In a recent blog post I mentioned several problems that can occur when a furnace is over-sized; the home can be less comfortable, the furnace will operate less efficiently, the furnace may short-cycle, and the life of the furnace will be reduced. While the focus of that blog post was on over-sized furnaces, all of these same conditions can occur when a furnace has insufficient air flow.

If a furnace can't pull enough air through the ductwork to dissipate the heat that's created, it will run too hot. Simple. While an over-sized furnace will typically have insufficient air flow, a few more things that affect air flow are undersized / insufficient ductwork, a dirty furnace filter, a clogged secondary heat exchanger, a clogged evaporator coil, or too many supply registers blocked off.

If you're curious about how your own system is operating, here's an easy inspection procedure that homeowners can conduct to help determine if there's a problem. With the thermostat set to 'heat', turn up the temperature at the thermostat about five degrees warmer than the current temperature. This should get the furnace to kick on. Now wait about fifteen minutes.

During this time, the furnace should run continuously without shutting off. If the furnace shuts off during this time, go check the thermostat. If the thermostat has already been satisfied, you may have an oversized system, or the thermostat might be located in a poor location, like right in front of a heat register. It's not normal for a furnace to raise the indoor temperature five degrees in less than fifteen minutes. If the furnace shuts off without satisfying the thermostat, it's short-cycling, which is a problem.

After the furnace has been running for about fifteen minutes, put your hand on the ductwork above the furnace. If everything is operating normally, the ductwork will be hot, but not uncomfortably hot. You should be able to leave your hand on the ductwork without feeling any pain.

If you're not satisfied with this basic test, get a meat thermometer and stick it into the ductwork above the furnace to check the temperature of the supply air, and then check the temperature of the return air as well.


he difference between these two numbers is what HVAC contractors and geeky home inspectors call "Delta T" (written "ΔT"), as in "temperature difference". This is also called the "temperature rise". To determine if your furnace has an acceptable temperature rise, just take a look at the specs on the furnace. Every furnace should have a label with some basic info about the unit, such as the model number, serial number, BTUs, and temperature rise.

The temperature rise will give a range; for example, the label pictured below says the temperature rise should be between 30 and 60 degrees. If the temperature rise is above 60 degrees, there's a problem.

If there's an excessive temperature rise, the first thing to check is the filter; a dirty furnace filter will obviously restrict air flow, making the temperature rise higher. If the filter is clean, go around the house and make sure the supply and return registers are open; closed registers = less air flow = higher temperature rise.

If it's neither of those things, you probably have a problem with your system. That's when it's time to call a good HVAC contractor out for the annual furnace inspection you've been putting off for the past several years. While the focus of this post has been on airflow problems with a furnace, an excessive temperature rise could also indicate a combustion problem.

Author: Reuben Saltzman, Structure Tech Home Inspections