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10-03-2009, 03:39 PM   #1
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## Cold air return questions

An addition was put on my house before I bought it, with a crawl space underneath. They extended the plenum and branched off three ducts for heat. The ducts run through the crawlspace, and the addition is always cold in the winter (Michigan). There are no cold air returns in the addition, but there is one small cold air return vent in the hallway connecting to the addition. I am wondering if I change one of the three heat vents to a cold air return (on an outside wall) and change the cold air return to a heat vent in the hallway (more centrally located) if that would help heat the room better? What are your suggestions?

Also, my basement is finished and has four heat vents, but there are no cold air returns in the basement. Would it be best to add a cold air return to circulate the air? How can you determine if your supplies and returns are balanced?

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10-03-2009, 04:14 PM   #2
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Scuba Dave might have a more better answer to this one, but I think if you talk to any HVAC trades They will most often tell you that whatever number of square inches of free area you have coming out of your furnace you must put back into your furnace to make it balanced. In other words ten 2 x 12 floor supplies (Redneck Math 101) equals 2 x12= 24 x 10= 240 sg. in.
of free area of warm discharge needs a 16 x 16 return (256 sq. in.) duct to your furnace to balance the load. The best bet is if you have the accessability to the return plenum is just to put a return register on the opposite side of the room for every supply register you have in your home or size one return for several supplies in a common area like a family room or a kitchen/ dining room.

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 10-03-2009, 04:35 PM #3 Old Newbie   Join Date: Aug 2009 Location: Ottawa Valley, Eastern Ontario, Canada Posts: 70 Rewards Points: 75 Is the crawlspace insulated and heated?

 10-03-2009, 07:13 PM #4 Member   Join Date: Aug 2008 Location: Northern Michigan Posts: 36 Rewards Points: 25 The crawlspace has styrofoam insulation along the walls. One of the ducts stops about two feet into the crawlspace and is open, which I guess "heats" the crawlspace. I have three larger cold air returns in the hallway centrally located in the house, up high on the walls, and then the one small vent in the other hallway next to the addition. There are none in the basement. I have a total of 11 heat vents upstairs and 5 downstairs.
 10-03-2009, 07:52 PM #5 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,747 Rewards Points: 6,458 Probably would do more harm then good. Since you would be putting less heat into the room.
10-03-2009, 08:44 PM   #6
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Quote:
 Originally Posted by beenthere Probably would do more harm then good. Since you would be putting less heat into the room.
A room will never be comfortable if the floor is cold.

 10-03-2009, 09:00 PM #7 Member   Join Date: Aug 2008 Location: Northern Michigan Posts: 36 Rewards Points: 25 Well one of the vents doesn't blow very warm or forceful, it is a long ways from the furnace and runs through the cool crawlspace. So I'm thinking if I change that one to a cold air return, and then change the cold air return in the hallway right next to the addition (which would not have much length from the furnace and not run through the crawlspace) that it will supply about the same amount of heat to the room, plus add a cold air return.
10-03-2009, 09:04 PM   #8
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Quote:
 Originally Posted by tinmanrob A room will never be comfortable if the floor is cold.
Removing a supply to add a return won't make a room warmer.

The floor is cold due to stratification. Caused by lack of air mixing. He has poor velocity, so the cold air on the floor is not mixed/entrained with the hot/warm supply air.

If the room needs 9,000BTUs of heat. And his 3 supplies are only suppling 7,000BTUs. Removing one and adding a return could lower his BTUs to that room to only 6,500 BTUs. Making the room colder.

10-03-2009, 09:06 PM   #9
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Quote:
 Originally Posted by ekulrenlig Well one of the vents doesn't blow very warm or forceful, it is a long ways from the furnace and runs through the cool crawlspace. So I'm thinking if I change that one to a cold air return, and then change the cold air return in the hallway right next to the addition (which would not have much length from the furnace and not run through the crawlspace) that it will supply about the same amount of heat to the room, plus add a cold air return.
Increase the size of that supply. Add insulation to those supplies.
Add a return to that room.

Don't delete any of the supplies in that room..

10-03-2009, 09:37 PM   #10
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Quote:
 Originally Posted by beenthere Removing a supply to add a return won't make a room warmer. The floor is cold due to stratification. Caused by lack of air mixing. He has poor velocity, so the cold air on the floor is not mixed/entrained with the hot/warm supply air. If the room needs 9,000BTUs of heat. And his 3 supplies are only suppling 7,000BTUs. Removing one and adding a return could lower his BTUs to that room to only 6,500 BTUs. Making the room colder.
Agreed.
Removing a supply is not an option.
However, insulating and heating the crawlspace will affect the comfort level of the room above.
In my neck of the woods, the building code requires this.
Even for bedrooms above a garage.

10-04-2009, 04:37 AM   #11
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Quote:
 Originally Posted by tinmanrob Agreed. Removing a supply is not an option. However, insulating and heating the crawlspace will affect the comfort level of the room above. In my neck of the woods, the building code requires this. Even for bedrooms above a garage.
Sorry. Thought you were trying to say to use a return to get the cold air off the floor.

 10-04-2009, 10:29 AM #12 Member   Join Date: Aug 2008 Location: Northern Michigan Posts: 36 Rewards Points: 25 Ok, I won't remove any of the supplies. If I leave the supplies and add a return to the addition, where is the best place to locate the return? Would it be best to use the current supply vent that doesn't blow very hot and is a long ways from the furnace on an outside wall. Or would it be best to place in near the center of the room, which is right next to the hallway with the small return already?
 10-04-2009, 10:33 AM #13 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 33,747 Rewards Points: 6,458 As long as its near the room. So that the air has an easy unobstructed path to it. position doesn't really matter. Returns on outsie walls are usually not a great idea though. I would work on trying to reposition where that supply is tapped into the duct system also. So it does get better air flow.
10-04-2009, 06:06 PM   #14
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Quote:
 Originally Posted by beenthere Sorry. Thought you were trying to say to use a return to get the cold air off the floor.
No need to apologize.
I guess my posts were not clear. (I'm learning)
I've been preaching for years that adding a return air in an area will not necessarily induce more supply.
I get those kind of calls constantly.
"I have a room that's cold in the winter, and I need you to install a cold air return"

10-04-2009, 07:15 PM   #15
An old Tradesmen

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Velocity and its air mixing benefits is not understood by many.

And sometimes, I get asked if installing a second return in a cold room will help. :D

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