HVAC glossary
Plain-English definitions for the heating, cooling, and air-quality terms you'll run into — so you can make confident decisions about your Baltimore-area home.
- 25C tax credit
- The former federal Energy Efficient Home Improvement Credit. It ended for equipment placed in service after December 31, 2025, so it no longer applies to new installations (verified July 2026). Installations completed by that date may still be claimable on that year's return - confirm with a tax professional.
- AFUE
- Annual Fuel Utilization Efficiency — the percentage of fuel a furnace turns into usable heat over a year. A 95% AFUE furnace converts 95 cents of every fuel dollar into heat. Higher AFUE means lower heating bills.
- Air handler
- The indoor unit that moves conditioned air through your ductwork. In a heat pump or electric system it pairs with the outdoor unit; it houses the blower and, often, the evaporator coil.
- BTU
- British Thermal Unit — a measure of heating or cooling energy. HVAC capacity is rated in BTUs per hour; the right number for your home depends on a proper load calculation, not square footage alone.
- Capacitor
- A small component that gives motors (like your AC's compressor and fan) the jolt they need to start and run. A failed capacitor is one of the most common, and most affordable, AC repairs.
- Compressor
- The heart of an air conditioner or heat pump's outdoor unit. It pressurizes refrigerant so the system can move heat. A failed compressor is a major repair and often factors into a repair-or-replace decision.
- Condensate drain
- The line that carries away water your AC pulls from humid air. In muggy Baltimore summers it works hard — a clogged drain can trip a safety switch and shut the system off, or cause water damage.
- Condenser
- The outdoor unit of a split-system AC or heat pump. It releases heat collected from inside your home (in cooling mode) and contains the compressor and condenser coil.
- Dual-fuel system
- A heat pump paired with a gas furnace. The heat pump handles mild weather efficiently, and the furnace takes over in deep cold — a popular, cost-smart setup for Maryland's mixed climate.
- Ductless mini-split
- A heating and cooling system with an outdoor unit and one or more wall- or ceiling-mounted indoor heads, connected by a small refrigerant line instead of ductwork. Ideal for additions, sunrooms, and older Baltimore homes without ducts.
- EmPOWER Maryland
- A statewide energy-efficiency program (delivered through utilities such as BGE) that can offer rebates on qualifying high-efficiency HVAC equipment. Eligibility and amounts change — ask us to check what's current.
- Evaporator coil
- The indoor coil that absorbs heat and humidity from your home's air during cooling. It's where condensation forms — and where ice can build up if airflow or refrigerant is low.
- Heat exchanger
- The furnace component that transfers heat from burning fuel to your home's air while keeping combustion gases separate. A cracked heat exchanger is a safety concern and usually means furnace replacement.
- Heat pump
- A single system that both heats and cools by moving heat rather than burning fuel. Modern heat pumps work well through Maryland winters and are highly efficient in our climate.
- HSPF2
- Heating Seasonal Performance Factor 2 — the current efficiency rating for a heat pump's heating mode. Like SEER2 for cooling, a higher HSPF2 means more heat per unit of electricity.
- Load calculation (Manual J)
- The industry-standard method for sizing HVAC equipment based on your home's size, insulation, windows, and orientation — not a rule of thumb. Correct sizing prevents short cycling and comfort problems.
- MERV rating
- Minimum Efficiency Reporting Value — how well an air filter captures particles, from about 1 to 16. Higher MERV traps more, but too-restrictive a filter can strain some systems; we help you match the right one.
- Refrigerant
- The fluid that carries heat through an AC or heat pump as it changes between liquid and gas. Low refrigerant signals a leak — simply 'topping off' without finding the leak is a temporary fix, not a repair.
- SEER2
- Seasonal Energy Efficiency Ratio 2 — the current standard rating for air-conditioner and heat-pump cooling efficiency. A higher SEER2 generally means lower summer cooling bills.
- Short cycling
- When a system turns on and off too frequently without completing a full cycle. It wears out parts and wastes energy, and is often caused by an oversized unit, a dirty filter, or a refrigerant issue.
- Static pressure
- The resistance air meets as it moves through your ducts — like blood pressure for your HVAC system. High static pressure (from undersized or blocked ducts) hurts efficiency and shortens equipment life.
- Thermostat
- The control that tells your system when to run. Smart and programmable thermostats can lower bills, but a miswired or failing thermostat is also a common cause of 'no heat' and 'no cooling' calls.
- Ton
- A unit of cooling capacity equal to 12,000 BTU/hour. A typical Baltimore-area home might use a 2-to-5-ton system, but the correct size comes from a load calculation, not square footage.
- Two-stage / variable-speed
- Equipment that can run at lower outputs instead of only full blast. It runs longer and quieter at gentler speeds, improving comfort and humidity control — a noticeable upgrade in humid Maryland summers.
- UV light (germicidal)
- An air-treatment device installed in your system that uses ultraviolet light to reduce mold and microbes on the coil and in the airstream — one option for improving indoor air quality.
- Whole-house dehumidifier
- Equipment that removes excess moisture from your entire home's air, not just one room. In Baltimore's humid summers it improves comfort, protects against mold, and lets you stay comfortable at a higher thermostat setting.
- Whole-house humidifier
- Equipment that adds moisture to dry winter air across your home, easing dry skin, static, and respiratory irritation while letting you feel comfortable at a lower temperature.
- Zoning
- Using dampers and multiple thermostats to heat and cool different areas of a home independently. Helpful for multi-level homes, additions, or rooms that never seem to match the rest of the house.