Part of our heating and cooling services.
Short answer
Sizing is where a furnace replacement goes wrong. Ductwork is where it goes wrong twice. Ducts installed decades ago were sized for that furnace and may not suit a new one, particularly if a heat pump is being considered later, since heat pumps move more air at a lower temperature.
That makes the duct question a decision about the next fifteen years instead of about this installation, and it is far cheaper to answer while the equipment is out than afterwards. Ask whether the ductwork was measured or assumed, what static pressure the new unit needs against what the existing system delivers, and whether the design leaves room for a heat pump if that is a plausible next step for the building.
Jobs this covers
- High-efficiency furnace replacement
- Gas, propane and oil furnaces
- Dual-fuel systems
- Blower and filtration upgrades
- Duct modifications
- Furnace and central A/C projects
Signs a furnace is due for replacement
- The furnace fails repeatedly
- Heat is uneven
- The existing unit is oversized
- Venting needs replacement
- A cooling system is being added
- The home needs better airflow or filtration
What service may include
A load calculation should set the size. Installation may include sizing, removal, equipment setting, fuel and vent connections, electrical and controls, duct transitions, filter setup, startup, combustion testing and permits.
What affects the recommendation and price
Undersized ducts limit any new furnace. Heat loss, duct condition, fuel line, chimney or sidewall venting, electrical work, cooling coil, filtration, access and efficiency level affect installed cost.
What is different around Burlington
Assessing the duct system during a furnace replacement is the cheapest time to do it, because it is the one moment when changing it is straightforward.
What a written quote should tell you
- Whether sizing came from a heat-loss calculation rather than the old unit's rating
- Whether the existing ductwork was measured and found adequate
- Whether the duct assessment considered a future heat pump, if that is on the horizon
- Venting, combustion air and condensate for a high-efficiency unit
- Startup readings, permits, and removal of the old equipment
| Question | Why it's cheapest to answer now |
|---|---|
| Sizing | From a heat-loss calculation, not the old unit's rating |
| Ductwork | Sized for the old furnace: may not suit a new one, especially before a future heat pump |
Common questions
How should a furnace be sized?
Use a building heat-loss calculation and confirm the duct system can move the required airflow.
Can I add central air at the same time?
Often. Coordinating the indoor coil, blower, electrical work and outdoor equipment can reduce duplicated work.
What is a dual-fuel system?
It combines a heat pump with a furnace and changes heat source based on controls, temperature and operating strategy.
Will a higher-efficiency furnace need new venting?
Frequently. Condensing furnaces commonly use different vent and condensate systems than older equipment.
What is a variable speed furnace and is it worth it in Vermont?
It refers to the blower motor, and often to the burner as well, being able to run at part load rather than only full on or full off. In a cold climate that is genuinely useful: a furnace that can run for long periods at 40% output holds a steadier temperature, moves air more quietly, filters more continuously and uses substantially less electricity for the fan than a single-speed unit. The trade-offs are higher purchase price and more electronics to fail. In a house with comfort complaints between rooms, the longer run times often do more than the efficiency rating does.
How do I know what size furnace I need?
From a Manual J heat loss calculation performed on your actual house, its insulation, air tightness, window area, orientation and volume, not from the size of the existing unit and not from square footage alone. This matters more in Vermont than in mild climates because the design temperature is low, so sizing errors are amplified. An oversized furnace short cycles, delivers uneven temperatures and blows uncomfortably hot air in short bursts. If a contractor proposes a size without measuring the house, ask on what basis, and treat a shrug as an answer.
Can I install a furnace myself?
No. Gas piping, venting, combustion air and electrical all converge, permits are required, and the same carbon monoxide risk applies as with any fuel-burning appliance. A furnace is also one of the appliances most often installed badly by professionals, which points to where a homeowner should actually spend effort: insisting on a Manual J heat loss calculation instead of a replacement in kind, insisting the ductwork is assessed, not assumed, and asking for the startup readings that show it was commissioned rather than merely connected.
What happens if the ductwork is not addressed when the furnace is replaced?
You install efficient equipment onto a distribution system that is throwing away part of its output, and you keep whatever comfort problems the house already had. Duct leakage in unconditioned space is a well-documented loss, and in Vermont, ducts in a vented crawlspace or an unheated attic are losing heat to the outdoors directly. Undersized returns are the other half: they starve the furnace of airflow, trip the high limit, and shorten the heat exchanger's life. Replacing the box without looking at the ducts is how a new furnace fails to fix the cold bedroom.
How long does furnace installation take?
A like-for-like replacement is commonly one day. It becomes two or more if the venting has to change, which it usually does when moving from an older atmospherically vented furnace to a condensing one that needs sealed PVC venting through a wall; if a chimney is being abandoned and a water heater is left orphaned on it, which is a real and frequently missed issue; or if duct modifications are part of the scope. Plan replacements for the shoulder seasons instead of mid-winter.
If the furnace is replaced and the chimney is abandoned, who checks the water heater?
It should be part of the installer's scope and it is worth naming explicitly in the contract, because it falls between trades and gets missed. When a furnace that shared a chimney is replaced with a direct-vented condensing unit, the gas water heater left behind is now venting alone into a flue sized for two appliances. It is oversized, cool, and prone to condensing and to poor draft, which can spill combustion products into the basement. The fix is usually a properly sized liner. Ask the question before the work, not after.