Return air grille vs duct distance
orange_cat
| Posted in General Questions on
I was just looking at the framed walls and noticed that return grilles are far apart from return air ducts. That makes no sense to me. I am attaching the image where they are at least close – but there are instances where they are 4 feet apart behind drywall and I do not even know if insulation is blocking the air flow or not.
Please help me understand. I am at a point of demanding they reset the return ducts (which on drawings DID meet the returns). But with partly install drywall it will be not a well received request.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
They're probably intending to use the stud bays themselves as the "duct" for the returns. This is commonly done, but less than ideal, since it causes negative pressure in the stud bay, which makes any air leaks worse.
Better is to have them put some sheet metal into the stud bay to make a "duct", instead of relying on the drywall. If doing this, it's important to seal off the area between the sheet metal and the studs, and at the top and bottom of the "duct" section of the stud bay. Basically you want to confine the air to the area of the "duct" section, and not let it leak out into the rest of the stud bay.
Best is to use a proper metal duct for the entire air pathway.
Bill
Thank you very much!
I'd be interested in what the HVAC drawings say. The 14x8 return with an area of 112sqin narrows to a 6 inch duct with an area of 19sqin.
Thermopan is used a lot as a cheaper alternative to metal. If they won't put metal in there I'd caulk where the thermopan meets the studs and tape the round duct junction. If you have panned joist returns, you may have similar issues, especially if plumbing or electrical trades come after HVAC and cut holes in them.
Is this a custom or production build?
Custom. I am really disappointed. I do not understand them all that well so do not even know which parts to copy paste here.
But it does say things like "INDIVIDUAL SLOT SHALL BE
CONNECTED TO MAIN
DUCT SEPARATELY.
CONNECTIONS FROM MAIN
DUCTWORK SHALL
TRANSITION INTO LINEAR
SLOT PLENUM. PLENUM TO
BE FIELD FABRICATED AND
COORDINATED WITH GC"
MECHANICAL CONTRACTOR TO
COORDINATE THE OPENING
SIZES IN THE STUD FRAMING
WALL WITH THE GC AND
FRAMER
Your HVAC drawings submitted for your permit should have a drawing for each floor and then a bunch of pages with the heat load or manual J calculation. See if you can post the drawings here. This is mine for the basement where the supplies and returns are for the first floor and basement. The default standard is panned joist returns. You can see that I asked for a metal duct for the return far away from the furnace to minimize losses. I allowed for panned joist returns in the unfinished area, where I can easily seal them up after
I'm not an HVAC engineer. If you can post your drawings, hopefully you get more responses.
Opps, sorry that was a previous version. It still shows a panned joist return. I made them change it to 14x10 metal duct.
Translating that "all caps" technobabble-speak into plain english will read something like this:
"Every vent opening will connect back to the main duct using a seperate run of duct. No two vents will share a single small size run of duct along the path back to the main duct."
"Connections to the main duct will be made with the appropriate transition fittings, and not randomly connected."
"Large ductwork connected to the furnace will be fabricated at the site in coordination with the general contractor (GC)."
"Mechanical contractor installing the furnace and ductwork will work with the general contractor and framer (framer = carpenter who builds the STRUCTURE of the house) to make sure holes are the right size for the duct and vents to fit."
That last one is important: you don't want the guys installing the duct to hack big holes through structural parts of the house. The framers will know what and where you can safely cut holes. The GC will settle any arguments between contractors.
There is a rule in the construction world for all the trades that basically goes "you don't F' with the structure". That means you don't just go randomly drilling big holes and cutting things, because you might weaken the structure and make things unsafe. That is a Bad Thing. I have seen plenty of times where this has been done anyway though, and the worst offenders are HVAC guys with holes for ductwork, and plumbers running toilet drains through joists.
Even in my own home here, I found a place where the HVAC guys during the original construction ~40 years ago had cut out a header and left the ends of two floor joists almost hanging free (only one end of the header was supported), and this caused sagging of the floor above. I jacked that back up, sistered in some extra lumber, and did a few other things to get those joists properly supported -- which greatly stiffened the floor above!
Bill
I do not see any big problems in the photo you posted. It could use some mastic.
I see a “high return” general consider an upgrade. The installer went out of his way to offset supply duct from the register this cost time and money so another upgrade as it allows for slower air flow at the register and that makes for quieter operation with better air flow.
Walta
They did not make a choice where to put exaust grilles. That decision was made by the architects, with engineers, and they were merely asked to build to drawings. That is why on the image above the grille is positioned as drawn - and the duct was never properly located to meet it. There is ample space behind this particular grille to move the duct. It is sheer lack of care.
Why do you want the duct directly behind the grill?
The offset reduces noise from the blower and reduces air velocity for more even air flow and less chance the grill will vibrate and make noise.
Walta
Because I want the return air to be pulled out of the room, and not from imperfectly insulated spaces between the studs and for all I know from the mechanical room at the lower floor connected to the same stud bay?
I did start with a genuine question whether this is something to worry about and the answers appear divided?
It seems very unlikely that a return duct would be installed in an exterior wall so I am assuming the insulation was installed for noise control. That being the case leakage into interior wall is virtually irrelevant.
Seems very unlikley they would install any ductwork if the stud bay was being used as a duct.
Walta
There is a duct for exhaust. Terminating about 2 feet above floor level. I can see it on the other side of the wall - it is an interior wall - which have not been insulated from the other side at that point.
The grille for exhaust air is about 7 feet above floor level.
When asked how does the grille connect to the duct (so exhaust air gets to the ERV down the line) - the space between was pointed to me.
This is pervasive in multiple locations - like the person running exhaust ducts did not read the plans that closely half the time, so they were vaguely in the right areas, but only a few times exactly right (or where I spotted this earlier and asked to have fixed).
All the ducts are in the interior walls or between floors.
It is typical for a floor return to offset the duct from the grill to keep debris out or at least somewhere where it can be reached.
As long as the air velocity is low, it doesn't matter much, with such a small duct you are fine. They really should have used metal and boxed in the top to where the register starts.
These are all wall registers, not floor registers and they are closer to the ceiling, than the floor.
However, the actual duct in one example is 4 feet lower than the grill and I cannot tell what is behind the drywall they installed in that location this morning - is there insulation in that wall?
That isn't a typical wall install. Normally you use oval pipe/stack duct with a stack head or panned bay for a return. Hope that is not an outside wall though, you have to be careful with those, best to line the whole bay with rigid that is sealed with canned foam before any ducting.
Provided the velocity is low, if the offset is 4" or 4' it won't effect the flow.
So your picture is looking up at high wall returns just below the ceiling? Is this a second floor system with ductwork in an attic? What's the space behind the wall that the 6" duct runs in?
Yes, looking up towards the ceiling.
This is the second floor, but the ductwork is not in the attic - there is no attic. There is a dropped ceiling in certain areas on the top floor (over bedroom closets, so invisible) where the ERVs are located and the ductwork. There is about 5 feet vertical by 3 feet horizontal space where these ducts are run and before insulation was added it was quite clear that there is ample room to reposition the ducts (which I did ask to have done back then and some - and at least some - the ones I specifically pointed to - were fixed.).
The "4 feet lower" example I gave has the duct running from floor level to the grille - between the studs - just not all the way up (as I thought it would). Again, it is just within 2x6 wall - nothing stopping the duct from reaching the grille higher up along the wall. It was just terminated early.
There is a different ERV on this level.
Ah, Ok. it sounds like you have a modern design home with a flat roof where there are mechanical chases and shafts to accommodate the HVAC runs for the second floor.
In that case, I wouldn't worry about that. If anything, I'd just try to seal up the cavity better. For my second floor, my 14x8 returns have short 6" solid ducting to the main return duct. 30x8 returns have two short 6" ducts.
A cosmetic issue you might have is what you see when you look through the registers when finished. If you plan on Aria vents, they will block any view inside the stud cavity (along with much of the air). If you use grills with slots, you may want to paint that area the wall color before you install the registers, especially if they are mud in.
A small fragment of gently sloping roof exist -not flat though - , the rest is sloped properly, but otherwise yes.
Thank you for the suggestion the paint.
Did not know about Aria (now Fittes)- not sure if I like the vents, I see performance data on air flow and throw, but concerned they still will just dump air into a room vs throw across room to create air circulation.
But I like the minimal reveal access panel and the flip-down for things like security control panel.
1. Is the current situation as in the photo you posted, or is the gyp now installed?
2. You are correct to be concerned, but I think your concern is about the wrong thing.
a. It does not matter that the grill will not align with the duct. This is often done on purpose to minimize noise.
b. What does matter is an efficient return air path. This is best achieved with a duct path with very little leakage, which generally requires complete sealed ducting for the entire path, and metal duct is preferred. Your photo indicates they are using the stud bay plus some thermopan material to construct a "duct." Even though this is a return air duct, it should still be efficient and should pull air from the room, which it is most likely to do since that is the path of least resistance. However, I would not rely on likely. I would require what amounts to a metal box with a rectangular hole where the frame for the return diffuser is, and a round hole where the duct is. The box should be the entire width of the stud bay, and the height that the thermopan partial box is. It should be sealed, especially where it meets the round duct and at the seams.
Regarding another commenters post regarding painting. I agree the inside should be painted. Typically the inside is painted flat black. You can do this yourself with a can of flat black spray paint, even after the gyp board is installed.