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johna1947
09-12-2005, 03:22 PM
My contractor tells me that the there is insufficient return air for my HVAC system. He calculated the total supply grill area and compared that to the return grill area. The house has 7 supply registers that equal 369 sqin and only 2 returns equaling 250 sqin. Is this the correct way to do this? Is there a more scientific way to determine of the system is getting insufficient return air supply? He wants to put in another return duct and 2 more registers but I'm not sure if he is shooting from the hip. The contractor comes highly recommended but this is a small job for him and he often doesn't retun my calls for days or blows me off if I ask a question. Any advice about how to check if he is doing this the right way would be appreciated.

John

suemarkp
09-12-2005, 03:35 PM
Although there will be a correlation between supply area and return area, I'd say that is not the way to figure it out. You need to know how much air your system needs to move in CFM. Once you know that, you have a chance at determining the required total duct area for the supply and return. Return ducts are usually slightly larger than supply ducts. Return grill area should be larger than return duct area, but how much larger depends on whether you have filters at the return grill or directly at the air handler.

Do you know how large your AC or air handler are going to be (in tons) or the proposed airflow CFM rate? The scientific way of calculating duct sizes is with the ACCA Manual-D.

BaldLoonie
09-12-2005, 03:44 PM
Just looking at grilles tells little. The entire duct system has to be looked at. Running 7 4" vents with huge grilles moves so much less air than 7 7" vents with same grilles.

What size are your supply & return runs? Most homes have 6" or 7" supplies. That would normally be 1.5 to 2 tons of air and 250 sq inches of return grille would do more than that if the return system is properly sized.

johna1947
09-12-2005, 04:40 PM
The whole system is a mess and needs to be up dated. The A/C unit is 2 ton and the return duct was made by nailing a piece of sheetmetal to the underside of the the basement floor joists. The duct space measure 14" by 10". The house is 85 years old so running new duct work upstais is a problem. The house is only on one zone althouth there are three bedrooms and a bath upstairs and no return. There are two retruns in teh living room and dining room which are on the same duct. The contractor wants to fabricate a new sheet metal duct to fit between the joists and he wants to add 2 registers and a new run of ductwork to the kitchen and living room. The suplys are a mass of flexible 6' ducts connected to old 8" metal dusts with reducers. The house is about 1200 sq ft and insulation is poor owing to the age of the house.

Irishmist
09-12-2005, 04:56 PM
With the number of concerns you have indicated, you really need a professional contractor to look at the entire system and make recommendations. If the existing contractor 'blows you off' when you have a question, then it makes it that much easier to 'kick him to the curb' and call another. Not sure who to call? Check with neighbors, friends, and family or business acquaintenaces to see who they use and have received good service from.

You definitely have some issues, not the least of which is the lack of return to the second floor. How best to resolve this issue can best be determined by a contractor via an on-site inspection. He will determine what, if anything, has to be done to modify your current system, and what is your best option to address the second floor.

Feel free to bring those thoughts and ideas back to this site for discussion. If something is off the wall, we can point that out, but keep in mind, we are trying to visualize the job, when someone else has the opportunity to see it first hand. Let us know where you are, and there may very well be a contractor from this site that can assist you. Most of us welcome the opportunity to work first hand, with those individuals that come onto the site with questions

johna1947
09-12-2005, 05:54 PM
Thank you. Sounds like good advice.

dx
09-13-2005, 01:35 AM
While you certainly have a number of issues, be advised that making a return duct by attaching sheetmetal to the bottom of the joists, when done properly, is not an issue. It is called joist spanning and every house in our area is done that way. Unless your local code prohibits it, there is nothing wrong with it.

cem-bsee
09-13-2005, 05:29 AM
so, get out the measuring tape, measure each room size and opening to the exterior. measure the insulation & note its type in the attic & walls.
then run the load calcualtion from this site --

then YOU will know where the heat losses are -- then one can calculate how much air is needed for each room -- then one can compute the duct size needed --

or get a contractor to do this --

frankly, the contractor seems to know how to get the ducts installed, but guessing as to the duct sizes needed --

start with the basics -0- what are your issues? too hot in bdrm? too cold? or ?

deejoe
09-13-2005, 08:47 AM
At last, the use of the joist space for Return air is appreciated, as DX also suggest is fine.
In our area it's referred to as "joist lining" and is very commonly used to carry return air along a joist path from the intake gille and into the main trunk, into the unit,etc. Its quite common and saves a lot of unnecessary r/a ductwork. Think of it as being the same as a vertical return which also uses the wall studs and the drywall(no metal duct) to afford a path downward to the horizontal "joist lined" return path.

core
09-13-2005, 08:52 AM
Originally posted by cem-bsee
so, get out the measuring tape, measure each room size and opening to the exterior. measure the insulation & note its type in the attic & walls.
then run the load calcualtion from this site --

then YOU will know where the heat losses are -- then one can calculate how much air is needed for each room -- then one can compute the duct size needed --

cooling CFM = Room Sensible Cooling Load(BTUH) / 1.1 x (Room Db - Supply Air Db)

Room Db = design temp
Supply Db = Supply Air (standard used value is 55Deg. F)

Core

ruud-man
09-13-2005, 08:59 AM
From what I gather, your house may have limited access for duct. If this is the case you may want to consider a small duct high velocity system.

These systems use 2" duct and 2" round outlets as supplies. The supplies can be integrated into most interior designs and be all but invisible.

The equipment is a little more expensive but the performance, especially humidity control, is excellent. When installed properly there is no noise from the supplies.

The high velocity (1500 fpm) at the supply induces room air and keeps the room air moving so there is little stratification.

As you may expect, these systems require specific training of the contractor. To find a contractor in your area you should call UNICO- 800-527-0896. They manufacture these systems and have a great deal of experience with older home renovations.

Best regards...