Every industry has its own vocabulary. HVAC and plumbing have more of it than most. This glossary is plain-English definitions for the terms you will see in contractor quotes, product spec sheets, and the rest of this Library. We have organized it alphabetically with cross-references where one term depends on another.

If something is missing or unclear, call us. 1-855-GET-WAHL.

A

AFUE (Annual Fuel Utilization Efficiency). The percentage of fuel energy your furnace or boiler converts to useful heat in your house over a full heating season. An 80% AFUE furnace turns 80% of the gas you pay for into heat. The other 20% leaves through the flue. Modern high-efficiency condensing furnaces run 95% AFUE or higher.

Air handler. The indoor blower unit for a heat pump or air conditioning system that does not use a gas furnace. Houses the evaporator coil, the blower fan, and the controls. Functionally takes the place a furnace would otherwise occupy.

Ambient temperature. Outdoor air temperature, typically measured a few feet above the ground in the shade. Heat pump capacity, AC efficiency, and several other HVAC ratings depend on ambient temperature.

Ampacity. The amount of electrical current a wire or breaker can carry safely. Relevant for HVAC because heat pumps and electric heat strips have high ampacity requirements that sometimes exceed older electrical panels.

Anode rod. A sacrificial metal rod inside a tank water heater designed to corrode preferentially, protecting the tank itself from corrosion. Should be inspected every 3 to 5 years and replaced when depleted to extend tank life.

B

Balance point. The outdoor temperature at which a heat pump can no longer fully heat the house and backup heat begins. For a properly sized cold-climate heat pump in Pittsburgh, the balance point is typically 5 to 20 degrees. For a standard heat pump, often 30 to 40 degrees.

BTU (British Thermal Unit). The unit of heat energy used in HVAC. One BTU is the heat needed to raise one pound of water by one degree Fahrenheit. Heating and cooling capacities are typically given in BTU per hour (BTU/hr). Residential furnaces are commonly sized 40,000 to 100,000 BTU/hr input. AC units are sized 18,000 to 60,000 BTU/hr (1.5 to 5 tons).

Burner. The component in a furnace, boiler, or water heater that mixes gas with air and ignites the mixture to produce heat. Single-stage burners fire at one output. Two-stage and modulating burners can vary their output.

C

Cast iron boiler. A boiler with a heat exchanger made of cast iron sections joined together. Very durable, typically 30 to 40 year service life. Common in older Pittsburgh homes.

CFM (Cubic Feet per Minute). The unit of air volume flow rate. AC and furnace blowers are sized in CFM. A typical residential blower moves 1,000 to 2,000 CFM. Heat pumps usually need more CFM per ton than gas furnaces because supply air temperature is lower.

CIPP (Cured-In-Place Pipe). A trenchless sewer repair technique. A resin-saturated liner is pulled or inverted into a damaged pipe, then cured in place to form a new pipe within the old one. Allows sewer rehabilitation without excavating the entire run.

Combi boiler. A combination boiler that handles both home heating (hydronic) and domestic hot water (on-demand) in a single unit. See our combi boiler page.

Combustion air. The air a gas furnace, boiler, or water heater needs to support combustion. Atmospheric appliances pull combustion air from the room. Sealed combustion appliances pull combustion air from outside through a dedicated pipe.

Condensate. The water produced when warm humid air contacts a cold AC coil (cooling mode) or when combustion gases cool enough in a condensing furnace or boiler. Must be drained to a suitable location.

Condenser. The outdoor unit of an AC or heat pump system. Contains the compressor, the outdoor coil, and the fan. The condenser rejects heat outside (cooling mode) or absorbs heat from outside (heat pump heating mode).

Condensing furnace/boiler. A high-efficiency gas furnace or boiler that captures heat from combustion gases that would otherwise leave through the flue. Achieves 90% to 98% AFUE. Requires PVC venting and a condensate drain.

COP (Coefficient of Performance). The efficiency rating for heat pumps in heating mode. Defined as heat output divided by electrical input. A COP of 3.0 means the heat pump delivers 3 units of heat for every 1 unit of electricity. Higher is better.

D

Damper. A movable plate in ductwork or near a vent that controls airflow. Zoning systems use motorized dampers to route air to specific rooms based on thermostat demand.

Defrost cycle. When a heat pump operates in heating mode at cold outdoor temperatures, frost forms on the outdoor coil. The unit periodically reverses (briefly running in cooling mode) to melt the frost. A normal part of heat pump operation in Pittsburgh winters.

Delta-T. The temperature difference between two points. In HVAC, often refers to the temperature difference between return and supply air across the indoor coil (cooling: 15 to 20 degree drop typical) or the temperature rise across a furnace heat exchanger (35 to 65 degree rise typical).

DHW (Domestic Hot Water). The hot water for showers, sinks, dishwashers, and laundry. Distinguished from the hot water in a hydronic heating system.

Drum trap. An older style of plumbing trap used under bathtubs in pre-1940 plumbing. Replaced in modern code by P-traps.

Ductless. Heating and cooling equipment that does not use ductwork. Mini-splits are the primary residential ductless category. See our mini-split page.

E

ECM (Electronically Commutated Motor). A variable-speed motor used in modern HVAC blowers and circulators. More efficient than older PSC (Permanent Split Capacitor) motors, particularly at part load.

EDR (Equivalent Direct Radiation). The unit used to size steam boilers. Measures the heat output capacity of steam radiators in square feet. Different radiator types have different EDR per linear foot or per section.

Energy Factor (EF). The efficiency rating for water heaters. Tank gas water heaters typically run 0.55 to 0.65 EF. Tankless run 0.82 to 0.99. Heat-pump water heaters run 3.0+. Higher is better.

Equalizer. A specific piece of near-boiler piping on steam boilers that maintains water level and pressure balance. Has to be installed per manufacturer specs.

ERV (Energy Recovery Ventilator). A whole-home ventilation device that brings outdoor air in and exhausts indoor air, with a heat exchanger that recovers both temperature and humidity. Used in tightly sealed modern homes that need controlled ventilation.

Evacuation (deep vacuum). The process of pulling a refrigeration system down to a very low pressure (under 500 microns) before charging with refrigerant. Removes air and moisture that would otherwise contaminate the system. Mandatory on every new AC or heat pump install.

Evaporator coil. The indoor coil where refrigerant boils, absorbing heat from the indoor air. In cooling mode, this is where the cool air is created. Typically sits on top of the furnace or inside an air handler.

F

Flue. The vent pipe that carries combustion gases away from a furnace, boiler, or water heater. Atmospheric appliances vent through metal flues and chimneys. Sealed-combustion condensing appliances vent through PVC.

Forced air. A heating and cooling system that distributes conditioned air through ductwork using a blower. The dominant residential HVAC type in postwar Pittsburgh.

G

Galvanized steel. A type of metal pipe coated with zinc to resist corrosion. Used in older Pittsburgh homes for supply piping (1900 to 1960 era). Develops mineral buildup over decades, reducing flow.

Gas valve. The component in a gas appliance that regulates gas flow to the burner. Solenoid valves open and close on demand from the controls. Modulating valves vary the flow rate.

GPM (Gallons Per Minute). The unit of liquid flow rate. Tankless water heaters and water pumps are rated in GPM. Plumbing fixtures have GPM ratings (shower heads typically 1.8 to 2.5 GPM, kitchen faucets typically 1.5 to 2.2 GPM).

H

Hartford loop. A specific piece of near-boiler piping on steam boilers that prevents loss of boiler water in case of a wet return failure. Required by code on all steam boiler installations.

Heat exchanger. The component in a furnace, boiler, or AC that transfers heat from one fluid or gas to another. Furnace heat exchangers transfer heat from combustion gases to room air. Boiler heat exchangers transfer heat from combustion gases to system water. AC coils are technically heat exchangers transferring heat from refrigerant to air.

Heat loss calculation. The engineering calculation that determines how much heat a building loses at design conditions, used to size heating equipment. Manual J is the residential standard.

Heat pump. A reversible refrigeration system that can both heat and cool by moving heat between indoor and outdoor air. More efficient than electric resistance heat. Cold-climate heat pumps maintain capacity in Pittsburgh winters. See our heat pump pages.

HRV (Heat Recovery Ventilator). Similar to an ERV but recovers temperature only, not humidity. Used in tightly sealed homes that need controlled ventilation in dry climates.

HSPF (Heating Seasonal Performance Factor). The efficiency rating for heat pumps in heating mode, averaged over a heating season. Federal minimum is HSPF 8.8 (HSPF2 7.5). Top units exceed HSPF 12.

Hydronic. A heating system that uses hot water as the heat transfer medium. Includes baseboard heat, cast iron radiator heat, and radiant floor heat.

I

IDWB (Indoor Dry-Bulb / Wet-Bulb). Indoor air temperature measurements used in HVAC sizing and commissioning. Wet-bulb measurement accounts for humidity.

Indirect water heater. A water heater that uses a boiler as its heat source. The boiler heats water in a coil inside the indirect tank. Very efficient and long-lasting, common pairing with cast iron boilers.

Inverter compressor. A compressor that runs at variable speed instead of single-speed. Allows the equipment to modulate output to match load, which improves comfort, efficiency, and equipment life.

L

Line set. The pair of copper refrigerant lines (and an insulating cover) connecting the outdoor unit of an AC or heat pump to the indoor coil. Sized by capacity and length.

Liner (chimney). A pipe inside a chimney that vents one or more appliances. Required when a chimney serves a smaller load than it was sized for (often the case after switching to a high-efficiency furnace or boiler).

Load calculation. See “heat loss calculation.” Sometimes called “Manual J” for the ACCA technical manual that defines the residential standard.

M

Manual J. The Air Conditioning Contractors of America standard for residential heating and cooling load calculation. Every proper HVAC sizing in a residential setting should be based on Manual J or equivalent.

MERV (Minimum Efficiency Reporting Value). The rating for air filters, measuring how well the filter captures particles. MERV 8 is basic, MERV 11 is mid-range, MERV 13 is high-efficiency. Higher MERV catches smaller particles but increases pressure drop. Most residential furnaces and air handlers work well with MERV 8 to 13 depending on blower design.

Mini-split. A ductless heat pump. See our mini-split buying guide.

Modulating. Continuously variable output. A modulating furnace, boiler, or heat pump adjusts its output across a range rather than running at fixed levels.

O

Outdoor reset. A boiler control feature that adjusts supply water temperature based on outdoor temperature. Major efficiency improvement, particularly for condensing boilers. We install on every condensing boiler.

P

PEX. Cross-linked polyethylene tubing used for plumbing supply lines. Flexible, durable, easier to install than copper in retrofit applications. Modern alternative to copper.

PRV (Pressure Reducing Valve). A valve that reduces water pressure entering a home to a safe level (typically 60 to 80 psi). Pittsburgh’s water main pressure can exceed 100 psi in some areas, which is too high for residential plumbing.

PSC (Permanent Split Capacitor). An older type of blower or pump motor. Single-speed, less efficient than ECM. Being phased out of modern HVAC.

Pro Press. Copper tubing with mechanical fittings (crimped instead of soldered). A modern alternative to soldered copper, especially in retrofit work.

PVC. Polyvinyl chloride plastic. Used for sealed combustion venting on high-efficiency furnaces and boilers, condensate drains, and some plumbing applications.

R

R-410A. The current standard refrigerant for residential AC and heat pumps. Being phased out in favor of lower-GWP refrigerants like R-454B and R-32 starting 2025.

Radiant floor heat. Hydronic heat distributed through tubing buried in or under the floor. Even warmth, no air movement, ideal pairing with modulating-condensing boilers.

Recovery rate. For water heaters, the rate at which the unit can heat new cold water to setpoint. Measured in gallons per hour. A 50-gallon gas tank typically has a recovery rate of 40 to 50 gallons per hour.

Refrigerant. The fluid that absorbs and rejects heat in an air conditioner or heat pump. Different refrigerants have different properties and environmental impacts.

Reversing valve. The component that allows a heat pump to switch between heating and cooling modes by reversing refrigerant flow.

S

SEER (Seasonal Energy Efficiency Ratio). The cooling efficiency rating for AC and heat pumps, averaged over a cooling season. Federal minimum was SEER 14, now SEER2 13.4 for northern climate zones. Top inverter units exceed SEER 22.

SEER2. The updated SEER testing standard implemented in 2023. SEER2 numbers are slightly lower than the old SEER numbers for equivalent equipment because the test methodology is more representative of real-world conditions.

Sealed combustion. Burners that pull combustion air from outside through a dedicated pipe and exhaust through another dedicated pipe. Eliminates draft and combustion-air issues. Standard on high-efficiency condensing equipment.

Service line. The water supply line from the municipal water main to the home. In older Pittsburgh, often lead. Replacement is sometimes coordinated with the water authority.

Static pressure. The pressure resistance in a ductwork system. High static pressure indicates restrictive ducts, undersized filters, or other airflow issues. Measured during commissioning.

Steam boiler. A boiler that generates steam (rather than hot water) for distribution to cast iron radiators. Common in pre-1940 Pittsburgh homes. See our steam boiler page.

Subcooling. The temperature difference between condensing temperature and actual liquid line temperature in a refrigeration system. Used in commissioning to verify proper refrigerant charge on TXV-equipped systems.

Sump pump. A pump that removes water from a sump pit in the basement. Critical in many Pittsburgh homes due to high groundwater and basement flooding risk.

Superheat. The temperature difference between evaporating temperature and actual suction line temperature in a refrigeration system. Used in commissioning fixed-orifice (piston) systems and as a TXV diagnostic.

T

TXV (Thermostatic Expansion Valve). A refrigerant metering device that adjusts flow to the evaporator coil based on superheat. Standard on modern AC and heat pumps. Older systems used fixed orifices (pistons) instead.

Two-stage. Equipment that operates at two output levels rather than single-speed or modulating. Common in mid-tier furnaces and AC units.

V

Variable-speed. Equipment that operates at continuously variable output. Highest efficiency tier for furnaces, AC, and heat pumps. Pairs with modulating burners and inverter compressors.

Vapor injection. A refrigeration technique used in cold-climate heat pumps to maintain capacity at low outdoor temperatures. The technology that makes modern cold-climate heat pumps actually work in Pittsburgh winters.

Venting. The system that removes combustion gases from a fuel-burning appliance. Atmospheric venting uses a chimney. Power venting uses a fan. Sealed combustion (high-efficiency condensing) uses PVC routed to a side wall.

VRF (Variable Refrigerant Flow). A commercial mini-split technology with one outdoor unit supporting many indoor heads (up to 30+). Mitsubishi City Multi is the leading line. Used in schools, offices, hotels, and multi-tenant residential. Wahl is one of the few Pittsburgh contractors that handles VRF.

W

Water hammer. The banging or shock wave that travels through plumbing when a fast-closing valve stops water flow suddenly. Solved with water hammer arrestors at the noisy fixtures.

Wet bulb. A temperature measurement that accounts for humidity. Wet-bulb temperature equals dry-bulb temperature when air is saturated (100% RH) and is lower than dry-bulb when air is dry. Used in HVAC sizing and commissioning.

Z

Zoning. Dividing a heating or cooling system into multiple independently controlled zones, each with its own thermostat. Allows different temperatures in different parts of the house. Standard zoning uses motorized dampers (forced air) or zone valves (hydronic).

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If something you have read in a quote or spec sheet is not in this glossary, call us and we will add it. 1-855-GET-WAHL. The glossary is meant to be a real resource for Pittsburgh homeowners, not a static list.