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 (*75Â°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 (*55Â°F*, for example).

In this case, the delta T is **20Â°F** (since 75Â°F – 55Â°F = 20Â°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 Temperature_{After Evaporator (Supply Duct)}**

*Quick Example:* Let’s say we measure 77Â°F temperature in the return duct (before the evaporator). After the evaporator (supply duct), we measure 56Â°F. What is the delta T in this case? Just use the equation above like this:

**Delta T = 77Â°F – 56Â°F = 21Â°F**

We can see that delta T is 21Â°F. This is normal delta T.

The *normal* delta T range is between **18Â°F and 22Â°F**. The HVAC school refers to this as “It should be 20Â°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 18Â°F to 22Â°F range, we have normal delta T. Here are the conditions for low and high delta T:

is when the measured delta T falls*Low*delta T**below 18Â°F**.is when the measured delta T breaches*High*delta T**above 22Â°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**, we will get*high*airflow. That’s because more air is moving across the evaporator coil, and it cannot be cooled adequately.*low*delta T - At
**very**, we will get*low*airflow. That’s because less air is moving across the evaporator coil, and it is cooled excessively.*high*delta T

*Low delta T example:* Let’s say that we measure 70Â°F incoming air temperature (return vent), but the outgoing air temperature (supply vent) is as high as 55Â°F. In this case, we have **15Â°F delta T** (since 70Â°F – 15Â°F = 55Â°F). This is a low delta T since it is below 18Â°F.

*High delta T example:* Let’s say that we measure 74Â°F incoming air temperature (return vent), but the outgoing air temperature (supply vent) is as low as 49Â°F. In this case, we have **25Â°F delta T** (since 74Â°F – 49Â°F = 25Â°F). This is a high delta T since it is above 22Â°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.