Temperature Testing
Supply and return air temperature measurement, delta-T analysis for heating and cooling modes, evaporator and condenser coil performance verification, and diagnosing equipment problems through temperature data.
- Measure supply and return air temperatures at the equipment and at registers to calculate delta-T
- Apply the 18-22 degree F cooling delta-T standard and the manufacturer's heating temperature rise range
- Diagnose low refrigerant charge, dirty coils, and airflow problems using temperature measurements
- Measure wet bulb temperature and calculate the sensible heat ratio for dehumidification assessment
Lesson 1
Delta-T Fundamentals - Cooling and Heating
What Delta-T Reveals
Delta-T (written as the Greek letter delta followed by T, meaning "change in temperature") is the temperature difference between the return air entering the system and the supply air leaving the system. It is the single most accessible diagnostic measurement in HVAC because it requires only two inexpensive thermometers or temperature probes.
In cooling mode, delta-T tells you how much heat the evaporator coil is removing from the air. In heating mode, it tells you how much heat the furnace or heat pump is adding to the air. Deviations from the expected delta-T range point directly to specific equipment or airflow problems.
Cooling Mode Delta-T
For a properly operating residential air conditioning system at 400 CFM per ton:
The expected cooling delta-T depends on the humidity of the return air:
- Dry conditions (below 50% RH): 20-22 degrees F delta-T. More of the coil's capacity goes to sensible cooling (lowering temperature) because there is less moisture to remove.
- Normal conditions (50-60% RH): 18-20 degrees F delta-T. A moderate amount of coil capacity goes to latent cooling (removing moisture).
- Humid conditions (above 60% RH): 14-18 degrees F delta-T. More coil capacity goes to latent cooling, leaving less for sensible cooling, so the temperature drop is smaller.
Interpreting Cooling Delta-T
Delta-T too high (above 22 degrees F):
- Most likely cause: Low airflow across the evaporator coil. When airflow is below 350 CFM/ton, the air moves too slowly across the coil and gets cooled too much. The supply air may be 48-52 degrees F instead of the target 55-58 degrees F.
- Other causes: Low refrigerant charge (the coil temperature drops but capacity decreases), oversized evaporator coil for the condensing unit.
- Consequences: Coil icing risk, compressor liquid slugging, potential compressor damage.
Delta-T too low (below 14 degrees F):
- Most likely cause: Low refrigerant charge causing the evaporator coil to warm up. The coil cannot absorb enough heat from the air.
- Other causes: Dirty condenser coil (high head pressure reducing capacity), excessive airflow (above 450 CFM/ton), restricted metering device, compressor failure.
- Consequences: System cannot meet cooling load, runs continuously, high energy bills.
Normal Delta-T (18-22 F)
Indicates: Proper airflow and refrigerant charge
Supply air: 55-58 F
Return air: 72-78 F
System status: Operating within design parameters
High Delta-T (Above 22 F)
Indicates: Low airflow (most common) or low charge
Supply air: Below 53 F
Risk: Coil icing, compressor damage
First check: Static pressure and airflow
Low Delta-T (Below 14 F)
Indicates: Low charge, dirty condenser, or compressor issue
Supply air: Above 62 F
Result: Cannot meet cooling load
First check: Refrigerant pressures and condenser condition
Heating Mode Delta-T (Temperature Rise)
Every gas furnace has a rated temperature rise range printed on the nameplate or rating plate (e.g., "Temperature Rise: 35-65 degrees F"). The measured temperature rise must fall within this range.
Temperature rise below the rated range:
- Airflow is too high (blower speed too fast, or ductwork less restrictive than expected)
- Furnace is under-firing (gas pressure too low, wrong orifice size)
- The furnace is too small for the duct system
Temperature rise above the rated range:
- Airflow is too low (dirty filter, restrictive ducts, blower speed too slow)
- Furnace is over-firing (gas pressure too high)
An out-of-range temperature rise is a safety concern. If the rise exceeds the maximum, the high-limit switch will trip, shutting down the burner. Repeated high-limit trips cause short-cycling and accelerate heat exchanger fatigue, which can eventually lead to cracking and CO leakage.
Normal cooling delta-T is 18-22 degrees F at standard humidity. High delta-T (above 22 F) usually indicates low airflow - check static pressure first. Low delta-T (below 14 F) usually indicates low refrigerant charge - check refrigerant pressures. In heating mode, temperature rise must fall within the manufacturer's rated range to ensure safe operation and prevent high-limit trips.