Air Conditioners and Heat Pumps; How are they Different?

Many homeowners casually call the HVAC system that cools their home in the summer an air conditioner. While that’s a proper description of its cooling function, in many cases the term is technically incorrect. Sometimes the contraption is actually an electric air-source heat pump, which cools your home when it’s warm outside and heats it when outdoor air is cold.

The two most common central heating and cooling residential set-ups in the U.S. and Canada are: 1) a furnace for heating, AC for cooling; and 2) an air-source heat pump for both heating and cooling.

So what’s the difference between an air conditioner and an electric heat pump? Here’s an explanation their similarities and differences.

How Heat Pumps and ACs Are Similar

First, let’s make it clear we’re talking about the most common type of heat pump in use today, the aforementioned electric air-source heat pump.

Among other types of heat pumps are water-source and geothermal, though they’re not as common as the air-source type. From this point on, you can assume this blog is talking about a central, forced-air, air-source heat pump when the term “heat pump” is used.

These are typically split systems, with one unit inside and one unit outside. The units trade their functions when moving from the heating season to the cooling season (and back again).

How Cooling Works in Both Systems

The cooling function of a heat pump works the same way as a standard air conditioner.

• Both systems employ refrigerant (aka coolant), a chemical solution that when heated or cooled, easily changes from a liquid to a gas and back to a liquid again. Refrigerant serves as the medium of heat exchange in an AC or heat pump.
• The refrigerant, as it flows through the evaporator coil inside the house, extracts heat energy from the interior air and transfers it outside, where condensation occurs in a condensing coil, and the heat energy is expelled into the air.
• Inside, now that heat has been removed, the chilled air is circulated through the house by a powerful fan. The evaporation and condensation processes allow for the key heat exchange to occur, extracting heat inside and releasing it outside, respectively.

That’s a simple description of the air conditioning process at work in both ACs and heat pumps, though there’s more to it than that. (We didn’t even talk about the outside compressor unit, without which air conditioning couldn’t happen. Often, when a broken AC requires a major repair or replacement, the compressor is the culprit.)

How a Heat Pump Is Different From an AC

The fundamental difference between an air conditioner and heat pump is that the former only cools the house (and removes moisture), while a heat pump is designed to provide both cooling and heating.

The switch from heating to cooling and back again takes place thanks to a reversing valve that, no surprise, reverses the flow of refrigerant when changing from cooling to heating and back again. When this switch takes place, the outside condensing coil becomes an evaporator coil and the inside evaporator coil becomes a condensing coil.

So in heating mode, a heat pump extracts heat energy from the outside air, then transfers it inside where it warms the interior air. The heated air is then distributed in the house by the same blower/duct/register system that circulates cool air in the summertime.

The heat pump will provide relatively efficient heating during the winter as long as outside temperatures don’t fall much below freezing for extended periods. A standard air-source heat pump that gets regular professional maintenance (as well as simple homeowner tasks such as changing the air filter) can be expected to produce three times the heat energy as the electrical power that’s running the machine (300 percent efficiency).

A standard gas furnace, meanwhile, will never achieve greater than 100 percent efficiency (though when natural gas is especially inexpensive, 90 percent efficiency can be less expensive than electricity operating at 300 percent efficiency).

It’s difficult to generalize about the comparative costs of gas and electricity, since their values fluctuate from year to year, month to month.

Auxiliary Heating in a Heat Pump

When it does get especially cold outside, most heat pumps will default to an emergency or auxiliary electric-resistance heating element that the heat pump is equipped with. This is the usual backup heating set-up, though dual-fuel heat pump systems employ a connected furnace for auxiliary heat.

Some dual-fuel systems allow the homeowner to manually switch from the heat pump to the backup furnace, if, for example, natural gas prices are much cheaper than buying electricity. As stated, that price difference can go a long way toward eliminating the efficiency advantage of heat-pump operation in the winter. Of course, during the cooling season, there’s no practical fuel alternative to electricity for cooling.

Heat Pumps and Air Conditioners Circulate Cooled Air Differently

Heat pumps and air conditioners also differ in how they circulate cooled air.

The interior unit of a split-system heat pump is often called an “air handler.” It contains the condensing/evaporator coil and a powerful fan for circulating the cold or warm air.

Central air conditioners are usually paired in a home with a gas furnace, and during the cooling season, the AC uses the furnace blower to circulate the air. In these systems, it’s important to continue inspecting the furnace filter on a monthly basis year-round, and replace it when it gets dirty.

Likewise, air filter replacement is important with heat pumps as well, since they also rely on a forced-air distribution system.

With split-system ACs, many homeowners cover their outside unit during the off-season, to protect its interior components from the elements. Sometimes this may be a dedicated cover, though it also can be something as simple as a plywood board weighted down by a heavy object. If you use a heat pump to both heat and cool your home, you definitely don’t want to cover the outside unit at any point in the year. This is an all-season unit.

Maintenance Is Essential

Whether your homes is cooled and heated by a heat pump, or cooled by an AC in the summer and warmed by a gas furnace in the winter,  reliable and regular professional maintenance is essential.

The best HVAC companies will arrange to perform maintenance twice yearly, once for your heating and cooling system’s cooling function and once for its heating function. That pro maintenance, along with basic homeowner maintenance (changing home air filters, etc.) will ensure more efficient and reliable performance in providing comfort in your home.

Your favored AC or furnace contractor in Milford, Indian Hill, Mason or wherever you live in the Cincinnati area can set you up with preplanned seasonal maintenance on your cooling and heating system.