Energy Vanguard Blog

Learn the Lingo - Air Conditioning Terminology & Tidbits

Posted by Allison Bailes on Wed, Jul 13, 2011

hvac air conditioner window units terminology lingoHave you heard your air conditioner repairman use terms you didn't understand? Are you a new home energy auditor or HERS rater trying to figure out how heating and cooling systems work? There's a lot of jargon in the world of HVAC (including those initials), and I'll demystify some of it for you here. There's no way I can get it all here, so I'll hit what I think are the most important. If I miss one you think should be included, let me know in the comments below.

As stated in the title, I'll focus only on air conditioning here.  Some of it overlaps with heating lingo, but much of it is specific to air conditioning. We'll do heating another time.

ACCA - Air Conditioning Contractors of America, the trade association for the AC industry. Check out their great consumer resources, including the Quality Installation Checklist you can use to select a good HVAC contractor.

Air flow - How much air your duct system moves. Air conditioners are generally designed to move about 400 cubic feet per minute (cfm) for each ton of AC capacity. In dry climates, that number will be higher (maybe up to 500 cfm/ton), and in humid climates it'll be lower (~350 cfm/ton).

Air handler - If you have a split system air conditioner, this is the box that contains the guts of the indoor piece. As its name implies, it contains the blower, but I usually include the heating and cooling components as well (i.e., evaporator coil, supplemental resistance heat, furnace) when I use the term.

Boot - The sheet metal transition piece that connects to the duct on one side and has a grille or register on the other.

Checking the charge - Determining how much refrigerant is in the system. When your AC guy puts his gauges on the system, he's measuring the pressure of the refrigerant to see if you have the right amount.

Compressor - The part of your air conditioner responsible for most of the noise. It sits in the outside part of your AC - the condensing unit - and raises the temperature and pressure of the refrigerant.

Delta T (ΔT) - Temperature difference. If all is working well in your AC, you should get about a 20° F ΔT when the air passes through the cooling stage of the refrigeration cycle.

Ductless mini-split - What the rest of the world uses for air conditioning. It's a split system heat pump that's smaller and (usually) has no ducts. The blower and evaporator coil are in the head, which is mounted on a wall or ceiling in the room you're trying to cool. (See Duct-Free Zone - The Advantages of Mini-Split Heat Pumps.)

EER - The efficiency rating used for window unit ACs and ground source (geothermal) heat pumps: Btu/hr of cooling divided by electricity input in watts, instantaneous.

Geothermal heat pump - The standard heat pump dumps or pulls heat to or from the outside air. A geothermal heat pump dumps or pulls heat to or from the ground or a body of water. Ground source heat pump (GSHP) is a better name for it.

Grille - The type of non-operable cover you see in return vents. (See register.)

Heat pump - An air conditioner that can run in reverse. In summer, it moves heat from inside to outside; in winter, it moves heat from outside to inside.

HVAC - Heating, ventilating, and air conditioning; the general name to cover the whole field. I usually pronounce each letter (H-V-A-C). Chris calls it H-Vac. Either's fine.

Latent heat - The heat you have to remove from the air to remove the moisture.

Line set - The two refrigerant lines that connect the condensing unit to the evaporator coil. The smaller, hotter, uninsulated copper tube is the liquid line. The larger, colder, insulated tube is the suction line.

Load calculation - Determining how much heat a house gains or loses through the building envelope, from duct losses, and by internal gains (people, appliances...). It's one part of the HVAC design process.

Media air cleaner - Bigger, better filters for your HVAC system. Made of materials (media) that do a better job of catching the small particles passing through your duct system. Can have big effect on static pressure (see below).

MERV - Minimum Efficiency Reporting Value, a rating for filters used in HVAC systems. Not all filters have a MERV rating. The higher the number, the more it catches.

Plenum - The box attached to either side of the air handler to which the other ducts are attached. On the return side, there may be a filter between the plenum and the air handler. On the supply side, the evaporator coil may be in a separate housing from the blower. Look for the box that the ducts are attached to. It's usually made of insulated sheet metal or duct board, though it could be plywood or other materials.

PTAC - Package Terminal Air Conditioner. What you see in most hotel rooms, it's the through the wall unit that contains all components of the AC in one box.

Refrigerant - The working fluid that carries the heat. Most current air conditioners use either R-22, which began its phase-out in 2010, or R-410a. Before the Montreal Protocol to protect the ozone layer, most AC refrigerants were CFCs. R-22 is an HCFC, and R-410a is an HFC.

Refrigeration cycle - The thermodynamic cycle that allows your air conditioner to pick up heat from inside the house and send it outside. See The Magic of Cold, Part 1 and The Magic of Cold, Part 2.

Register - The type of cover you see in supply vents. It has operable louvers that allow you to control the air flow. Occasionally you'll see a register on a return vent. Usually this happens when the return and supply vents are the same size, and a homeowner pulled them out to paint and then put them back in the wrong places.

Return - The side of the duct system that pulls air from the house back to the air handler to be conditioned again. You can identify return vents by taking a piece of tissue and seeing if it gets pulled in or blown out when you hold it in front of the grille or register.

Reversing valve - The part responsible for the difference between a heat pump and an air conditioner.

SEER - The efficiency rating used for central air conditioners: Btu/hr of cooling divided by electricity input in watts, averaged over a whole cooling season. 13 SEER is the minimum you can get now.

Sensible heat - The heat you have to remove from the air to lower the temperature.

Sensible Heat Ratio (SHR) - The ratio of the sensible cooling load or capacity to the total (sensible + latent) cooling load or capacity.

Split system - An air conditioner with one box outside (the condensing unit) and one box inside (the air handler and evaporator coil), connected by the refrigerant lines.

Static Pressure - Something that almost never gets checked but has a big effect on the efficiency and effectiveness of your air conditioner. It's the pressure inside your duct system and is often too high to get the proper air flow.

Supply - The side of the duct system that pushes conditioned air back into the house. Supply vents have cool air coming out of them when the AC is on.

Tons of air conditioning - Not the weight of the air conditioner but the capacity. One ton of air conditioning capacity is equal to 12,000 Btu/hour. So, a 3 ton air conditioner can remove 36,000 Btu from your home if it runs for an hour. (Actual capacity is not the fixed quantity it might seem, however. See David Butler's guest post on Manual S equipment selection.)

Trunk - A big duct attached directly to the plenum from which the branches go out to the vents


Photo at top by Jan Tik from, used under a Creative Commons license.

Tags: HVAC, design, heating & cooling distribution