Problem guide
How to change a thermostat
Swapping a standard low-voltage thermostat is a common owner job. Line-voltage and heat pump wiring are where it goes wrong.
Get quotes Repair cost calculator
Written by HYRE HVAC Research Desk Primary-source research, data analysis and fact checking
The short answer
To change a thermostat, first confirm it is a low-voltage (24-volt) model, switch off power to the furnace or air handler at the breaker, photograph and label the wires, then move each wire to the matching terminal on the new thermostat. Check whether the new one needs a C-wire for power.
Stop and call a pro if the old thermostat is line-voltage (thick wires, often for baseboard heaters), if you have a heat pump with unclear wiring, or if the labels do not match.
Check before you buy or start
Most central HVAC thermostats are 24-volt, with thin, colored wires. Thermostats for electric baseboard heaters are often line-voltage (120 or 240 volts), with thick wires and wire nuts. Line-voltage swaps are electrical work; call a licensed electrician or HVAC contractor.
Many smart thermostats need a common (C) wire for steady power. Look for a wire on the C terminal, or a spare wire tucked in the wall.
Gas furnace and AC, heat pump, dual fuel or multi-stage equipment each need the right thermostat and settings. Check compatibility before buying.
Swapping a low-voltage thermostat
Switch off the breaker for the furnace or air handler (and the service switch). Confirm the old thermostat screen goes blank.
Take a clear photo of which colored wire goes to which letter.
Use the stickers that come with the new thermostat, by terminal letter, not by wire color. Colors are not always standard.
Keep the wires from falling back into the wall; wrap them around a pencil if needed.
Match each labeled wire to the same terminal. Follow the new thermostat’s instructions for heat pump O/B and C terminals.
Choose the right system type in setup (conventional or heat pump, stages, fuel). Test heating, cooling and fan.
Common terminal letters
| Letter | Usually means |
|---|---|
| R, Rh, Rc | 24-volt power from the transformer |
| C | Common, completes the 24-volt circuit for steady power |
| W, W1, W2 | Heat (first and second stage) |
| Y, Y1, Y2 | Cooling compressor (stages) |
| G | Fan |
| O/B | Heat pump reversing valve |
| Aux, E | Auxiliary or emergency heat on heat pumps |
Always follow the wiring diagram for your equipment and the new thermostat’s instructions; labels vary by maker.
Stop and call a pro when
That is a line-voltage thermostat. Electrical work.
A wrong connection can blow the furnace control fuse or damage the board.
Adding one may mean running a wire or fitting an adapter at the furnace control board, inside the cabinet.
O/B set wrong, or auxiliary heat miswired.
Is a new thermostat worth it?
DOE’s consumer guidance notes that setting temperatures back while you sleep or are away saves energy. A programmable or smart thermostat makes that automatic. Our thermostat savings calculator lets you estimate the saving for your own schedule and prices.
Get connected
Tell us about the work
Three short steps: the job, your home, and how to reach you. Nothing goes to an HVAC company unless you agree.
A person reads it and replies, free and with no obligation. Matching is not live yet, so this is not a quote. If you smell gas or a CO alarm sounds, leave the house and call 911 or your gas utility first.
Get HVAC Options
A person at HyreHVAC reads the request and replies. Matching with contractors is not live yet, so this is not a quote.
Questions
Can I change my thermostat myself?
What is a C-wire?
Do I need to turn off power to change a thermostat?
Why does my new thermostat not turn on the heat?
Will any thermostat work with a heat pump?
Written and audited by
HYRE HVAC Research Desk
Primary-source research, data analysis and fact checking
We are a research desk, not a sales floor. We read the federal microdata file, the statute or the manufacturer data sheet ourselves, and we publish the figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify.
The counts below are generated from the published pages themselves, last counted October 26, 2026, and they are what we have actually published rather than what we intend to.
- 18
- studies published
- 15
- federal sources read and cited
- 15
- studies published with their full dataset as CSV
- 51
- jurisdictions reproduced against EIA’s own tables
How this desk works
- Primary sources only. Federal data comes from the agency that collects it, in the file that agency publishes. We do not cite an article that cites a source; we download the source and compute the figure ourselves.
- We validate against the agency before we publish. First, we use each federal microdata file to reproduce the agency’s own published tables. Our cooling research reproduces EIA’s state estimates and standard errors for all 51 jurisdictions. That check caught a variance formula that was off by a factor of four.
- Every estimate carries its uncertainty. These are survey figures, not counts. Standard errors are computed from the replicate weights the federal file supplies and printed beside the estimate. An estimate too imprecise to publish is reported as such rather than printed.
- Nothing is typed by hand. Prose, tables and charts all read from one dataset built by script, so a number in a sentence and the same number in the table below it cannot disagree.
- We publish the data, not just the conclusion. 15 of our 18 studies offer the full computed table as a CSV download on the page, so you can check the analysis or disagree with it. A study without a row-level dataset gets no download link and claims none in its structured data.
- We correct in public. Where we have published a figure wrongly we fix the figure, rewrite any analysis that rested on it rather than patching the number underneath it, and leave a dated correction note on the page.
- We do not install or sell HVAC equipment, and we take no payment for placement, ranking or a favorable mention. Nobody buys a position on this site.
Data as of Sources checked October 26, 2026. Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
HyreHVAC is a publisher. We do not install, repair or inspect HVAC equipment and hold no contractor licence. Nothing here replaces an on-site diagnosis by a licensed contractor. Sources are cited with the date they were checked.