Heat Pump Mysteries

I smelled fall in the air yesterday morning for the first time. The cold is on its way. Let’s take a few minutes and consider heat pump capacity and take a look at how heat pumps will act on those colder mornings this winter. Did you know in some parts of the country, there are days when a three-ton heat pump will deliver less than 12,000 BTU?

Diminished Capacity
Heat pumps can be like a fair weather friend. When you really need them, they’re just not there for you.

As the temperature outside drops, so does the heating output of an air-to-air heat pump. When it’s 55F outdoors, you can expect the traditional 36,000 BTU of heat from a typical three-ton heat pump. When it drops to 35F outside, its heating capacity will drop to around 24,000 BTU. And when the mercury hits zero, the typical three-ton heat pump is lucky to deliver 12,000 BTU of heating into the building.

In addition to heating capacity reduction, as the outdoor temperature declines, so does the efficiency of heat pumps. I was studying one manufacturer’s heat pump specifications last week. At 65F outside, the EER was rated at 16.2; at 32F it dropped to 10.9, and at -10F, it fell to 5.0.

Don’t get me wrong, heat pumps are terrific as long as we understand what they do and size and apply them accordingly. In certain areas of the country, a heat pump is the only way to go. Down to a certain outdoor temperature they’ll blow the socks off of electric heating and even outperform gas heating in certain regions of the country.

Remember the purpose of this article is to help technicians and designers become aware of the changing conditions so we can understand and deal with these issues when we face them in the field.

The cause of all these mysteries is physics. As outdoor conditions change, heat transfer properties change. And, the equipment simply responds to the laws of nature.

Because temperature and energy costs vary so much around the country, we’ll stay away from determining set points and energy savings issues in this article. Let’s stick to the basics today.

Manufacturer’s Engineering Data
So, where do we find out what to expect of a heat pump under different operating conditions?

Each manufacturer publishes engineering information that reveals what each piece of equipment will do under certain conditions. The problem is that many of us are unaware of what this valuable information holds for us in the field. This knowledge will help us solve problems our customers may have had for many years.

When it comes to heat pumps, each manufacturer publishes tables that help us see what the equipment is doing, or will do under certain operating conditions. The information provides a delivered BTU rating, under a certain set of conditions. The conditions range from 65F to below zero outdoors based on the required airflow at 70F degrees return-air temperature.

More important information includes expected temperature rise through the equipment (usually at 400 CFM per nominal ton), as well as the equipment rated kW and Amp draw under different conditions. The COP rating is also included at the various outdoor conditions.

This valuable information can act as a map to guide us through troubleshooting and problem solving as we work in the field and as we design systems in anticipation of live operating conditions the system will face through its life of service.

Heat Strip Capacity
In many areas of the country, back-up or auxiliary heat is needed when outdoor temperatures drop below a certain point, and as the heat pump capacity diminishes and can no longer keep up with the building heat loss.

Heat strips are rated in Kilowatts and the capacity is usually listed on the nameplate of the equipment.

Heating capacity is based on exact electrical characteristics, but generally you can multiply the heat strip rated kW by 3413 to find the expected heating output. For example 10 kW x 3413 indicates these heat strips should deliver 34,130 BTU of heat.

As far as this article goes, let’s keep it simple. When designing for heating verify the heat loss of the house using ACCA manual J. There is a point in many regions of the country where as it gets colder, the heating capacity of the heat pump alone may be inadequate to heat the building. At this point, auxiliary heat is added to the diminished capacity of the heat pump to make up the difference.

For you southern folks, heat strips are sometimes used for more than the defrost cycle. You lucky guys!

Take the time to download your manufacturer’s heat pump engineering data from the Internet. Keep it in a place where you can study it from time to time and become aware of what these marvelous machines can and can’t do, depending on the weather outside.

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 a free NCI Generic Heat Pump Performance Table, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.

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