A New Way to Check Heat Pump Heating Capacity

When a heat pump’s charge and airflow are correct, it should deliver near 100% of its rated Btus. A heat pump in heating mode transfers sensible heat from the outside air to the indoor air through the refrigerant circuit.

When you measure and calculate these Btus, you will use the sensible heat formula, CFM x ∆t x 1.08. This formula uses fan airflow (in cfm) and measured dry bulb temperature change (∆t) of an operating system and then applies a multiplier (1.08).

Step One: Gather Equipment Data

Before you test, you need to gather some equipment data. It will provide the targets you're aiming for as you calculate sensible heat from your measurements. Start at the equipment and gather the following information:

• Equipment manufacturer name
• Model and serial numbers
• Air handler fan speed settings
• Indoor fan tonnage
• Outdoor unit rated tonnage

Once you have this information, find the heat pump heating capacity table and air handler fan table. You will need both of them to perform this check. These documents are often with the manufacturer's literature, or you may need to Google the equipment manufacturer's name and first three or four digits of the model number with the word “specifications.” You'll usually get the correct document on the first page of your search. If you can't find the manufacturer's specifications, NCI has generic heat pump heating tables and fan tables you can use in a pinch.

Step Two: Measure Total External Static Pressure

The second step is to measure the air handling equipment's total external static pressure (TESP) with a manometer. This step will get you vital information necessary to plot fan airflow using the air handlers fan table in the third step. Before you measure TESP, check the condition of the blower wheel and indoor coil. If they are dirty, your TESP measurements will look too good to be true. You can use other airflow measurement techniques to measure fan airflow, such as a duct traverse. It will get you very close when system conditions are dirty.

Install two test ports where air enters and exits the air handling equipment. Some air handlers include a factory filter, while others don't. If you're unsure how the manufacturer rated the air handler, you can find out in the footnotes of the air handler's fan table.

NCI has found it more accurate to measure your pressures after the filter and in the supply plenum to keep your readings consistent. If you measure one pressure at a time, add these two readings together. If you use two hoses, you can read TESP directly on the display of your manometer. Then, record your measurement to use in the next step with the fan table.

Step Three: Plot Fan Airflow

Next, it's time to plot fan airflow. Before you measure it, you need to know what the heat pump's target airflow is. Use the industry standard of 400 cfm per ton and multiply it times the tonnage from the heat pump model number. For example, if you have a 2-ton heat pump, you should be within ±10% of 800 cfm (400 x 2 = 800) moving across the indoor coil.

Once you know the required fan airflow, identify the fan speed setting from the blower speed taps on either the board or a controller. Next, intersect your fan speed and TESP reading from step two on the fan table. Where the lines meet is the approximate amount of fan airflow the air handler is moving. You now have your first piece of information to use in the sensible heat formula.

Step Four: Measure Dry Bulb Temperature Change

Next, measure the air handler temperature rise with only the heat pump running. No heat strips should be on during this test. If they energize, your temperature change will be higher than usual. Temperature measurement works best when you use a fast-acting digital thermometer. Dual probes or wireless probes make this step even quicker and increase measurement accuracy.

Ensure the heat pump has run for at least 10-15 minutes before you measure air temperatures entering (return) and exiting (supply) the air handler. Avoid any heat source affecting your readings like a duct leak, motor heat, or bypass humidifier. If you use a single thermometer, start with the return temperature and then quickly measure your supply temperature.

Subtract the two temperature readings to determine the air handler temperature rise or delta t (∆t) — the second piece of information in the sensible heat formula.

Step Five: Calculate Delivered Heating Btus and Compare to Equipment-Rated Btus

Now that you have airflow (cfm) and temperature rise (∆t), you can finish the sensible heat formula. This calculation allows you to determine the delivered heating Btus of the heat pump. Next, you need to determine the rated heat pump heating Btus. This value is what you will compare your delivered heating Btus against.

Measure the outdoor ambient temperature entering the heat pump and then plot it on its heating capacity table to see how many Btus the heat pump should deliver at the conditions you're testing for.

Once you have your measured heat pump heating Btus and rated heat pump heating Btus, you can apply them in the formula to see how close you are. A heat pump with proper airflow and refrigerant charge should deliver ±10% of the rated heating Btus.

For example, you measure a four-ton heat pump system on a 55°F day delivering 21,033 sensible Btus. Referring to the heat pump heating capacity tables, you find the rated heat pump heating Btus under current conditions is 36,900 Btus. Divide your measured Btus by the equipment-rated Btus to find the percentage of rated capacity. The finished formula would appear as follows:

21,033 measured Btus ÷ 36,900 rated Btus
= 57% of rated heat pump heating capacity

If the heat pump was heating correctly, it should deliver close to 36,900 Btus. However, since this heat pump is operating at 57% of its rated heating capacity, more troubleshooting is needed.

With a percentage of rated capacity this low, your next step would be to determine which of the values in the sensible heat formula are outside of the equipment manufacturer’s specifications. Ask yourself two questions:

1.Is fan airflow within +/- 10% of required airflow at 400 cfm per ton?

2.Is the temperature change (∆t) near manufacturer specifications at the outdoor temperature tested?

Next Steps

While this method is not perfect, it allows you a simple way to see how well the system performs compared to equipment ratings. It also offers a simple number your customers can relate to.

Try these five steps the next time you get a call about a heat pump that isn't performing well. First, check to see that the equipment is delivering near its rated Btus. Then, if the equipment is operating as designed, it's time to dig deeper into the duct system and its influence on system performance. Remember, heat pumps are susceptible to duct temperature losses, just like a cooling system.