Reference & Specs
The verified numbers behind every diagnosis this tool gives. The AI is grounded in this same data, so you can check its work. One rule throughout: the data plate on your actual equipment always overrides a generic range.
Measurement 01
Temperature split (Delta T)
The most useful measurement a homeowner can take. Subtract supply air temperature from return air temperature after the system has run 15 minutes.
| Reading | What it means | Where to look |
|---|---|---|
| 16–22°F | Normal cooling split | No action needed |
| Below 16°F | Not removing enough heat | Low refrigerant charge, dirty condenser coil, failing compressor — pro diagnosis |
| Above 22°F | Not enough air across the coil | Dirty filter, dirty evaporator coil, closed vents, weak blower — start DIY |
Measuring it correctly
Keep the supply probe out of direct line-of-sight of the evaporator coil — radiant cold skews the reading low. High indoor humidity naturally pulls the split toward the bottom of the range; that alone isn't a fault. These targets assume roughly 400 CFM per ton of airflow; humid regions often commission at 350 CFM/ton, which shifts the targets.
Heating — temperature rise
Gas furnace temperature rise commonly falls in a 30–60°F band, but the correct range is printed on your furnace's data plate as a min/max (usually about a 30-degree spread), and that value governs. Rise above the plate maximum means restricted airflow and will eventually trip the high limit. Rise below the minimum means excessive airflow or an underfiring gas issue.
Regulation · Current as of 2026
The refrigerant change you need to know about
The biggest shift in residential HVAC in twenty years took effect this year, and most repair advice online hasn't caught up to it.
| Refrigerant | GWP | Safety class | Status |
|---|---|---|---|
| R-22 | — | A1 | Production and import ended 2020. Service only from reclaimed stock, expensive. |
| R-410A | 2,088 | A1 (non-flammable) | No longer permitted for new installs. Existing systems legal to run and service. |
| R-454B | 466 | A2L (mildly flammable) | Current standard for new equipment. |
| R-32 | 675 | A2L (mildly flammable) | Current standard for new equipment. |
Manufacturers stopped producing new residential AC and heat pump equipment charged with R-410A, under the EPA's AIM Act Technology Transitions Rule.
New residential system installations must use a refrigerant with GWP of 700 or less. R-410A no longer qualifies for new installs.
Four things this changes for you
1. Your existing R-410A system is still perfectly legal to own, run, and repair — the rule restricts new equipment, not servicing installed ones. 2. You cannot retrofit an R-410A system to R-454B or R-32; they aren't drop-in compatible, so converting means full replacement. 3. R-410A prices are climbing as supply tightens, which shifts the repair-vs-replace math on an older system with a significant leak. 4. A2L refrigerants are mildly flammable — keep ignition sources away from a suspected leak on newer equipment. All refrigerant work still requires EPA Section 608 certification regardless of type.
One more thing worth knowing: refrigerant isn't consumed in normal operation. A system that's "low on refrigerant" has a leak. Adding more without repairing the leak is a temporary fix you'll pay for again.
Diagnostics
Furnace flash codes — read this before you Google yours
Flash codes are not universal
The same number of flashes means different things on different brands. On many Goodman furnaces 3 flashes indicates a pressure switch stuck open; on some Carrier units 3 flashes indicates a rollout or limit switch trip. Acting on the wrong brand's chart sends you down a completely wrong path. The authoritative source is the legend printed on the label inside your furnace's access panel or blower door. Read that first.
| Pattern | Common meaning (verify against your label) | Frequent root cause |
|---|---|---|
| Slow steady | Normal standby, no call for heat | — |
| Rapid | Normal — call for heat active | — |
| Solid on / off | Control board or power problem | Check power, then pro |
| 1 flash | Often ignition failure or flame sense | Dirty flame sensor, failed ignitor |
| 2–3 flashes | Often pressure switch fault | Clogged condensate drain, blocked flue or intake, cracked hose |
| 4 flashes | Often open high-limit switch | Dirty filter, blocked returns |
| 5 flashes | Often flame sensed when it shouldn't be | Pro diagnosis — gas valve |
Count carefully. Watch at least three complete cycles before concluding — mistaking 3 flashes for 4 leads to entirely different troubleshooting. And a rollout switch trip is not the same as a limit trip: it means flame escaped the burner box. Shut down and call a pro immediately rather than resetting.
The technician's shortcut
Sequence of operation — where it fails tells you more than what failed
This is the single most useful diagnostic idea on this site. Instead of guessing at parts, find the step where the startup sequence stops.
| Step | What should happen | If it stops here |
|---|---|---|
| 1 | Thermostat calls for heat | Thermostat, batteries, C-wire, breaker, door switch |
| 2 | Inducer / draft motor starts | Control board, inducer motor, thermostat wiring |
| 3 | Pressure switch proves airflow, closes | Clogged condensate drain, blocked flue or intake, cracked hose |
| 4 | Ignitor glows | Cracked ignitor, control board |
| 5 | Gas valve opens, burners light | Gas supply, gas valve — pro only |
| 6 | Flame sensor proves flame | Burners light then die in 3–7 sec = dirty flame sensor. Classic. |
| 7 | Blower starts after short delay | Blower motor, capacitor, control board |
Cooling is simpler: thermostat calls → contactor pulls in → outdoor fan and compressor start together → indoor blower starts. Outdoor fan running without the compressor, or the reverse, narrows the fault immediately. The blower-on delay after ignition is normal and exists to prevent cold air at the vents.
Airflow
Filters & MERV
| MERV | Best for |
|---|---|
| 1–4 | Minimal filtration — protects equipment only |
| 8–11 | Sweet spot for most homes |
| 13+ | Better filtration, but only with a system and ducts sized for the added static pressure |
Higher MERV is not automatically better — a filter more restrictive than your system was designed for chokes airflow, which can freeze the evaporator coil and overwork the blower. Change 1-inch pleated filters every 1–3 months (monthly with pets); 4–5 inch media filters run 6–12 months. The airflow arrow points toward the blower.
Electrical
Capacitors
The most common cause of "compressor hums but the fan won't spin" and "clicks but won't start." Typical life is 5–10 years; heat and power surges shorten it.
- Failure signsBulging or domed top, oily residue, rust, split casing
- RatingPrinted in microfarads (MFD/µF) plus voltage. Dual-run caps show two values, e.g. 45+5 MFD (compressor + fan)
- ReplacementMust match MFD and meet or exceed voltage rating
- SafetyHolds a dangerous charge after power is off — call a pro
While you wait for service
Shut the system off at the thermostat. Running a humming compressor with no fan airflow can overheat and destroy it.
Planning
Lifespan & repair-vs-replace
| Component | Typical service life |
|---|---|
| Central AC / heat pump | 12–15 years |
| Gas furnace | 15–20 years |
| Capacitor | 5–10 years |
A common heuristic: if repair cost × system age in years exceeds replacement cost, replacement is usually better value. Treat it as a rule of thumb, not a rule. A single failed capacitor or contactor on an otherwise healthy 10-year-old system is normally worth repairing.
On efficiency ratings: SEER2/EER2/HSPF2 replaced the older SEER/EER/HSPF test procedures in 2023. SEER2 numbers read slightly lower for equivalent equipment, so comparing your old unit's SEER to a new unit's SEER2 overstates the improvement.
Prevention
Seasonal maintenance
- Monthly–quarterlyCheck and change the filter
- SpringClear 2 ft around the outdoor unit, rinse the condenser coil inside-out with power off, flush the condensate drain, test cooling before the first heat wave
- FallNew filter, test heat early, verify CO detectors, clear flue and intake terminations of debris and nests
- Year-roundKeep landscaping, fencing, and dryer vents clear of the outdoor unit
Don't fully cover the outdoor unit in winter
Trapped moisture promotes corrosion and full covers invite rodents. A top-only cover is the usual compromise. Heat pumps must stay uncovered — they run all winter.