Heating and cooling is the largest energy expenditure of the average home, so it makes sense to reduce energy bills by getting efficient equipment to do that heating and cooling for you—especially when you’re building a “Pretty Good Home” that incorporates passive solar design and super insulation.
When building such a home in the southeastern US, you can’t get more efficient than an air (or ground) -source heat pump for heating your home efficiently, and affordably. And if you’re going to have a heat pump, odds are you are going to have cooling too, because it’s hot out, and an air conditioner is just a heat pump running in reverse. (There’s some simple physics behind why air source heat pumps are so efficient a choice in mild climates.) A high efficiency heat pump and air conditioning isn’t the only energy efficient choice in this climate and it isn’t the only green choice either, but as far as cost to install and operating costs are concerned, it becomes one of the most practical choices. An air conditioner is also an important consideration in a climate that is particularly humid—as ours often is—because the process of cooling air with an air conditioner also dehumidifies it, keeping excess moisture from building up in the house to damage interior materials and making the occupants much more comfortable.
There are three ingredients to an efficient heating and cooling system worthy of the performance of a “Pretty Good House” and only the first ingredient is the efficiency of the unit itself. Toward that end; however, look for a heat pump with a heating season efficiency of 9.0 HSPF or greater, and a cooling season efficiency of 16 SEER or greater. There will be a premium to be paid for units that are quite this efficient compared to the base efficiencies available, but considering that this system will last a decade or two, the ongoing efficiency is worth it.
The second ingredient is getting a system that is properly matched to your home design. There is a lot of physics involved in the heat flow of your home, and things like how much insulation, what type of windows, what direction your windows are facing, matter a great deal, especially when you’ve built a home that has more efficient windows and more insulation than average. Heat pumps and air conditioners are typically sized in “tons,” a measure of the amount of heat or cool they can produce, and homes are given a system with a certain number of tons of capacity based on how much heat and cool the HVAC installer determines that they’ll need. A larger home will require more tons than a smaller home, and a well insulated home will require fewer tons than a same-sized home that does not have as much insulation. Getting the precise amount of tons that you need right, without overdoing it, is important since installing a 3 ton system costs more than installing a 2 ton system or even a 2.5 ton system. An over-sized system uses more energy than is needed to get the home to the desired temperature, and can also be uncomfortable. Air conditioners that are overpowered for their homes will turn on, bring the house to temperature quickly, and turn off again, waiting for the house to warm up substantially before turning on very briefly once again. This short amount of run time doesn’t give the air conditioner enough time to properly dehumidify the air, as well as cool it, making the occupants continue to feel warm even though the air has been cooled.
The solution is to make sure the equipment has been sized to match the needs of the home by asking your contractor to perform what is called Manual J calculation, which looks at the actual home design to calculate how much heating and cooling energy the home actually needs. Any HVAC contractor should be able to do one if asked to, but may not automatically do one otherwise—and just because a Manual J calculation is done doesn’t mean that the energy efficiency of the home has been fully considered, especially if the heating and cooling contractor doesn’t know much about what’s going on in the rest of the house. It can also be a good idea to enlist a third party energy professional to the cause, and look to get your home certified under a program such as Energy Star for Homes. Under most green programs, heating and cooling system design must be highly scrutinized to ensure the energy efficient features of the home are taken into account, and that the system is not over-sized.
The third ingredient is the way that the heated and cooled air is distributed, typically through air ducts connecting to all the rooms in the house. This duct-work is often run through an attic, crawlspace, or basement—and may or may not be installed in a way that is airtight and avoids being overly restrictive of airflow. You could have the most energy efficient heat pump in the world—but if it’s forcing air into your house through ducts that leak half of their content into your crawlspace and tie themselves in knots on the way to your living room, that energy efficient heat pump will have to operate way more than it needs to in order for you to actually feel warm air.
This, again, is where a third party certification program such as Energy Star can help out, as these programs also often involve visual inspection and testing of duct-work by an energy professional.
Really, as far as heating and cooling goes, even those who don’t want to certify (and remember, the idea behind a “Pretty Good House” is to build green without following the dictates of a certification) do receive an important quality assurance benefit from hiring a third party professional. Oversight on Manual J calculations and duct-work installation are two common green building certification program requirements that can make a big difference in how the home performs.