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Learn the Lingo – Air Conditioning Terminology & Tidbits

Air Conditioning Lingo And The Green Curmudgeon

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.

Have 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.

ACCAAir 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.

This Post Has 18 Comments

  1. Nice post, although I prefer
    Nice post, although I prefer my definitions. (Obviously I had some free time this morning.) 
    HVAC – Does anyone want to buy a vowel?  
    Refrigeration cycle – Bike riding in cold weather 
    Refrigerant – The wind that blows on you when riding your bike in the winter 
    Line set – Two or more lines drawn on a piece of paper. Crayon is acceptable. 
    Load calculation – The difference between your weight with and without a big box or bag on your shoulder 
    Tons of air conditioning – How much most people think they need to keep their house cool 
    Air handler – Magician who can juggle invisible cubes of air 
    Checking the charge – Looking at the balance on your credit card 
    Return – Desk at Target where you bring back the crap you didn’t need in the first place 
    Supply – Your pot dealer 
    Plenum – “Lots”, Variation on “plenty” 
    Trunk – Think you took all your stuff to camp in. 
    Boot – Thing that gets stuck in your butt when you piss someone off 
    Register – That think we did after standing in a long line at the beginning of the college semester 
    Grille – What you cook food on outdoors, I prefer my Big Green Egg 
    MERV – Deceased talk show host 
    Media air cleaner – Big speakers that deafen attendees at concerts 
    Delta T (ΔT) – Fraternity that wouldn’t let you pledge because you weren’t geeky enough 
    SEER – What you say when you notice a particularly attractive woman, usually preceeded by “I” 
    EER – Expression of confusion, usually a delay tactic in conversation 
    ACCA – Noise preceeding Heimlich maneuver 
    Heat pump – What the guys lifting weights outdoors do 
    Reversing valve –  
    Geothermal heat pump – Lifting weights under water or underground 
    Compressor – A client who stiffs you on a payment 
    Split system – When someone steals the equipment from the jobsite before or after it is installed. 
    PTAC – Does anyone want to buy another vowel? 
    Ductless mini-split – When someone steals a small piece of equipment  
    Sensible heat – Heat that doesn’t act like your crazy spouse. 
    Latent heat – Heat that is buried deep in your psyche 
    Sensible Heat Ratio (SHR) – The relationship between a normal person and your crazy spouse 
    Air Flow – Kind of like yoga, only either hotter or colder. 
    Static Pressure – When someone really fat sits on you and won’t get up. 

  2. It appears you left out a key
    It appears you left out a key word in your Refrigeration cycle definition: The thermodynamic cycle that allows your air conditioner to pick up (heat) from inside the house and send it outside.

  3. Carl: Wow!
    Carl: Wow! I should’ve just let you write the article to begin with. I love those. I think my favorite is the one you wrote for ACCA.

  4. Carl, you really need to get
    Carl, you really need to get a real job… That was great. Hope to see you at Camp.

  5. Carl, 

    These are f#$ing Brilliant!! 
    Sensible heat – Heat that doesn’t act like your crazy ex-spouse.  
    Latent heat – Heat that is buried deep in your psyche  
    Sensible Heat Ratio (SHR) – The relationship between a normal person and your crazy ex-spouse  
    This should be a 5 minute presentation at Summer Camp! 

  6. Allison, sorry to interrupt
    Allison, sorry to interrupt the fun (actually, I’m just envious because I have no sense of humor), but I’d like to make one tiny elaboration on your definition for Media Air Cleaner.  
    Although there’s nothing wrong with 5″ media filters, they tend to be installed directly adjacent to the return opening of air handler or furnace, thus creating an unacceptable restriction on airflow. Even if the blower has enough capacity to overcome the static drop, it will use more energy to do so. 
    The only way I know to mitigate this is to install a double-wire cabinet with two filters. My rule-of-thumb is that the surface area should be at least 1.5 times the area of the return opening. This requires two transitions. Transitions take up a lot of space. 
    Better (and much less expensive) to use 1″ pleated filters at the return grilles. This not only increases the surface area, but is often more convenient for the homeowner. Also protects return ducts from dust (although return ducts obviously need to be tight).

  7. In previous comment, should
    In previous comment, should have said “double-wide” cabinet.

  8. David:
    David: Actually, the fun didn’t start till Carl commented. My post above is mostly pretty serious. I did mention that media air cleaners can have a big effect on static pressure but didn’t really say how or what to do about it. Thanks for elaborating!  
    I wrote that static pressure is “something that almost never gets checked,” and media air cleaners can drive it sky high if the installer doesn’t allow extra area, as you suggest. Since they don’t measure the static pressure, they may never even know that they just killed the air flow.

  9. David: Ah,
    David: Ah, I was wondering what a double-wire cabinet was. I was picturing the upper filter being held by a set of two wires in some kind of housing I’ve never seen before. ;~)

  10. Pressure drop numbers are
    Pressure drop numbers are published for many if not all media filters, and they are not too bad if you keep the filter face velocity down. The problem I see with the 1-inch filter recommendation is, those all too often are undersized and their pressure drop can be worse than the media type filters. HVAC pros rave and rant against the 3M Filtrete brand in particular, they are often associated with too-high static pressure stressing the mechanicals. I am not saying I agree with this scorn of 3M. Things would be a whole lot clearer if people would just measure static pressure as a normal test.

  11. Great point about velocity
    Great point about velocity Mark. I should clarify that the reason to use 1″ filters is this: Unlike media filters, 1″ filters can be used in filter grilles. With multiple return grills, the surface area will be much larger than a central filter, thus the static drop at each filter almost disappears (think about resistors in parallel).  
    Some of the 5″ filters do have good static performance, but you’re still somewhat limited in the surface area. My 1.5 rule-of-thumb is a bare minimum to get decent performance.  
    With grille filters, you can design to achieve very low face velocities. Pros who dis 1″ filters are simply not thinking along these lines.

  12. M. Johnson
    M. Johnson wrote: 
    Things would be a whole lot clearer if people would just measure static pressure as a normal test. 

  13. David, while I agree with
    David, while I agree with your comments on using 1″ media in the return air grilles in order to increase area, I have found that, working in an area with many senior citizens, the homeowners cannot get to the grilles to change the filters. Many require an 8′ ladder, or are placed where they are inaccessible due to some heavy wall unit being placed directly below. I had one homeowner who wasn’t even aware of one filter after living there for three years. Place the filters where they can be easily maintained, and there will be fewer problems with air flow. By the way, I know that most people think the elderly live in St. Petersburg FL. They do – but their parent live in Citrus County!

  14. Walter, accessibility is a
    Walter, accessibility is a great point. However, if the air handler is in a crawl space or attic, a central filter may be even less accessible, no? In cases where unit is inside (as it should be), then a central filter may indeed be more accessible.  
    Media filters aren’t the problem. The problem is that the typical HVAC system is starved for air due to poor duct design/installation practice. Jamming a media filter against the blower return opening is just asking for trouble.  
    Airflow problems are greatly exacerbated in larger and/or more efficient homes with right-sized equipment — less blower power to cover more square footage. Using filter grilles is just one of the techniques I rely on. I also avoid ceiling returns whenever possible (but for different reasons). In smaller, inefficient homes with oversized equipment, blower power is usually not an issue.

  15. Allison, 

    I’m not sure who your audience is, but here are a couple of terms you want to add to your list: 
    1. CFM 
    2. BTU 
    3. Enthalpy 
    4. Velocity Pressure 
    5. Themostatic Expansion Valve (TXV) 
    6. Cap tube (fixed orifice) 
    7. Condensing unit 
    8. Evaporator coil (I noticed where you mentioned these two items under split system but you may want to explain what each one does.) 
    I agree with your comment that air flow is the most neglected test that service techs overlook; especially with the newer higher efficiency system, air flow is critical. 

  16. Steve: The
    Steve: The main audience I wrote this one for is homeowners and new HERS Raters/energy auditors who don’t know the basics. I definitely should go back and add cfm and BTU. I mentioned both of those in other definitions but should add them outright to the list. 
    I explained static pressure and velocity pressure in this article: 
    Rats to You, Daniel Bernoulli! – Understanding Air Pressure 
    And I just recently wrote two articles on the refrigeration cycle, the second of which covers evaporator coils and condensing units. The first one is in plain English: 
    The Magic of Cold, Part 1 – How Your Air Conditioner Works 
    The Magic of Cold, Part 2 – Intermediate Air Conditioning Principles

  17. Reverse cyle aircondition
    Reverse cyle aircondition with automatic interlocks 
    Meaning of this

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