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What is a furnace?

A furnace heats air and sends it through ducts to room registers. ENERGY STAR’s residential fuel-burning definition covers units below 225,000 British thermal units per hour of fuel input. Unlike a boiler’s water-and-pipe system, furnace ductwork can also distribute central air conditioning—a useful distinction when describing replacement work.

InstaBids is an AI-run marketplace for homeowners and property managers: describe a home job once, by chat or through your own AI assistant, add photos, and get multiple written quotes on the same scope, compared apples to apples, with no phone calls or home visits before you pick.

At a glance

By InstaBids editorial team with AI research agents. Updated

What this means for your home project

  1. Look for air registers supplied by ducts when describing how heat reaches your rooms. Source: Vermont Energy Saver, read 2026-11-05.
  2. Request a written exhaust-routing scope stating whether the proposed high-efficiency sealed-combustion furnace uses a new wall vent or chimney lining. Source: Vermont Energy Saver, read 2026-11-05.
  3. Ask the heating contractor to size the proposed furnace after accounting for planned building-efficiency improvements; request that sizing basis in the proposal. Source: Vermont Energy Saver, read 2026-11-05.
  4. Compare the warranty documents for each proposed furnace and record their differences before choosing. Source: Vermont Energy Saver, read 2026-11-05.
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What is the difference between a furnace and a boiler?

Vermont Energy Saver helps you identify your existing distribution system and its cooling compatibility before describing a replacement (read 2026-11-05). “Furnaces heat air (“forced hot air”), distribute it through ducts and deliver it to registers throughout a building.” (source) “Boilers heat water and distribute it (either as water or steam) through pipes that feed radiators.” (source) “A primary difference between a furnace and boiler is that a furnace system’s ductwork can also be used to deliver cool air from a central air conditioner.” (source)

What is AFUE on a furnace?

ENERGY STAR explains annual fuel utilization efficiency, or AFUE, so you can interpret the fuel-conversion percentage when comparing furnace proposals (read 2026-11-05). “For the exact definition of AFUE, refer to the federal test method 10 CFR Part 430, Appendix N to Subpart B.” (source) “In general, the percentage of the heat in the incoming fuel which is converted to space heat instead of being lost.” (source)

What is a sealed combustion furnace?

Vermont Energy Saver explains which air supply to discuss when a proposal specifies a sealed-combustion furnace (read 2026-11-05). “A sealed combustion furnace or boiler will bring outside air directly into the burner and exhaust flue gases (combustion products) directly to the outside, without the need for a draft hood or damper.” (source) “Furnaces and boilers that are not sealed-combustion units draw heated air into the unit for combustion and then send that air up the chimney, wasting the energy that was used to heat the air.” (source)

When is it time to replace a furnace?

ENERGY STAR lists these reasons to discuss furnace replacement with a professional, rather than treating age alone as a diagnosis (read 2026-11-05). “Certain telltale signs indicate it's time to consider replacing heating and cooling equipment, or improving the performance of your overall system.” (source) “Your furnace or boiler is more than 15 years old.” (source) “Your equipment needs frequent repairs and your energy bills are going up.” (source)

What else should I look for when buying a furnace?

ENERGY STAR names an oil-furnace specification and equipment components to request; Vermont Energy Saver adds a warranty comparison (read 2026-11-05). Under ENERGY STAR’s residential furnace criteria, “All oil-fired furnaces must be certified to the UL 727 B20 specification.” (source) “Unit must include burner(s), heat exchanger(s), blower(s) and connections to heating ducts.” (source) “You'll want to choose a dependable unit and compare the warranties of each furnace or boiler you’re considering.” (source)

What drives the cost of a furnace replacement?

Vermont Energy Saver identifies sizing and exhaust-routing decisions to clarify in the written furnace replacement scope (read 2026-11-05). “Before buying a new furnace or boiler—or modifying your existing unit—consider first making every effort to improve the energy efficiency of your building and having a heating contractor size your furnace to meet the new, reduced heating load.” (source) “High-efficiency sealed-combustion units generally produce acidic exhaust gases that are not suitable for old, unlined chimneys (there are also other potentially harmful gases in the byproducts of combusion in a furnace or boiler), so the exhaust gases should either be vented through a new wall vent or the chimney should be lined.” (source)

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