When is a differential too high and what could a very high differential indicate?

[Gene South]

A trend I see after several hundred inspections is that on small houses (2000 sq feet of less), I almost always see a high differential. 20 degrees is not uncommon. Sometimes I can see a differential of 22-23 degrees. Usually anything above 20 causes me to suspect the unit actually may need service. Today I had a house of 1800 sq feet, and got a 22-23 degree differential. The unit appeared to be in great condition, actually pretty new condition, well maintained and a clean filter. The ducts also appeared to be clean. When is a differential too high and what could a very high differential indicate? I am measuring the temperature using a infrared thermometer at the main return and a sampling of room vents.

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[James Duffin]

I would guess low air flow across the coil or low refrigerant charge.

[Ted Menelly]

A trend I see after several hundred inspections is that on small houses (2000 sq feet of less), I almost always see a high differential. 20 degrees is not uncommon. Sometimes I can see a differential of 22-23 degrees. Usually anything above 20 causes me to suspect the unit actually may need service. Today I had a house of 1800 sq feet, and got a 22-23 degree differential. The unit appeared to be in great condition, actually pretty new condition, well maintained and a clean filter. The ducts also appeared to be clean. When is a differential too high and what could a very high differential indicate? I am measuring the temperature using a infrared thermometer at the main return and a sampling of room vents.

It sounds like you are taking the measurements way to soon. If you walk into a home and turn the ac down and start taking measurements you have not allowed the system to balance at all. You don't have to wait an hour but at least a short time for a decent amount of the moisture in the air to be condensed.

When I take measurements like that I always have much less a differential. If these are almost new homes with high efficiency units then I get much less differential than that.In the new homes I am finding 12 to 15 degrees. The older homes have a bit higher differential but not up into the 20s usually 15 to 18 degrees. Of course if I just let the system run for just long enough to cool the ducts and system in the attic the differential is pretty high but again there has been no balance yet. It is to soon. Even if you take it before and after the evaporator cool you will have a much higher differential after just starting the system.

[Rod Butler]

HVAC systems are usually sized with a 20 deg temp differential. I have to admit that in all of the houses that I have lived I have NEVER Seen a 20 deg temp drop.

I agree with James that the airflow might be too low. That being said if the coil is not freezing I am not sure it should be reported as irregular or suspect.

There are multiple lugs on a direct drive motor and the cooling lug should be on HIGH.

Is the condensing unit matched size to the blower coil/furnace?

[Jerry Peck]

I can only say that before I stopped taking TD as it is rather meaningless on the scope of things the HI checks for an a/c system, I frequently found TDs of 20 to 21 degrees, that was 'normal'.

The TD rule-of-thumb when I first started HI was 15-20, then changed to 16-22, and then there was no real documentable (is that even a word? ) evidence that any TD meant anything as low air flow (caused by many things) can give a false high TD and that high air flow (caused by some things) can give a false low TD and that neither reflected the 'condition' of the a/c system (i.e., neither the AHU nor the CU), as the causes of those varying reading could be, and were at times, in the duct system.

[Bruce King]

TD discussions are getting even more useless, especially if the type of refrigerant (R22 or R410) is not stated and the type of air handler (variable speed vs single speed) is not stated. Also whether or not the house is vacant or occupied, humidity level, if the house is heat soaked and if direct sun is on the house.

[Jerry Shipman]

There are multiple lugs on a direct drive motor and the cooling lug should be on HIGH.

Not necessarily. It depends on the heating and cooling load. Many furnaces have higher heating speed requirements than cooling. Ex: in cooler climates, smaller a/c units may be matched with larger heating units. This is quite common, and the heating airflow needed is higher than the cooling airflow required.

Many systems have dehumidification function designed into the controls. During this dehumidification, the blower speed is reduced in an effort to create a colder coil, creating more condensation and removing moisture from the home. This reduced airflow, as you would expect, will also raise the TD from return to supply.

Jerry P.
Thank you for your post. I've been arguing for years that TD cannot be used to evaluate the condition of a system, and using this method often causes undue expense and frustration for everyone involved. Proper system charge evaluation is achieved by taking liquid and suction line temperatures and pressures; both outside an HI's responsibility and expertise.