How can you know if your existing home’s heating and cooling systems are efficient?

New homes are often modeled in software before they are constructed, and in many states this model is required to result in a certain level of heating and cooling efficiency. But after a home has been constructed (with varying levels of quality), the smart thermostat has been programmed (or not!), and the occupants settle into their own specific behavioral patterns (are all the windows shut when the heater is on?), that original model no longer applies and you have to take another approach.

One approach is to hire a professional HVAC auditor. They will come to your home and check the condition of your HVAC equipment and thermostat settings, the levels of insulation, the levels of infiltration (often using a “blower door test”), and the quality of the ducting if you have a forced air system. This can cost from a few hundred dollars to well over a thousand depending on where you live and the size or complexity of your home.

Another approach is to use a good “virtual audit” tool like HomeIntel. They can assess your home’s heating and cooling efficiency for you automatically by analyzing your utility data and local weather data. But if you don’t have access to such a tool in your area below are some high level instructions on how to do it yourself.

What units used?

A home’s heating or cooling efficiency can be expressed in terms of BTUs (energy used) per square foot (size of home) per degree day (a measure of the local climate). Let’s describe each of these in more detail:

• BTUs: This is a unit of energy just like kWhs (used for electricity), therms (used for natural gas), and calories (used for food energy). In this case, we want the total amount of electric and heating fuel energy used to heat (or cool) your home over a period of time (e.g. one year). First you’ll need to estimate how much of your electric and fuel use goes toward heating or cooling your home, then you’ll need to convert kWhs and therms (used by utilities) to BTUs.
• Square foot (sf): This is the size of the floor space in the conditioned areas of your home. Normally places like the garage and the attic are unconditioned (not heated or cooled), so these spaces are excluded from this total. A typical single family home might be 2100 sf, and this information is often publicly available for free on real estate sites like Zillow.
• Degree days (hdd and cdd): Heating and cooling degree days are unique units used to measure how much heating (hdds) and how much cooling (cdds) a building needs in a particular location or climate to keep it’s interior comfortable for the occupants. Read more about them, or collect them for your home for free here.

What values are good?

Using these units, the DOE has said that a “modern efficient home” should have a heating efficiency around 5 BTU/sf/hdd (assuming a fuel-burning furnace with AFUE of 80%) and a cooling efficiency around 1.5 BTU/sf/cdd (assuming an air conditioner or heat pump with a COP of 2.7). The terms AFUE and COP are measures of device efficiency, and can be converted to each other.

For example, a home with a heating efficiency of 8 BTU/sf/hdd is doing pretty well, whereas a home using 20 BTU/sf/hdd needs a lot of work!