HVAC glossary.

How much of a furnace's fuel actually becomes heat in your home, expressed as a percentage.

AFUE measures the percentage of a gas furnace's annual fuel input that is converted into usable heat. An 80% AFUE furnace wastes 20% up the flue; a 96% AFUE condensing furnace wastes only 4%. Minnesota code requires 90%+ AFUE on most new residential installs.

The current US efficiency standard for central air conditioners; higher is better.

SEER2 replaced SEER in 2023 with stricter test conditions. The 2023 federal minimum for northern states is 13.4 SEER2; high-end systems exceed 22 SEER2. A 1-point SEER2 increase typically reduces cooling costs by 6–8%.

The efficiency rating for the heating mode of a heat pump.

HSPF2 measures heat-pump heating efficiency across a season. The 2023 federal minimum is 7.5 HSPF2; cold-climate heat pumps in Minnesota typically rate 8.1–10.5 HSPF2. Higher HSPF2 means more heat per kilowatt-hour, especially below freezing.

A heat pump engineered to deliver rated heating capacity in subfreezing temperatures.

Cold-climate air-source heat pumps from Mitsubishi, Trane, Carrier, and Daikin maintain rated capacity down to about -5°F and continue producing useful heat to -15°F or below — making them practical for Minnesota winters when paired with a backup gas furnace (dual-fuel).

A heat pump paired with a gas furnace; the thermostat picks the cheaper fuel automatically.

A dual-fuel (hybrid) system uses an electric heat pump for mild heating and switches to a gas furnace at a 'balance point' (typically 15–25°F in Minnesota) where gas becomes cheaper per BTU. Reduces annual heating bills 20–40% versus gas-only.

The room-by-room math that determines what size HVAC system your home actually needs.

Manual J is the ACCA standard for sizing residential heating and cooling. It factors square footage, insulation, window orientation, infiltration, and climate zone. Skipping Manual J — sizing by 'rule of thumb' — leads to short-cycling, humidity problems, and premature failure.

The unit of heat used to size furnaces, AC, and heat pumps.

1 BTU is the heat needed to raise 1 lb of water by 1°F. HVAC equipment is rated in BTU/hour. A typical 2,000 sq ft Minnesota home needs 60,000–80,000 BTU of heating and 24,000–36,000 BTU (2–3 tons) of cooling — but only Manual J gives the real number.

A unit of cooling capacity equal to 12,000 BTU/hour.

One ton of cooling removes 12,000 BTU of heat per hour. Residential AC and heat pumps range from 1.5 to 5 tons. The term comes from the cooling effect of a ton of melting ice over 24 hours.

A furnace whose burner and blower run at multiple stages instead of just on/off.

Variable-speed (or modulating) furnaces fire at 40–100% of capacity based on demand. They run longer at lower output, giving more even temperatures, quieter operation, and 5–15% lower fuel use than single-stage furnaces.

An AC or heat pump that delivers conditioned air without ductwork via wall, ceiling, or floor units.

Ductless mini-splits use a small outdoor unit connected to one or more indoor 'heads' by refrigerant lines through a 3-inch wall hole. Ideal for additions, finished basements, sunrooms, and homes without ducts. Mitsubishi, Daikin, and Fujitsu lead the residential market.

The chemical inside an AC or heat pump that carries heat between indoor and outdoor coils.

R-410A is the legacy residential refrigerant being phased out in 2025 due to high global-warming potential. New systems installed in 2025+ use R-454B or R-32, which have ~75% lower GWP. Equipment is not cross-compatible.

The metal chamber inside a furnace where combustion heat transfers to your home's air.

The heat exchanger separates combustion gases from breathable air. A cracked heat exchanger leaks carbon monoxide and is grounds for immediate furnace replacement. Inspected with a camera scope during every tune-up.

A small metal rod that proves the burner is lit; the most common no-heat failure point.

The flame sensor uses flame rectification to confirm ignition. When it gets coated with soot or oxidation, it can't detect the flame and shuts the burner down — causing intermittent or no heat. Cleaning takes 10 minutes; replacement runs $80–$160 installed.

The glowing ceramic element that lights the gas burner in modern furnaces.

Hot-surface ignitors (HSI) heat to ~2,500°F to ignite gas. They typically last 5–10 years and are one of the most common failure parts. Replacement is straightforward but the part is fragile — touching it with bare hands shortens its life.

Where the water created by a high-efficiency furnace or AC coil drains out of the system.

90%+ AFUE furnaces and AC evaporator coils produce 1–3 gallons of water per day. A clogged condensate trap or drain causes water leaks, error codes, and short-cycling. Cleared during annual maintenance.

How well an air filter captures airborne particles. Higher MERV = more filtration but more resistance.

MERV ranges from 1 to 20. Residential filters typically run MERV 8 (basic dust), MERV 11 (allergens), or MERV 13 (smoke, viruses). MERV 14+ requires a system designed for higher static pressure — installing one in a stock furnace causes airflow problems.

A device that exchanges stale indoor air for fresh outdoor air without losing heat.

HRVs transfer heat; ERVs transfer heat AND humidity. In tight Minnesota homes (especially post-2015 builds), an HRV/ERV is what makes mechanical ventilation work without dumping heat in winter or humidity in summer.

The resistance to airflow inside ductwork; the 'blood pressure' of an HVAC system.

Static pressure is measured in inches of water column. Most residential systems are designed for 0.5" w.c. Anything above 0.8" causes blower strain, noise, and premature failure. Caused by undersized ducts, dirty filters, or restrictive coils.

The ACCA standard for sizing ductwork to deliver the right airflow to each room.

Manual D follows Manual J. Without proper duct sizing, even a perfectly sized furnace will under-heat back bedrooms and over-heat near the supply. Most retrofit ductwork problems trace back to skipped Manual D.

Federal tax credit covering up to $2,000 of a qualifying heat pump installation.

Section 25C of the IRA gives homeowners a 30% federal tax credit (capped at $2,000 for heat pumps, $600 for furnaces/AC, $150 for energy audits) on qualifying equipment installed 2023–2032. Stacks with Xcel Energy and CenterPoint utility rebates.