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Old 02-19-2020, 12:01 PM   #1
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Help with basement vent layout


Hey guys,

I am in the process of finishing my basement and I am trying to figure out the HVAC vent layout so the basement stays comfortable.

It's a split level house with a walk out basement, so the long exterior wall is 100% above grade.

Hopefully some more knowledgable HVAC guys can chime in and tell me if my plan will work.

I attached the blueprint with dimensions. I am finishing it according to what the builder envisioned on the plan.

There is currently 2 vents already cut into the trunk coming out of the ceiling, those I marked with blue X's on the blueprint. They are 6" pipes. Probably just to pass code for an unfinished basement.

What I am thinking of doing is moving those 2 vents further out towards the exterior wall closer to the sliding walk out door and the window - which would be like 4 feet out from where they are now (I marked those in green) and then adding a third 6 inch vent where the third green mark is.

Then for the bedroom I would add a 6 inch vent in the ceiling above that window, and a cold air return at the bottom of wall (where the green line is).

And then one more cold air return where the other green line is at the back of the short hallway. Would that one cold air return be enough for the entire basement family/rec room area? Or should I add one more where I marked a red line against the wall by the bathroom?

Should I add the third 6 inch vent or do you think he 2 already there will be adequate? We are also installing an electric fireplace that will help a small amount in heating the area.

Also, with ceiling vents (I am going to use the round diffuser style covers) do I still want to put those about a foot or so from walls above windows? Or should I keep them closer to the center of the room where they are now? And if so maybe I can just use the 2 that are already there?

Thanks for any advice!

I will also add a 4? or 5? or 6 inch? vent for the bathroom.
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Old 02-19-2020, 12:19 PM   #2
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Re: Help with basement vent layout


I like your plan. I've seen a lot less provision for a finished basement and still work well.

It's worth moving those 2 registers closer to the outside wall and door if it isn't a big deal.

Run a 6" to the bathroom, you can always damper it. If there is some obstruction making a 6" tough to get in, use whatever will work of the smaller choices. It doesn't take much for a bathroom.

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Old 02-19-2020, 12:23 PM   #3
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Re: Help with basement vent layout


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I like your plan. I've seen a lot less provision for a finished basement and still work well.

It's worth moving those 2 registers closer to the outside wall and door if it isn't a big deal.

Run a 6" to the bathroom, you can always damper it.
Awesome, thanks for the reply! Do you think that second cold air return (marked in red) is necessary?

I have read that you can never have too much air return (or at least its hard to have too much) but the reason I am hesitant is because if I add 2 then I would need to add three returns total (including the bedroom return) and the return trunk for the entire rest of the house only has 4 total. So I would almost be doubling it and I don't know if that will unbalance the system....

For what it's worth I would only be adding 3 total supply runs (bathroom, bedroom, and one more to the main family room)

Or, do you think it would be better just to use the 2 registers already installed in the main area of the basement, leave them towards the center of the room, and then I would only have to add 2 supply lines (bathroom and bedroom) and add 2 cold air returns (bedroom and one for the main basement area).

The only reason I would go this route is if there is concern about adding too much to the current unit. The house was just built last year though, and I have read that newer construction usually sizes the HVAC units with the future completion of the basement in mind (especially for split levels).

What do you think?

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Old 02-19-2020, 12:44 PM   #4
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Re: Help with basement vent layout


I've not seen much advantage of supply registers being in close proximity to windows unless the blower system runs 24/7.
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Old 02-19-2020, 12:54 PM   #5
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Re: Help with basement vent layout


*******

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Awesome, thanks for the reply! Do you think that second cold air return (marked in red) is necessary? No, it's not necessary. Could be a problem depending on the static of the upstairs return.

I have read that you can never have too much air return (or at least its hard to have too much). That's true in most situations but not in yours.

For what it's worth I would only be adding 3 total supply runs (bathroom, bedroom, and one more to the main family room) Sounds good.

Or, do you think it would be better just to use the 2 registers already installed in the main area of the basement, leave them towards the center of the room, and then I would only have to add 2 supply lines (bathroom and bedroom) and add 2 cold air returns (bedroom and one for the main basement area). No, you should add the other supply and register. Also move the present center room registers close to the outside wall and change the type to adjustable vanes to direct the flow away from the wall.

The only reason I would go this route is if there is concern about adding too much to the current unit. The house was just built last year though, and I have read that newer construction usually sizes the HVAC units with the future completion of the basement in mind (especially for split levels). I doubt you have to worry about that... the system is practically supporting that area already.

What do you think?
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Old 02-19-2020, 01:05 PM   #6
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Re: Help with basement vent layout


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I've not seen much advantage of supply registers being in close proximity to windows unless the blower system runs 24/7.
I agree and wouldn't change them in this case if it wasn't for the sliding door that will no doubt produce some good cold air flow into the room, which will tend to creep across the floor and make things uncomfortable. Directing the register vanes steeply downward will help to entrain that cold air with some warm air and possibly minimize the problem.

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Old 02-19-2020, 01:21 PM   #7
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Re: Help with basement vent layout


Awesome, thanks so much for the advice surferdude2! That really helps.

Two more quick questions. I will move the top 2 vents that are already there out closer to the exterior wall, is a foot from the wall standard? Or what would you suggest? And should the one above the sliding glass door be directly above the door or can I leave it offset a bit?

The reason I ask is because if it would be okay to keep it offset then it will save me the work of cutting a new hole in the truck and I can use the current hole and not have to bother with adding 90's.

And second, for the third vent that I will run (the one in the lower right of the picture) there will be a sectional couch over there (it will be the TV area) so would it be okay to keep this one further back from the window/wall and use a circular diffuser there? Just so I don't have to worry about heat/AC blowing on people sitting on the couch?

EDIT: One more questions I thought of that would be great if you could answer:

1) Should I insulate the supply runs? The builder didn't insulate anything but I would hate to seal up the ceiling and then have a pipe sweat and create mold on the ceiling. But maybe it wouldn't sweat because the entire house is climate controlled with an air exchanger to keep humidity in check...

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Old 02-19-2020, 01:57 PM   #8
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Re: Help with basement vent layout


One other question...

For the cold air return, is a 6 inch takeoff connected to a 6 inch oval connected to a 10 X 6 stack okay? The rest of the returns are 6 inch, but I measured the stacks in the rest of the house and they are all 12 X 6. But every store around me only has 10 X 6 stacks in stock and I don't really want to order it so hoping a 10 X 6 is fine?
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Old 02-19-2020, 02:04 PM   #9
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Re: Help with basement vent layout


Quote:
Originally Posted by tjuser23 View Post
Awesome, thanks so much for the advice surferdude2! That really helps.

Two more quick questions. I will move the top 2 vents that are already there out closer to the exterior wall, is a foot from the wall standard? Or what would you suggest? And should the one above the sliding glass door be directly above the door or can I leave it offset a bit?

The reason I ask is because if it would be okay to keep it offset then it will save me the work of cutting a new hole in the truck and I can use the current hole and not have to bother with adding 90's. It isn't that critical, do it the easy way if looks acceptable from the standpoint of symmetry.

And second, for the third vent that I will run (the one in the lower right of the picture) there will be a sectional couch over there (it will be the TV area) so would it be okay to keep this one further back from the window/wall and use a circular diffuser there? Just so I don't have to worry about heat/AC blowing on people sitting on the couch? That's good thinking. You can use one of the circulars you took out.

EDIT: One more questions I thought of that would be great if you could answer:

1) Should I insulate the supply runs? The builder didn't insulate anything but I would hate to seal up the ceiling and then have a pipe sweat and create mold on the ceiling. But maybe it wouldn't sweat because the entire house is climate controlled with an air exchanger to keep humidity in check...
Not necessary unless you have an extremely damp basement which would surprise me since you have a pretty new build.
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Old 02-19-2020, 02:08 PM   #10
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Re: Help with basement vent layout


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One other question...

For the cold air return, is a 6 inch takeoff connected to a 6 inch oval connected to a 10 X 6 stack okay? The rest of the returns are 6 inch, but I measured the stacks in the rest of the house and they are all 12 X 6. But every store around me only has 10 X 6 stacks in stock and I don't really want to order it so hoping a 10 X 6 is fine?
That's acceptable.
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Old 02-19-2020, 02:13 PM   #11
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Re: Help with basement vent layout


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That's acceptable.

Great! Thank you so much for taking the time to answer my questions, you have helped immensely, I really appreciate it!
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Old 02-19-2020, 02:17 PM   #12
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Re: Help with basement vent layout


When you get this all installed and running, and assuming you have a stairwell door, you can close it down to a small size crack and smoke test the air flow.

It wouldn't hurt to do that test now for comparison to later. The present test may surprise you.

Best regards, SD2
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Old 02-19-2020, 03:12 PM   #13
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Re: Help with basement vent layout


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When you get this all installed and running, and assuming you have a stairwell door, you can close it down to a small size crack and smoke test the air flow.

It wouldn't hurt to do that test now for comparison to later. The present test may surprise you.

Best regards, SD2
Interesting, how would I go about doing the smoke test? And what would I look for?

This sounds interesting and like something I want to give a shot
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Old 02-19-2020, 03:45 PM   #14
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Re: Help with basement vent layout


There are several types of smoke tests and that many and more products sold to create the smoke. Some of the smoke generators are expensive.

However, for your purpose, an incense stick, a cigar or cigarette, a Juul or anything else you can MvGyver up to make a little non toxic smoke will do. Holding that smoke source close to the door crack (") will tell you something about the air balance between the upstairs and basement.

Smoke Generator Matches

I would expect that it may be slightly negative (with the system running) in the basement before you make any changes. If the smoke barely drifts into the door crack from upstairs to down, it's fine. When you finish the modifications, we would hope it remains that way or even slightly positive, smoke drifting slowly towards upstairs, will be OK.

This test is pretty crude and by no means worth relying on to design or make major modifications to a system. It is only useful to see that you didn't make any major change to what was likely a well designed system initially.

If the imbalance pulls the door shut or resists closing it, you most assuredly have a problem.

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Old 02-20-2020, 07:48 AM   #15
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Re: Help with basement vent layout


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Originally Posted by surferdude2 View Post
There are several types of smoke tests and that many and more products sold to create the smoke. Some of the smoke generators are expensive.

However, for your purpose, an incense stick, a cigar or cigarette, a Juul or anything else you can MvGyver up to make a little non toxic smoke will do. Holding that smoke source close to the door crack (") will tell you something about the air balance between the upstairs and basement.

Smoke Generator Matches

I would expect that it may be slightly negative (with the system running) in the basement before you make any changes. If the smoke barely drifts into the door crack from upstairs to down, it's fine. When you finish the modifications, we would hope it remains that way or even slightly positive, smoke drifting slowly towards upstairs, will be OK.

This test is pretty crude and by no means worth relying on to design or make major modifications to a system. It is only useful to see that you didn't make any major change to what was likely a well designed system initially.

If the imbalance pulls the door shut or resists closing it, you most assuredly have a problem.
Awesome, thanks for the info! I did the smoke test last night and it definitely doesn't move the door at all. The smoke very slightly got sucked through the crack in the door into the basement, so it sounds like that is good news and hopefully it will stay close to that when everything is done.

Thanks again for all the valuable advice!
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