One of the biggest energy expenses in the home is heating and cooling. Upgrading older, inefficient equipment to 95-97% efficient equipment can result in huge energy savings. Let's break this down:
There are several different ways to heat a home. Most commonly seen are split systems- where the furnace (either powered by electricity or gas, diesel or propane) and a separate air conditioner. Also seen are package units (heat and air together in one metal box) or mini-splits which have an outdoor condenser but also indoor wall mounted outlets that provide heat and cooling. Some homes may have a wall furnace and some are heated by propane, diesel, kerosene, or even wood.No matter what form of furnace you have they all have an efficiency rating. For gas furnaces there is a percentage followed by the letters AFUE, which stands for Annual Fuel Utilization Efficiency. Some older furnaces can be as low as 65% efficient, with newer gas furnaces getting as high as 97% efficient. For electric units the number will be a whole number with a decimal, such as 7.8, followed by the letters HSPF. This stands for Heating Seasonal Performance Factor and is very similar to AFUE. On these types of units the range is 5.6 for really old models, up to 9.3 for some of today’s ultra-efficient options. Even less efficient are electric baseboard resistance type heaters which are about 3.14 HSPF.
Just like heating there are several different types of air conditioners. In a split system, the condenser is the part that is outside, usually on a patio or back yard. These split systems have a coil that is part of the furnace and both the condenser and coil work together to cool the air, which is then distributed into the house via the fan that is part of the furnace.
There are also air conditioners that are part of packaged units, room air conditioners that go through a wall or in a window, as well as portable air conditioners that are on wheels and can be moved from room to room. The efficiency of all air conditioners can be easy to figure since they are all powered by electricity.
SEER is the Seasonal Energy Efficiency Ratio and EER is the Energy Efficiency Ratio. These are values that have been assigned to compare how efficient the air conditioner is. The SEER addresses the efficiency of the air conditioning unit itself, while the EER is a rating applied to the combination of equipment including the coil that is inside the furnace. Older models average 8 SEER/6 EER, while newer high efficiency models can get up to 17 SEER/12 EER, if not higher. Some mini splits are in the 30 SEER range.
Another thing you will hear about with air conditioning is the tonnage. This is a way to classify how “big” your air conditioner is. Having an under-sized or over-sized unit can lead to comfort problems inside the home. Too small and the house will never cool down. Too large and the air conditioner will keep turning itself on and off (known as short cycling) which can burn out the unit well before it’s time and cause the compressor to freeze.
Most HVAC systems we find in modern homes are ducted systems- the hot or cold air goes through the “air handler” which is part of the furnace, and is then delivered to the rooms in the house. Each room has a supply register, usually on the ceiling or the floor, which is how the air gets into the room. Then somewhere else in the house is a larger register known as the return. This is how the air gets back into the duct system in order to be heated or cooled again.
Ducts leak. Old ducts, new ducts, metal ducts, flex ducts. All ducts leak. The goal is to get them to leak as little as possible. The air in the duct system is air that you have paid to condition, to make hot or cold, and you want as much of it as possible to get into the home. Studies have shown ducts leak an average of 25-40%, even in new homes. Energy conservation has just never been a priority during the construction of the home and installation of the system. Just install a larger furnace or a/c and duct leakage won’t be noticed. These days are gone. We need to have all the heated or cooled air delivered to the appropriate rooms.
There are a few things to consider with ducts- what are they made out of, how much insulation to do they have, and where are they leaking. Let’s take each of these in turn.
Metal ducts- the older ducts are made of metal and have many elbows/joints/turns in order to get where they need to go. At each turn or joint (every 4-5 feet) there is a potential for duct leakage. This never used to be an issue as a larger furnace was installed to compensate for the high duct leakage. The old way of sealing these joints was to use cloth-backed duct tape. That’s right, the grey stuff that we all know and love to use for projects around the house. Well, it’s been found over time the adhesive on cloth backed duct tape breaks down and it simply falls off. So the joints that might have been sealed when installed are no longer sealed.
Flex ducting- flex ducting is preferable because it comes in long lengths, so there are no leakage points other than at the plenum or the register. As the name implies, it us flexible and easily changes direction around obstacles or upward to the register without extra fittings which would be a potential leakage point.
Insulation- duct insulation has been around since about the 1950’s. Heating and cooling contractors and duct manufacturers realized that sending conditioned air through very hot or very cold metal would significantly alter the temperature before the air arrived at it’s destination. So not only did wrapping ducts become popular, but companies started making ducts that already had insulation on them when they left the factory. In the beginning there was just a light covering, maybe an R-2 value which is one layer of ¾” fiberglass duct wrap. Then R-4.2 became popular. Now the standard is to use ducts that are R-8. R-8 is several inches of fiberglass between the duct and the outside. This level of insulation is enough to provide a good thermal break without being too bulky to work with or take up too much space in a tight crawlspace.