How Much Heat Is That Heat Pump Really Delivering?

Have you ever found yourself standing outside on a cold day, looking at a heat pump and wondering how much heat the unit is really delivering? If so, it may be time for you to come up with some better daydreams. However, the heat pump question is still a good one.

If you step back and take a look at what constitutes good heat pump system design and service, you’ll find that you need accurate system performance information to effectively select and maintain heat pump systems. You’ll also find that by adding this knowledge to your personal database, you’ll move far ahead of the industry in your day-to-day work, and be able to provide solutions and solve field problems you may be overlooking.

Good Engineering Data

Each manufacturer provides engineering data for every heat pump unit they manufacturer. All you have to do is to call and ask for it. If they tell you they don’t have this information, you should consider buying equipment from another distributor — it sounds like all you have is a box flogger with little or no knowledge about the industry or your business.

Nearly all of the tables that provide heat pump performance data list the outdoor temperature on the left, and the indoor temperature along the top of the table. Correction factors are often given for airflow from 350 cfm per ton to 450 cfm per ton also. Your job is to plot current temperature and airflow conditions to discover the rated Btu output of the heat pump.

How to Get the Necessary Field Test Numbers

Let’s look at the work it takes before you can plot Btu output on a heat pump performance table. First, you need to measure outdoor air temperature. This sounds easy, but there are a couple of things to be aware of. Some outdoor units may be near a building or have a restriction to the airflow due to an overhang. So, it’s best to measure air temperature entering the outdoor equipment to increase accuracy. A traverse of the entering outdoor unit air temperatures may be needed under certain conditions.

Next, measure the entering air temperature to the indoor coil. When measuring system performance, we normally measure the average return air temperature at the return grilles. However, in this case you should measure the air temperature where it enters the air handler. This will allow you to more accurately interpret heat pump Btu output.
Finally, identify the cfm per ton the unit is moving. Measure the total external static pressure of the system and plot the system airflow on the manufacturer’s fan performance chart to calculate cfm per ton.

Now that you have the air temperatures and the fan airflow, you’re ready to find the rated heat pump Btu delivery.

Determine Equipment Rated Btu Delivery

Using the outdoor and indoor temperatures on the manufacturer’s Heat Pump Performance Table, plot the rated Btu output.

You may need to extrapolate to be accurate within the body of the table (for example, if you have an entering air temperature of 73F, and the table only provides for 70F or 75F). This is an acceptable practice and is necessary to interpret the table accurately.

Just a note: Be sure the heat strips are off during this heat pump testing. If auxiliary heat kicks in during testing, you’ll discover the performance tables don’t quite match up with your field testing.

And here’s a tip: Collect the manufacturer’s data for the heat pumps you regularly install and service. Make copies of the heat pump heating performance data. Hand them out to your installers, salespeople, and service techs.

Since older heat pump performance data may be extremely hard to come by, we have created a Generic Heat Pump Performance Table that technicians may use in the absence of engineering data (see the contact information at the end of the article). Many have found this table more than adequate for service calls when current heat pump data is not available, although actual manufacturer’s data is always preferred.

Field Measure System BTU Output
Once you know the BTU delivery the system is supposed to produce, you can compare against a quick field test of the system’s actual Btu output.

At the National Comfort Institute, we have a service program called HeatMaxx™ being performed across the country, and we’re finding the typical installed heating system is producing only 60% to 70% of rated capacity. The low performance is mostly due to poorly performing duct systems.

To field measure approximate system Btu output takes only about 10 minutes. Measure the average temperature at the system’s return grilles, and then measure the average temperatures at the supply grilles. Subtract these two temperatures to find the net temperature change through the system.

Next, accurately measure the system’s total external static pressure and plot the fan airflow on the manufacturer’s fan performance data. This is the airflow of the fan.

To find system delivered Btu, simply multiply the fan airflow times the temperature change through the system times the sensible BTU constant of 1.08. The answer is the approximate Btus the system is delivering into the home.

Finally, compare the rated heat pump Btu delivery to the measured Btu delivery, and see how well your heat pump systems perform.

Look a little deeper into the performance of the heat pump systems that you service, install, and sell, and get a new view of your job and how well you really serve your customers.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in no cost Generic Heat Pump BTU Output Tables, contact Doc at [email protected] or call him at 800/633-7058. NCI’s website is found at www.nationalcomfortinstitute.com.

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