Whether individual homeowners know it or not, virtually all American homes have a heat pump in them. Now, when most people think about a heat pump they think of something that actually heats their home. However, in reality, heat pumps are used for lots of things other than heating homes. For example, a refrigerator is a heat pump since it takes warm air out of the interior of the fridge and pushes (pumps) that heat out through the fridge vents. That's exactly the same thing a home heating pump does, except the home heat pump takes heat out of the air, the ground or in some cases water and pumps it into your home to heat it. However, heat pumps used for home heating have an extra feature refrigerators don't - home heat pumps can be run in reverse so they pump hot air out of your home in the summer, doing double duty by cooling your home as well as heating it.
A little background on heat pumps
Heat pumps work on the concept that heat always runs 'downhill', or in other words heat always flows from a warmer area to a cooler area, exactly the same principle an air conditioner uses to cool your home. So, if you think of running an air conditioner in reverse, you can understand how a heat pump works. The heat pump takes warmth from a heat source that could be the outside air, the ground under and around your home, or in some cases a nearby pond or lake, captures it, then carries it into your home to warm it.
Heat pumps aren't a new idea, in fact they've been used in North America and around the world for years. Besides the refrigerator example, heat pump technology is also used for example, in refrigerated trucks and train cars, where they pump the warm air out of the interior the same way a large refrigerator would work.
It took a while for the technology to be improved/adapted to provide home heating and perhaps more importantly, the price points to become more acceptable for consumers to start thinking about heat pumps as a viable home heating option.
How heat pumps work
Heat pumps don't generate heat themselves, they gather heat from an external source, with different designs of heat pumps gathering their heat from various sources. Air source heat pumps are probably he most common type of heat pumps, and gather latent heat in the outside air to provide the heat for your home. While it's difficult to believe there can be any warmth in the January air in Minnesota, there really is some heat out there - (amazingly some Canadian studies have found that even at 0° F, outside air still contains a large percentage of the heat it had at 70°F).
Other heat pump designs gather their heat from alternate sources. Ground source or geothermal heat pumps literally use the ground around your home (where the underground temperature below 4' remains relatively stable winter and summer), and water source heat pumps gather their heat from ponds or lakes.
All heat pump systems consist of five basic components –
- a way to gather the heat from the source,
- a 'refrigerant' to carry the heat (used to be Freon but in recent years more ecologically friendly 'refrigerants' have been developed.
- a compressor that increases the temperature by compressing the refrigerant and also pumps the refrigerant though the system
- a fan to blow the heat into a duct/distribution system and throughout your home
- tubing to carry the refrigerant throughout the system.
Simplistically this is how a heat pump actually works
All heat pump systems are closed loop systems, so starting on the outside, refrigerant is pumped through coils of tubing where it gathers heat from the heat source (air, water, or ground). As heat is picked up the liquid refrigerant expands into a warm vapor that is then carried to a compressor where while it's being compressed, the temperature goes up. The now hot refrigerant is then carried to coils inside your home where a fan blows over the hot tubing, spreading the heat into the distribution system while cooling the refrigerant. After giving up its heat, the now cool refrigerant is carried back out to the heat source where it once again picks up heat and starts the process all over again.
Are heat pumps more energy efficient than other heating systems?
Since they only move heat rather than generating it, heat pumps are surprisingly energy efficient. Just think about it, a furnace needs to turn fossil fuels or electricity into heat before it can distribute it, while a heat pump simply gathers existing heat then moves it into your home.
However, the efficiency of any heat pump is impacted by the difference in temperature between the heat source and the final temperature – the larger this difference, the less efficient the unit. In heat pump terminology the difference between the temperature at the 'source” and the delivered temperature - the 'sink' is known as ' 'lift'. The higher the lift the more energy is required to achieve it and the less energy efficient the heat pump is.
Can you measure the Energy efficiency of heat pumps?
There are at least two accepted ways to measure the energy efficiency of a heat pump
One 'measurement' is the Coefficient of Performance (COP) - a measurement of the ratio of the change in temperature of the output compared to the amount of energy needed to achieve that change. In simple terms, it tells you how much energy was needed to achieve a particular increase in temperature. For example, a COP of 3 means a heat pump can produce 3 units of heat energy for each single unit of electric energy consumed. In favorable situations some heat pumps can achieve a COP rating of 4 making them much more energy efficient than even a high efficiency furnace that operates at 95% efficiency.
Another measures of efficiency for heat pumps is the HSPF (Heating Seasonal Performance Factor) that compares a heat pumps estimated seasonal heating output in BTU's compared to the amount of electrical energy it consumes in watt -hours. The higher the HSPF the more energy efficient the heat pump is. Since 2005 all new heat pumps are required to have an HSPF of at least 7.7 and some units can achieve a rating of 10 (a unit with an HSPF of 8 or more is considered high efficiency and is eligible for a US Energy Tax Credit).
So, heat pumps are proven technology and energy efficient can I actually use one anywhere in the country? In an existing home?
Heat pumps can be used anywhere in the country to provide both heating and cooling - however, realistically in areas where winter temperatures commonly go below freezing, heat pumps likely won't be able to provide enough heat for your home to feel comfortable all the time, and you'll need some form of supplemental heating.
Similar to a central air conditioner heat pumps can be installed in existing homes that have some type of forced air heating – natural gas, propane, oil or electric. If your home has some other form of heating (electric baseboards, hot water radiators), you would need to install ducts to distribute the heat coming from a heat pump.
If you're considering a heat pump for your home there are some things you do need to consider
If the winter temperature in your area commonly falls below 40°F in your area, you won't be able to use a heat pump as your single source of home heating. Many people choose a gas or oil furnace to provide supplemental heat primarily because a furnace will more efficiently maintain the inside temperatures when there is a large difference between the inside and outside temperatures. A dual system like this will provide maximum efficiency since it uses the most efficient way to heat your home relative to the temperature differential (and you will have the advantage of the heat pump's capability to cool your home in the heat of the summer).
Type of heat pump
Air source heat pumps are the most common choice and the least expensive to install, since there is no need to dig up your yard to install the large coils of tubing to gather heat from underground. However, air source heat pumps have an inherent problem in that the outside part of the unit tends to frost up in cold weather, forcing the unit to use some of its heat to thaw itself out, rather than heating your home reducing it;s efficiency. Geothermal units don't have an external component that can freeze (all the heat pump's components except for the tubing are in your home) so this isn't a problem with them.
Ground source /Geothermal heat pumps – Gather the heat from the ground around your home where the temperature is relatively stable winter and summer (and much warmer than the temperature of the outside air in the winter), making them more efficient than air source designs. The temperature increase (the lift) needed to heat your home is less for a ground source heat pump than an air source unit.
The major downside to geothermal units are the costs associated with the digging and excavation to install the underground tubing. For an existing home, these cost can be substantial depending on the topography and actual layout of your property. Obviously it's possible to minimize these costs by installing a ground source system while a home is under constriction.
Sizing of the unit
Similar to a furnace a heat pump needs to be sized properly to work most efficiently in your home. An oversized unit will cycle on and off repeatedly, and won't properly dehumidify your home making it feel drafty and cold in the winter. An undersized unit will have trouble providing enough heat in the winter (and won't properly cool in the summer either).
While understanding the basics of how a heat pump works and the pros and cons of different designs will help a homeowner make an informed decision, actually determining the proper size and type of heat pump for your budget and area of the country is probably a job that should be done in consultations with a qualified heating contractor