HVAC Delta-T calculator

What Delta-T actually tells you about an HVAC system

Delta-T is the temperature difference between return air entering the evaporator and supply air leaving it. On a cooling system the formula is straightforward: return dry bulb minus supply dry bulb. On a healthy 3-ton AC with correct charge, healthy ductwork, and a clean evaporator, Delta-T sits between 17 and 22 degrees Fahrenheit for most homes at normal humidity. It is one of the three core air-side diagnostic numbers every HVAC technician should check on every service call.

The value of Delta-T is not in the number itself. It is in what the number tells you when it is wrong. A low Delta-T points to one cluster of problems (low charge, fouled coil, high airflow). A high Delta-T points to a completely different cluster (dirty filter, restricted return, blower failure). Comparing actual to target Delta-T is the fastest way to narrow a "the AC is not cooling" service call from a dozen possibilities to two or three.

Why the "20 degree rule" is wrong most of the time

Generations of HVAC techs have been taught that 20°F Delta-T is the target for any AC. That rule of thumb is only correct when indoor relative humidity sits between 45 and 55 percent, which is roughly the AHRI rating condition. In real homes, indoor humidity varies from below 30 percent in dry climates and during winter operation to 65 percent or higher in humid coastal homes during cooling season.

The target Delta-T moves opposite to humidity. At higher indoor humidity, the evaporator does more latent work (removing moisture) and less sensible work (cooling air temperature), so the sensible Delta-T drops. At lower humidity the evaporator does almost no latent work and Delta-T rises. A few common targets:

• Very humid (60+ percent RH, ~71°F wet bulb): 15 to 18°F target Delta-T
• Mid humidity (45–55 percent RH, ~63°F wet bulb): 18 to 22°F target
• AHRI standard (51 percent RH, 67°F wet bulb): 17 to 21°F target
• Dry (30–35 percent RH, ~59°F wet bulb): 20 to 23°F target
• Very dry (under 30 percent RH, ~55°F wet bulb): 22 to 25°F target

Always measure return wet bulb with a digital psychrometer before deciding what target Delta-T to chase. Charging a system to a 20°F split on a humid Florida day will overcharge it, and on a dry Arizona day will undercharge it.

How to measure Delta-T correctly without skewing the result

A bad measurement procedure can throw Delta-T off by 4 to 8 degrees, which is enough to misdiagnose a healthy system. Use this procedure:

• Return reading at the return grille, not at the air handler. Plenum readings get contaminated by heat from the air handler cabinet and from any nearby ducts.
• Supply reading at the closest supply register to the air handler. Reading further downstream loses heat to duct walls and gives a lower (artificially good) Delta-T.
• Use a digital psychrometer for the wet bulb. Sling psychrometers work but are slower and error-prone if the wick is dirty.
• Run the system 15 to 20 minutes at steady indoor and outdoor conditions before reading. Delta-T swings widely during the first 5 to 10 minutes of operation.
• Check the filter first. A dirty filter throws Delta-T high and looks identical to a refrigerant problem on the gauge but the fix is free.

Low Delta-T: charge problem or airflow problem?

Delta-T below the target range means the evaporator is not removing enough heat per pound of air moving across it. Three causes account for almost every low-Delta-T case:

• Low refrigerant charge. Verify with superheat and subcooling. High superheat with low subcooling is the classic undercharge signature.
• Blower running too fast. Too much air moves across a properly-loaded evaporator, dropping Delta-T because each cubic foot spends less time absorbing heat. Common after a thermostat upgrade where blower speed defaults changed, or after a filter swap where someone "fixed" airflow by maxing out blower speed.
• Reduced sensible capacity. Dirty evaporator coil, partially blocked indoor coil airflow path, or a compressor losing efficiency. Less common than the first two, but real.

Below 12°F Delta-T is critical. The system is barely cooling. Stop and diagnose immediately before adding refrigerant. Causes at this level include severely low charge, dead compressor, frozen evaporator coil, stuck reversing valve on a heat pump, or wildly oversized blower.

High Delta-T: it is almost always airflow

Delta-T above the target range means too little air is moving across the evaporator. The coil is removing plenty of heat from the small amount of air that passes through, but the total cooling delivered to the home is reduced. Most homeowners notice this as "the AC runs forever and the house never gets cool enough."

In order of how often each cause shows up on service calls:

• Dirty air filter. Check this before anything else. A heavily loaded filter cuts airflow by 30 to 50 percent.
• Blocked return grille. Furniture in front of return, dust on the grille, or the grille undersized for the system.
• Closed supply registers. Homeowners often close registers in unused rooms, which causes the air handler to fight high static pressure.
• Restricted return ductwork. Crushed flex, closed dampers, or undersized return ducts. Confirm with our static pressure calculator.
• Blower wheel fouling or failure. A dust-loaded squirrel cage moves 20 to 40 percent less air than a clean one. Pull it and inspect any time Delta-T is high without an obvious filter or duct cause.

Above 28°F Delta-T in cooling, the evaporator coil is at risk of freezing within 30 to 60 minutes. Shut the system off and find the airflow problem before running it again.

Heating mode Delta-T: very different targets

Heating Delta-T uses the same formula (supply minus return) but the target ranges look completely different because each heating technology delivers heat differently:

• Gas furnace: 40 to 70°F Delta-T. Modern condensing furnaces specify their target on the nameplate. Always go with the manufacturer's value over any rule of thumb.
• Heat pump: 15 to 30°F Delta-T. Drops with outdoor temperature because heat pump capacity drops. At the system balance point, Delta-T may legitimately fall below 15°F before backup strips kick in.
• Electric resistance / strip heat: 25 to 50°F Delta-T depending on the number of strips energized. Confirm against the strip ratings and the actual measured amperage.

Gas furnace Delta-T above the nameplate range is a serious safety issue. The heat exchanger can crack from overheating, releasing carbon monoxide into the supply air. If you measure 80+°F Delta-T on a residential gas furnace, shut it down until airflow is restored.

Combining Delta-T with superheat and static pressure

Delta-T alone tells you something is wrong but rarely tells you exactly what. Combining it with two other measurements gives you a clean diagnostic matrix:

• Delta-T low + high superheat: refrigerant undercharge. Add refrigerant after verifying with subcooling.
• Delta-T low + correct superheat: high blower speed or fouled evaporator. Check blower speed and inspect coil.
• Delta-T high + correct superheat + high static pressure: airflow restriction (filter, return duct, or blower). Find the restriction.
• Delta-T high + correct superheat + normal static pressure: blower speed too low. Bump it up one tap.
• Delta-T high + low superheat + high subcooling: overcharge AND low airflow. Fix airflow first, then recheck charge.

For the complete air-side workflow, use this calculator together with our superheat, subcooling, and static pressure tools.