In HVAC, delta T measurement is used to confirm adequate heat transfer at the indoor evaporator coil. We are going to look in-depth at what is delta T in HVAC, how we measure it, and what low delta T and high delta T mean.
Namely, delta T in heating and cooling is the temperature difference between:
- Temperature of air before it hits cold indoor evaporator coils. This is our indoor temperature (75°F, for example), and…
- Temperature of air after it passes over cold indoor evaporator coils. This is the lower temperature air conditioners provide to cool our homes (55°F, for example).
In this case, the delta T is 20°F (since 75°F – 55°F = 20°F). Here is a sketch that explains HVAC delta T visually:
In essence, the delta T is the temperature decrease of the air when it passes over the evaporator coils. It is a very useful measurement for air conditioner diagnostics, because it can:
- Primarily tell us if there is something wrong with the evaporator heat exchange (dirty coils, low or high airflow, etc.).
- Secondarily to confirm if the AC is system overcharged or undercharged (based on superheat and subcooling measurement).
Let’s have a look at how we measure and calculate delta T, and what delta T temperatures we are looking for. We will also explain what causes low delta H and high delta T:
Measuring And Calculating Delta T In HVAC
To calculate delta T, we have to measure these two temperatures:
- Air temperature is taken 2-3 feet upstream from the evaporator coil (this is in the return duct).
- Air temperature is taken 2-3 downstream from the evaporator coil (this is in the supply duct).
To calculate delta T, we use this formula:
Delta T = Air Temperature Before Evaporator (Return Duct) – Air TemperatureAfter Evaporator (Supply Duct)
Quick Example: Let’s say we measure 77°F temperature in the return duct (before the evaporator). After the evaporator (supply duct), we measure 56°F. What is the delta T in this case? Just use the equation above like this:
Delta T = 77°F – 56°F = 21°F
We can see that delta T is 21°F. This is normal delta T.
The normal delta T range is between 18°F and 22°F. The HVAC school refers to this as “It should be 20°F, of course” lazy rule.
Now, normal delta T doesn’t necessarily mean that everything is alright with your AC unit. Since this is a complimentary method, we cannot definitely say that system has the right charge, airflow, and so on. The primary methods are subcooling (here) and superheat measurements (here).
That’s because the TXV valve can compensate quite well in order to normalize the delta T. However, the “cost” of this compensation is usually high pressure on the high side of the AC unit. This can result in:
- Lower energy efficiency. Example: 16 SEER unit operates as 14 SEER unit.
- Strain on compressor and potentially shorter lifespan. High pressure on the high side will put additional strain on the compressor, and an overworked compressor = not as a long-lasting compressor.
Nevertheless, it is very important to understand what low delta T and high delta T mean:
Low Delta T And High Delta T
If the delta T is within the 18°F to 22°F range, we have normal delta T. Here are the conditions for low and high delta T:
- Low delta T is when the measured delta T falls below 18°F.
- High delta T is when the measured delta T breaches above 22°F.
The main reason why we get low delta T or high delta T is quite clear (but the culprit is a bit more complex to figure out). Namely, it all has to do with airflow:
- At very high airflow, we will get low delta T. That’s because more air is moving across the evaporator coil, and it cannot be cooled adequately.
- At very low airflow, we will get high delta T. That’s because less air is moving across the evaporator coil, and it is cooled excessively.
Low delta T example: Let’s say that we measure 70°F incoming air temperature (return vent), but the outgoing air temperature (supply vent) is as high as 55°F. In this case, we have 15°F delta T (since 70°F – 15°F = 55°F). This is a low delta T since it is below 18°F.
High delta T example: Let’s say that we measure 74°F incoming air temperature (return vent), but the outgoing air temperature (supply vent) is as low as 49°F. In this case, we have 25°F delta T (since 74°F – 49°F = 25°F). This is a high delta T since it is above 22°F.
We can see that HVAC technicians use delta T to primarily evaluate airflow across the evaporator coil (indirectly linked to heat exchange). When we measure non-normal delta T, we have to find a culprit for too high (low delta T) or too low airflow (high delta T).
Hopefully, you now have a better understanding of what delta T is in HVAC. It’s quite a useful measurement that helps us with AC diagnosis and repairs.