 600 at 12 VA leaves 16 VA for the gas valve and thermostat. Aprilaire allows a separate 10 VA minimum transformer, never wired into the blower circuit (Aprilaire 600 instructions), a point our humidifier installation guide covers.
Hydronic zoning is where the math bites: on nameplate ratings, two Taco 571 heat motors total 43.2 VA, past a 40 VA transformer. Our zone valve explainer compares motor and heat motor draws.
What are the symptoms of a bad HVAC transformer?
The classic bad HVAC transformer symptom is a dead control circuit: blank thermostat, no heat, no cooling, no 24 VAC at R and C. Goodman lists four other causes for that reading: an open fuse, bad wiring, a bad splice and an open door interlock switch.
- Nothing runs and the thermostat is blank. A wired thermostat that loses R and C goes dark, the same picture covered in our thermostat troubleshooting guide.
- A low-voltage fault code. Resideo’s S9200U1000 flashes 3+1 for “Low 24V” and tells the technician to verify voltage under every thermostat mode, including with the outdoor contactor pulled in.
- Cooling that will not start. ABB’s 24 V contactor coil has an 18 V maximum pickup voltage, so a sagging transformer may leave the outdoor unit idle; see why an AC will not turn on.
- An open-fuse code. Carrier’s code 24 and Resideo’s 3+2 both mean the 24 V fuse has opened, which points to a short downstream, not to the transformer.
How do you test an HVAC transformer?
Testing an HVAC transformer means reading AC volts at R and C, then at the primary leads if the secondary is dead. Goodman’s procedure says R to C “must read 24 VAC”, and Resideo’s board accepts 19 to 30 Vac. The primary check is a live line-voltage measurement, so it is technician work.
Safety: line voltage is present during this test
Goodman warns to disconnect all power before servicing wiring and that multiple power sources may be present. The reading is taken with the door switch closed and line voltage live. Without live-work training and equipment, stop at the fault code and call a licensed technician.
- Read and record the furnace board’s LED fault code first, then look it up on the door diagram.
- With power off, lift the thermostat wires from R and C at the board (our thermostat wiring reference shows the terminals). This takes field wiring out of the circuit.
- Restore power with the door interlock closed and measure R to C on the AC volts scale. Goodman expects 24 VAC.
- If R to C reads zero, measure at the transformer secondary leads. Goodman: 24 volts at the transformer but about 13 volts at R and C means a blown fuse.
- If the secondary is dead, a technician checks the primary for line voltage. Line voltage on the primary with none on the secondary means the transformer “is inoperative. Replace.”
- Reconnect the loads one at a time and re-measure in heat, cool and fan, as Resideo directs, to find the circuit that drags voltage down.
Why does a furnace transformer keep blowing?
A furnace transformer or fuse that keeps blowing has a short or overload on the 24 V side. Goodman says to replace the transformer only after checking for an electrical short, and Carrier reads an open secondary fuse as “a short in low-voltage wiring”. A new part on a shorted circuit fails again.
Correction: the second failure is not a bad batch
Repeat failures get blamed on cheap parts. The maker manuals say find the short: Resideo’s 3+2 code names limit, rollout and pressure switch wiring shorted to chassis, and a possibly shorted gas valve.
A 3.0 A fuse sits well above the 1.67 A full-load current of a 40 VA transformer, so it opens on a hard short but will not necessarily open on a steady overload between those currents. That overload, such as an added zone valve, heats the transformer instead. Fuse replacement is covered in our furnace control board article.
Work through the circuits the maker codes name: the safety chain through the limit switch, rollout and pressure switches, then the gas valve coil, then the thermostat and outdoor unit cable, which Goodman’s test isolates by lifting R and C.
If the line-voltage breaker trips instead, the fault is on the primary side; see why an HVAC breaker keeps tripping.
How do you match a replacement 24V transformer?
Match three things: primary voltage, VA and mounting. The new 24V transformer needs a primary tap for the supply voltage, a VA rating at or above the old one and the connected load, and a mount and lead style that fits the cabinet. Multi-tap units cover 120, 208 and 240 V in one part.
| Model (maker data) | VA | Primary | Mounting and leads |
|---|---|---|---|
| Resideo AT140A | 40 VA | 120, 120/208/240 or 208/240 Vac by variant | Multisystem mount; fixed 1/4 inch quick-connects |
| White-Rodgers 90-T40F3 (MARS 50354) | 40 VA | 120/208/240V | Foot mount, open construction, leads |
| White-Rodgers 90-T40M3 (MARS 50304) | 40 VA | 120/208/240V | Multi-mount with plate, leads |
| White-Rodgers 90-T75C3 (MARS 50321) | 75 VA | 120/208/240V | Foot mount, manual reset, leads |
White-Rodgers color-codes its leads: black is primary common, white is 120V, red is 208V and orange is 240V; on the secondary, blue is common and yellow is 24V (White-Rodgers catalog sheet). Use the tap matching the supply and follow the door diagram.
Resideo lists the AT140A as NEC Class 2 and UL 1585, identified Class 2 not wet, Class 3 wet (Resideo AT140A). A 75 VA Class 2 unit helps when a smart thermostat, humidifier and zone valves share one secondary. It does not fix a short.
How we researched this
Researched October 2026 from ten documents, all cited: Goodman, Resideo, White-Rodgers, ABB, Taco, Aprilaire and Carrier literature, 29 CFR 1910.308 and a Sola/Hevi-Duty Class 2 note. Maker figures are maker specifications, not our tests; VA totals are our arithmetic. Excluded: prices (no federal series), contractor blogs, and Reddit, because this is an electrical safety topic.
Frequently asked questions
What is the average cost to replace an HVAC transformer?
We publish no figure because no federal or national-lab price series exists. The cause matters more than the part: Goodman’s service manual says to replace the transformer only after checking for an electrical short, since a new transformer on a shorted circuit can fail the same way.
Where is the transformer in a furnace?
Inside the cabinet, wired to the integrated control behind the blower compartment door, which is where Goodman’s test procedure starts. Goodman tells technicians to consult the wiring diagram on that door for the 115 volt and 24 volt connections before touching any leads.
What does 40VA mean on a furnace transformer?
It is the transformer’s capacity in volt-amperes. At 24 V, 40 VA is about 1.67 A of continuous load. One Taco 571 zone valve at 0.9 A uses 21.6 VA, more than half of it, while a held-in ABB contactor coil uses 6.5 VA.
Can I install a bigger VA transformer?
Yes, if the primary voltage and mounting match and it stays Class 2, which caps the nameplate at 100 VA. White-Rodgers’ 90-T Class 2 line runs from 40 VA to 75 VA at 24 V. A bigger unit adds capacity for accessories; it does not cure a short.
What voltage should a 24V HVAC transformer read?
Goodman’s test says R to C must read 24 VAC. Resideo’s S9200U1000 control accepts 19 to 30 Vac and flashes code 3+1 for low 24 V, with an instruction to recheck the voltage while the outdoor contactor is energized.
Can a bad transformer stop the AC from turning on?
Yes. The outdoor contactor coil runs on the same 24 V secondary. ABB lists an 18 V maximum pickup voltage for its 24 V coil, so a dead or sagging transformer can leave the condenser idle while the thermostat calls for cooling.
Sources
- Goodman. “Service Manual RS6612022R1.” https://mobile.goodmanmfg.com/mobileapp/stellent/pdf/infoPdf/Lit/RS6612022R1.pdf. Accessed October 2026.
- Resideo. “S9200U1000 Integrated Furnace Control, 69-2075EF.” https://www.cochranesupply.com/media/assets/product/documents/Honeywell/S9200U1000.pdf. Accessed October 2026.
- Resideo. “AT140A1000 40 VA Transformer.” https://resideo.com/us/en/pro/products/gcs/transformers/at140a1000-40-va-multisystem-mount-transformer-at140a1000-u. Accessed October 2026.
- White-Rodgers. “Class 2 Transformers, 90-T Series.” https://media.onlinecomponents.com/productfiles/mf-wr/90-t40m1-white-rodgers-datasheet-128676267.pdf. Accessed October 2026.
- ABB. “Definite Purpose Contactors.” https://library.e.abb.com/public/790af83132a143be97b838939fa15958/1SXU100114C0201_Definite%20Purpose%20Contactors_DGT.pdf. Accessed October 2026.
- Taco. “571 and 572 Heat Motor Zone Valves, 100-3.1.” https://res.cloudinary.com/dzhpkjoqj/image/upload/Taco/Taco571-2-Spec_100-3.1. Accessed October 2026.
- Aprilaire. “600 Series Installation Instructions.” https://documents.alpinehomeair.com/product/Aprilaire%20600%20Series%20Installation%20Instructions%2001.08.pdf. Accessed October 2026.
- Carrier. “58MVP Instructions, page 19.” https://www.manualsdir.com/manuals/63165/carrier-58mvp.html?page=19. Accessed October 2026.
- eCFR. “29 CFR 1910.308.” https://www.ecfr.gov/current/title-29/subtitle-B/chapter-XVII/part-1910/subpart-S/subject-group-ECFR6e1d0b3d01a4c22/section-1910.308. Accessed October 2026.
- Sola/Hevi-Duty. “NEC Class 2 Power Circuits and Power Supplies.” https://hallaweb.jlab.org/tech/Detectors/public_html/hall_a/miscellaneous_info/sola_introNECclass2_rg.pdf. Accessed October 2026.
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