Return Air Duct Placement
I just started construction on a new home and am working with my HVAC contractor to spec out my equipment and design the system. I am trying to build a very tight home with icynene walls & roof deck keeping HVAC equipment in conditioned attic built on slab. it is a 2 story home with mechanical located in the attic (3rd story).
my question is about where to place cold air returns? my HVAC contractor is insisting that they need to be placed near the floor and an energy rater/consultant is telling me that it doesn't make that much difference in a tight house. It may be difficult and add expense to put them near the floor of the first floor.
also this house has all 9' ceilings, no vaults, etc. it is located in mixed-humid environment (memphis, tn).
can anyone help me to understand this a little better?
Often debated on this site,not everyone agrees.
So you'll get different opinions.
Mine is, if the supply side is designed correctly,it makes no difference. This is what Manual D ,from www.acca.org states,and Man. D is the ANSI Industry standard for duct design.
Additionally,18 years ago, when our home was built I thought differently,so I have tried it,no difference .
In the area that I worked,it was pretty standard practice to have both high and low returns with a single damper grille down low so as to engage or disengage that grille according to season.
Now if you think about it nthe area of the house that you are most concerned with is from the floor to the stat level of about five foot.
In the summer you wantb to reploace warm air with cool air.Where is thatb warm air,its up high.
In the winter you want to replace cold air with warm air.Where is that cold air,its near the floor.
So the best way is still to have both high and low in the same stud space controled with single damper grilles which need to be adjusted seasonally.
With the equipment in the attic,the dampered grille has to be the high grille.Also the grilles need to be installed upside down from what you would normally installm it.
Would you not agree that if you have two return ducts and a damper (or 2 per room, etc), that you must size each of them to fully satisfy max airflow? I read from this board there are so many houses built which do not have enough capacity even without dual returns.
Originally Posted by REP
Regards -- Pstu
First, you can never have too much return air. More importantly, you should have a return in each and every bedroom. Having said that and assuming the bedrooms are on the 2nd floor, I'll assume the bedroom returns will be on the ceiling, as well as the supply outlets. So that solves that issue on the 2nd floor.
As for the 1st floor, warm air rises and I'll hope that two things are designed into the sytem with a furnace on the 3rd floor. 1. You're installing a modulating or 2-stage furnace that is zoned 1st and 2nd floors at a minimum and 2. the selected contractor has done a thorough load analysis and designingn to Manual 'D' specs. If #2 is answered in the affirmative, then he already knows that Manual 'D' states that a return air grille can always be located on the ceiling and sometimes for heating purposes on a floor.
Sincer residential blowers are not all that powerful, I NEVER spec a 3rd floor furnace to service down 2 floors. Only AC only applications will I do that. I much prefer a furnace in the basement/crawl and a furnace in that attic. As an alternative and one I've used many times, I'll put a furnace ON the 2nd floor, do the 1st floor immedidately below from the ceilings and the 2nd floor imediately above from the floors. That requires the use of a mechanical space and allowance for a trunk but it provides nice even heat for all seasons at all levels.
If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!
i do not have any options for hvac locations. the house is narrow 2 story with only attic space on top. no crawl or basement. the hvac contractor has done a head load - manual J, but i'm not sure about the manual d. i'll have to find out.
the way he usually handles 2 stories is to still put the returns on the floor of two story bedrooms and supplies in ceilings...same for first floor.
the manual J came back at 3.5 tons for my house. I am looking at different options for furnaces, etc but am not sure what we're using yet. high efficiency with 14 SEER minimum but i haven't met with him again to look at the options he is putting together for me. we are planning a zoned system for upstairs & down. from everyone i've talked to a well insulated and tight house will work well with zoned system??
i haven't heard about it being a bad idea to have mechanical on 3rd floor servicing 1st floor but it makes sense. how big of a deal is it going to be if house is well built and using quality hvac equipment & ductwork installation? i dont have any other options.
My normal instalation would see a 6" supply to a 10x6 register per room and to that I would use a 14x6 or 14x8 RA grille using one stud space.If there were 2 registers then I would use two RA grilles and use two stud spaces.
Originally Posted by pstu
As far as where to put them,in heating,warm air rises to the ceiling and the cooler air is below the t stat.so in that situation the RA grille should be down low so it can suck that cooler air back to furnace.
In cooling,again the warmer air is at the ceiling so you want the RA grille to suck that warmer air out of the room so that it can be cooled at the indoor coil.