It's great that you've pointed out the problem that many customer's and even many contractor's don't seem to realize,. I was on a job recently that the return was so small that when the customer installed (pleated)1inch "High efficiency"filtersthey were pulled out of the R/A grill and up into the ductwork. The question I have is when you talk about 200 sq.in per ton, Are you taking into account the free area or "K" factor, because each one of those louvers is a restriction, and if you were to slide them all to one side and add up the area they take up, it's significant, one third or better of the grill's area, which is why the manufacturer's have to calculate the amount of free area for each Supply and Return Grille and register
I had a 2.5 Ton Unit Residential A/C. I went to a 3 Ton Unit. Does this mean My grill is too small? I noticed an opening at the front side of grill. It doesn't close totally like before. Thank You!
Great video, just changed out my louvered door for full 6-panel door, & installed a 24x24 in the wall & 8x30 grill above the door to give 816 for my new 4-ton unit. My contractor tried to tell me that the 24x24 would be fine, but it was easy to add the grill above my new door. Looks much better. Sounds quite now Thanks!
Thanks for the video. I've been having issues with my old system and not 1 person mentioned to me that the size of the return needs to be increased. I have a 24"x16" 384 sq.inches when I need 600 for my 3 ton unit. This is half my problem
Thank you very much for a such valuable information. I have an open concept home and my living room is 16 feet tall. I have two ac units. One downstairs and one upstairs. If I need to add an extra return does it matter if it's at floor level or ceiling level? Thank you.
Residential is sometime harder. Going behind some one else can be tough. But you can make your money faster. I would try that less starting Capital. Good luck.
Great series of videos, Dave! I have a quick question. I would like to finish my basement but I have one room where the return duct, leading to the furnace, is hanging down 9" and would make for a very unattractive bulkhead and low overhead. If this basement duct is 9" X 20" can I change that to 5" X 36" ? It's the same amount of square inches and those extra 4" of overhead room would really make a big difference to the look of the finished room.
Someone capped a floor return for a 3 ton unit. It has been pulling crawl space air into a open flash pan under house, then out to the packaged unit. I believe I will have to have a 20 x 30" filter grille installed in the floor
Or just use the formula for the cfm each grill will allow. width x length x 2 will also let you know how much cfm is allowed thru that grill. Ex. 20x25x2= 1,000 cfm which is good for a 2 or 2.5 ton a/c.
I have 228 inches of grill space but it’s using my wall space which is only 3 inches deep. Will increasing the size of the grills actually help any? There’s two. A 14 x6 and a 24 x6. Edit: I plan on adding jump ducts for the bedrooms and a return vent in the finished basement
@@michellemajako What size is your AC system? The size of your inside walls is only going to carry about 120 cfm in each bay. So, according to caculations. thats barely enough for a 1.5 ton ac. Increasing the size of the grills won't help much. Added in new returns in other rooms will definately help.
@@smacleod69 Three tons!! Added a 14x12 (I think that was the size) return grille and connected the register box with an 8 inch starter collar in the basement. There are two output registers down there. The basement is warmer this winter. Plan is to add returns to the bedrooms because we need more return than just the hallway and the new one downstairs. It has to be hard on the system. Also, the bedrooms just do not stay warm/cool. No idea how the company who put this system in thought that size was a good idea. This house is only a 1K square foot ranch, too. Thank you so much for your answer. Did leave the hallway returns that size.
Good demo and video sir. I think my return is very undersized, but I don't see any adverse symptons. I have a 3 ton unit with an 18x18 return mounted in an 18x18x20 plenum, connected with a 12in x 3 foot flex to the unit. According to the calculation that's liike 1/2 the size needed? Yet there is no whistling or even observable wind noise, and airflow seems to be excellent. THe return is in the tongue & groove hallway ceiling so its hard to make it larger.
I should also mention- the air-handler opening itself is about 20x20 that that's only 400 in2 - sized for a 2-ton unit. So even if I went to 24x24 return grille, it reduces to 20x20 at the handler? ALso the 12in flex area is pi x 36 =~ 113in2 ! - sized for a 1/2 ton!
Malabar Spyder Yes, but the air-handler opening and the flex duct do not have filters in them, so the air freely flows through them. To insure adequate air flow through the filter, the area of the filter must be greater than that of the duct and air-handler opening.
QUESTION: What if you have a 2 or 4in filter? Does that “take away” square inches? Doesnt a bigger filter create more static pressure and affect the return numbers?
Thanks for the video Its really helpful, I just want to as that you told Thumb rule for sizing Return Grill that 200 sq inch = I TR Is it standard of ASHRAE Or SMACNA??? Secondly is it valid for all type Units for example VRF Indoor , AHU etc??
Thanks for the great information. Could an AC start forming ice on the coils if it is starving for air? My 4 ton unit has 1 return duct register of 13x21.5 sq inches (280 sq inches) approx I've had the HVAC tech stop by and check for refrigerant leaks, or issues with the TXV and they all look good. He is recommending to add additional return air registers in the house to help with reducing the ice buildup.
If you get this reply, ice on the indoor coil is EXACTLY what you should expect from a unit that is starving for air. If more warm air was flowing over the coils, they would be kept above freezing. The engineers who design these units assume that the indoor coil will soak up as much heat as the outdoor coil can dissipate.
Niyati Abhi Gupta Hope you didn't have that done. All you needed was to increase the size of the existing return duct. Literally splice it, and add to it. Having just a couple extra feet that the air has to travel can make the difference, especially a couple degrees in which 33°F at the coil would not create ice. Either that or he was actually talking about a dump or bypass... oh well.
Mr David, I currently have a 2.5 ton unit. I'm stepping up to a 3 ton. My return air is 274 sq inches. Should u install another 275 sq inch return grill? Does my ductwork have to be taken into account? Thanks
My system has an appropriately-sized grill but immediately inside it is a much smaller round duct. I'm amazed by the widespread incompetence in the HVAC industry.
Dave, my home A/C was originally a heat pump, and it was a 5 ton unit. It produced too much moisture in my home,and the new A/C system was downsized to a 4 ton, but not a heat pump. The filter that was from the original system was huger, two 16x25 filters sitting side by side under one grill. I wanted to change it to a single filter and wanted to install a 24x24, but according to your calculation, a 24x24 would not be big enough. The trouble is that I can't find anything bigger in a Home Depot. To further complicate things, the actual open behind the grill is smaller than the actual filters. The actual opening is 25x25. So did the installer skip giving me a properly sized opening in favor of leaving it the way it was, and cutting costs?
I have a 5 ton unit I know I need a thousand square inches of return grill right now I only have about four hundred square inches and I am whistling I change the blower motor for the guy and that's when it saw that whistle he said it never whistled before if I open up another return give another 600 square inches do you think I'll take care of my problem ty for all your videos
It seems that is " A rule of thumb ".. We have to go back and check manufacturer's specifications. moreover registers/supply air outlets concept will be different because of many constraints Air Velocity, Sound level , etc.. Although the demonstration is very helpful .. Thanks
The house we bought has three returns with filters at each return. The air handler itself has a place to put a filter. Is it better to have the filters at the returns, is it better to just have a single filter at the air handler or does it not really matter? I question if all the filters add restriction, making the return more inefficient (they appear sized right for 3 ton system, 2 - 12x24 and 1 - 12x12). Thanks.
To get the "tons". The number you get will tell you how many tons the opening is good for. For example, I have 588, which divided by 200 is 2.94, so I am good for a 2.94 Ton AC unit. I have no idea what the size of my unit is because I can not find any markings on the unit in my house. I am going to assume it is probably 2.5 according to Sq foot/ton calculators on the internet.
thank you, in hawaii and my ac is 3 ton , gettin gonly about 324sq in" return thats squealing, my poor ac is starving of air so im going to go install another grill,,,, does it matter how long the duct is ? thank you so much for you video ,helped me alot, i got quoted 710. to run a new return and a 12" duct about 20 feet. is that accurate? thank you again ! Sarah
What if am using media filters? Calculations will be different. And if is for commercial installation also will be different. Had to consider the fresh air, so less static will be at the return grill.
Hello! Great video one question how would you translate this into cfm's? Example I have a 4 ton unit with only an 8x12 grill it's freezing up and flowing ice into the ducts. I know I need more air but honestly I would like to understand the difference between cfm's and free air. Thanks in advance
OK here is a back of the envelope swag:) : (1) Old rule of thumb is 400 cfm per ton thus 4*400 =1600 cfm. ( cubic feet of air per minute) (2) Grille area minimum = 4 tons *200 inches squared is 800 sqin . 800/144= 5.56 sqft too. (3) For fun the air velocity is 1600 cubic feet per minute / 5.56 sqft = 288 ft per minute. Ok 288/60 = 4.8 ft per second.
I live in a mobile home that has a 3.5 ton heat pump on it. My furnace sits in the hallway, and the door/grill is the only return I have because it free pulls from there. The opening of the grill only measures 17” x 23”, which is way too small for a 3.5 ton system. My calculations using this method bring me to the conclusion that I could just cover up that door somehow, and install a 24” x 30” filter grill. Would that work in this situation?
Loved your video,lots of good information! I currently have 4 ton unit with a24×24 air filter. I plan to replace the unit and have received some quotes with 4 and 5 ton units. The 5 ton unit quote indicated the reason for a5 ton was because of living room ceiling of 15 feet, is reason enough to go with a 5 ton unit?.
Have them do a manual J calculation. That will take into effect total heat load. Remember - you are conditioning cubic footage not square footage. The number of windows, insulation, shade, etc all come into play. And yes - I realize your comment is six years old - this is for anyone that finds this later…
My heating system is in a mechanical room. I have a large return grate in the wall outside of room next to the heater. There is a filter on the unit but no duct connecting the grate to the unit. Is it important to have this short run from grate to unit ran through duct? Probably 2 feet from the grate to the unit inside the mechanical room.
You seem to be talking about about the velocity across the air filter? Why not mention the can depth (typically connected to round flex) and the duct size of the return air going to the unit? Just wondering.
Hi David, my AC is 3.5 ton unit while the furnace is 4 ton unit. The house is about 2,500 sq. ft divided into 3 zones and I have only 2 returns with the total of 744 sq. inches. Do you think having another return will help the air flow since one of the zone now has much lower flow? Thanks in advance.
what would be the effects of having an undersized return? I've only got a 24×18 return so obviously that needs to be enlarged, tech never mentioned it, learning on my own at this point. I've had major sweating issues with my air handler, I've done everything to treat the attic and it stays under 90°f up there. Could the small return be contributing to the sweating issues?
Grilles and registers don’t have 100% free area. Most manufacturers state the free area of their grilles. It is usually around 80%. You can’t use the outside measurements, because of the space occupied by the louvers. You must adjust for the grilles free area. In other words 10x 20 grille is 200 square inches x 80% = 160. I know this sounds nit picky, but if the rated velocity thru the grille is exceeded, they can become noisy.
I was running with your intake measurement based upon unit of tons. i.e. for every ton one needs 200 square inches of intake grill. But then my HVAC neighbor argues that an intake grill size does not matter as long as it is the same size as the units intake. In other words you will not be able to gain any more air using a register that is larger than what the unit comes with. So for example I have a 2.5 ton unit. according to your estimate I would need 200 inches per ton or a grill that is at least 500 square inches or a grill that is a 20 x 25 or greater. But he argues the return on the unit itself is only say a 12 x 17 (before any flex or conduit) or 204 square inches before it is connected to the house? How can I justify this? How can you get more air out of a larger return grill over what your original units intake capacity is?
The fact is there are a few factors. And yes the leavers in the grill are definetly a factor the length of run bend fitting and all these favors come into play as well as the head pressure on the other side of the air handler. This is where a water column manometer comes in handy. But we have these general rules of rheum that help with in the field quality installation. But as a rule there is no limit to the size of return. You don't want a return making much if any negative pressure. So the return needs to be open and letting air pass freely.
My understanding is that the larger opening is because of the filter. Air filters reduce the amount of air that gets into the system. Having a larger filtered opening allows more air air to pass through the filter. If you have one of those cheap nylon filters that don't really filter anything in your system, you are probably fine. If you have one of those high filtration filters in your system, you are going to damage your unit. Most pros that I have heard from say to remove the filter from the AC unit and use filtered return air grills.
Does the 200 sq inch rule work for just a heating system as well? I don't have ac (tons) just heat. Not sure what the BTUs are but the unit puts out quite a bit. Thanks
Wish I could run my return numbers by you. I have OCD bad and I keep building my return then taking it back down. I have a 5 ton unit self contained. On the return side I wanted to filter every room. (Remember I have bad OCD) I have two 20x20 return grills hooked to 12 inch round metal duct in the hall way. I have another 20x20 hooked to a 12 inch in the ceiling of my cathedral ceiling. I have six 14x14 hooked to 6inch round metal duct each. . On the CFM side I'm guessing I'm ok but I nervous I don't have enough return since I wanted to filter each room independently. So my math shows three 12 inch 1500cfm. Six 6inch 600 cfm. That's showing 2100 cfm. My question is with all those filters will 2100 by rule of thumb math be enough on that 5 ton unit. And by the way I have no flex in my system all round metal duct work. My supply from unit 20 inch my return 20 inch round.
Question, does the design of the air return flex matter? Square versus round? Also, if a return is closer to the air handler does that mitigate the need for the rule of thumb 200" per ton? :-)
Dave, Im considering starting my own business. Im a journeyman sheet metal mechanic with my local. Work is slow and i mainly worked on commercial projects. Whats the biggest difference between residential and commercial work?
I'm in Texas, been in a.c. field 18yrs (and still don't know everthing, no a.c. guy should ever tell you this) anyways I've always divided for gas units by 144 and electric by 215 not 200 ,that's per Airflow icb, is this correct?
So if i have a 2 ton a/c unit would i need 400 square inches of return a/c grille space? If i am suppose to have 400 square inches for a 2 ton a/c unit will a 25 x 16 return a/c grille work since it comes out to 400 square inches?
I got a new return grille 20x20 and it's alittle smaller than my current old one and the filter is snug also I've searched for wider dimensions but none that goes to 23 on the outside, It's not in yet I need help on what to do
if you have a 3 ton let say you need at least 18 inch, then a 10 in another part of house make sure your return grills are large enough for the air flow look at my video on sizing
I have a 16x20 grill on 3 ton. Iam trying to put my own unit in and all the runns. My house is only less than 2000 square feet house one story with 2 bathrooms 3 bedroom s. All my rooms are about 14 x14 except my bathrooms and laundry room their 7x8. Whats the best size duck's do i need 6 in or bigger
hey. i have a question .recently i had installed 2.5 tonn rheem unit for heat and AC and there is only 1 return duct which is 12 by 14 and followed by 16 by24 main return built in unit filter. can it provide enough air volume for this unit? so it can work properly
So you're only putting 2 return grills in for a whole 3 ton system? Or are you calculating each room (outside of bathrooms and Kitchen) to know what size return grill you need?
It all depends. The size of the AC unit, the amount of room to install the grill. You could install a single grill if you have the space for it. The size of the house really has nothing to do with it as long as the air has a path back to the AC unit.
Been doing this 15 years ...best explanation ive ever heard.
It's great that you've pointed out the problem that many customer's and even many contractor's don't seem to realize,. I was on a job recently that the return was so small that when the customer installed (pleated)1inch "High efficiency"filtersthey were pulled out of the R/A grill and up into the ductwork.
The question I have is when you talk about 200 sq.in per ton, Are you taking into account the free area or "K" factor, because each one of those louvers is a restriction, and if you were to slide them all to one side and add up the area they take up, it's significant, one third or better of the grill's area, which is why the manufacturer's have to calculate the amount of free area for each Supply and Return Grille and register
I had a 2.5 Ton Unit Residential A/C. I went to a 3 Ton Unit. Does this mean My grill is too small? I noticed an opening at the front side of grill. It doesn't close totally like before.
Thank You!
Great video, just changed out my louvered door for full 6-panel door, & installed a 24x24 in the wall & 8x30 grill above the door to give 816 for my new 4-ton unit. My contractor tried to tell me that the 24x24 would be fine, but it was easy to add the grill above my new door. Looks much better. Sounds quite now Thanks!
thanks i have a test over this on Thursday and my teachers are horrible at explaining that. u just taught me more than my teacher did all class period
thanks for the comment
Thanks for the video. I've been having issues with my old system and not 1 person mentioned to me that the size of the return needs to be increased. I have a 24"x16" 384 sq.inches when I need 600 for my 3 ton unit. This is half my problem
You have the best videos for learning the proper techniques for HVAC repair and troubleshooting. Thank you for sharing.
thank you man. i wish my contractors knew it when the did the job
thanks for schooling us with this quick, easy to understand, fun to watch tutorial. You're awesome.
I hope you find this video helpful...thanks for watching from Jones Air Conditioning & Electric in Naples, FL!
Thank you very much for a such valuable information. I have an open concept home and my living room is 16 feet tall. I have two ac units. One downstairs and one upstairs. If I need to add an extra return does it matter if it's at floor level or ceiling level?
Thank you.
Excellent Tortola. I never had to replace an Air Conditioner Air Handler. Àlways a first!
I hope it's a 2 million dollars a year. I will be here for support.
Great video! Easy to understand, quick and to the point. Thanks for posting!
Residential is sometime harder. Going behind some one else can be tough. But you can make your money faster. I would try that less starting
Capital. Good luck.
I wanted to apologize about the Monkees and the pirate comment. I have been watching HVAC videos and started watching yours, good stuff, God bless.
really like your video s.. appreciate the knowledge.. How would u size the proper flex return for that grill?
Thanks for this video, but need another help please tell me how to calculate supply air grill size to consider supply cfm?
Great series of videos, Dave! I have a quick question. I would like to finish my basement but I have one room where the return duct, leading to the furnace, is hanging down 9" and would make for a very unattractive bulkhead and low overhead. If this basement duct is 9" X 20" can I change that to 5" X 36" ? It's the same amount of square inches and those extra 4" of overhead room would really make a big difference to the look of the finished room.
very good infomation , When you say generally 200 sq. in. , could or would there be any reason for a change
The right way
Someone capped a floor return for a 3 ton unit. It has been pulling crawl space air into a open flash pan under house, then out to the packaged unit. I believe I will have to have a 20 x 30" filter grille installed in the floor
Awesome Dave, but could you also address R/A duct size.
Or just use the formula for the cfm each grill will allow. width x length x 2 will also let you know how much cfm is allowed thru that grill.
Ex. 20x25x2= 1,000 cfm which is good for a 2 or 2.5 ton a/c.
I have 228 inches of grill space but it’s using my wall space which is only 3 inches deep. Will increasing the size of the grills actually help any? There’s two. A 14 x6 and a 24 x6.
Edit: I plan on adding jump ducts for the bedrooms and a return vent in the finished basement
@@michellemajako What size is your AC system? The size of your inside walls is only going to carry about 120 cfm in each bay. So, according to caculations. thats barely enough for a 1.5 ton ac. Increasing the size of the grills won't help much. Added in new returns in other rooms will definately help.
@@smacleod69 Three tons!! Added a 14x12 (I think that was the size) return grille and connected the register box with an 8 inch starter collar in the basement. There are two output registers down there. The basement is warmer this winter. Plan is to add returns to the bedrooms because we need more return than just the hallway and the new one downstairs. It has to be hard on the system. Also, the bedrooms just do not stay warm/cool. No idea how the company who put this system in thought that size was a good idea. This house is only a 1K square foot ranch, too. Thank you so much for your answer. Did leave the hallway returns that size.
Hi David jones, thanks for the video, what I'd like to know is what type of duct are you using in your design, is it flex, metal or fabricated board?
Good demo and video sir.
I think my return is very undersized, but I don't see any adverse symptons. I have a 3 ton unit with an 18x18 return mounted in an 18x18x20 plenum, connected with a 12in x 3 foot flex to the unit.
According to the calculation that's liike 1/2 the size needed? Yet there is no whistling or even observable wind noise, and airflow seems to be excellent. THe return is in the tongue & groove hallway ceiling so its hard to make it larger.
I should also mention- the air-handler opening itself is about 20x20
that that's only 400 in2 - sized for a 2-ton unit. So even if I went to 24x24 return grille, it reduces to 20x20 at the handler? ALso the 12in flex area is pi x 36 =~ 113in2 ! - sized for a 1/2 ton!
Malabar Spyder Yes, but the air-handler opening and the flex duct do not have filters in them, so the air freely flows through them. To insure adequate air flow through the filter, the area of the filter must be greater than that of the duct and air-handler opening.
Building a home on my own.
This will help.
Thanks!
David you are the greatest.... Thanks for sharing your knowledge.......
QUESTION: What if you have a 2 or 4in filter? Does that “take away” square inches? Doesnt a bigger filter create more static pressure and affect the return numbers?
Thanks for the video Its really helpful, I just want to as that you told Thumb rule for sizing Return Grill that 200 sq inch = I TR Is it standard of ASHRAE Or SMACNA??? Secondly is it valid for all type Units for example VRF Indoor , AHU etc??
great job.thanks for all your wisdom.you are planting seeds into million dollar business!!!
Thanks for the great information. Could an AC start forming ice on the coils if it is starving for air? My 4 ton unit has 1 return duct register of 13x21.5 sq inches (280 sq inches) approx I've had the HVAC tech stop by and check for refrigerant leaks, or issues with the TXV and they all look good. He is recommending to add additional return air registers in the house to help with reducing the ice buildup.
If you get this reply, ice on the indoor coil is EXACTLY what you should expect from a unit that is starving for air. If more warm air was flowing over the coils, they would be kept above freezing. The engineers who design these units assume that the indoor coil will soak up as much heat as the outdoor coil can dissipate.
Niyati Abhi Gupta Hope you didn't have that done. All you needed was to increase the size of the existing return duct. Literally splice it, and add to it. Having just a couple extra feet that the air has to travel can make the difference, especially a couple degrees in which 33°F at the coil would not create ice. Either that or he was actually talking about a dump or bypass... oh well.
What about converting to a 4 inch deep filter does that change the math...4x the filtration area or is the math just about the opening? Thanks
Can you make a video on how to size diffusers? Do you size grills with return ducts in the same way you size grills with no return ducts
Thanks for making this video.
Mr David, I currently have a 2.5 ton unit. I'm stepping up to a 3 ton. My return air is 274 sq inches. Should u install another 275 sq inch return grill? Does my ductwork have to be taken into account? Thanks
My system has an appropriately-sized grill but immediately inside it is a much smaller round duct. I'm amazed by the widespread incompetence in the HVAC industry.
Dave, my home A/C was originally a heat pump, and it was a 5 ton unit. It produced too much moisture in my home,and the new A/C system was downsized to a 4 ton, but not a heat pump. The filter that was from the original system was huger, two 16x25 filters sitting side by side under one grill. I wanted to change it to a single filter and wanted to install a 24x24, but according to your calculation, a 24x24 would not be big enough.
The trouble is that I can't find anything bigger in a Home Depot.
To further complicate things, the actual open behind the grill is smaller than the actual filters. The actual opening is 25x25. So did the installer skip giving me a properly sized opening in favor of leaving it the way it was, and cutting costs?
I have 4 ton roof packaged unit with 20x30 intake. Seems to be working fine with 600 inches. What do you think?
I have a 5 ton unit I know I need a thousand square inches of return grill right now I only have about four hundred square inches and I am whistling I change the blower motor for the guy and that's when it saw that whistle he said it never whistled before if I open up another return give another 600 square inches do you think I'll take care of my problem ty for all your videos
you are on the right track
David r I
It seems that is " A rule of thumb ".. We have to go back and check manufacturer's specifications. moreover registers/supply air outlets concept will be different because of many constraints Air Velocity, Sound level , etc.. Although the demonstration is very helpful .. Thanks
David, what size of duct should I put for on those return grills?
The house we bought has three returns with filters at each return. The air handler itself has a place to put a filter. Is it better to have the filters at the returns, is it better to just have a single filter at the air handler or does it not really matter? I question if all the filters add restriction, making the return more inefficient (they appear sized right for 3 ton system, 2 - 12x24 and 1 - 12x12). Thanks.
hey sir ...why we divide area of return duct by 200?
To get the "tons". The number you get will tell you how many tons the opening is good for. For example, I have 588, which divided by 200 is 2.94, so I am good for a 2.94 Ton AC unit. I have no idea what the size of my unit is because I can not find any markings on the unit in my house. I am going to assume it is probably 2.5 according to Sq foot/ton calculators on the internet.
I will, keep in touch. Thanks
I will work on it.. Thanks
thank you, in hawaii and my ac is 3 ton , gettin gonly about 324sq in" return thats squealing, my poor ac is starving of air so im going to go install another grill,,,, does it matter how long the duct is ? thank you so much for you video ,helped me alot, i got quoted 710. to run a new return and a 12" duct about 20 feet. is that accurate? thank you again ! Sarah
What if am using media filters? Calculations will be different. And if is for commercial installation also will be different. Had to consider the fresh air, so less static will be at the return grill.
What about calculations 20x30=600x2 cfm=1200/400cfm per ton=? Is this ok calculation?
Sir can u pls explain about slots calculation of linear plenum
Hello! Great video one question how would you translate this into cfm's? Example I have a 4 ton unit with only an 8x12 grill it's freezing up and flowing ice into the ducts. I know I need more air but honestly I would like to understand the difference between cfm's and free air. Thanks in advance
OK here is a back of the envelope swag:) :
(1) Old rule of thumb is 400 cfm per ton thus 4*400 =1600 cfm. ( cubic feet of air per minute)
(2) Grille area minimum = 4 tons *200 inches squared is 800 sqin . 800/144= 5.56 sqft too.
(3) For fun the air velocity is 1600 cubic feet per minute / 5.56 sqft = 288 ft per minute.
Ok 288/60 = 4.8 ft per second.
We always need to have a laugh I.am always joking around. Thanks for watching.
Great video David !! Take care, Brett
how did you come up with 200? basic rule of thumb number just curious
I live in a mobile home that has a 3.5 ton heat pump on it. My furnace sits in the hallway, and the door/grill is the only return I have because it free pulls from there. The opening of the grill only measures 17” x 23”, which is way too small for a 3.5 ton system. My calculations using this method bring me to the conclusion that I could just cover up that door somehow, and install a 24” x 30” filter grill. Would that work in this situation?
does your general rule of 200 sq inches per ton take into account the k-factor of the grille?
Hello sir! Great job! What do you mean by "per ton". Is it in kilograms or pounds, do you count a ton?
This is the rating on the AC unit.
How about without the filter? How do you calculate that?
Loved your video,lots of good information! I currently have 4 ton unit with a24×24 air filter. I plan to replace the unit and have received some quotes with 4 and 5 ton units. The 5 ton unit quote indicated the reason for a5 ton was because of living room ceiling of 15 feet, is reason enough to go with a 5 ton unit?.
no you're wayyyy off
Have them do a manual J calculation. That will take into effect total heat load. Remember - you are conditioning cubic footage not square footage. The number of windows, insulation, shade, etc all come into play. And yes - I realize your comment is six years old - this is for anyone that finds this later…
Is cutting the register necessary if I have wood studs where the register needs to go?
Nice video David👍👍
thanks
My heating system is in a mechanical room. I have a large return grate in the wall outside of room next to the heater. There is a filter on the unit but no duct connecting the grate to the unit. Is it important to have this short run from grate to unit ran through duct? Probably 2 feet from the grate to the unit inside the mechanical room.
You seem to be talking about about the velocity across the air filter? Why not mention the can depth (typically connected to round flex) and the duct size of the return air going to the unit? Just wondering.
Hi David, All the return air vent on the upper level of my home is positioned close to the ceiling, is that the correct or not?
You most likely are referring to "supply vents" not "return air vent"
well job sir.. proud of your skills..
I have Two( 20 by 20s ) for a 5 ton central unit.
Do all air returns have to be that size.. ie adding a return in a bedroom?
Another way is 2.7 cfm per sq. in, if a filter grill go 2 cfm per sq. in.
Thank you for your sharing. I want to know which book this comes from (Manual Q, or manual D...)? thanks again
Hi David, my AC is 3.5 ton unit while the furnace is 4 ton unit. The house is about 2,500 sq. ft divided into 3 zones and I have only 2 returns with the total of 744 sq. inches. Do you think having another return will help the air flow since one of the zone now has much lower flow? Thanks in advance.
It's same for supply grill also?
what would be the effects of having an undersized return? I've only got a 24×18 return so obviously that needs to be enlarged, tech never mentioned it, learning on my own at this point. I've had major sweating issues with my air handler, I've done everything to treat the attic and it stays under 90°f up there. Could the small return be contributing to the sweating issues?
Grilles and registers don’t have 100% free area. Most manufacturers state the free area of their grilles. It is usually around 80%. You can’t use the outside measurements, because of the space occupied by the louvers. You must adjust for the grilles free area. In other words 10x 20 grille is 200 square inches x 80% = 160. I know this sounds nit picky, but if the rated velocity thru the grille is exceeded, they can become noisy.
I was running with your intake measurement based upon unit of tons. i.e. for every ton one needs 200 square inches of intake grill. But then my HVAC neighbor argues that an intake grill size does not matter as long as it is the same size as the units intake. In other words you will not be able to gain any more air using a register that is larger than what the unit comes with. So for example I have a 2.5 ton unit. according to your estimate I would need 200 inches per ton or a grill that is at least 500 square inches or a grill that is a 20 x 25 or greater. But he argues the return on the unit itself is only say a 12 x 17 (before any flex or conduit) or 204 square inches before it is connected to the house? How can I justify this? How can you get more air out of a larger return grill over what your original units intake capacity is?
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Michael Ormandy mm
The fact is there are a few factors. And yes the leavers in the grill are definetly a factor the length of run bend fitting and all these favors come into play as well as the head pressure on the other side of the air handler. This is where a water column manometer comes in handy. But we have these general rules of rheum that help with in the field quality installation. But as a rule there is no limit to the size of return. You don't want a return making much if any negative pressure. So the return needs to be open and letting air pass freely.
My understanding is that the larger opening is because of the filter. Air filters reduce the amount of air that gets into the system. Having a larger filtered opening allows more air air to pass through the filter. If you have one of those cheap nylon filters that don't really filter anything in your system, you are probably fine. If you have one of those high filtration filters in your system, you are going to damage your unit. Most pros that I have heard from say to remove the filter from the AC unit and use filtered return air grills.
My house has a 5 ton unit and the return grill is 20x25 , so I need to add one more grill correct?
Does the 200 sq inch rule work for just a heating system as well? I don't have ac (tons) just heat. Not sure what the BTUs are but the unit puts out quite a bit. Thanks
Great video. Thanks
You should have your own online school to teach us.
, , when putting the air grill back onto the wall doesn't make a difference if the slots are facing down or up?
Wish I could run my return numbers by you. I have OCD bad and I keep building my return then taking it back down. I have a 5 ton unit self contained. On the return side I wanted to filter every room. (Remember I have bad OCD) I have two 20x20 return grills hooked to 12 inch round metal duct in the hall way. I have another 20x20 hooked to a 12 inch in the ceiling of my cathedral ceiling. I have six 14x14 hooked to 6inch round metal duct each. . On the CFM side I'm guessing I'm ok but I nervous I don't have enough return since I wanted to filter each room independently. So my math shows three 12 inch 1500cfm. Six 6inch 600 cfm. That's showing 2100 cfm. My question is with all those filters will 2100 by rule of thumb math be enough on that 5 ton unit. And by the way I have no flex in my system all round metal duct work. My supply from unit 20 inch my return 20 inch round.
Yes you can, it will be ok
What is general rule for supply grill ?? Or any formula
Question, does the design of the air return flex matter? Square versus round? Also, if a return is closer to the air handler does that mitigate the need for the rule of thumb 200" per ton? :-)
Dave, Im considering starting my own business. Im a journeyman sheet metal mechanic with my local. Work is slow and i mainly worked on commercial projects. Whats the biggest difference between residential and commercial work?
excellent video I would like to work for your company and learn how to design systems of air-conditioning
Does that figure in all the slates?
Thanks, Brett
I'm in Texas, been in a.c. field 18yrs (and still don't know everthing, no a.c. guy should ever tell you this) anyways I've always divided for gas units by 144 and electric by 215 not 200 ,that's per Airflow icb, is this correct?
So if i have a 2 ton a/c unit would i need 400 square inches of return a/c grille space? If i am suppose to have 400 square inches for a 2 ton a/c unit will a 25 x 16 return a/c grille work since it comes out to 400 square inches?
Thanks for nice advice.
I got a new return grille 20x20 and it's alittle smaller than my current old one and the filter is snug also I've searched for wider dimensions but none that goes to 23 on the outside, It's not in yet I need help on what to do
Use 16x16 if possible
My house is 1800 sq ft what size unit do I need and what size return
if you have a 3 ton let say you need at least 18 inch, then a 10 in another part of house
make sure your return grills are large enough for the air flow
look at my video on sizing
I have a 16x20 grill on 3 ton. Iam trying to put my own unit in and all the runns. My house is only less than 2000 square feet house one story with 2 bathrooms 3 bedroom s. All my rooms are about 14 x14 except my bathrooms and laundry room their 7x8. Whats the best size duck's do i need 6 in or bigger
How about sizing supply registers?
How much can you oversize the supply and return ?
hey. i have a question .recently i had installed 2.5 tonn rheem unit for heat and AC
and there is only 1 return duct which is 12 by 14 and followed by 16 by24 main return built in unit filter. can it provide enough air volume for this unit? so it can work properly
What duct work would the 600 sq in grille plunem?
.What is the proper way to hang a return.........louvers up or down
Thanks David
So you're only putting 2 return grills in for a whole 3 ton system? Or are you calculating each room (outside of bathrooms and Kitchen) to know what size return grill you need?
It all depends. The size of the AC unit, the amount of room to install the grill. You could install a single grill if you have the space for it. The size of the house really has nothing to do with it as long as the air has a path back to the AC unit.
Can you have too much return air on a system?
what about size of the flex in the grill
How do you measure cold air return for filter