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 gas 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. That's why replacing a very old furnace can cut a heating bill noticeably even before any rebate.
What size really means
Furnaces are rated in BTUs per hour. Common home sizes run from about 40,000 to 120,000 BTUs. 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" from an old 80,000 BTU 80% furnace to a new 80,000 BTU 96% furnace actually adds heat output, which can make an already oversized system worse. That's one more reason to size by load calculation.
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 cost more, and they make the most sense in homes with comfort problems or longer winters, like those on the ridge.
The orphaned water heater problem
When an 80% furnace and a gas 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 good installer checks the water heater's draft after every high-efficiency furnace install and recommends a liner when it's needed.
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. It often lets us bring the right part on the first trip.
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.
- Inducer and blower motors tend to last much longer, but bearings wear and get noisy before they fail.
- Heat exchangers are built to last the life of the furnace, but overheating from dirty filters, poor airflow or oversizing can crack them early.
Gas line and electrical checks
Older homes sometimes have gas piping sized for smaller appliances. When we add capacity, we check the pipe size and gas pressure so the new furnace gets the fuel it needs. We also check that the furnace has its own properly sized circuit and a working service switch. These checks are boring, and skipping them is how new furnaces end up with nuisance problems.
