It’s the middle of winter, and you’re probably getting calls from homeowners who want larger HVAC equipment. Their current system runs constantly and can’t keep a few rooms warm and comfortable. When most contractors receive these calls, they investigate and often find correctly sized equipment for the home’s heating needs that is operating fine.

So, what could hinder an HVAC system from heating a home on those coldest days? While there are many factors, you can frequently find one source of trouble when you measure system temperatures. Let’s look at how you can use a return grille and supply register temperature measurement to uncover one source of uncomfortable rooms.


Equipment and System Assumptions

A common assumption made in our industry is that equipment heating output and system heating output (the latter of which includes the duct system) are the same. This is rarely the case in real-world conditions. When a manufacturer rates their equipment, it is in a perfect laboratory environment. Yet once the equipment gets installed in a customer’s home, everything changes. Most installation conditions are far from perfect.

One major component of any installation to consider is the duct system. It determines system performance more than the heating equipment. If you focus too much on the equipment, you may overlook a potential reason the system can’t maintain comfortable conditions. Don’t fall into the mindset of believing the equipment is the system.


Measure Equipment Temperatures

Before you can test, you need a good digital thermometer that reacts to temperature changes quickly and measures to a tenth of a degree. As with any test instrument, you get what you pay for, so choose wisely. I love Bluetooth temperature probes for these tests. You can instantaneously review temperature readings from multiple places. If you want to speed up your testing, these probes are a wise choice.

To measure equipment temperatures, turn the equipment to heating mode and let it run for 10 to 15 minutes. If testing a heat pump, make sure the electric heat strips are off. Multi-stage gas furnaces need to operate in the highest heating stage. Failure to consider these conditions could lead to inaccurate temperature measurements.

Next, measure return and supply temperatures at the indoor air handling equipment. Record the temperatures and then subtract the return air temperature from the supply air temperature to determine the equipment’s temperature rise (∆t). Assure it is within range and record your readings.

It’s important to note that temperatures depend on airflow. If the airflow is high or low, it influences ∆t.


Measure System Temperatures

Next, measure system temperature rise (∆t) in the most uncomfortable room. If you have Bluetooth temperature probes, you can place one in the return grille and the other in the supply register. If there are multiple supply registers, you can use extra probes or measure temperature from the supply register farthest away from the air handling equipment.

If you don’t have the luxury of Bluetooth probes, you can still use your digital thermometer to test. Measure the return grille temperature first, then measure the farthest supply register second. Be sure you don’t take too long between measurements; your readings could be off substantially. Once you have your readings, subtract them to determine system ∆t.

If the duct system is well insulated with minimal leakage, the equipment and duct system temperature rise should be close. There will be some differences, but there shouldn’t be more than a 10% temperature change across the duct system.



Let’s say you have a three-ton heat pump operating in heating mode at a 30° outside ambient temperature. You measure temperatures at the air handler and find your supply air temperature is 88.5° and return air is 70.2°. This equals an 18.3° equipment ∆t (88.5 – 70.2 = 18.3).

Next, measure temperature at the farthest supply register and return grille of the most uncomfortable room. In our example, the supply register temperature reading is at 79.7° and the return grille temperature is 70.5°. This equals a 9.2° system ∆t (79.7 – 70.5 = 9.2).

To determine the percentage of duct system temperature loss, divide the 9.2° system ∆t by the 18.3° equipment ∆t (9.2 ÷ 18.3 = .50). After you move the decimal point two places to the right, you discover a duct system temperature loss of 50% to this room. As a result, the temperature available at the air handler doesn’t make it to the room.

How would you handle this if you measured similar readings?


Move Beyond the Box

This test will surprise you when you calculate how much temperature a duct system will lose, even in conditioned spaces such as a basement. It also presents you with a new opportunity that most of your competition will walk past while trying to sell larger equipment or booster fans.

The most applicable repair for duct temperature loss is more duct insulation. Be careful as you choose from various insulation options available. Some insulation types offer high promises but don’t perform well in the field. The test we went over can also help you verify insulation effectiveness once repairs are complete.

If you consider adding duct insulation, be sure the duct system delivers proper airflow at an acceptable total external static pressure. Insulating a leaky or undersized duct system is a waste of time and money. The system will need additional repairs to work as intended.

I would also encourage you to consider more than the HVAC system. Don’t assume the room insulation is right and working as designed. You can get a glimpse into insulation effectiveness with an infrared thermometer or a thermal imaging camera. If the air temperature in the room is 72° and the wall temperature is 68°, you’ll have a hard time maintaining comfortable conditions even with duct system improvements.

The rubber meets the road when you test for yourself. Document what you find as you perform this simple test and help your customers understand the need to choose better options.