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HVAC Return Air Ducting

HVAC Return Air Ducting
Air Conditioning Sizing - Tips on Ductwork

There is nothing more uncomfortable than lying in bed on a hot summer night sweating. Tossing and turning does no good. What's worse, you know that the first floor or basement of your house is much more comfortable. Does it have to be this way? Absolutely, positively NOT!

My house and lots of others that I have built and remodeled are very comfortable, even in the hottest weather. The reason is simple. My HVAC contractor, Richard Anderson, knows the importance of return air ducting AND sizing correctly both supply and return air ducts. This is the key to comfort.

Existing Hot Houses

I'll bet that 3 out of 5 people who read this bulletin have an existing home that has no or inadequate return air ducts. Don't worry, we are going to solve those problems. It isn't always going to be easy or pretty, but it can be done.

Return air ducts are necessary in just about every room. However, there are two rooms that I never place return air ducts - kitchens and baths. Let's try to keep the garlic odors and hair spray in these rooms if you don't mind! The reason for return air is simple. Your furnace is simply a recirculating pump. Instead of pumping water, it is pumping air. We need to get the air back to the pump. Furthermore, without a return air duct in each major room, the supply duct air has to "push" all of the air in the room out of the way as it makes its way towards you. The return air ducts help pull the air as it is being pushed.

Where Can You Put Them?

If you have an existing ranch style home, installing return air ducts in each room can be done fairly easily. You do what we do in new homes. You use the wall cavities as the duct. Think of it. Drywall that is nailed to 2x4s is really a duct - a box with 4 sides. All you have to do is cut out the floor immediately below the wall cavity as well as the bottom wall plate. Once you have created the opening, sheet metal will allow you to connect this to your existing return air system.

Two story houses can be more of a challenge. You have to figure out a way to get return air from the ceilings of the second floor to the basement or furnace room.

Do you have a laundry chute that you don't use that often? Can it be sacrificed for the cooling cause? We have used these with tremendous success.

Sometimes, you can use a kitchen pantry closet or a hall closet. We have often been able to put a duct in one corner of the pantry or closet. This duct extends up to the second floor where it often lines up with an interior hallway wall.

Often you can collect the ceiling air through a network of flexible pipes in an attic. These pipes join together in one central location that permits you to run a duct down to the basement or a second floor closet.

As a last resort, you may have to put a duct in the corner of a room(s). These can be drywalled easily. To effectively disguise it, you may build another one several feet down the wall. Shelves can be put between these two "stacks". If they are in a child's room, they can often be painted in a decorative manner as if it was intended for them to be there.

It was common in many older homes to have projections in rooms such as this. These bump outs often were fireplace chimneys as they rose through to the roof.

Balancing Return Air Flow

If you want to really have a way to get the most bang for your return air buck, think of installing grill covers on the wall that have operating louvers. This will allow you to choke down rooms possibly on the first floor that are drawing too much air. This in turn creates a greater suction in the rooms that really need the cool air. This principle is no different than dampers which are used on supply lines to regulate air flow.

The grills with louvers are inexpensive and effective. On an older existing home that is being retrofitted for central air-conditioning, they are a must.

Reversed Air Flow On Old Homes

Do you have a house that is approximately 80 years old? If so, your supply ducts may be on inside walls. You may have a single giant return air grill in the floor or at the bottom of a staircase. This is all wrong.

Supply ducts should be located on outside walls, preferably under windows and near doors. The intent is to wash the exterior walls with heated or cooled air. You are trying to combat the heat or cold at its source.

The single giant return air grill in the floor balances the load at the furnace motor, but does nothing to promote cross ventilation in each room.

What, you don't think you can match the hardwood flooring? Rubbish! Go to a first floor closet and get the flooring from there. A good hardwood installer can make the repair and no one will be the wiser. Who cares if the flooring inside the closet doesn't match.

I wish you luck in your effort to stay cool. Don't hesitate to contact me if I can help.


Air Conditioning Sizing Considerations

Residential air conditioners, both central and window units, are like shoes. They come in various, different sizes. Buy the wrong shoe and you will be uncomfortable. It is no different with air conditioning.

Bigger Is NOT Better

So you really want your house cold. What are you going to do? Buy a monster 5 ton central unit? That could be a mistake. If you put in a unit that is too powerful (produces too much cooling), it will short cycle. It will turn on, blast out vast quantities of cool air, and then shut off. The result will be improper dehumidification. You will be cool and clammy. The air conditioner needs to run for a sufficient time to squeeze out the humidity that has seeped into your house. You achieve the highest comfort when the air conditioner removes as much humidity as possible from your interior air.

How Are They Sized?

To properly size an air conditioner, one has to perform a heat gain calculation. In other words, you must determine how fast heat is getting into your house and what things inside the house are contributing to heat gain. To perform the calculation, you need to know how much insulation is in your exterior walls, how much is in your attic, the size and type of glazing for each window, the type of construction of each window, extra special lights or cooking appliances that generate heat, compass direction that each wall of your house faces, etc. In other words, you have to do some work to get the equipment sized right. Tables and charts have been around for years that convert these measurements into BTUS. Once you know how many BTUs your house is gaining, you can purchase the correct sized air conditioner.

When purchasing a unit, ask for these calculations! Don't just buy the same size unit you now have. Maybe it was not sized right. Maybe you have upgraded your windows or insulation. Maybe you added a small or mid sized room addition or remodeled an attic space.

Replacing An Existing Unit - BEWARE!!

If you have an existing house which needs a new central AC unit, be careful! For you to achieve the published efficiency and performance that is sold to you by the salesperson, the interior coil housed in the furnace must be compatible and matched for size! If it is not, the outdoor unit will struggle to keep your house cool. It is a question you must ask the installer. Make him/her prove to you that the interior coil will handle the outside unit. Remember, the inside coil could have been wrong from the beginning! The original furnace contractor could have downsized one size to save money. The AC will work, but not as well as it should have!


Column B143

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35 Responses to HVAC Return Air Ducting

  1. Good information, thanks. I am considering remodeling my basement. My basement has a cold air return that services the first floor. It runs across the basement ceiling, perpendicular to the ceiling joists, sitting right below them. This return spans my entire basement. during the remodel I could just box it in, but the size of the return after boxing and then covering with sheetrock or paneling would really create a low-point in the room. The return is rigid foam about 12 in x 12 in, so its bottom point is below the main beam that supports the first floor joists. Are there any options for changing the shape of that return to a lower profile, so it doesn't come down a full 12 inches?

    • I have the exact same question. The return ducts in my basement are exactly 1/2" below the top of my head (ask me how I know). I'm hoping that a shallower, wider duct would give me an equivalent volume of airflow with less frontal lobe damage. Could I swap my 8" x 14" duct with 5" x 22-1/2", or are there fluid dynamics issues that would make this not an equivalent amount of airflow? Thanks!

  2. I am remolding a 97bottom year old farm house and i put central heat and air in it so i was wondering if you can put the cold air return duct in the ceiling in one corner and the actual duct in the ceiling in the other corner.

  3. i live in an 2 bedroom 1200 sq ft hi rise condo in atlanta ga. the hvac unit is in the center of the apt and supply lines are install up high ( since AC is predominately needed due to climate and the heat is easily retained from the concrete mass. the closet is only slightly larger than thewidth/ depth of the hvac unit. suplly is facing upward. the return is located on the bottom of the unit directly facing the floor. directly underneath the unit is a 50 gallon electric water heater. the space between hvac and water heater below is 3 inches. even if i switched out water heater to something like point of se on demand i am at a loss of how to run return lines from the two bedrooms. the living room is fine as this closet is in the living room. is there a way to use the skinny tube return?

    • Jim, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  4. I have a two story home. The duct work for the return upstairs goes nowhere. The main floor has no duct work and only runs through the joists in the wall is this normal?

  5. I have bought my house and the bed room was added but there is no return vent.. The room is cold almost 10 degrees less than the rest of the house ,,
    Should I open ahole in the bottom of the closet that lead to the living room and but admell fan to blow the cold air from the bottom to act like return
    ..

    As I know cold air stay down and hot up what I think ?

  6. We have a 7year old house, very little or no cold air return upstairs. The registers are there, but they never cut through the floor to the basement. They, heartland homes, placed plumbing pipes through them and braced them at the floor with 2x4's, blocking everything. I ran a snake with a camera down them. Is this legal or correct or do I have a solid complaint against them? I'd rather not cut through all my newly painted bedrooms on my dime. Really frustrated with the mechanics of this builder.

  7. I just did an inspection on a contemporary house built in 1985. It is a 2 story house. The heat is hot water / base board. There is central AC. There is only one Return (which is on the second floor directly under the air handler). What are the implications of this? Also, when we had the fan running the air coming through the vent was VERY loud. It was a wooshing sound. Is this normal? how can we preven this?

  8. I live in a single floor ranch house in the northeast (Western NY State). I'm building a fully insulated, airtight, multi-use soundproof room in my basement. It will mainly be a home theater/lounge type area. The space is not a pure rectangle but almost. The last 10 feet of the room shifts over about 6 feet. In total it's about 3450 cubic feet of area. I have a capped off 6" supply run stubbed out about 2 feet from the furnace plenum that was intended for heating/cooling the basement.

    I plan to "y" or tee that stub and run two separate supply runs. Each will use a damper door for seasonal flow control adjustment. Each run will supply one end of the room. Given that, I expect I'll need one return line for each supply line introduced to the room.

    I've been told I don't need return air vents in the room but I'm not buying that. I've read that the returns should be 1.5 times the supply volume to assure proper air flow but I've also read they should be equal. I assume that refers more to the trunk line volumes so I was simply expecting to run two 6" return lines to the room from the existing cold air trunk to match the two 6" supply lines. I thought I'd place the return registers opposite each other (about 15 feet apart) midway between the supply registers so they each draw from the center of the room. Is this a plausible location?

    I may be over simplifying things but will using two 6" heat/AC supply runs and two 6" return lines be sufficient for my room volume? I'm not looking to rework my HVAC system I just want to make sure that I can heat and cool the room to a reasonable comfort level. I don't have zone heating which means the thermostat is upstairs so this new room is going to be at the mercy of the upstairs living area but hopefully, I can use the dampers to achieve a decent comfort level for watching movies or just hanging out with friends.

    Any advice as to whether I'm on the right track or not?

  9. Its a rule in hvac if you have any unit change out say from three ton to now a three and a half ton ac that the return air duct is at least 18 inch to 20 inch return remember its only going to give what it gets so if you have no velocity out of the registers threw out of the home look to insure that its not running on too small of return air flow duct and filter grill air can

  10. When ever you change out a split system unit were the condenser or out door coil is changed in order to get the seer rating seasonal energy efecancy rateing value of this the indoor coil must be matched to the new condenser and or change the metering device in some cases this may be posible but its allways best replace indoor air handler and evap coil to get top performance out of the new seer ratio wich is nothing more than a larger out door coil.

  11. I had an addition put on to back of house and the return air vent was along the baseboards on the wall underneath a window, this ultimately became the opening into the addition, thus closing off the return vent. I would like to place a floor register for return air just next to the doorway using the floor joists as the return in the basement ( which is how they currently are). Problem is the space I want to put new return falls right next to current return bay. How can I transition the new return vent into the old bay ( joist are 2 x 10 ).

  12. I have a 4yr old two storey home in Ontario Canada, I am finishing the basement with 1 Family room - 1 Office/Bedroom - 3pc washroom plus workshop and utility room areas. Ive installed the heating ducts already but I need to know if its essential to install cold air returns in all or some of these basement spaces, if so does it need 5" ducting or will 4" be sufficient. Also in the basement should the side wall duct grills be at the top or bottom of the walls, in the rest of the house on the ground floor the return air registers are at floor level while on the top floor bedrooms the return air registers are at the top of the walls.
    Any assistance appreciated.

    Rgds; John/

  13. I have 6 inch supply line for HVAC in two new bedrooms but can only put in 5 inch lines for return without some major remodeling how much problem will this create?
    Thank you.

  14. We have a 2 story house with a crawl. Recently we installed
    heat pump duct work. the installer installed duc vent into floor not wall.
    Is this ok? Should the duc be installed above the floor for better
    operation? thanks nancy

  15. I have a two-story house with a walk out basement. The basement is 10° colder in the summer then the rest of the house can I put a cold air return in the basement? I would like to circulate the cold air from the basement to the rest of the house it's a finished basement.

    • Yes, you can do this to capture the cooler air. Just be aware that in a few days the basement will be hot and humid. What's more, you could set up a condensation issue in the basement as you introduce lots of humidity on the cooler wall and floor surfaces. Mold follows condensation.

  16. Multiple return ducts? So, bought house. It's 30 years old. Looks like the added the central air later on in it's lifetime. Blower unit is in the attic . It's a 2 story raised ranch. In the attic, it looks like a quick , lazy job. One end of the attic, is the unit with a 2x 2 feet intake duct in the ceiling of the hallway just under the unit. The outgoing air duct goes from one end to the other of the attic with several off shoot ducts to the outer walls of the upper rooms. But is also just holes in the ceiling of the upper floor. Just along the outside wall. Is this correct installation. I myself, think not. We're should the intake duct or ducts be located. And the out going air ducts. Should they be in ceiling or walls? Upstairs, downstairs, etc... Cheers!

  17. I had a new 3-ton unit installed in 2010. It replaced a 2.5 ton. My house is 1200 sq ft ranch with ducts in the crawl space. The original duct work is 14 inches and I have been told I need to increase to 16 inches. I have broken straps and sagging and it looks like something has been chewing on the cover. Do I need to repair the 14 inch or tear it all out and install 16 inch? My return in floor is 14X20. Is this correct size for a filter?

  18. Another possible return air duct is an abandoned chimney. I have a 90 year old house with a chimney running up the center of the house. It was knocked below the roofline when the roof was shingled last. I knocked it down even more, built a cap for it, and ran insulated flex duct to each bedroom, then made a connection between the chimney and return duct in the basement.

  19. Hi, I have a two story house one year old and the bedrooms upstairs are colder in the winter and hotter in the summer by close to 10 degrees depends which room. Of course the builder sends his HVAC company to tell you everything is working within spec, the house temp not comfortable at all. I noticed the the return is using the cavity which is drywall and joist but doesn't have sealant anywhere just an opening. How do I know if the problem is with the return? Another thing that I have noticed is I have few ducts for the heat/cold are running on the outside walls. Please let me know what you recommend.

  20. Tim, we just bought a 47 yr old ranch in St. Louis, MO. It's 1000 sf, 3 small bedrooms and 2 baths (one very tiny). There is only one air return vent in the hallway; it's 14" x 30". Should I replace it with vents in all the other rooms? 3 Bedrooms and living room? We have a kitchen but no dining room. Those are all the rooms. The vent in the hallway is huge and almost touches the two bedroom doorways on either side of it. I want to put new 1x4 trim around the doors but it won't fit because of the big vent. Thanks.

    • If you want the best heating and cooling experience, you'll have your supply registers on exterior walls and a return air in each room on the opposite wall. The air is PULLED through the room. You NEVER put a return air in a kitchen or bathroom.

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