On a cold, wintry morning, when a home needs the most heat, a heat pump’s heating capacity is low. That afternoon, when outdoor temperatures warm up, not as much heat is needed and the heat pump capacity is much higher. Let’s take a look at how understanding this simple fact can make or break your diagnostics.
In the morning, a heat pump may deliver only 5,700 Btu per ton and have a 13ºF temperature rise. That afternoon its capacity may increase to 13,400 Btu per ton with a
31ºF temperature rise.
A newer technician may think this system has big troubles. However, a master technician understands it’s a normal system response to outdoor air temperature changes. A technician can refer to the manufacturer’s heating capacity tables to interpret heating capacity and temperature rise, then correctly diagnose and repair system problems.
Gather System Data
As an HVAC professional, an important part of your job is to identify what the equipment capacity and temperature rise should be at the time you’re diagnosing a customer’s problem. To do this requires collecting a few bits of information.
First, identify the manufacturer and equipment model number. Then go online to the manufacturer’s website and lookup its heating capacity tables.
Second, measure indoor and outdoor temperatures. All manufacturer specifications refer to the indoor and outdoor temperatures that the equipment was tested at when rated in the laboratory. Compare the design test temperatures to the temperatures you measure in the field.
Third, determine fan airflow. You can do this in a few minutes by measuring the air handler’s total external static pressure (TESP) and verifying the fan speed setting. Use these two pieces of information to plot fan airflow on the manufacturer fan table.
Fourth, find equipment capacity under current operating conditions by plotting the system’s operating temperatures and fan airflow on the manufacturer’s heating capacity table.
Heat Pump Heating Performance
The heating performance table for each heat pump tells the story of how a heat pump’s heating capacity changes under different operating conditions.
You’ll soon see it is impossible to diagnose system temperature and capacities without this table to help interpret your field measurements.
For learning purposes, we have provided an abbreviated table showing the heating capacity and temperature rise of a typical heat pump with 400 cfm per ton and 70º F entering air dry bulb temperature.
Work from left to right and observe what happens to the rated heating capacity per ton and the temperature rise as the outdoor temperature increases.
Hopefully, after studying this table, the scenario used earlier in the article makes more sense.
If you serviced this heat pump on a 20ºF morning with the heat strips off and measured a 13º F temperature rise over the equipment, then you’d know you were on target.
However, if the outdoor temperature was 50ºF and you measured a temperature rise of 20º F, when you knew it should be 31º F, you would know the heat pump wasn’t heating as it should.
Without this information, odds are you would make a false diagnosis and recommend a solution that would do little to satisfy your customer.
Outdoor temperature and fan airflow significantly change heat pump heating capacity. By the way, similar principles apply to cooling equipment capacity as it also changes with outdoor temperature. But that’s an article for next spring.
How often have you misdiagnosed a system because you didn’t check the specifications and winged an assumption? Don’t worry, you’re not alone. Today is the time to improve your heat pump diagnostics.
Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free Heat Pump Heating Capacity and Dt Table, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.