AFUE, in plain terms
AFUE stands for annual fuel utilization efficiency. It's the share of the fuel you buy that ends up as heat in your home. An 80% furnace turns 80 cents of every fuel dollar into heat, and the rest goes up the chimney. A 96% furnace keeps 96 cents. Older furnaces from the 1980s and earlier often ran in the 60% range, and older oil boilers can be similar, which is why replacing a very old system can cut a heating bill noticeably.
What size really means
Furnaces are rated in BTUs per hour. The right number comes from a load calculation, often called a Manual J. It looks at square footage, insulation, windows, air leaks, ceiling heights and your location. Size by the old furnace's label and you often repeat an old mistake. Oversized furnaces heat in short bursts, wear their parts faster, and leave some rooms hot and others cold.
60,000 vs 80,000 BTU furnaces
Furnace labels list input BTUs, the fuel burned per hour. The heat you actually get is the output, which is input times efficiency. An 80,000 BTU furnace at 80% AFUE delivers about 64,000 BTUs of heat. The same 80,000 BTU input at 96% delivers about 76,800. A 60,000 BTU furnace at 96% delivers about 57,600. So "upgrading" to the same input rating at higher efficiency adds heat output, which can make an already oversized system worse.
Single-stage, two-stage and variable-speed
A single-stage furnace runs at full blast every time it turns on. A two-stage furnace runs on a lower setting most of the time and goes to high on the coldest days, so it's quieter and more even. A variable-speed blower adjusts airflow in small steps, which helps with uneven rooms and filtration. These upgrades make the most sense in homes with comfort problems.
The orphaned water heater problem
When a furnace and a gas or oil water heater share a masonry chimney, the furnace's hot exhaust helps the chimney draft. Replace the furnace with a 95% model that vents through PVC, and the water heater is left alone in a flue that's now too big for it. The exhaust can cool, condense and spill back into the house. A careful installer checks the water heater's draft after every high-efficiency furnace install.
Filters: what to buy
Use the size printed on your current filter's frame. For most homes, a pleated filter in the MERV 8 to MERV 11 range balances clean air and airflow. Very restrictive filters can choke an older furnace with a small return, so ask before going higher. Thick media filters of 4 to 5 inches last longer and restrict airflow less, but your furnace needs a cabinet built for them.
Reading the blinking light
Most furnaces made in the last 25 years have a small LED on the control board, visible through a window in the lower door. It flashes a code that points to the problem, such as a pressure switch fault, a failed ignition or an open limit switch. The code chart is usually printed on the inside of the door. Write down the flash pattern before you call.
The oil burner reset button
Oil burners have a red reset button on the primary control. Press it once. If the burner doesn't stay lit, don't keep pressing. Each press can pump oil into the combustion chamber, and that can cause a puffback or fire when it finally lights.
How long parts usually last
- Hot surface ignitors are wear items and often fail in the first decade or so.
- Flame sensors collect residue and need cleaning, sometimes yearly.
- Blower capacitors weaken with heat and age, and often fail on the first cold start of the season.
- Circulator pumps on boilers often last many years, then fail suddenly.
- Heat exchangers are built to last the life of the furnace, but overheating from dirty filters, poor airflow or oversizing can crack them early.
