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HVAC & plumbing terms, in plain English
Plain-English definitions of the heating, cooling, plumbing, and air-quality terms you'll hear on an estimate — so you can ask sharper questions and make confident decisions.
Cooling
Compressor The pump at the heart of an AC or heat pump that pressurizes refrigerant to move heat. It's the most expensive single component — which is why a failed compressor on an older system usually triggers the repair-vs-replace decision. Condenser (Outdoor Unit) The outdoor half of a split system, where refrigerant releases the heat collected inside. Its coil needs clear airflow — cottonwood fluff, grass clippings, and crowding shrubs are its enemies. Ductless Mini-Split A heat pump system with a compact outdoor unit and one or more wall-mounted indoor heads, connected by refrigerant lines instead of ductwork. The go-to answer for additions, finished attics, and row-house rooms the ducts never reached. Evaporator Coil The indoor coil where refrigerant absorbs heat and moisture condenses out of your air. A dirty or frozen evaporator coil is behind many 'AC runs but doesn't cool' calls. Heat Pump An air conditioner that runs in both directions: it cools in summer and, by reversing the refrigerant flow, extracts outdoor heat to warm the house in winter. Modern cold-climate models stay effective well below freezing — a real option in the Mid-Atlantic. R-454B The lower-global-warming-potential refrigerant replacing R-410A in new residential equipment. It's mildly flammable (A2L class), so it comes with updated handling and equipment standards — one reason new-system installs are certification-sensitive. Refrigerant The working fluid that carries heat between the indoor and outdoor coils. Modern systems use R-410A or the newer low-GWP R-454B; older refrigerants are being phased out, which raises recharge costs on aging systems. SEER2 Seasonal Energy Efficiency Ratio 2 — the current federal efficiency rating for air conditioners and heat pumps in cooling mode, measured under more realistic duct-pressure test conditions than the old SEER standard. Higher numbers mean less electricity per unit of cooling. Short Cycling When a system starts and stops in rapid bursts instead of running steady cycles. Causes range from oversizing to a clogged filter or failing part — and in summer it guts dehumidification, which is why short-cycling homes feel clammy. Tonnage (Cooling Ton) A measure of cooling capacity: one ton = 12,000 BTU/h, historically the melting power of a ton of ice per day. Home systems typically run 1.5–5 tons — and the right number comes from a load calculation, not square footage alone. Zoning Dividing a home into independently controlled temperature areas, via duct dampers with multiple thermostats or via multiple ductless heads. It solves the classic 'freezing downstairs, roasting upstairs' pattern of multi-story homes.
Heating
AFUE Annual Fuel Utilization Efficiency — the percentage of your fuel that becomes usable heat over a season. An 80% furnace wastes 20 cents of every gas dollar up the flue; modern condensing furnaces reach 95%+. Balance Point The outdoor temperature at which a heat pump's output exactly matches a home's heat loss. Below it, supplemental heat runs — electric strips or, in dual-fuel systems, the gas furnace. Carbon Monoxide (CO) An odorless, colorless gas produced by incomplete combustion. Any home with gas appliances needs working CO detectors on every level — and combustion equipment inspected annually, which is a core purpose of a heating tune-up. Dual-Fuel System A heat pump paired with a gas furnace as backup: the heat pump carries the mild majority of the season efficiently and the furnace takes over automatically below the balance point. A strong fit for Mid-Atlantic winters. Flame Sensor A thin metal rod that confirms the burners actually lit; if it can't sense flame within seconds, it shuts the gas off. A dirty flame sensor is one of the most common 'furnace starts then dies' causes — and one of the cheapest fixes. Heat Exchanger The sealed metal passages inside a furnace that keep combustion gases separate from the air blown into your home. A cracked heat exchanger can leak carbon monoxide into the airstream — the one furnace failure that's a genuine safety emergency. High-Limit Switch A safety sensor that shuts the burners down if the furnace overheats — most often tripped by a clogged filter or failing blower. Repeated limit trips are a symptom to diagnose, not a switch to bypass. Hot Surface Ignitor The glowing element that lights a modern furnace's burners (replacing standing pilot lights). Ignitors are wear items — brittle with age and a classic first-cold-snap failure. HSPF Heating Seasonal Performance Factor — the heating-mode efficiency rating for heat pumps (HSPF2 under current test rules). It's the winter counterpart to SEER2; higher means cheaper heat per kilowatt-hour. Inducer Motor A small fan that proves the flue can draft safely before the gas valve is allowed to open. If it fails or its pressure switch doesn't close, the furnace refuses to start — by design. Two-Stage / Modulating Equipment Furnaces and ACs that can run at partial capacity instead of full-blast-or-off. Longer, gentler cycles mean steadier temperatures, quieter operation, and better summer dehumidification than single-stage equipment.
Plumbing
Backflow Prevention Devices that keep water flowing only from the supply into your home — never backwards into the drinking system. Required for irrigation and certain fixtures, and one reason modern homes are 'closed systems' that need expansion tanks. Check Valve A one-way valve — on a sump discharge line it stops ejected water from draining back into the pit. When it fails, the pump re-pumps the same water on a loop until it burns out. Galvanized Pipe Zinc-coated steel supply piping common before the 1960s. It corrodes shut from the inside over decades — falling pressure and rusty morning water are its signature — and it's a whole-system replacement candidate rather than a patch-forever material. GFCI Protection (Wet Areas) Ground-fault circuit interrupters cut power in milliseconds when current leaks — required by code near sinks, in bathrooms, and around sump pumps and laundry areas. If a sump pump trips a GFCI repeatedly, diagnose the pump; don't move it to an unprotected outlet. Main Water Shutoff The valve that stops all water to the house — usually where the service line enters, near the meter. Every adult in the home should know where it is; a seized shutoff during a burst pipe turns a mess into a disaster. P-Trap The U-shaped bend under every fixture that holds a plug of water to block sewer gas from entering the home. A dry trap (common in rarely-used basement floor drains) is the usual culprit behind a mystery sewer smell. PEX Cross-linked polyethylene — the flexible plastic supply piping used in most modern repipes. It fishes through walls with minimal demolition, tolerates freezing better than rigid pipe, and connects with fewer joints. Polybutylene (PB) Gray plastic supply piping installed roughly 1978–1995 that proved failure-prone and was the subject of major class actions. Insurers may surcharge homes that still have it; it's a replace-on-discovery material. Pressure-Reducing Valve (PRV) A valve on the main line that knocks municipal supply pressure down to a safe household range (usually 50–70 psi). Chronic high pressure hammers fixtures, appliances, and water heaters — a failed PRV shows up as banging pipes and shortened equipment life. Sewer Lateral The buried drain line carrying everything from your house to the public sewer main — and the homeowner's responsibility, not the utility's. Root intrusion and collapsed clay laterals are the classic five-figure surprise in pre-1980 homes; a camera inspection sees it before you buy. Sump Pump A pump in a basement floor pit that ejects collecting groundwater away from the foundation. In the Chesapeake region's clay soils it's often the only thing standing between a summer storm and a flooded basement. Water Hammer The bang or thud in pipes when fast-closing valves (washing machines, dishwashers) slam moving water to a stop. Arrestors absorb the shock; chronic hammering also points at excess pressure worth checking.
Water Heaters
Anode Rod A sacrificial metal rod inside a tank water heater that corrodes on purpose so the steel tank doesn't. Once it's consumed — typically 4–6 years in — the tank itself starts corroding. Replacing it mid-life is the single best tank-life extender. Dip Tube The internal pipe that delivers incoming cold water to the bottom of the tank so hot water draws from the top. A broken dip tube mixes cold straight into the outlet — the classic 'my showers suddenly got short' failure. Expansion Tank A small air-cushion tank that absorbs the pressure spike created when a water heater heats water in a closed plumbing system. Required by code in most local jurisdictions on modern installs — its absence is a common inspection flag. Heat-Pump (Hybrid) Water Heater An electric water heater that pulls heat from the surrounding air instead of generating it resistively — a fraction of the operating cost. It wants space and tolerable noise (a basement fits), and it dehumidifies that space as a bonus. Recovery Rate How quickly a tank water heater reheats after depletion, in gallons per hour. Two tanks of identical size can behave very differently on a busy morning — recovery is why. Sediment Flush Draining water (and the mineral sediment it carries) from the bottom of a tank water heater. Sediment blankets the burner area, cutting efficiency and causing the rumbling 'popcorn' sound of an aging tank — an annual flush fights both. T&P Relief Valve The temperature-and-pressure safety valve on every water heater that vents before the tank can over-pressurize. It must discharge through a pipe toward the floor — and a T&P that drips constantly is diagnosing a pressure problem for you. Tankless Water Heater An on-demand unit that heats water as it flows instead of storing it — endless supply, 15–20-year lifespan, wall-mounted footprint. The trade-offs are install cost and flow-rate limits during simultaneous use, plus gas-line and venting requirements in retrofits.
Indoor Air Quality
Dew Point The temperature at which air becomes saturated and moisture condenses. Chesapeake summer dew points in the 70s are why AC coils here work overtime and why basements sweat — it's a better mugginess gauge than relative humidity alone. Duct Sealing Closing the leaks in supply and return ductwork (mastic, tapes, or injected sealant). Typical homes leak a meaningful share of conditioned air into attics and crawl spaces — sealing recovers capacity you already paid for and evens out room-to-room comfort. HRV / ERV Heat- and Energy-Recovery Ventilators bring in fresh outdoor air while exchanging heat (and, for ERVs, moisture) with the outgoing stale air. They solve the tight-renovated-house staleness problem without throwing conditioning away. Media Filter Cabinet A 4–5 inch deep pleated filter housing installed at the air handler. The extra surface area captures far more particulate than 1-inch filters without restricting airflow — and changes twice a year instead of monthly. MERV Rating Minimum Efficiency Reporting Value (1–20) — how well a filter captures particles. MERV 8 handles dust; MERV 11–13 captures allergy-relevant fine particles. Ratings above ~13 need a deep media cabinet, not a standard 1-inch slot, or they choke airflow. Relative Humidity (RH) The percentage of water vapor air holds relative to its maximum at that temperature. Homes feel and stay healthiest at 40–55% in summer and 30–45% in winter; sustained readings above 60% invite mold and dust mites. Static Pressure The resistance the blower pushes against in your duct system — HVAC's blood pressure. High static (undersized returns, choked filters, crushed flex duct) strains motors and starves rooms of airflow; it's measurable in minutes with the right ports. UV Coil Light An ultraviolet lamp mounted over the evaporator coil to keep biological growth off the perpetually wet coil and drain pan. Valuable for system health and musty odors in humid climates — but it does nothing for dust or pollen. Whole-Home Dehumidifier A ducted unit that removes moisture from the whole house independently of cooling demand, draining automatically. The finisher for damp basements and shoulder-season stickiness that the AC alone can't solve.
Whole-Home
BTU British Thermal Unit — the energy needed to raise one pound of water by 1°F. HVAC capacity is expressed in BTU/h; 12,000 BTU/h equals one ton of cooling. Condensate Drain The line that carries away the water your AC wrings from the air — often gallons a day in a Chesapeake summer. Algae-clogged condensate lines cause ceiling stains and shutdowns; keeping them clear is a standard tune-up item. Load Calculation (Manual J) The room-by-room math (ACCA Manual J) that determines how much heating and cooling a specific house actually needs, accounting for insulation, windows, orientation, and infiltration. It's the difference between sized and guessed equipment. Smart Thermostat A connected thermostat with scheduling, remote control, and runtime analytics. Beyond convenience, its real values are catching abnormal runtimes early and running fan circulation strategically for filtration and even temperatures.
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