When you think about it, "pump" is a funny word isn't it? Heat Pump is also funny, because the name is a little misleading. When you see Heat Pump, you only think about "heating", but it can also be used to cool. So really, it should be called a Heat/Cool Pump. The most common form of a heat pump is the air-source heat pump, and that will be the focus of this article. There are other types of heat pumps, like the geothermal heat pump, which we touched on in September of 2009.
HowStuffWorks.com has a great article on the Heat Pump which includes a nice animation of the heat pump heating process. I do wish they would focus more on how it can cool in the summer months. Basically, a heat pump in heating mode just acts like an air conditioner in reverse. In the summer, an Air Conditioner uses a heat transfer fluid to absorb your home's hot air. A blower moves your home's hot air over coils that contain fluid that is cooler than your home's air. As the air passes over these coils some heat that is in the air is absorbed by the fluid and thus the air temperature is lowered (the air loses heat, which results in a lower temperature) and this cooler air blows into your home. A heat pump works in a similar way, except that instead of your home's heated air being transferred to the fluid in air conditioning mode, a heat pump uses heat that is in outside air and transfers that heat into your home.
The state of California has a good description of air conditioners and heat pumps on their Energy Quest site (even though it looks like it is made for kids it has some great explanations):
A heat pump is an air conditioner that contains a valve that lets it switch between "air conditioner" and "heater." When the valve is switched one way, the heat pump acts like an air conditioner, and when it is switched the other way it reverses the flow of the liquid inside the heat pump and acts like a heater.
The DOE Energy Savers Heat Pump guide has a great introduction to what a Heat Pump is and where it works:
For climates with moderate heating and cooling needs, heat pumps offer an energy-efficient alternative to furnaces and air conditioners. Like your refrigerator, heat pumps use electricity to move heat from a cool space into a warm, making the cool space cooler and the warm space warmer. During the heating season, heat pumps move heat from the cool outdoors into your warm house; during the cooling season, heat pumps move heat from your cool house into the warm outdoors. Because they move heat rather than generate heat, heat pumps can provide up to 4 times the amount of energy they consume.
My biggest question with this opening paragraph is what defines a moderate climate? While this isn't defined by Energy Savers in their heat pump section, they do touch on it in the Heating with Electrical Resistance section:
If electricity is the only choice, heat pumps are preferable in most climates, as they easily cut electricity use by 50% when compared with electric resistance heating. The exception is in dry climates with either hot or mixed (hot and cold) temperatures (these climates are found in the non-coastal part of California; the southern tip of Nevada; the southwest corner of Utah; southern and western Arizona; southern and eastern New Mexico; the southeast corner of Colorado; and western Texas). For these dry climates, there are so few heating days that the high cost of heating is not economically significant.
But I think Energy Savers was a bit conservative in listing areas where heat pumps don't work so well. From their air-source heat pump page they state:
When outdoor temperatures fall below 40°F, a less-efficient panel of electric resistance coils, similar to those in your toaster, kicks in to provide indoor heating. This is why air-source heat pumps aren't always very efficient for heating in areas with cold winters. Some units now have gas-fired backup furnaces instead of electric resistance coils, allowing them to operate more efficiently
Let's face it, it has been a cold winter all across the country, and I'm sure many areas that may be considered "moderate" have had long periods of time where the temperature was below 40°F. So for the areas which haven't been that cold this winter (which only include the following: Miami, Hawaii, Puerto Rico --> this is a non-scientific list) I'm sure a heat pump is a good idea, but what about the rest of the country, where it has been freezing? In other words, it is great if heat pumps work well 95% of the year, but if they consume much more energy (by relying on electrical resistance heating) in the 5% of the year that is very cold, would it make more sense to rely on another form of heating/cooling for your home?
Highlighting the case that air-source heat pumps may not be the best solution for some locations is the experience of a Mapawatt team member, Patrick. He and his girlfriend just moved to Asheville, N.C. and they have wall-mounted air-source heat pumps.
When we moved up to Asheville on December 23rd and into our new apartment I was concerned about heating the 1400 sq ft apartment during the cold months that lie ahead. The apartment sits at the bottom level of a 5 story 19th century converted hospital and comes equipped with 3 electric wall heat pumps placed strategically throughout the apartment. With pets and myself working from home during the day it was a requirement to heat the space to a comfortable temperature at all times of the day (68° F). Not knowing what to expect I decided to wait until my first bill arrived in order to know what steps I should take to heat the space efficiently. However, I was quite surprised after receiving a bill from the utility company several days after moving in. It seems as though I moved in at the end of the billing cycle and this particular bill was for 5 days of service. Minus the connection fee, the bill totaled $51 for 5 days of service. Nearly 528 kWh were consumed during those 5 days. At that rate, I was looking at a $300 utility bill for a full month of service!
Unlike the bedroom and kitchen, which have other apartments above them, the living room has our upstairs neighbor's porous concrete porch above it. It seems that, in addition to the 2 arrays of windows in the living room, the fact that there is a porch exposed to the outside air above this room (and not other heated apartments) is the reason this space is so hard to keep heated. Now I have blocked off the living room and shut down the heater that was constantly running and only run the heaters in the bedroom and kitchen. Additionally, I plan to shut down the heat pumps in all rooms and instead run water/oil filled radiant heaters to heat the rooms.
What Patrick's story shows is that while air-source heat pumps may work great in some areas, they may not work well in areas that get fairly cold. Relying on my old post on finding your city's temperature history I was able to see that the average temperature in Asheville for Dec. 23-28 was right around 36 degrees; which is below the 40 ° temperature that Energy Savers suggests for heat pump effectiveness! Considering Patrick's extremely high bills over this time it makes me wonder if a heat pump makes sense in climates that may experience bouts of weather under the 40 degree mark.
With that said, heat pumps do work well in many areas of the country (we're still working to define those areas, and if you have a heat pump please tell us where you live and how it works in the comments below!). When you are considering buying a heat pump there are three main terms you will need to know and I am taking the definitions straight from Energy Star's website:
- Seasonal Energy Efficiency Ratio (SEER), is most commonly used to measure the efficiency of a central air conditioner. The higher the SEER, the more efficient the system. SEER measures how efficiently a cooling system will operate over an entire season.
- Energy Efficiency Ratio (EER) is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (95oF). The higher the EER, the more efficient the system.
- Heat Seasonal Performance Factor (HSPF) is the most commonly used measure of a heat pumps heating efficiency. The higher the HSPF, the more efficient the heat pump.
Luckily Energy Star has an air-source heat pump buying guide that provides great detail on what to look for when considering purchasing a heat pump.
Heat pumps can be a much more efficient way to heat and cool your home, but I'm not going to tout their benefits for all locations. Too often I'll read articles promoting an efficient product with total disregard for where that product will be put into place. Energy related to heating and cooling is totally dependent on the region where that heating/cooling takes place (this may provide a little insight as to what puts the "Map" in "Mapawatt").
When considering if a heat pump is right for you, do some research into how well they perform in your geographic location, and then figure out the best one to buy if it looks like it is the best option!
Here's another link on the basics of a Heat Pump.