Because of the inefficiencies of generating electricity by burning coal or gas, along with some transmission losses, it takes about 3 units of source energy to produce 1 unit of site energy. In other words, utility grid power is about 33% efficient. When fossil fuels are burned on-site, conversion losses are much lower. The site-to-source conversion factor for natural gas is 1.092 (1.092 units of source energy for every 1 unit of site energy). The ratio is 1.158 for fuel oil and 1.151 for propane.
Let’s compare the real efficiencies of electric baseboard heat and a run-of-the-mill gas furnace with an annual fuel utilization efficiency (AFUE) of 78%. When the site–source ratio of 1.092 is applied, the 78% AFUE furnace is 70% efficient on a source basis, meaning that for every 100 Btu of energy potential in the raw fuel, 70 Btu are delivered in the form of heat.
On a site basis, electricity is roughly 100% efficient because virtually all of the site energy is converted into heat. But on a source basis, gas is more than twice as efficient, roughly 70% compared to 30%.
Comparing site and source energy gives a better picture of heating efficiency, and a more accurate measure of the environmental impact of using different types of fuel.
Oversized heating equipment costs more initially and will not operate as efficiently as a correctly sized appliance. Equipment that’s too small won’t provide adequate heating. So the goal should be to be to install equipment that’s chosen carefully on the basis of actual heating loads rather than guesswork.
The standard for many years has been “Manual J,” published by the Air Conditioning Contractors of America, which allows HVAC contractors to do the necessary heat load calculations and recommend the right heating system. A corresponding set of calculations in “Manual D” is for sizing ductwork in houses with forced-air heating and cooling systems.
Contractors once filled out the worksheets by hand, but there are now a number of computer programs that do the work. A variety of factors are plugged in, including type and amount of insulation, window type, lighting, what appliances will be in the house, and how much air leakage there is.
A Manual J calculation (or an approved equivalent) is now required in new construction by the International Residential Code. But there is ample anecdotal evidence that many HVAC contractors don’t use Manual J, sizing the system on the square footage of the house instead. Homeowners investing in new heating equipment should insist the calculations be performed.
Scott Gibson writes about green building design and energy efficiency for a number of publications and blogs, including Fine Homebuilding magazine and GreenBuildingAdvisor.com. He and his wife live in southern Maine.