Heat pumps often have a bad reputation in colder climates. Unfortunately, much of the bad publicity might be misplaced. Many contractors hear about cold air blowing from the supply registers and want nothing to do with it. What's odd is these same concerns also happen in milder climates where a heat pump should work great.
So, why would a properly sized and charged heat pump system struggle to condition a home? While there are many potential issues, you can quickly discover one of them by measuring duct system temperatures. Let's look at how a return grille and supply register temperature measurement can uncover a key reason heat pumps have a bad reputation.
Equipment and System Assumptions
A common assumption in our industry is that heat pump and system heating output are the same. This assumption is rarely valid in real-world conditions. Manufacturers test and rate heat pumps in a perfect laboratory environment. However, once you install them in a customer's home, every variable used to rate them changes.
One component to look at in every installation is the duct system. It determines heat pump system performance more than the heat pump itself.
One component to look at in every installation is the duct system. It determines heat pump system performance more than the heat pump itself. If you're too focused 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 – it isn't.
To isolate equipment from the system, you need to measure temperatures from both the equipment and the system. These measurements will help uncover hidden duct installation issues that affect heat pump system performance and your customer's comfort.
Measure Air Handler Temperature Rise
Before testing, you'll need a good digital thermometer that quickly reacts to temperature changes and measures to a tenth of a degree. So, an analog (dial-type) thermometer isn't the best choice for this type of testing. As with any test instrument, you get what you pay for, so choose wisely.
I'm a big fan of wireless probes paired with an app like MeasureQuick™
To measure air handler temperature rise, turn the thermostat to heating mode and let the equipment run for 10 to 15 minutes. If the air handler has electric heat strips, ensure they are off. Leaving the heat strips on can lead to deceptively high temperature readings and hide heat pump issues.
It's important to note that temperature rise on an air handler paired with a heat pump depends on fan airflow and outdoor ambient temperature.
Next, measure return and supply temperatures at the air handling equipment. Record the temperatures, then subtract return air temperature from supply air temperature to determine air handler temperature rise (∆t).
It's important to note that temperature rise on an air handler paired with a heat pump depends on fan airflow and outdoor ambient temperature. Both influence the supply air temperature discharged from the air handling equipment. Two relationships to keep in mind are:
1. As fan airflow decreases, temperature rise increases.
2. As outdoor ambient temperature decreases, temperature rise also decreases.
Most manufacturers have tables in their installation manuals or specifications that list the appropriate ∆t based on airflow and outdoor ambient temperature ranges. Make sure you know the operating conditions, and then record your readings.
Measure System Temperatures
Next, measure the system temperature rise. An easy way to do this is to measure and record air temperatures from the supply register and return grille farthest away from the air handler.
Be sure you don't take too much time between measurements, or your readings could be off substantially. Once you measure the farthest supply register and return grille temperature, subtract them to determine system temperature rise. Using two wireless probes lets you see live system temperature rise and reduces the time involved.
If your duct system has great insulation and minimal leakage, the temperature rise of the equipment and duct system should be very close. There will be some differences, but duct system temperature loss shouldn't exceed 10% of the air handler's temperature change. Here's an example of how to determine this.
You measure a three-ton heat pump operating in heating mode at a 30°F outside air temperature. You also measure temperatures at the air handler and find that supply air temperature is 88.9°F and return air temperature is 70.7°F. These measurements mean there is an 18.2° temperature rise (88.9 – 70.7 = 18.2) across the air handler.
Next, measure temperatures at the farthest supply register and return grille.
You find that the supply register temperature reading is 80.6°F, and the return grille temperature is 71.5°F. This equals a 9.1° system temperature rise (80.6 – 71.5 = 9.1).
To determine the percentage of duct system temperature loss, divide the 9.1° system temperature rise by the 18.2° air handler temperature rise (9.1 ÷ 18.2 = .5). After you move the decimal point two places to the right, you have a duct system temperature loss of 50%!
How would you address this problem if you measured similar readings on one of your own installations?
Look Beyond the Box
This test will surprise you when you see how much temperature loss there is through the duct system. It also presents you with a new opportunity that most competitors will pass. They're too busy trying to sell larger equipment or bashing heat pumps.
The most appropriate duct temperature loss repairs are adding duct insulation and duct sealing. Temperature testing helps guide you to which side of the duct system needs the most help. Choose wisely from the various insulation options available.
Don't assume a duct system inside the conditioned space is safe from this problem. If you hook up a heat pump to an old, leaky sheet metal duct system with no insulation, you become part of the reason for heat pumps' bad reputation. That might sound harsh, but it's an issue our industry needs to consider and solve sooner than later.
The most appropriate duct temperature loss repairs are adding duct insulation and duct sealing. Temperature testing helps guide you to which side of the duct system needs the most help. Choose wisely from the various insulation options available. Some insulation types offer high promises but fail in extreme conditions. Repeating the temperature test we reviewed can also help 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 (TESP). Insulating a leaky or undersized duct system wastes money and effort. You may often find the system needs additional repairs to operate as intended.
You can see the real problems for yourself when you test and uncover duct temperature loss. This simple diagnostic test can help customers understand why their comfort problems probably have more to do with their duct system than their heat pump. So, if you're tired of hearing people criticize heat pumps, maybe it's time to look past the equipment and focus on the system.
David Richardson serves the HVAC industry as Director of Training for the National Comfort Institute, Inc. (NCI). NCI specializes in training focusing on improving, measuring, and verifying HVAC and Building Performance.
If you're an HVAC contractor or technician interested in learning more about duct temperature testing, contact David at ncilink.com/ContactMe or call 800-633-7058. NCI's website, www.nationalcomfortinstitute.com, contains free technical articles and downloads to help you improve your professionalism and strengthen your company.