Heat pump vs. air conditioner
First, an air conditioner is a type of heat pump. It “pumps” heat from the inside of a building to the outside to cool the indoor space. The critical difference between the terms is that a heat pump can work in the other direction1 as well. While an air conditioner is only useful for cooling, a heat pump can switch modes and move heat from the outside into the building.
It’s this switching between modes that makes a heat pump more attractive in climates that have hot summers and cool winters. Of course, there is a bit more to the story…
Moving heat around
Heat pumps and air conditioners rely on the same physical laws to operate. They use a system that compresses a refrigerant (which raises the temperature), allows this compressed gas (or liquid) to cool to ambient temperature, and then lets it expand (which lowers the temperature). This takes advantage of the properties of gasses which cool when they expand quickly. This is the same physical process that makes the outflow from a spray canister cool – the compressed contents spray out and expand.
In air conditioning systems this happens with three main elements:2
- Compressor – takes the low pressure refrigerant and compresses it, which raises the temperature.
- Condenser – Allows the high temperature, high pressure refrigerant to cool to ambient temperature (for AC, the outside temperature).
- Evaporator – the cooling coils where the refrigerant is allowed to expand and cool (for AC, this is inside the house).
A heat pump works this way as well, but switches the function of the condenser and evaporator. One way to think of this is that when it’s in heating mode, the heat pump is cooling the outside as it heats the inside. Functionally, you could do this with a window air conditioner by mounting it backwards in the window – now, the hot side is in and the cool side is out.
Besides cooling, air conditioning also dehumidifies the air. By removing moisture and making the air drier, we sweat more efficiently. This means we feel cooler when the air is drier, even if the measured temperature is the same.3
The cooling coils of an air conditioner will naturally remove water from the air. This is the same mechanism that causes water to bead up on a cool glass. This condensate is then allowed to drain away – usually to the outside (for a window air conditioner) but can be drained (for split systems or central air).
Another important point is the thermal insulation between the two “sides” of an air conditioner or heat pump. To most efficiently keep the hot side hot and the cold side cold, there has to be a good barrier so that air doesn’t just mix between the two. You can imagine how inefficient an air conditioner would be if it were just sitting in a field and running. The coils on each side would be hot or cold, just as they are meant to be, but the air temperature wouldn’t change. The separation between “outside” and “inside” is important.4
This last concept is why split systems (where the condenser and compressor are completely outside and away from a building) are more efficient than window air conditioners. Because the hot side and cold side in a window unit are right next to each other, the insulation cannot be as good as a split system. This is also why refrigeration lines are insulated. Heat losses can occur if the lines the refrigerant is moving through are not well insulated.
The process of compressing and moving refrigerant around in a cycle costs energy. Furthermore, the temperature difference you are trying to overcome also affects how hard a heating or cooling system has to work. It is easier to go from 75 degrees to 70 degrees (5 degree difference) than it is to go from 85 degrees to 80 (the same number of degrees). The reason for this is because the amount of heat moved in each cycle depends on the difference between the temperature of the refrigerant at the condenser coils and the outside temperature. The result is that for hotter climates, it costs more (per degree) to cool a building than in cooler climates.
The same rule applies when using a heat pump in heating mode. The colder it is outside, the more work the unit has to do to heat the inside.5
Because climate matters so much, heat pumps and air conditioners are rated based on the area of the world they will be used. The SEER (Seasonal Energy Efficiency Ratio) takes this into account.6 It also explains why heat pumps usually have a lower SEER rating in heating mode than in cooling mode.
When cooling a building, the difference between outside temperature and inside is usually no more than 30 degrees, and usually closer to 20 (90 degrees outside, 70 inside). In heating mode, the temperature difference can be much greater (30 degrees outside, 70 inside), depending on the area of the country you are in. With the greater difference in demand, the efficiency falls.
Finally, the size of the unit makes a difference on the SEER rating you can expect. An undersized unit will have to cycle more to move the same amount of heat around. More cycles mean less efficiency. Properly matching the size of a unit to the expected demand is then an important consideration – especially as energy costs rise.
At some point, the efficiencies of a heat pump (in heating mode) fall below that of other types of heating. For this reason, heating systems will often have a supplemental source of heat – either electrical (most common) or fossil fuels.
Types of heat pumps
Air conditioners usually run with a refrigerant that evaporates easily in ambient temperatures and compresses into a liquid under high pressure. But in extreme environments, a heat pump may have to operate where the outside temperature is very low. For this reason, other types of heat exchange may be used.
The normal type is called an air-source heat pump, where the outside coils are either heated or cooled by the air around them. But in Alaska (for example) the outside air may be extremely cold in the winter. In this case, a better solution is to take heat from another source – the ground. 7
Although it seems odd to extract heat from frozen ground, at some depth, the soil is actually warmer than the surface air. This concept also works in other parts of the country, where the ground is relatively cool in the summer and relatively warm in the winter. These are called geothermal systems.
Geothermal systems use coils that are buried in the soil at a depth where the temperature remains fairly constant. Also called Ground Source Heat Pumps (GSHP), these systems are more than 50% more efficient when heating and 20% more when cooling. The efficiencies come from the temperature differential mentioned above – the ground temperature is less variable than the outside air temperature.
Another potential source for harvesting heat is with solar panels. These use solar energy during daylight hours for heating during the winter (with geothermal or other ballast to store heat for nighttime) and use solar energy for electricity to run traditional units in the summertime. Other heat pumps are marketed as “solar ready” because they tie into solar power panels.9
Almost any source can be used to “mine heat” with a heat pump. In large buildings, the exhaust from the already heated interior (plus body heat from occupants) can be mined to regain some heat and reduce operating costs. Deep geothermal can be tapped (where temperatures are quite high), hot springs can be used and even bodies of water that differ from air temperature (in the right direction—cooler in summer, warmer in winter). Some systems even use a heat exchange with the incoming water supply when water from the local utility company is considerably warmer or cooler than the outside air temperature.
Which is the best option?
Choosing between an air conditioner alone or a dual-function heat pump is largely a mixture of cost and convenience. In areas of the country where the summers and winters are not extreme, a heat pump allows both heating and cooling without switching systems. In other parts of the country, where demand for either heating or cooling is high, a heat pump would have to be part of a mixed system, usually on the heating side.10
GSHP is making inroads and looks to be an up and coming technology. Because of the efficiencies of geothermal and rising energy costs, these systems show a great advantage over traditional central air or standard heat pumps. The downside is the installation costs, so they find their best applications in new construction rather than switching existing systems.
For areas of the country where summer heat is the main problem, an air conditioner (split or central) is still the most popular option. Heating is minor and for short periods making the additional expense of a heat pump unwarranted.
There are several factors to consider when pricing a heat pump over an air conditioner. The first is the cost of electricity in your area. If electricity is much more expensive than other heating options, an air conditioner for cooling and a furnace for heating are probably best. Also consider how cold your winters are. If they are often below 40 degrees, you will likely need supplemental heat anyhow and electricity may be too expensive.
The next consideration is the price differential between a heat pump and an air conditioner. The base prices are usually not much different (perhaps 5% of the cost) and installation is slightly higher with a heat pump combination. But there’s a hidden cost. Since a heat pump, unlike central air, runs year round, the unit’s life span will be half as long.11 Some HVAC companies will also charge more for servicing heat pump and warrantees will reflect this harder usage.
On the plus side, you may qualify for a tax incentive from your State by installing a new, energy efficient heat pump. Geothermal systems still have a federal tax credit as well.
One tip is to visit a new development in your area. Generally, developers will opt for the best package on new construction and these will reflect local energy costs and climate conditions. Since systems are outside, a little poking around will give you the names of models in use (and sometimes installation company information). Actual costs can be found by talking to homeowners. Most are perfectly willing to tell you the good and the bad about their system.
A second resource is HVAC companies in your area. Balance the information you receive from them against what you find on the Internet to make your decision. HVAC companies may have an incentive to sell models they service, but most will recommend products that are decent, if only because they have to service them under warranty.
Air conditioners are cheaper to purchase and operate than heat pumps (except for geothermal systems, which are more expensive to install, but much cheaper to run). Heat pumps have the advantage of providing heat at a more efficient rate than other types of heat (within a moderate temperature range). They cannot supply all the heat needed for very cold winters (temp less than 30 degrees for a substantial period), except for GSHP, which taps ground heat and performs well in a greater range of temps.
Since the cooling capacities between the two are about the same (remember, an AC unit is a “one way” type of heat pump) it’s the heating side that pushes the decision one way or the other. Thinking of the choice in these terms will help solve any dilemmas and make the complex differences manageable.