Quick answer: Delta T (ΔT) is the temperature difference a system produces or moves across — the return air minus the supply air on an air conditioner, or the return water minus the supply water on a chilled-water coil. A healthy split-system air conditioner shows an air-side ΔT of about 8–11 °C (15–20 °F). A chilled-water system is usually designed around a water-side ΔT of 5–6 °C (10 °F).
What Delta T means in HVAC
“Delta” is the Greek letter used for a difference, so ΔT is simply the difference between two temperatures. In air conditioning and heating it turns up in three places, and knowing which one is being talked about avoids most of the confusion:
- Air-side ΔT — the drop in air temperature across a cooling coil, measured as return air temperature minus supply air temperature. This is the number a technician reads at the grilles to judge whether a unit is cooling correctly.
- Water-side ΔT — the rise in water temperature across a chilled-water coil, measured as return water minus supply water. This sets how much flow a chiller plant needs.
- Room ΔT — the difference between indoor and outdoor design temperatures, used when estimating a heating or cooling load.
All three come from the same idea: a heating or cooling system carries energy by changing the temperature of air or water, and ΔT is the size of that change.
The Delta T formula
ΔT links three things: the load (how much heat is moved), the flow rate (how much air or water carries it), and the temperature difference itself. Fix any two and the third follows.
Air (metric): Q (kW) = 1.23 × airflow (L/s) × ΔT (°C)
Air (imperial): Q (BTU/h) = 1.08 × CFM × ΔT (°F)
Water (metric): Q (kW) = 4.2 × flow (L/s) × ΔT (°C)
Water (imperial): Q (BTU/h) = 500 × GPM × ΔT (°F)
Cooling tons = GPM × ΔT ÷ 24The constants come from the density and specific heat of air and water. The 1.23 (air) and 4.2 (water) figures are the amount of energy needed to raise one unit of flow by one degree. You do not have to memorise them — the point is that load, flow and ΔT always move together.
Enter any two of load, flow rate and temperature difference for air or water — the calculator returns the third.
Open the HVAC Delta T Calculator →What is a good Delta T on an air conditioner?
For a direct-expansion split or packaged unit, measure the dry-bulb temperature of the air entering the indoor coil (return) and the air leaving it (supply). The difference is the air-side ΔT, often called the temperature split.
| Air-side ΔT | What it usually means |
|---|---|
| Below 8 °C (14 °F) | Airflow too high, low refrigerant charge, or a dirty coil |
| 8–11 °C (15–20 °F) | Healthy — the unit is cooling as designed |
| Above 12 °C (22 °F) | Airflow too low, blocked filter, or a failing blower |
The reading depends on indoor humidity, so use it as a health check rather than an exact target. In humid conditions the split runs a little lower because some of the coil’s capacity is removing moisture instead of dropping the air temperature.
Why Delta T tells you so much about a system
Because load, flow and ΔT are locked together, the ΔT you measure reveals whether the flow and the load are matched. Two failure patterns cover most field problems:
- Low ΔT — the air or water is moving too fast to pick up much temperature change, or the coil is not delivering its capacity. On the air side this points to over-fast fans or weak cooling; on the water side it points to valves passing too much flow.
- High ΔT — the flow is too slow, so each unit of air or water is asked to carry more than its share. On the air side this is usually a dirty filter or a failing fan; on the water side it can be a blocked strainer or a throttled valve.
Chilled-water Delta T and why it matters
A chilled-water plant is normally designed for a supply temperature around 7 °C and a return around 12 °C, giving a design ΔT of about 5 °C (or a 10 °F rise in imperial units). That design ΔT decides the pump flow for the whole building.
| Design ΔT | Flow for 100 tons cooling | Relative pump energy |
|---|---|---|
| 4 °C (7 °F) | ~343 GPM | High |
| 5.5 °C (10 °F) | 240 GPM | Baseline |
| 8 °C (14 °F) | ~171 GPM | Low |
A wider design ΔT means less flow for the same cooling, which means smaller pipes, smaller pumps and lower pumping energy. This is why modern plants push for higher ΔT. To turn a cooling load and ΔT into a flow rate in litres per second or GPM, use the chilled-water pipe sizing calculator.
Low Delta T syndrome
The most common chronic fault in large chilled-water systems is low delta T syndrome: the return water comes back cooler than design, so the ΔT shrinks. When that happens the pumps have to move far more water to deliver the same cooling, and the plant runs out of pumping capacity long before it runs out of chiller capacity.
Typical causes include coils that are dirty or undersized, three-way valves that let water bypass the coils, control valves that fail open, and setpoints that keep valves wide open. The symptom is always the same — flow climbs, ΔT falls, and pump energy rises — so tracking ΔT across a plant is one of the quickest ways to spot wasted energy.
Example — sizing air flow from a cooling load
A room needs 5 kW of sensible cooling and you want a 10 °C air-side ΔT. Rearranging the air formula for flow:
- Airflow = Q ÷ (1.23 × ΔT) = 5 ÷ (1.23 × 10) = 0.41 m³/s, about 407 L/s or 862 CFM.
- If you widen the ΔT to 12 °C, the flow drops to 339 L/s — a smaller fan and duct for the same cooling.
- If the ΔT falls to 8 °C, the flow climbs to 508 L/s — a bigger fan and more energy.
The same trade-off runs through every air system: a bigger ΔT buys smaller ducts and fans, but there is a floor because supply air that is too cold causes cold draughts and condensation at the diffusers.
Example — chilled-water flow from tons
A chiller delivers 100 tons at a design ΔT of 10 °F. Using the tons formula:
- Flow = tons × 24 ÷ ΔT = 100 × 24 ÷ 10 = 240 GPM.
- If the real ΔT in the field is only 6 °F, the flow needed for the same 100 tons becomes 400 GPM — a 67% jump in pump work for no extra cooling.
That single comparison is the business case for chasing ΔT problems: the pump energy penalty is large and continuous.
Design Delta T targets by system
Every part of an HVAC system has a ΔT it is designed around. Knowing the usual target tells you at a glance whether a reading is reasonable.
| System | Typical design ΔT |
|---|---|
| Cooling coil, air side (comfort cooling) | 10–12 °C (18–22 °F) |
| Chilled water | 5–6 °C (10 °F) |
| Condenser water (to cooling tower) | 5–6 °C (10 °F) |
| Low-temperature heating (radiators) | 10–20 °C |
| Heating hot water (older systems) | 11 °C (20 °F) |
| Underfloor heating | 5–7 °C |
Modern designs push these ΔT values wider wherever they can, because a bigger difference always means less flow, smaller pipes and less pump or fan energy for the same heat moved.
Working in imperial units
Much field equipment is labelled in CFM, GPM, BTU/h and Fahrenheit, so the imperial forms of the formula are worth keeping close:
Air: BTU/h (sensible) = 1.08 × CFM × ΔT (°F)
Water: BTU/h = 500 × GPM × ΔT (°F)
Tons of cooling = GPM × ΔT (°F) ÷ 24Example: a coil handling 2,000 CFM with a 20 °F air split removes 1.08 × 2,000 × 20 = 43,200 BTU/h of sensible heat, about 3.6 tons. On the water side, 40 GPM at a 10 °F rise carries 500 × 40 × 10 = 200,000 BTU/h, or 40 × 10 ÷ 24 = 16.7 tons.
Sensible, latent and why the air split changes
The air-side formula with the 1.23 (or 1.08) constant only counts sensible heat — the part that changes air temperature. A cooling coil also removes latent heat when it condenses moisture, and that part does not show up as a temperature drop. In humid weather more of the coil’s capacity goes into drying the air, so the measured temperature split is smaller even though the coil is working hard. This is why an air-side ΔT is a health indicator, not an exact measure of total cooling. For the full sensible-and-latent picture, the psychrometric calculator shows both.
Troubleshooting with Delta T
| Symptom | Likely cause | Where to look |
|---|---|---|
| Air ΔT too low | Fan too fast, low refrigerant, iced or dirty coil | Charge, coil, fan speed |
| Air ΔT too high | Airflow restricted | Filter, blocked coil, failing fan, closed dampers |
| Water ΔT too low | Excess flow or coil bypass | Valve position, balancing, three-way valves |
| Water ΔT too high | Flow too low | Strainer, throttled valve, air in the line, pump fault |
How Delta T sets pipe and duct size
Because flow follows directly from ΔT, the design ΔT decides how big the pipes and ducts have to be. Widen the ΔT and the flow drops, so the same load runs through smaller pipes and ducts at lower velocity. That is why a chilled-water plant designed for a 6 °C rise uses noticeably smaller mains than one designed for 4 °C. Once you know the flow, size the water pipe with the chilled-water pipe sizing calculator and the air path with the duct size calculator.
How to measure Delta T correctly
- Let the system run steadily for 10–15 minutes before reading, so temperatures settle.
- Measure air temperatures in the duct, not at the room, and keep the sensor out of direct line with the coil to avoid radiant error.
- Measure water temperatures at wells close to the coil, on both the supply and return sides.
- Read return and supply at the same moment; a delay between the two readings distorts the difference.
- Note the indoor humidity when judging an air-side split, because it shifts the expected value.
Key points to remember
- ΔT is the temperature difference a system moves — return minus supply.
- Load, flow and ΔT are tied together: fix any two and the third is set.
- A healthy air-conditioner air split is about 8–11 °C (15–20 °F).
- Chilled-water plants are usually designed for a 5–6 °C (10 °F) rise; a wider ΔT saves pump energy.
- Falling ΔT with rising flow is the classic sign of a wasteful, poorly matched system.
To size the water pipe once you know the flow, use the chilled-water pipe sizing calculator, and for the cooling load itself see how to calculate a cooling load.
Frequently asked questions
What is a good delta T for an air conditioner?
About 8 to 11 C (15 to 20 F) between the return air and the supply air. A lower split suggests too much airflow or low refrigerant; a higher split suggests restricted airflow.
What does delta T mean in HVAC?
It is the temperature difference a system moves, measured as return minus supply, on either the air side or the water side.
What is the delta T formula?
For air, load in kW = 1.23 x airflow in L/s x delta T in C. For water, load in kW = 4.2 x flow in L/s x delta T in C. Rearrange to find any unknown.
What is the ideal chilled water delta T?
Plants are usually designed for a 5 to 6 C rise (about 10 F), with supply near 7 C and return near 12 C. A wider delta T lowers pump energy.
What causes low delta T?
Dirty or undersized coils, control valves passing too much flow, and bypass paths. Flow rises while delta T falls, which is known as low delta T syndrome.