How much does a transformer cost?
$189–$320 installed. The transformer itself is $25–$60; the rest is labor and the time to verify why it failed in the first place.
San Antonio · Transformer Replacement Repair · Family-Owned Since 1996
Your Thermostat Is Dead And The System Won't Respond. It's Usually The 24V Transformer.
The transformer steps your 120V house power down to 24V to run the thermostat and control circuit. When it fries, nothing in your HVAC system will turn on — but it's an inexpensive fix.
Typical price
$189–$320
Time to fix
45–75 minutes
Urgency
High — system is fully down
In plain English — what's going on with your transformer replacement
Every furnace and air handler has a small transformer that converts your house's 120V down to the 24V used by your thermostat and all the control wiring. When it dies, your thermostat goes dark, nothing responds, and it can look like the whole system failed.
Common failure causes: a shorted thermostat wire (the most common — a wire grounded out somewhere and burned up the transformer), an overheated control board pulling too much current, or simple age (transformers last 15–25 years usually).
Replacement is one of the cheaper fixes — $189 to $320 — but the real work is finding why it died. If you just swap the transformer without finding the short, the new one fries in minutes. We always meter the secondary side for shorts before installing a new transformer.
What it costs in San Antonio
$189–$320 — Replacement is quick. Tracing the original short adds time on some calls. Our $59 diagnostic is waived when you approve the repair, so you only pay for the fix itself.
Symptoms you'll notice at home
If two or more of these sound familiar, it's probably your transformer replacement.
Thermostat screen is blank or unpowered
No response from the system to any thermostat command
Burnt smell from the air handler or furnace cabinet
Visible scorch marks or melted plastic on the transformer
Just had a new thermostat installed and now nothing works
Outdoor unit comes on but indoor blower doesn't (or vice versa)
Why transformer replacements fail in San Antonio
- Shorted thermostat wire — most common, usually staples through wire insulation in the wall.
- Failed control board pulling too much current.
- Bad zone control board overloading the transformer.
- Improper thermostat wiring during a recent install (R and C swapped).
- Lightning surge through the low-voltage circuit.
How we diagnose and replace it
Here's exactly what a Carnes and Sons tech does at your house — no mystery, no upselling.
- 1
Check primary and secondary voltage
Primary side should read 120V. Secondary should read 24V. No primary means a fuse/breaker issue, no secondary means dead transformer.
- 2
Find the short before replacing
We disconnect the secondary leads and test resistance to ground on each thermostat wire color. If anything reads short, we trace and fix it FIRST — otherwise the new transformer dies immediately.
- 3
Install matched VA rating transformer
Modern systems with ECM blowers and zoning often need 50–75 VA transformers, not the old 40 VA. We size it right for what's hooked to it.
- 4
Verify all 24V loads
Test thermostat, contactor, gas valve, inducer, ignition — everything that pulls on the 24V circuit — to confirm nothing else is wrong.
What Your Neighbors Are Saying
Get Your Transformer Replacement Diagnosed Today
Tell us what's happening. We'll text you back in minutes — no call center, no robo-calls.
Transformer Replacement questions San Antonio homeowners actually ask
Why did my transformer go bad?
Almost always a shorted thermostat wire somewhere, or a failing component pulling too much current. We always find and fix the root cause — swapping the transformer alone means it'll just fail again.
Can I replace the transformer myself?
Possible if you're comfortable with line voltage and meter readings. Risks: shock hazard from 120V, installing without finding the underlying short (new transformer fries instantly), wiring it backward and damaging the control board. Cheap fix, but easy to make expensive.
How long does a transformer last?
15–25 years normally. If yours died sooner, something else in the system is overloading it and needs investigation.
Will my thermostat work right after?
Yes — once the 24V is back, the thermostat lights up immediately and the system responds to commands. We test cool, heat, and fan-only before leaving.
Is there a fuse on the transformer?
Many control boards have an inline 3A or 5A automotive-style fuse on the 24V side. Sometimes that's all that's blown ($4 fuse fix) and the transformer is fine. We always check that first.
What if you find the short isn't fixable?
Rare — almost every short is traceable to a specific wire that we can repair or rerun. If a wall-buried wire is unreachable, we'll run a new low-voltage line along the easiest path. Always fixable.
Could a power surge cause this?
Yes — lightning especially. After a surge-related transformer failure we recommend a surge protector. $385 to install one prevents recurring damage to transformers, boards, and motors.
Serving transformer replacement repairs across the greater San Antonio area
We're based in Stone Oak and roll trucks across San Antonio and the surrounding Hill Country every day.
Carnes and Sons Air Conditioning
License TACLB29435E · Family-Owned Since 1996
401 E Sonterra Blvd Suite 375
San Antonio, TX 78258(210) 600-5091
San Antonio, TX 78258(210) 600-5091
Mon–Sat: 7AM – 8PM · Emergency same-day service
Cities & neighborhoods we serve
San Antonio · Stone Oak · Alamo Heights · Hollywood Park · Hill Country Village · Shavano Park · Castle Hills · Leon Valley · Helotes · Boerne · Fair Oaks Ranch · Bulverde · Timberwood Park · Garden Ridge · Schertz · Cibolo · Selma · Universal City · Live Oak · Converse · Windcrest · New Braunfels
Talk to a real person about your transformer replacement
No call center. The owner or one of his sons picks up.
(210) 600-5091