Quick answer
Differential temperature (delta T) is the difference between the temperature of the air entering a furnace, heat pump or AC (return air) and the air leaving it (supply air), measured in degrees Fahrenheit.
In cooling it is often called the temperature split; in heating, the temperature rise. It is a quick health check, but only meaningful alongside airflow, humidity and the maker's range.
Quick facts
The key facts about Differential Temperature (Delta T), with sources:
- Formula
- Delta T = return air temperature − supply air temperature (cooling); supply − return (heating) 3
- Typical cooling split
- Generally 14–23°F, varying with blower type and load 1
- Where to find your range
- Furnace rating plate 3
- Humidity effect
- Higher RH lowers the cooling split 1
- Measure
- Near the equipment, not at distant grilles 1
Key takeaways
- Delta T is the temperature change across the equipment.
- In cooling, splits generally fall between 14°F and 23°F, but vary with humidity and blower setup 1.
- For furnaces, the right rise is printed on the rating plate; one Rheem line lists 40–70°F to 50–80°F by model 2.
- Measure near the equipment; grille readings mislead 1.
- A split outside the range is a clue to check airflow and charge, not a diagnosis by itself.
What delta T measures
Every heating or cooling system changes the temperature of the air passing through it. Delta T (the Greek letter Δ means "change") is how many degrees that is. In a furnace, Bosch defines temperature rise simply as "the temperature difference between supply and return air" 3.
In cooling, the split depends on how much of the coil's work goes to cooling versus drying. HVAC School explains that when humidity is higher, more latent heat is removed and "the delta T will decrease" 1. A small split on a muggy day can be normal.
Delta T ties directly to airflow. For a fixed amount of heat, less air means a bigger temperature change, and more air a smaller one. That is why a high furnace rise or a large cooling split often points to low airflow 4.
Where to measure
- Return air: in the return duct just before the furnace or air handler
- Supply air: in the supply plenum just after the coil or heat exchanger, far enough for air to mix 1
- Not at room grilles: ducts gain and lose heat on the way 1
- Supply − return (heating) or return − supply (cooling) = delta T
How a technician takes a reliable reading
- Let the system run for several minutes so temperatures settle.
- Use calibrated probes in the return and supply ducts near the unit 1.
- For furnaces, keep the supply probe out of direct line of sight of the heat exchanger so radiant heat does not skew it.
- Record indoor humidity in cooling, since it changes the expected split 1.
- Compare with the maker's range and with airflow, superheat and subcooling readings 1.
Delta T by system type
| System | Name used | Typical guide | Source |
|---|---|---|---|
| Central AC / heat pump cooling | Temperature split | Generally 14–23°F | 1 |
| Gas furnace | Temperature rise | Rating plate range, e.g. 40–70°F | 2 3 |
| Heat pump heating | Temperature rise | Usually lower than a furnace; varies with outdoor temperature | Maker's data |
| Variable-speed systems | Either | Shifts with speed; less useful alone | 1 |
Worked example: checking a cooling split and a furnace rise
Example only, with illustrative readings compared against published ranges.
| Reading | Math | Compared with | Takeaway |
|---|---|---|---|
| AC: return 76°F, supply 57°F | 76 − 57 = 19°F | 14–23°F general range 1 | Normal |
| AC on a very humid day: return 76°F, supply 61°F | 15°F | Lower end; higher RH lowers the split 1 | Likely fine; check humidity |
| AC: return 76°F, supply 48°F | 28°F | Above range | Suspect low airflow; check filter and coil for ice |
| Furnace: return 68°F, supply 124°F | 56°F | 40–70°F on that Rheem model 2 | Inside range |
| Furnace: return 68°F, supply 148°F | 80°F | Above 40–70°F | Low airflow; limit switch may trip |
Using the sensible heat formula, a 19°F split at 1,200 cfm means about 1.08 × 1,200 × 19 ≈ 24,600 BTU/h of sensible cooling 4.
If airflow were really 900 cfm, the same split would mean far less cooling.
That is why HVAC School says delta T alone "can't tell you that much"; it needs airflow and refrigerant readings beside it 1.
Where you will see the term
- On a furnace rating plate: "temperature rise range" 3.
- In start-up and commissioning reports.
- On service invoices: "split 19°F" or "rise 55°F."
- In smart thermostat or sensor apps that log supply temperature.
Why techs use it, and why it can mislead
Useful because
- Quick and cheap to measure.
- A furnace rise outside the rating plate range is a clear red flag 3.
- Shows trends over time on the same system.
The "20-degree rule" and its limits
Many people learn that a good AC split is 20°F. HVAC School calls that rule of thumb "too simple to be a hard and fast rule," and notes many rules of thumb only hold for systems running 400 cfm per ton 1.
Field data shows why. DOE's review found low airflow in 50% to 93% of systems studied and refrigerant charge outside the maker's range in many others 5. Both change the split. A system with low airflow and low charge can show a "normal" split while delivering much less cooling.
Where cost comes in
Delta T itself costs nothing to read, but it often decides what a repair visit chases next: a filter, a blower setting, a duct fix, a charge check or a coil cleaning. Asking for the readings on your invoice helps you judge the fix.
See what drives repair prices in the repair cost calculator.
What happens when delta T is out of range
- Check the filter, returns and supply vents.
- Measure airflow or static pressure.
- On cooling, look for ice and check superheat and subcooling.
- On furnaces, check gas input and blower speed; Bosch notes the right rise is usually reached at rated input with the as-shipped blower speed 3.
- Adjust blower speed or fix the restriction, then re-measure.
What owners can do
- Change filters on schedule; dirty filters raise furnace rise and change the AC split.
- Keep vents open and returns clear.
- Ask your technician to record delta T at each tune-up so you can see trends.
- Use a simple thermometer at a nearby supply grille for a rough trend only, knowing it reads differently from a duct probe 1.
When the reading points to a problem
- Furnace rise above the rating plate range: overheating risk; call a technician 3.
- Very large cooling split with weak airflow: possible frozen coil. See frozen evaporator coil.
- Very small split on a dry day: possible low charge or a compressor issue.
- Split falling year over year: dirty coil, airflow or charge drift.
Where the rules come from
No code sets a single delta T. For furnaces, the binding number is the maker's: Bosch tells installers to operate within "the furnace's intended temperature-rise range" with external static pressure in the allowed range, as listed on the rating plate 3.
Rheem's tables add that its airflows above 0.5 inches of static pressure are for reference only 2.
For cooling, makers publish charging and performance data instead of a fixed split. Industry quality-installation practices check delta T alongside airflow and charge at start-up.
Delta T vs related readings
| Reading | What it shows |
|---|---|
| Delta T | Temperature change of the air |
| Superheat | How fully refrigerant boiled in the coil |
| Subcooling | How fully refrigerant condensed |
| Static pressure | Resistance to airflow |
| Airflow (cfm) | How much air is moving |
Common misconceptions
- Myth A good AC split is always 20°F.
- Reality It varies with humidity and setup; 14–23°F is a general range 1.
- Myth A bigger split means better cooling.
- Reality It can mean low airflow and less total cooling 4.
- Myth Grille temperatures are good enough.
- Reality Ducts change temperatures on the way 1.
- Myth Every furnace should rise the same amount.
- Reality Each model has its own rated range 2.
Climate and the split
When it matters to you
It matters when your home is not cooling or heating well, after a new install, and at tune-ups. Ask for the delta T with the airflow and refrigerant readings, not alone.
Next step: compare your system's age and condition with the equipment lifespan estimator, and read what a full commissioning check covers.
Questions about Differential Temperature (Delta T)
What should the temperature split be on my AC?
HVAC School puts the general range at 14 to 23 degrees Fahrenheit, varying with blower type and load. On very humid days the split is often at the low end because more of the coil's work goes to removing moisture. Use it with airflow and charge readings.
What is a normal furnace temperature rise?
It depends on the furnace. The rated range is printed on the rating plate. For example, Rheem's FG6RA models list ranges from 40–70°F up to 50–80°F depending on size. A rise above the range usually means low airflow.
How do you measure delta T?
Measure return air just before the unit and supply air just after it, in the ducts, after the system has run for several minutes. Subtract one from the other. Readings at room grilles are less accurate because ducts gain and lose heat.
Is a 20 degree split good?
Often, but not always. The 20-degree rule is a rough guide that assumes typical airflow and humidity. HVAC School calls it too simple to be a hard rule. A technician compares the split with humidity, airflow and refrigerant readings.
What does a high delta T mean?
In cooling, a high split often means low airflow, which can lead to a frozen coil. In heating, a rise above the rating plate range usually means low airflow or overfiring, which can trip the limit switch. Check the filter first.
What does a low delta T mean?
In cooling, a low split can mean high humidity, which is normal, or low refrigerant, a dirty condenser or a compressor issue. In heating, a low rise can mean high airflow or low gas input. A technician checks which.
Does humidity affect delta T?
Yes. When indoor air is humid, more of the AC coil's capacity goes to removing moisture, so the air temperature drops less. HVAC School notes higher relative humidity tends to lower delta T.
Why is my supply air warm in cooling?
A small split with warm supply air can mean the outdoor unit is not running, low refrigerant, a dirty outdoor coil or a failed compressor. Check that the outdoor unit is running and the filter is clean, then call a technician.
Can I check delta T myself?
You can get a rough idea with a thermometer at a return grille and a nearby supply vent, knowing it reads differently from duct probes. Use it to spot trends, and leave precise readings and any equipment adjustments to a technician.
Sources
- HVAC School, What Should the Air Delta T Be? (Air Temperature Split), retrieved .
- Rheem, FG6RA Series 80+ AFUE Gas Furnace Technical Specifications, retrieved .
- Bosch, 96% AFUE Gas Furnace Installation, Operation and Maintenance Manual (04.2020), retrieved .
- Green Building Advisor, Converting Heating and Cooling Loads to Air Flow Needs, retrieved .
- U.S. DOE Building Technologies Office, Residential HVAC Installation Practices: A Review of Research Findings (June 2018), retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The cooling range and humidity effect are HVAC School's training guidance; furnace rise ranges are copied from Rheem's spec sheet and the rise definition from Bosch's manual. The worked example compares illustrative readings with those published ranges and applies the 1.08 sensible formula.
Suggest a correction. We fix errors and say what changed.