Air conditioners are rated with a SEER number that indicates how efficient they are. Higher numbers means better performance. We have a related system for furnaces, though it’s a little different. Heaters use the AFUE, Annual Fuel Utilization Efficiency.
Fuel to Useful Heat
In any combustion system, there’s just about guaranteed waste. In a car, there’s wasted heat and wasted work, where we don’t use all the energy generated by combustion. In furnaces, we concern ourselves with wasted fuel. Depending on the design of the burner, the ignition system, and the fuel type, some amount of fuel will be exhausted without having been burnt.
Determining the fuel lost for this efficiency is generally done in lab tests when the furnace is being designed. The manufacturers measure how much fuel goes in, capture the exhaust, and use lab equipment to detect how much fuel is in the exhaust. This fuel is lost because it just never got the chance to burn in the combustion chamber. Some fuel sources burn slower, sometimes a spark or flame doesn’t propagate through all the fuel that’s injected into the combustion chamber. It may still burn in the exhaust, but by then it’s too late. Whatever the case may be, this lost fuel never heats your facility.
The AFUE scale is therefore a percentage scale. We use a range of 30%-100%. Anything under 30 is basically useless. You’re just spraying a geyser of gas in the air and by chance, some of it burns. This sort of combustion often leads to black, sooty smoke. If you see your furnace outputting black, sooty exhaust, it’s basically shooting your expensive fuel out into the atmosphere. It’s in desperate need of emergency servicing. It could be a health and fire hazard.
Furnace efficiencies are dependent on two things above all others: fuel type and design. An electric furnace will have a 100% efficiency rating, because every watt of power going in becomes heat. It can’t exhaust any of its ‘fuel’ into the atmosphere unburned. This does not necessarily mean that an electric furnace is going to be better or cheaper, it just means this particular rating won’t help you compare a fuel based system to an electric one. Other fuels like gas and oil have different ratings. Gas fired systems often get higher AFUE ratings. This is mainly down to gas’s incredibly high flameability. Gas burns fast from the tiniest spark. Oil on the other hand burns slower and can be far harder to light. There’s a reason oil burners use extremely high voltage igniters.
To illustrate this difference in flameability, we have a video on gasoline (petrol for the UK) vs. diesel (essentially the same as heating oil). I wanted to record us doing this demonstration for real, but Sales Guy Scott said we couldn’t possibly do our experiment far enough from his jeep for comfort, without flying to another country. Our Insurance also said if we blew up Scott’s jeep from the experiment, they wouldn’t cover it… SO, DO NOT TRY THIS AT HOME. And… enjoy this video someone else made.
The Bottom Line
Pay close attention to the energy rating on your furnace. And pay attention to the type of fuel available to you when choosing a new furnace. Bare in mind, these numbers aren’t the see all/end all either. Some fuels produce more heat per gallon than others. Suppose Fuel A produces 10 kilowatts of heat per gallon and Fuel B produces 5 kilowatts of heat per gallon. Even if you can only burn Fuel A at 80% efficiency, it’ll still produce more heat per gallon than Fuel B at 100% efficiency.
There’s more to fuel efficiency than the efficiency number. Consider your fuel source, burner type, and other essential factors when choosing a furnace. If you see black smoke coming from your furnace exhaust, contact your HVAC Contractor immediately. It may be necessary to shut down your heating system for safety reasons.