return near entrance door and height

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Old 07-23-20, 02:47 PM
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return near entrance door and height

My HVAC system is oil burning furnace/air and I live in MD. I have a return in foyer that I want to move. I have concerns about the location and height. The duct comes from basement and is 12" round. It was attached to underside of floor and a rectangular hole was cut in floor. metal ductwork was fabricated and place on top of the floor and put the TOP of the 10" wide x 12" high opening 13" from the ceiling. The ceiling height when it was installed was 90". I have removed the drop ceiling and now the height will be 105", which will place the top of the opening 28" from ceiling if I use same piece of ductwork. Should it be higher? Efficiency? The opening in other rooms are about 10" from ceiling. I know some use lower openings in winter and higher in summer, so probably not critical but wanted thoughts. Also, I will be moving it and where it can go is limited. One location on side of a closet puts it near the main house entrance, about 5-6 foot away, and facing the door. Didn't know if that is best location? I could have it face into room, but that would definitely require new ductwork due to limited areas where I can bring the duct up from below due to joist bay widths and other ducts, a header, etc.
 
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Old 07-26-20, 01:37 AM
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Please send an image or sketch showing the return grille/duct. It'd be helpful to have the furnace and outdoor unit M/N's as well.

Return location is not critical so long as the supply outlets are balanced and properly sized. Contractors who don't balance their systems and/or use "One Size Fits All" for supply outlets (4x10x6 everywhere) must provide high/low, etc. to improve comfort issues derived by the improper supply design and failure to balance.

Contractors note the "Famous 16 Words" - which, when you think about it, require balancing:



 
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Old 07-27-20, 01:04 PM
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I recall when installed the guy talking about manual J? for duct size calculations? All new ductwork was installed with the new units as the location of the furnace was changed and an addition doubling house was added. first picture is the furnace. 2nd is the the duct. It is just sitting on floor right now. you can see where the old ceiling was located (where drywall on wall ends). The 3rd picture is outdoor unit. The 4th is the indoor coil. all units are carrier. looks like first picture there twice and couldn't figure out how to remove one.






 
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Old 07-27-20, 07:45 PM
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120MBH - You said you live in MD, but the furnace is sized for AK! Hopefully your installer downsized the 0.75.

Based on that indication of technical expertise, do go for a high return.

The 12x10 return has a Manual D capacity of 550 CFM. You need at least twice that, so hopefully there's another return duct or two somewhere.




 
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Old 07-27-20, 10:09 PM
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by downsizing the 0.75, you mean nozzle? I can't recall off the top of my head but I think it was not 0.75, may be 0.65 nozzle.

I have 3 more returns. This furnace only supplies my first floor and basement. This return is on first floor. It is very open floor plan. There is another return in the open space within a 2x6 stud cavity with 14x14 grill. There is another one in a room with a door also within 2x6 stud cavity but with 14x8 grill. Is there a reason for different grill sizes when the cross section of the ducts is the same? (2x6 stud cavity -14x5.5=77 cubic inches, which is larger than 14x8 or 14x14). In basement there is one return in ceiling-14x14 grill with 16x7 duct

you previously mentioned using 4x10x6 supplies everywhere. So the supplies are 4x10 with 6" round flex for all of them! but with wide open floor plan and thermostat in center, comfort really isn't issue. except basement cooler in winter for sure.

If facing door is not an issue, maybe I can just get extension made for this duct to get it up near ceiling. If not preferred, I guess I could also have horizontal extension to the opening to have it facing into the room.

thank you.
 
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Old 07-28-20, 06:49 AM
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Yes, nozzle size. Not sure what Carrier permits, but you should reduce the nozzle size as much as you can, to promote more continuous operation in cold weather.

As for the return branch ducts, it looks like there's plenty of capacity, and the grilles are oversized. That's consistent with the level of technical expertise mentioned earlier (as is 4x10x6). Do make sure there're "Low Resistance Return Paths" throughout (Exception: Existing door undercuts should be adequate for small rooms).

Please send an image showing the main return duct at the furnace - Often where there's a significant restriction. Give us the size of that duct as well.

One thing: "16x7" is an odd size that'd have to be fabricated locally. 16x8, on the other hand, is on the shelf at supply houses. Please double check that measurement.

"Facing door" is no prob.

Industry Standard requires the Basement to be on its own zone.





 
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Old 07-28-20, 01:08 PM
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So this got me thinking because it illustrates how lack of - or improper - training influences what some contractors do. I developed a spread sheet commenting on sizing, do note the difference between round and flex.

As always, these comments are not meant to reflect unfavorably on the many knowledgeable, highly trained and skilled contractors out there - May their tribe increase.
 
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Old 07-29-20, 11:57 AM
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OK, had chance to measure other ducts. So the 16x7 is about 16 1/8 x 6 7/8. Definitely NOT 16x8. that duct collect the returns from the two wall bays and the basement. The first thing it collects is the two wall bays. They are adjacent bays with one open into main open area and the other open on other side of the wall into a room with a door. it then collects the foyer return. I think it is panned in crawl space where it collects foyer duct and exits crawl space into a 16x10 duct that then as turns to go to furnace it changes into 20x10. this is about 8' from furnace.

given open plan on first floor, the only issue is there is another room with a door that does NOT have a return. having a huge gap under the door is not option. it does have a drop ceiling so it I ever got enough motivation I suppose I could run a jumper into the foyer.

Your pdf says that foyer duct with 2 10x14 grilles? mine isn't open on 2 sides and the current opening for grille is 10x12. Are you saying I should have 2 10x14 grilles? That would be hard unless have one high and one low.open at same time. or do something like I said with extending the duct horizontally but using the existing top opening and the one off the extension. They actually put a high and low and then blocked off the low. using the low would be a little tricky. That duct would be on subfloor. Then there would be baseplate around it. on other side of wall there would be 3/4" hardwood and 5 1/2" baseboard. so 6 1/4" The bottom of that opening is at 7" so with the outside of the grilles extending lower, it would be close to baseboard. if that is what you are suggesting, I should probably raise that duct up some off subfloor.

not sure I understand the whole multi level thing. well Of course hot air rises , just not sure What are they suggesting? My house is very old and originally had two systems because the previous owner rented upstairs and had own system When I put on addition, which added a basement, I remember the guy talking about the 3 levels being issue and we ended up with 2 systems. the furnace for basement and first floor, and a heat pump in attic for the 2nd floor. Is that what they are saying? each floor needs own system? So I should have 3?


 
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Old 07-30-20, 12:59 AM
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I hate panned joist spaces, fortunately they're prohibited today. Do seal them against leakage using a UL listed duct sealant. Be especially careful to seal all around the junctions of ducts and panned joist spaces. Seal all return duct joints in the room/space containing the furnace, too.

The 16x7 (!!!) has a 500 CFM capacity at 700 FPM, so you might want to check what's connected to it using my table. 16x10 is 750 CFM and 20x10 is 900 CFM, a problem.

Just put a 14x6 transfer grill in the wall of the room without a return and you'll be fine: 14x6 on each side of wall, connected through a short 14x6 duct (AKA "sleeve").

Use a 10x30 grille, and provide turning vanes behind it so airflow is uniform across the face, to minimize air noise. Mount it high.

Please send an image showing the connection of the mail return duct to the furnace, to include the duct and the entire furnace.

The furnace should have a Zone Damper setup with two zone thermostats, one per floor. Don't sweat that for now, just understand it'll take seasonal flow adjustments to have a hope of Basement comfort with the system as is.
 
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Old 07-31-20, 07:55 AM
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I found Installation Instructions for the furnace, learned 0.75 is the smallest it's listed for. I also noted a huge mistake some undertrained contractors make: He selected the furnace because of its "5 ton" air moving capacity. Guys who do that know nothing about airflow or static pressure, and have been taught to always oversize the air mover "to be safe".

As always, there are a great number of skilled, highly trained contractors out there who wouldn't make such a basic mistake. May their tribe increase.

You might want to call your contractor and request the 120 be replaced with a 105. You can still have 3 tons' airflow, and heating comfort will improve considerably.


 
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Old 08-10-20, 11:10 PM
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Sorry, been busy with other repairs!

furnace installed over 10 years ago so don't think contractor going to replace with smaller unit.

the 16x7 collects the foyer duct (Not sure of size where goes through 4x6 seal plate.) They form a panned joists, 2x8 but spacing may not be 16oc. Comes out of joist space as 16x10 then just increases to 16x20. Appears to further increase in size before comes down to floor and enters filter box. Don't think answered question about whether need to open the foyer duct to 2 openings as appeared to mention 2 grilles in your chart.


 
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Old 08-10-20, 11:16 PM
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Forgot, there is only one thermostat. On main level. None in basement. basement ceiling not finished south do have access to a lot of the dampers. Others are in crawlspace. Not convenient to adjust every season. Ok in summer. Cold in winter. May need to put supplemental like baseboard in basement. Something just to turn on when using room, not all the time.
 
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Old 08-11-20, 12:13 AM
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See #9 on the grille.

What's the "drop" (vertical return duct at unit) size? What's the air cleaner size? What size hole is cut into the side of the furnace? Thanks!
 
 

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