Need tips on more return air/ducts

[kenny71]

I am preparing to replace my AC probably sometime next week or so and I have gotten some good advice here. I’m going with a 3-ton dual fuel, but this thread is about ducts.

My return duct is in the kitchen (yes, kitchen, I know that is a bad place but everyone says it was the only place). In the wall opposite the return is the gas fireplace. So when you look in the return, you see the back of the fireplace. Standard 20x25 grill, but inside the opening in the wall cavity, the back of the fireplace significantly reduces the size of the opening. The tech put a velocity meter on a vent, it measured 450 (not sure of the units, cfm?), then we removed the cover from the furnace blower and that vent shot up to 775. This furnace is trying to move the air, there’s just no air to move. So I need more return air but enlarging the current one is not an option.

I’ve been told that a register or two in the ceiling, run the duct down through a closet is a possibility.

Has anyone ever heard of using a wall cavity (space between wall studs, drywall front and back) as a return? Just install a register towards the top of the wall, cut out the floor board, attach sheet metal and you’ve got a vent, right?

Any ideas on how/where to run additional return ducts? House is rancher over a full basement, split unit, furnace is in the basement.

[lolson]

return air inlets shall not be located within 10 feet of any appliance fire box, solid fuel burnning appliance or draft diverter in the same enclosed room equal to the return air taken from the space.

cold air return opening shall not be located in a closet, bathroom, toilet room, kitchen, garage, mechanical room, boiler room or furnace room.

codes i go by

low air flow is not an option:eek:

[pstu]

A return in each bedroom is an excellent idea. I gained a lot of capacity that way in my own house.

>>Has anyone ever heard of using a wall cavity (space between wall studs, drywall front and back) as a return?
>>Just install a register towards the top of the wall, cut out the floor board, attach sheet metal and you’ve got a vent, right?

This is actually fairly common but is widely criticized as a bad idea. If you really want to run thru the wall then you can put a rectangular duct in the wall and be airtight. The biggest problem is the wall tends to leak at the top and bottom, therefore you can suck attic or basement air right into the return, generally a poor thing to do. Doing it right will cost more money and you will need multiple such returns due to the duct being 3-inch by 15-inch or something like that. 45 sqin will not carry a lot of air.

A 3-ton AC will typically need 400 cfm/ton or 1200 cfm total airflow. If your return is flex duct one recommendation is to stay below 600 feet/min air speed (hard pipe has higher limits). That means either two 14-inch return ducts for 308 sqin or one 20-inch for 314 sqin. One 12-inch return will add 113 sqin, a 10-inch 79 sqin, an 8-inch 50 sqin.

If your tech measured 450 feet/min air *speed* that is not a bad thing but if he measured 450 cfm that is so crazy bad that I would have it checked again. I am a homeowner not a pro so would like to hear what some pros think of this reading that you report.

I believe in many regions a kitchen return is against code and the theory is a kitchen fire might spread more rapidly to the rest of the house. Also that certain kinds of cooking might lead to grease droplets clogging the filter, or worse getting past the filter into the duct system. I say this just so you can think about how to minimize the downside in your own house.

Some of my own experiences with return air paths, FWIW. I have put a return in a laundry room which I now know is supposed to be bad, but cannot imagine how it is bad in my house. I once asked for a bathroom return (near sink but far from farts) but my tech talked me out of it, then I put in a transfer duct which ran from a shower area thru a closet to the open part of the house. That would seem to follow the letter of the law but violate the principle, yet is seems to have no ill effects and lots of benefit. I am still scratching my head wondering why closet returns are such a bad thing -- unless there is the potential to pull a lot of dust laden air across ones stuff in the closet. There are some times when it pays to analyze just why the code says what it does. However I understand your kitchen return exists just because there was no choice.

Your simple answer may just be to get a couple flex duct returns mounted in the ceiling, starting with the master bedroom.

Hope this helps -- Pstu

[zzonko]

A return in each bedroom is an excellent idea. I gained a lot of capacity that way in my own house.

>>Has anyone ever heard of using a wall cavity (space between wall studs, drywall front and back) as a return?
>>Just install a register towards the top of the wall, cut out the floor board, attach sheet metal and you’ve got a vent, right?

This is actually fairly common but is widely criticized as a bad idea. If you really want to run thru the wall then you can put a rectangular duct in the wall and be airtight. The biggest problem is the wall tends to leak at the top and bottom, therefore you can suck attic or basement air right into the return, generally a poor thing to do. Doing it right will cost more money and you will need multiple such returns due to the duct being 3-inch by 15-inch or something like that. 45 sqin will not carry a lot of air.

A 3-ton AC will typically need 400 cfm/ton or 1200 cfm total airflow. If your return is flex duct one recommendation is to stay below 600 feet/min air speed (hard pipe has higher limits). That means either two 14-inch return ducts for 308 sqin or one 20-inch for 314 sqin. One 12-inch return will add 113 sqin, a 10-inch 79 sqin, an 8-inch 50 sqin.

If your tech measured 450 feet/min air *speed* that is not a bad thing but if he measured 450 cfm that is so crazy bad that I would have it checked again. I am a homeowner not a pro so would like to hear what some pros think of this reading that you report.

I believe in many regions a kitchen return is against code and the theory is a kitchen fire might spread more rapidly to the rest of the house. Also that certain kinds of cooking might lead to grease droplets clogging the filter, or worse getting past the filter into the duct system. I say this just so you can think about how to minimize the downside in your own house.

Some of my own experiences with return air paths, FWIW. I have put a return in a laundry room which I now know is supposed to be bad, but cannot imagine how it is bad in my house. I once asked for a bathroom return (near sink but far from farts) but my tech talked me out of it, then I put in a transfer duct which ran from a shower area thru a closet to the open part of the house. That would seem to follow the letter of the law but violate the principle, yet is seems to have no ill effects and lots of benefit. I am still scratching my head wondering why closet returns are such a bad thing -- unless there is the potential to pull a lot of dust laden air across ones stuff in the closet. There are some times when it pays to analyze just why the code says what it does. However I understand your kitchen return exists just because there was no choice.

Your simple answer may just be to get a couple flex duct returns mounted in the ceiling, starting with the master bedroom.

Hope this helps -- Pstu


WoW

I guess you have now graduated?

[kenny71]

A return in each bedroom is an excellent idea. I gained a lot of capacity that way in my own house.

How did you run them? Wall cavity or ceiling?




A 3-ton AC will typically need 400 cfm/ton or 1200 cfm total airflow. If your return is flex duct one recommendation is to stay below 600 feet/min air speed (hard pipe has higher limits). That means either two 14-inch return ducts for 308 sqin or one 20-inch for 314 sqin. One 12-inch return will add 113 sqin, a 10-inch 79 sqin, an 8-inch 50 sqin.

You kind of lost me here. You mentioned cfm and feet/min. What is the standard unit of measure and what am I trying to achieve? The bottom line is that a 3-ton needs 1200cfm of return air, correct? Number and size of ducts do not matter as long as I get 1200 cfm. Is there such a thing as too many return ducts? The air mover is only going to move a certain amount anyway, right? When you say 8-inch return or 12-inch return, do you mean square as in 8x8 or 12x12?

Edit: It just dawned on me that you are referring to round duct. I'm an idiot sometimes :rolleyes:


If your tech measured 450 feet/min air *speed* that is not a bad thing but if he measured 450 cfm that is so crazy bad that I would have it checked again. I am a homeowner not a pro so would like to hear what some pros think of this reading that you report.

I'm sure I just messed up the units. feet/min, cfm, I'm not sure.

Thanks for your help.

[Shophound]

I’ve been told that a register or two in the ceiling, run the duct down through a closet is a possibility.

Has anyone ever heard of using a wall cavity (space between wall studs, drywall front and back) as a return? Just install a register towards the top of the wall, cut out the floor board, attach sheet metal and you’ve got a vent, right?

Any ideas on how/where to run additional return ducts? House is rancher over a full basement, split unit, furnace is in the basement.

Wall cavities are very difficult to seal well for return air purposes. I would not use them.

With your furnace in the basement, are the supply registers mounted in the floor, sidewall, or ceiling?

Is the kitchen return low near the floor or high in or near the ceiling?

Cfm is determined by Manual J sensible load and calculated cooling factor. This is used to determine room by room cfm requirements. Ducts should be sized to deliver sufficient air to remove the calculated heat gain per room. 400 cubic feet per minute (CFM) per ton (12,000 BTUH) is a baseline standard for moving air across a cooling coil, but is not set in stone. Depending on what your structure sensible heat ratio is, cfm per ton can vary considerably either side of 400. If you live in a humid region, blower speeds can be set lower for higher humidity removal ability. More arid regions can flow higher amount of air over the cooling coil and realize greater sensible capacities.

If you have ceiling supplies, and they all are located near the inside walls of each room, insist on one-way curved blade registers that throw the air along the ceiling toward an outside wall. This, combined with an adequate return path, will assure good mixing of room air with conditioned air...hard to beat the comfort good air circulation patterns bring. High sidewall registers can be aimed toward the ceiling to achieve a similar effect (not more than 15 degree deflection upward). Floor registers should throw air straight up the wall, not into the room.

Lots of technical mumbo jumbo above, it likely seems to you, but it's a basic sketch of how proper equipment sizing and good air distribution is done.

[kenny71]

Wall cavities are very difficult to seal well for return air purposes. I would not use them.

With your furnace in the basement, are the supply registers mounted in the floor, sidewall, or ceiling?

Is the kitchen return low near the floor or high in or near the ceiling?


Supply registers in the floor on exterior walls usually below a window.

Kitchen return is low, 8" or so above floor.

Seems like I could install a few ceiling registers, come together in the attic, down through a closet, tap into main kitchen return, connect to furnace.

Do return ducts have to "neck down" like supply ducts to keep the velocity at a required level? (I'm sure neck down is not the proper terminology, but you know what I mean)

[pstu]

Shophound is giving you fine professional advice. I live in Texas where flex duct in attic is nearly universal, my returns added are all flex duct to a ceiling return. For practical purposes, you cannot have too much return, I have overkill for return capacity on one of my two systems. Don't think there is much benefit to keeping velocity up on returns. I hope the pros agree with me that your plan for ceiling returns is a good plan.

Measuring "ESP" (External Static Pressure) I understand to be one useful way to check up on how adequate your return ducts are. Supplies too. If the ESP number is going to be high then a VS (variable speed) air handler will do the job better. My VS air handler has normal ratings up to 0.9 inch w.c. (water column), far above the 0.5 max for my earlier air handler.

Best of luck -- Pstu

P.S. I am really glad to hear you had a load calc done and are replacing all the ducts to be suited for your larger size AC. Sounds like you are going about this the right way, better than 95% of the work I hear about.

[kenny71]

P.S. I am really glad to hear you had a load calc done and are replacing all the ducts to be suited for your larger size AC. Sounds like you are going about this the right way, better than 95% of the work I hear about.

I have a friend who says he can get me a unit at cost and I have another friend who says he can do this or that, blah, blah, blah. I am chosing to go the professional, warranty, done right the first time route here. I want it done, I want it done right and I don't want to have to do this again for 10-15 years.

Thanks for all your advice.

[HPLearner]

Isn't there some kind of flexible ducting available that could line the cavity for an airtight seal? In an inside wall, you wouldn't worry about condensation, just air leakage.

Seems like an ideal way to add a return to an area (br) where the only return is a doorway, although limited in size as you pointed out.

And no return is allowed in a bathroom because of f**ts and moisture? Doesn't it help to dry the room out quickly?


A return in each bedroom is an excellent idea. I gained a lot of capacity that way in my own house.

>>Has anyone ever heard of using a wall cavity (space between wall studs, drywall front and back) as a return?
>>Just install a register towards the top of the wall, cut out the floor board, attach sheet metal and you’ve got a vent, right?

This is actually fairly common but is widely criticized as a bad idea. If you really want to run thru the wall then you can put a rectangular duct in the wall and be airtight. The biggest problem is the wall tends to leak at the top and bottom, therefore you can suck attic or basement air right into the return, generally a poor thing to do. Doing it right will cost more money and you will need multiple such returns due to the duct being 3-inch by 15-inch or something like that. 45 sqin will not carry a lot of air.

A 3-ton AC will typically need 400 cfm/ton or 1200 cfm total airflow. If your return is flex duct one recommendation is to stay below 600 feet/min air speed (hard pipe has higher limits). That means either two 14-inch return ducts for 308 sqin or one 20-inch for 314 sqin. One 12-inch return will add 113 sqin, a 10-inch 79 sqin, an 8-inch 50 sqin.

If your tech measured 450 feet/min air *speed* that is not a bad thing but if he measured 450 cfm that is so crazy bad that I would have it checked again. I am a homeowner not a pro so would like to hear what some pros think of this reading that you report.

I believe in many regions a kitchen return is against code and the theory is a kitchen fire might spread more rapidly to the rest of the house. Also that certain kinds of cooking might lead to grease droplets clogging the filter, or worse getting past the filter into the duct system. I say this just so you can think about how to minimize the downside in your own house.

Some of my own experiences with return air paths, FWIW. I have put a return in a laundry room which I now know is supposed to be bad, but cannot imagine how it is bad in my house. I once asked for a bathroom return (near sink but far from farts) but my tech talked me out of it, then I put in a transfer duct which ran from a shower area thru a closet to the open part of the house. That would seem to follow the letter of the law but violate the principle, yet is seems to have no ill effects and lots of benefit. I am still scratching my head wondering why closet returns are such a bad thing -- unless there is the potential to pull a lot of dust laden air across ones stuff in the closet. There are some times when it pays to analyze just why the code says what it does. However I understand your kitchen return exists just because there was no choice.

Your simple answer may just be to get a couple flex duct returns mounted in the ceiling, starting with the master bedroom.

Hope this helps -- Pstu

[pstu]

Isn't there some kind of flexible ducting available that could line the cavity for an airtight seal? In an inside wall, you wouldn't worry about condensation, just air leakage.

Seems like an ideal way to add a return to an area (br) where the only return is a doorway, although limited in size as you pointed out.

And no return is allowed in a bathroom because of f**ts and moisture? Doesn't it help to dry the room out quickly?
I have seen such ducting in catalogs at least. Using that should eliminate the leakage problem, leaving only the issue of capacity. You can only move air so fast (I think 700-1000 feet/min), so your 45 square inches can only flow so much CFM. But I agree that seems like a good professional solution to a bedroom return.

For bathrooms, it sure does help to dry it out. The conventional wisdom is exhaust fans will expel both moisture and fart smells, I have confidence they will remove all the smells and part of the moisture. My idiosyncratic notion is a dehumidifier deserves to be right there where the moisture is highest, attacking a problem at its source. But that is just me speaking and not ACCA or anything professional.

Hope this helps -- Pstu

[menaphnx]

you can you a wall as a return duct be sure to insulate the wall and seal it up good