HVAC: The Parts of a Residential Heating and Cooling System
Basic Upflow HVAC System
Welcome to the wonderful world of HVAC. You are now among the elite few who will be providing the systems that heat and cool our homes. Being the new guy is never easy when working in the trades so I've been putting together a few items that can help you get up to speed on things. I've already given you a brief introduction to the industry and a list of tools to familiarize you with those of the trade. Now, I will give you a quick run down of the overall system; what the different basic parts of the system are and how it works.
I will be using a rather typical forced air, high efficiency gas furnace and split central air conditioning system for our exercise. This is an up-flow system meaning the heating or cooling air will be disbursed upward from the top of the air handling unit as opposed to a down-flow or horizontal type install. The terminology and function of the system though is quite the same.
HVAC System Components
Supply Air Ducting (Coil Case, Plenum, Straight Duct)
Supply Air Runs
Return Air Ducting (Tap, Drop, Boot and Straight Duct)
Return Air Runs
Adaptions (Transitions, Elbows (90),
Let's Start At The Furnace
In photo #1, you can see the "set" furnace unit. Set meaning the unit is fixed to the spot chosen for the final location of the furnace. Before we move through the system, notice photo #2. This photo shows you how the furnace is divided into 2 cabinets...the burner cabinet (top) and the blower cabinet (bottom). This is reversed in a down-flow system where the duct work is located below the unit. A horizontal unit will be set up the same as an up-flow most often and just laid onto its' side. The modifications to a horizontal unit have more to do with drainage than cabinet arrangement. (Note: A high efficiency furnace creates condensation in the flu pipes. These pipes drain back toward the furnace and from there, discharged to a drain in the floor or a pump of some sort.) Now back to photo #1.
This set unit is raised slightly off the ground by cork/rubber pads (Vibration Pads) that prevent noise that would be created by metal vibrating on concrete. Often bricks will be added as well to raise the unit further off the ground; preventing water from entering the unit should mild flooding occur in the basement.
Located directly on top of the furnace is the air conditioning systems evaporator coil. This is what makes this a split system...the coil is "split" from the condensing unit that is located outside the home in most cases. Refrigerant is pumped into the evaporator coil where the pressure is changed thus changing the boiling point of the refrigerant. This process, simplified, makes the coil cooler than the air passing over it. Like a cold glass of water on a warm day, the coil pulls the humidity from the air and cools it at the same time. Again, there's more science behind it than that but you get the idea. The coil must be installed level or just slightly toward the coil drain outlet so that the collected condensation can be drained off properly, just as in the high efficiency furnace. (Photo #2 is of a factory cased coil. Coils can be purchased uncased but ultimately must be cased in some fashion to contain the air to the system.)
On top of the coil is the plenum. This is the first piece of duct work in the system that all others in the supply ducting will originate from. This could be a plenum with one opening big enough to carry the entire amount of needed supply air when located at the far end of a home or it could be a "T" plenum. A "T" plenum is used when the furnace is more centrally located in the home (suggested) and the openings on either side are sized to carry the amount of air required on each side of the home. There is also a box plenum. This type of plenum is used in a "spider" system where all heat runs are fed directly from the box and there is no "trunk-line". There are a few other various types but they are not as common and we'll stop at these 3 examples.
Lastly, the piece of metal attached to the side of the unit you see in photo #1 is called the return air "boot". This piece is where the air will finally return to the blower cabinet. We'll come back to this as we follow the airflow through the system.
Once The Air Leaves The Furnace (Air Handler)
Think of a furnace as your lungs. What goes out of the unit must be brought back into the unit in order to function properly and increase the unit's lifespan. The air distributed into the home is referred to as "supply air". The supply air is forced by the blower (fan) through the heat exchanger and/or evaporator coil to be "conditioned". Then through the plenum it goes an into our main supply duct. (Trunk-lines, photo #3.)
The supply duct is sized based on the amount of air it must move. This trunk sizing will shrink as you move down the line since air will be fed out to individual "supply runs" (photo #5)along the way. Fittings called "transitions" (photo #4) are used to make this size adjustment; thus the name transition. (Transitioning from one size to another.) These fittings are also used on the "return air" side of the system as the same idea applies in reverse when bringing air back to the system. You see in photo #4 that transitions are not the only type of duct fitting. Various fittings are used to maneuver around obstacles or rise and lower to avoid beams and piping as well. The possibilities are endless but these basic terms and diagrams cover the majority of what you'd see in the field or average home.
NOTE: See the handle on the pipe in photo #5...this is the handle of a "damper". Dampers are used to close off air flow to specific areas of a home. If the handle is in line with the flow of air, it is open. If positioned sideways, it is closed. Sometimes dampers are used in the trunk-line as well to shut down whole portions of a home or to just balance the airflow better.
Once the air is pushed through the trunk-lines and into the supply runs, it will be released through the register "boot". These boots come in many various shapes and sizes depending on what is needed based on the room above in relation to the structure of the home. Though they very in size and shape, they fall into one of 3 main categories. Straight boots, ell boots and end boots.
Now that the conditioned air has reached its' destination and kept us cool or warm, it must head back to the furnace. Remember, it's like a lung...imagine breathing out all day without inhaling...not good. Though the fan is capable of inhaling and exhaling simultaneously, it must do both or it will "starve" for air and burn up long before its' time.
The air will leave the room via return air registers that draw the air back down the wall and into return air bays. These bays are simply the space created by the joists in the basement which are "panned off" (photo #7) to seal the air going back to the return air trunk. (photo #3)
NOTE: Sealing off return air is important. You don't want to draw unpleasant air from the basement, bathrooms, or kitchens throughout the entire house.
The last piece of our puzzle for now is the "flue" or chimney. In photo #8 I've provided an example of the flu piping used in a high efficiency system. This PVC piping is usually 2 -3 inches in diameter and typically vents out the side of the home. (Vertically through the roof is possible though.) This is the piping that will condensate and is to be installed pitched downward toward the furnace. (1/4" per foot is recommended.) In a lower efficiency system, steel piping is still used and usually vented into a masonry chimney and out through the roof. (Note: A chimney should be lined with a steel liner or at least clay tile. Flu gas is corrosive and will deteriorate the mortar over time.)
Educate Yourself On Your HVAC
Were you able to learn something from this information that you didn't know?
Did you know that a ton of air has nothing to do with its' weight? In HVAC, the word ton is derived from the old days. They would measure the amount of cooling "power" based on how much cooling could be provided by melting a ton of ice in a 24 hr. period. How did they make ice without refrigeration? Well that's another story.
A Final Tid Bit
The amount of air needed in a system is usually sized based on the amount of air conditioning that the home requires. The furnace and A/C are sized to match and the fan is usually equipped to operate at various speeds as the amount of air needed to heat the home and the amount needed to be moved to cool the home are normally different. The speeds are based on CFM (cubic feet per minute)
With that in mind, air conditioning is the most finicky part of the system and is used to size the ducting. Air conditioning units are measured in tons. One ton of A/C will require 400 CFM (2 tons, 800 CFM and so on) of air to be moving across the evaporator coil in order to function at full capacity without complication or stress on the system. With this information, the duct work can begin to be sized from the furnace on down the line. If you start with 1200 CFM and over the first 8 feet of duct you give off 200 CFM to supply runs, you will then need to transition to a smaller size duct to maintain the same airflow.
Understand that this is a very basic explanation of the system and it's functions. As a "newbie", I don't want to get to scientific and freak you out. This information is intended to remove that dear in a headlights look from your face when your crew leader asks for what he feels is a very basic request. You'll win a couple brownie points and perhaps avoid having your toolbox nailed to the floor as a practical joke....well, I can't promise that but at least you'll have saved some face.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Does the return air duct to the furnace (ours sits directly above the furnace) need to be fire-proofed? Or is a plywood/stud/drywalled return air "box" sufficient
No need to fireproof that I can see based on your description and really a return can be enclosed by anything. It just needs to be sealed.