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ekulrenlig 10-03-2009 02:39 PM

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?

Grampa Bud 10-03-2009 03:14 PM

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.

tinmanrob 10-03-2009 03:35 PM

Is the crawlspace insulated and heated?

ekulrenlig 10-03-2009 06:13 PM

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.

beenthere 10-03-2009 06:52 PM

Probably would do more harm then good.
Since you would be putting less heat into the room.

tinmanrob 10-03-2009 07:44 PM

Quote:

Originally Posted by beenthere (Post 335967)
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.

ekulrenlig 10-03-2009 08:00 PM

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.

beenthere 10-03-2009 08:04 PM

Quote:

Originally Posted by tinmanrob (Post 335995)
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.

beenthere 10-03-2009 08:06 PM

Quote:

Originally Posted by ekulrenlig (Post 336006)
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..

tinmanrob 10-03-2009 08:37 PM

Quote:

Originally Posted by beenthere (Post 336009)
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.

beenthere 10-04-2009 03:37 AM

Quote:

Originally Posted by tinmanrob (Post 336025)
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.

ekulrenlig 10-04-2009 09:29 AM

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?

beenthere 10-04-2009 09:33 AM

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.

tinmanrob 10-04-2009 05:06 PM

Quote:

Originally Posted by beenthere (Post 336105)
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"
:huh:

beenthere 10-04-2009 06:15 PM

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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