Learning Center
The HVAC Glossary
Every term a quote, a spec sheet, or a technician might use — defined in plain language, linked to the guide that explains it.
- AFUE (Annual Fuel Utilization Efficiency)
- The percentage of fuel a furnace or boiler converts into usable heat over a season. An 80% AFUE furnace loses about 20% of fuel energy up the flue; modern condensing models reach 95–98%.
- How a gas furnace works
- AHJ (Authority Having Jurisdiction)
- The local authority — typically your city's building department — that enforces codes and issues permits. Each Des Moines-metro city administers its own.
- HVAC permits in the metro
- Air handler
- The indoor unit that moves air through your ducts — blower, coil, and controls in one cabinet. In furnace-based systems, the furnace itself acts as the air handler.
- Anode rod
- A sacrificial magnesium or aluminum rod inside a tank water heater that corrodes in place of the steel tank. Consumed in 3–5 years; replacing it is the cheapest way to extend tank life.
- How long do water heaters last?
- Auxiliary (backup) heat
- The secondary heat source — electric resistance strips or a gas furnace — that assists a heat pump during deep cold. Its configuration determines winter operating cost.
- Do heat pumps work in Iowa winters?
- Balance point
- The outdoor temperature below which a heat pump can no longer carry the full heating load (thermal balance point) — or below which gas becomes cheaper to run (economic balance point). Dual-fuel systems switch fuels here.
- Heat pump vs. furnace for Iowa
- BTU (British Thermal Unit)
- The basic unit of heating and cooling capacity. One BTU raises one pound of water 1°F; residential equipment is rated in thousands of BTU per hour (e.g., a 60,000 BTU furnace).
- C-wire (common wire)
- The thermostat wire that provides continuous 24V power. Many smart thermostats require one; older Des Moines homes often lack it, which is a solvable install detail.
- Smart thermostat installation
- Cold-climate heat pump
- A heat pump engineered (inverter compressor, enhanced refrigerant circuit) to maintain meaningful heating capacity at sub-zero outdoor temperatures — the class that made heat pumps viable in Iowa.
- Do heat pumps work in Iowa winters?
- Commissioning
- The measured verification that a new system performs to spec — refrigerant charge by superheat/subcooling, airflow, temperature split, staging. The step that separates installed from installed correctly.
- AC installation: what to expect
- Compressor
- The pump at the heart of every AC and heat pump that pressurizes refrigerant to move heat. The most expensive component — its failure usually drives the repair-vs-replace decision.
- AC repair vs. replace
- Condensate
- Water produced when an AC coil dehumidifies air, or by a condensing furnace extracting extra heat from exhaust. It must drain somewhere — blocked condensate lines are a common service call.
- Condenser (outdoor unit)
- The outdoor half of an AC or heat pump: compressor, condenser coil, and fan. It rejects the heat collected indoors to the outside air.
- Condensing furnace
- A 90%+ AFUE furnace with a second heat exchanger that captures heat from exhaust gases until water vapor condenses. Vents in PVC through a sidewall and produces condensate.
- Furnace replacement cost drivers
- Dual-fuel system
- A heat pump paired with a gas furnace: the heat pump handles most of the season efficiently, the furnace takes over below the balance point. The hedged configuration for Iowa winters.
- Heat pump vs. furnace for Iowa
- Ductless mini-split
- A heating/cooling system with an outdoor compressor and slim indoor wall units connected by refrigerant lines — no ducts. Each indoor head is its own independently controlled zone.
- What is a ductless mini-split?
- Economizer
- A commercial rooftop-unit feature that cools with outdoor air when conditions allow instead of running the compressor — 'free cooling' when it's mild outside.
- Light commercial HVAC
- Evaporator coil
- The indoor coil (usually atop the furnace) where refrigerant absorbs heat and moisture condenses out of household air. Coil leaks on aging systems are a classic replacement trigger.
- Flame sensor
- A safety probe that confirms furnace burners actually lit. A dirty flame sensor is the most common cause of a furnace that starts, runs seconds, and shuts down.
- How a gas furnace works
- Heat exchanger
- The sealed metal passages inside a furnace that transfer combustion heat to household air while keeping exhaust gases separate. A cracked heat exchanger is a safety issue and usually ends the repair conversation.
- Furnace repair vs. replace
- Heat pump
- An air conditioner that runs both directions: it moves heat out of your home in summer and into it in winter. Delivers 2–3 units of heat per unit of electricity in most conditions.
- Do heat pumps work in Iowa winters?
- Hydronic heating
- Heating that circulates hot water (or steam) through radiators, baseboards, or floor tubing instead of blowing air — the system older Des Moines neighborhoods were built around.
- How boiler heat works
- Inverter (variable-speed) compressor
- A compressor that modulates output continuously instead of cycling on/off. The technology behind cold-climate heat pumps, quieter operation, and better dehumidification.
- Load calculation (Manual J)
- The room-by-room engineering calculation of how much heating and cooling a home actually needs, based on insulation, windows, orientation, and air leakage. The correct basis for equipment sizing — square-footage rules of thumb are not.
- AC installation: what to expect
- MERV rating
- Minimum Efficiency Reporting Value — a 1–16 scale of how well a filter captures particles. MERV 11–13 balances real filtration with airflow for most residential systems.
- Air purifier vs. filter vs. UV
- Refrigerant
- The working fluid that carries heat through an AC or heat pump. It circulates in a sealed loop and is not consumed — 'needing a top-off' means there's a leak to find and fix.
- RTU (Rooftop Unit)
- A packaged commercial unit — heating, cooling, and blower in one cabinet on the roof. The standard equipment for storefronts, offices, and light commercial buildings.
- Light commercial HVAC
- SEER2 (Seasonal Energy Efficiency Ratio 2)
- The current federal test standard for AC and heat pump cooling efficiency (successor to SEER, with more realistic test conditions). Higher numbers mean less electricity per unit of cooling.
- Short cycling
- Equipment starting and stopping in rapid cycles instead of running steadily. Causes include oversizing, dirty filters or coils, and failing controls — it raises bills and wears parts fast.
- How central AC works
- Single-stage / two-stage / modulating
- How finely equipment can adjust output: single-stage is all-or-nothing; two-stage adds a low setting; modulating adjusts continuously. Higher staging runs longer and gentler — steadier temperatures and better dehumidification.
- Static pressure
- The resistance your duct system presents to airflow — HVAC's blood pressure. High static (undersized returns, restrictive filters) strains blowers and starves rooms.
- Superheat / subcooling
- The refrigerant measurements technicians use to verify correct charge. If a charge was set without measuring these, it was guessed.
- What a tune-up should include
- T&P valve (temperature & pressure relief)
- The safety valve on a water heater that releases dangerous over-pressure. It should be tested during maintenance and never capped or plugged.
- How long do water heaters last?
- Zoning
- Dividing one duct system into independently controlled areas with motorized dampers and multiple thermostats — or achieving the same result with ductless heads per room.
- HVAC zoning guide
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