Bob77064
Elite Member
The house that I used to live in, we replaced the flex duct with rigid. The rigid ducts flow alot more air, and there is no problem with it collapsing.
HVAC Question: Rigid metal vs. flex duct
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Gary Fowler
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I think the HVAC guys that recommend flex are just lazy and prefer the flex or they have not idea on how to do ridged. Thay said, I have the flex in my new house and it works OK I suppose but I think they lines need to be larger when using flex. Putting in a return air duct is a good idea also and not many contractor put one in each room which causes some problems in rooms that are closed door all the time unless you leave a big gap under the door.
Do some commercial HVAC design at work.... Won't use flex for anything except the last three or for feet, usually just to connect from a nearby rigid trunk line to the register. The friction value per foot of flex is much higher than that of sheet metal duct, so it reduces flow unless you upsize it a lot.
Bird
Rest in Peace
I would have never thought about a builder NOT putting return air ducts in places that would prevent that problem, but I have a nephew whose mother lives with them, they had a new house built, and she complained about the temperature in her room for a long time before my brother saw that the problem was simply no return air duct in her room. So apparently it does happen.
Qapla
Veteran Member
Keep in mind that any return air, be it a duct or vented under/through the door, needs to be filtered before it returns to the evaporator coil. This may be in the form of small filters at the return or it can be in a central location where the various returns converge.
Leaving an air-gap under the door or placing a louvered grate in a door or having a transom window above the door will work for return air if running a separate duct is not practical.
Also, in the OP he stated that all lines would run through on area making ceiling vents in the down areas and floor vents in the up area. Personally, I would re-think this idea. Not only do floor vents tend to collect debris (dirt, pens, toys, etc) cold air goes down. So A/C is not as efficient when coming out of floor vents as it is when it comes from above.
However, if you need heat more of the year than cool, floor vents may be more efficient, though that still collect undesirable objects and limit furniture placement.
Leaving an air-gap under the door or placing a louvered grate in a door or having a transom window above the door will work for return air if running a separate duct is not practical.
Also, in the OP he stated that all lines would run through on area making ceiling vents in the down areas and floor vents in the up area. Personally, I would re-think this idea. Not only do floor vents tend to collect debris (dirt, pens, toys, etc) cold air goes down. So A/C is not as efficient when coming out of floor vents as it is when it comes from above.
However, if you need heat more of the year than cool, floor vents may be more efficient, though that still collect undesirable objects and limit furniture placement.
Bird
Rest in Peace
Yep, I was quite surprised when an old friend of mine showed me 4 or 5 different sized filters for the different sized returns in his house. Thank goodness my house has a single, huge Aprilaire 2200 filter. It's not cheap, but only needs changing once a year.
wedge40
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My crawl space is 8" so when the house was built they used flex duct. That is the worst stuff in world. They have codes against the dumbest stuff and if it were up to me this junk would be outlawed. I've replaced the stuff twice in the last 20 years because the mice eat through it and use the insulation to build nests. If you have the space and money use all metal.
Just my :2cents:
Wedge
Just my :2cents:
Wedge
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Nothing wrong with floor vents if the system is designed properly. You get good throw out of them and the cool air shoots up to the ceiling and stirs up all the air in the room anyway.
Of course a properly designed system isn't that common in residential construction. You aren't going to get that good throw with a minimum required cfm fan and thirty feet of under sized flex duct.
Of course a properly designed system isn't that common in residential construction. You aren't going to get that good throw with a minimum required cfm fan and thirty feet of under sized flex duct.
greasemonkeyok
Veteran Member
"Flex" = moves when system turns on and turns off = eventually wears out. Cheap to install and a trained chimpanzee can do it. That's why some HVAC guys push it.
Ed of all trades
Elite Member
The placement of the vents makes a big difference too. In two of our bedrooms the vent is by the door and the heat or ac just flows out the door and does little good in the room. Vents should always be far away from doors, if that is the return and near a window, more heat transfer than walls. and again on the other side of the room from the return.
