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Old 11-16-2011, 02:57 PM   #1
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Pitiful low air flow to master bedroom


We bought a 1980s 1.5 story house a few years ago.

Since we've had the house the air flow to the bedroom has been *terrible*. I don't have an accurate way to measure but it's at least 25% or lower compared to the other vents in the house.

The house has a basement that is finished except for the utility room.

Here's how the ducts are set up:

The main supply plenum for the basement and main floor is in a soffit in the basement (perpendicular to the joists). A large flex supply run goes up through a chase to the second floor to a second supply plenum for the second floor vents.

There is a return in the basement, main floor, and second floor.

For the basement and main floor, each vent has a flex run from the main supply plenum running parallel to the joists to the vent. In the second floor attic each vent has a flex run to the vent. Flex to each vent is 6" round. Also, each flex run has a damper installed just after it comes of the supply plenum.


For the "bedroom in question" we have two vents. There is a flex run from the end of the supply plenum to each vent. One flex run is about 20ft, the other is 26ft. The flex runs to the rest of the vents on the main floor and the basement is about 15ft each.

Oddly enough, our bathroom has decent air flow. The two vents in our bathroom are connected at the same point on the supply plenum -- directly on the opposite side. The flex run for those two vents is a little shorter at about 14ft and 20ft but the distance is comparable.


So now with all that info --whew -- I'm wondering if the flex runs to our bedroom are merely damaged? If the flex was crushed would it be noticeable?

I think I've looked at the obvious things already. The vents are fully attached at the supply plenum and the dampers to both vents are fully open. I'd played with different positions on the dampers and know they are working. I'd have to open a soffit to see if the boot was fully attached so I haven't done that.

Any advice?

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Old 11-16-2011, 04:13 PM   #2
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Pitiful low air flow to master bedroom


Here's a quick drawing. The floorplan is very open and very tall ceilings in the living room, master bedroom, and master bath.

I didn't include the upstairs area but can do that if needed.

The AH is the blue box and the grey is the main supply plenum.

Each vent is red.

For the returns, they are very near the AH.

There is a 16x20 return in the basement.

Two 20x20 returns on the main floor -- one on the floor and the other above it near the ceiling.

The return for the second floor is 10x14.
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Old 11-16-2011, 04:28 PM   #3
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Pitiful low air flow to master bedroom


By code (at least here in my area) each run should have a damper in it.....usally located very close to the main trunk ....is it closed or open for that run?
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Old 11-16-2011, 04:38 PM   #4
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Pitiful low air flow to master bedroom


The dampers are fully open to those two flex runs.

And the damper to the main plenum is also fully open.

I've also confirmed they're operating correctly.
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Old 11-16-2011, 04:42 PM   #5
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Pitiful low air flow to master bedroom


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Originally Posted by Rewound98 View Post
The dampers are fully open to those two flex runs.

And the damper to the main plenum is also fully open.

I've also confirmed they're operating correctly.
What I would do next is to try to inspect the run.......flash light and mirror, or even have a drain company out and have them run a camara down the run. You did say that the celing in the basemnet is finished.......so taking the run down is out of the question. Is there a un-blocked return air in the bedroom that has bad air flow?
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Old 11-16-2011, 04:52 PM   #6
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Pitiful low air flow to master bedroom


Yes, it's finished.

I've cut holes in the sheetrock to get to the dampers already where the run starts from the supply plenum.

I suppose I may have to cut holes in the garage soffit and inspect the boots also.


There isn't a return in the bedroom.

I just find it odd there's enough air flow from both bathroom vents which is even further from the return.
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Old 11-16-2011, 05:13 PM   #7
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Pitiful low air flow to master bedroom


so the garage ceiling is the access to the bedroom run out like you said the bathroom is opposite the bedroom take off did you block the bathroom vents to see if the bedrom takes it....having a bedroom over an unheated garage pulls on the heated bedroom keep that in mind once you get the air problem solved.....how's the cooling guess its hot in the summer. could be one take off from the end of the truck and split to the registers....if you have floor registers on that main floor drop old newspapers on top of the ones nearest the furnace and see if the air starts to move to the end of the trunk..even block the bedroom bathroom to see if it might be ripped or disconnected should be blasting out...do it with the system in FAN/ON only if you have the stat selection.usually you have one supply tapped to supply the registers in a room not multipule truck connections keep in mind heating runs a slower in LO/MED LO speed while cooling is alway HI speed air

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Old 11-17-2011, 03:22 PM   #8
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Pitiful low air flow to master bedroom


Thanks for the advice!

The run to these two vents isn't split. Each has its own run from the plenum. I've verified that.

So here's what I did. I turned the FAN on and did some more testing.

The airflow does get a hair better when I close off run closer to the air handler but it's nowhere near where it should be.

I think I have two problems.

First, the flex looks like it's compressed where it goes through the wall into the garage soffit. It's hard to tell but I think that's the case. The flex also isn't very tight and it's height changes going into and out of the penetration. That could introduce a lot of turbulence right?

Second, I just noticed the vent to the stairway has no damper AND it's directly off of the air handler. Blocking that vent noticeably adds more velocity to the rest of the system.


So here's what I propose to do:

1. Add a damper for the flex run to the stairs and reduce the air flow to that vent.

2. Replace the flex run to the two vents in the bedroom.

I'm thinking of running a rigid duct until it gets into the garage soffit. From there I'll using the existing flex.


At worst we definitely should have a damper for that flex run to the stairs. It's coming directly off of the air handler!

How does that sound?
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Old 11-17-2011, 03:27 PM   #9
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Pitiful low air flow to master bedroom


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Originally Posted by biggles View Post
so the garage ceiling is the access to the bedroom run out like you said the bathroom is opposite the bedroom take off did you block the bathroom vents to see if the bedrom takes it....having a bedroom over an unheated garage pulls on the heated bedroom keep that in mind once you get the air problem solved.....how's the cooling guess its hot in the summer. could be one take off from the end of the truck and split to the registers....if you have floor registers on that main floor drop old newspapers on top of the ones nearest the furnace and see if the air starts to move to the end of the trunk..even block the bedroom bathroom to see if it might be ripped or disconnected should be blasting out...do it with the system in FAN/ON only if you have the stat selection.usually you have one supply tapped to supply the registers in a room not multipule truck connections keep in mind heating runs a slower in LO/MED LO speed while cooling is alway HI speed air
Yeah that unheated garage definitely affects the heating/cooling of that room and is soon on the list to address. Right now we're just hoping to get a bit more airflow in there.
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Old 11-17-2011, 08:04 PM   #10
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Pitiful low air flow to master bedroom


I added a damper and things are MUCH better.

I'll hold off on replacing that flex run for now and see how things are.


So, I'll hijack my own thread here.

While adding the damper I saw what is either a lot of dirt or hopefully not mold.

This is the main and secondary plenum above the air handler and a view down.

How bad is this???

We seem to have terrible allergies. There is no mystery why now, eh.
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