Furnace Repair & Installation

A furnace is defined by two independent choices: the fuel it burns and how many output levels its burner has. Natural gas is not run to every part of Maine — it follows the utility pipeline network, mostly in and around the larger service areas — so homes outside those corridors typically heat with oil or propane instead, both delivered by truck and stored on-site.

The staging question is separate from fuel. A single-stage furnace has one burner setting: full fire or off. A two-stage furnace has a high and a low firing rate and runs on low most of the time, only stepping up to high on the coldest days or right after a big setback. A modulating furnace adjusts its firing rate in small increments across a wide range instead of jumping between fixed steps, matching output to the house's actual heat loss at that moment.

AFUE (annual fuel utilization efficiency) is the percentage of fuel energy that ends up as usable heat in the house rather than going out the flue. The U.S. Department of Energy sets a legal minimum AFUE for every new residential furnace sold, and it varies by fuel type and furnace category. Under the current federal rule (10 CFR 430.32), the AFUE floor for furnaces manufactured from November 19, 2015 onward is shown below.

A separate DOE rule raises the minimum for non-weatherized and mobile-home gas furnaces to 95% AFUE — effectively requiring condensing technology — for units manufactured on or after December 18, 2028. Oil furnace minimums are not changed by that rule. Homeowners shopping today can still buy furnaces at the current floor, but anything installed well before that date should be sized and vented with the newer condensing standard in mind if a replacement is likely before the furnace's expected service life is up.

A home with existing ductwork has three reasonable paths, and the right one depends on the fuel already available, the age of the ducts, and how much the household cares about summer cooling.

Furnace only makes sense when the ducts and furnace are both in good shape, the home has no interest in central air conditioning, and fuel cost is not pushing toward electrification. A pure heat pump replacement removes the furnace (or keeps it only as an emergency backup) and moves all normal heating and cooling to the heat pump. Efficiency Maine notes that many cold-climate heat pump models keep producing heat down to –22°F or lower, and Maine customers have reported them running as the only heat source through wind chills near –49°F — but that depends on correctly sizing the equipment to the house, not just picking a model off a spec sheet.

Dual fuel sits between the two: a heat pump does most of the heating and cooling, and the furnace switches on automatically only below a set outdoor temperature or during a cold snap, when the furnace can recover the house faster or when electricity is pricier per unit of heat than the fuel already in the tank. The one setup detail that matters in any combined system: Efficiency Maine warns that where a heat pump and another heating system are controlled by separate thermostats, poor placement can let one system's thermostat block the other from running — not a concern with a heat pump running alone, but worth getting right in a dual-fuel design.