Cold air returns (or lack thereof...)

Umby

I have a 3 bedroom ranch on a crawl (with attic) that is approx. 1200 sq. ft. NONE of the six total rooms have c/a returns. However there is a large one low on the wall of the hallway that leads to all of these rooms and the furnace is on the the other side the wall, approximately in the center of the house. Regarding the supply side of the system: there is no main trunk, just six 4" flex ducts (R6)running straight from over the furnace to the six supply regs, and they lay on the floor of the attic. All supply registers are 3" off the floor on the interior wall of every room. I just had a new 75K BTU and 2 ton furnace and central air installed using this existing duct system. The problem is the longest run, about 10', is to the farthest bedroom, and it is this room that just doesn't seem to cool off. I have NO dampers in any of these six supplies to play with other than at the registers themselves. Is this a cold air return problem, is it a supply duct problem, or is it both? BTW, in the newer part of the house, even farther from the furnace, the temps are perfect. But these rooms are supplied by 6" flex through their ceilings and c/a returns at the floor...

housedocs

I'm not an expert in HVAAAC, but it seems to me that the supply ducts are a bit small. What did your installer have to say about the duct system?

__________________
All you need is ignorance and confidence and success is assured. - Mark Twain

Umby

He hinted that they seemed small. But because the I was personally replacing the metal duct with flex, all of it 4"; I don't think he wanted me starting another register replacement project to change those from 4" to 5" or 6".
What about lack of cold air returns?

murray

so you were replacing 4" pipe with 4" flex.....and there is a/c????the returns in the hall is typical .this system will probably freeze up .a 2 ton a/c coil is good for 800-1000 cfm. 4" flex is equivilant to 3" pipe. you would need 20 -4" runs to roughly size that system.there must be at least 4-6"runs ??? 3" doesnt even show on my ductulator increase them to 6" flex with dampers at the plenum ,then you can balance .does the room in question get full sun ?if so it needs more condition.if the runs down the walls r 4' ,install 6" wisper grills in the ceilings

Umby

I already cut out a larger hole in the plenum for the new 6" take-off, but the hole ended up being 7.25" which took up more room than I expected. I don't believe I can cut any more big holes like that one because I have little or no room now. I then put in a new supply register in the ceiling of the room in question (which gets little or no sun). Now when the air is on, I have to lean into the door to get it to open! I need to cut a c/a return on the bottom of the door. I would like to put at least one if not two more 6" supplies to other rooms, but now my plenum is getting full...

murray

you only need about a 1"-1 1/4" under cut on the door to get a good return.30 square inches is good for 100-150cfm of return and this is plenty of return for any typical residential use of a room .if the square foot of the room is the size of a family room or large master bedrm then add a return And even if you cut a 6" hole for the pipe ,it is better than a 4"flex..If you have to lean into the door to get it to open from a 6" run ,this makes no sense ..are you growing something in this room???if so, you dont need to change the air that many times,use an ionizing air filter,and change the air ten times per hr...ha ha ha ..there is allways a way to get more air.you can remove the runs in the plenum and add a small main trunk up top,then add your runs to the main ,this will carry more preasure and less restriction ,and if you do that ,add a return trunk line ,and add a return in the ceiling ,this will make the a/c more effiecient

meiersenterprises

I had a very similar problem a few years back. The master bdrm was always hot. I put a cut-through return vent OVER the bdrm door and that solved the problem. It was only one of those small vents from HDepot - I want to say 8" but im not sure. If you don't have some kind of a return in each room, you won't have consistent air flow and temp. Shaving the door at the bottom isn't a bad idea but that kindof looks ugly. The vent I installed was quite neutral and un-noticeable. Good luck!

Umby

If I have a ceiling supply register and an over-the-door return register, would these work together efficiently? I have heard of the main trunk idea before, I should look into it, thanks.