Positive Displacement? Was I wrong to cover this opening?

Old 11-12-15, 05:38 AM
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Positive Displacement? Was I wrong to cover this opening?

This is my air handler. It's at the top of my house in a finished yet unconditioned "attic". As you can see at the bottom of the picture there is a gap where the air handler is placed.

So last summer I was trying to get my A/C to work better and thought that this gap in a previously unused system was due to accommodating the machine to the construction and should covered since it was sucking relatively warm air from the upstairs and none of the returns seemed to be sucking much. So I covered it up and everything worked but the returns still didn't seem to be sucking any air.

This year as I cranked up the AC for the first time (southern hemisphere). I tried for the first time opening the damper to the supply ducts in the garage and nothing came out. I was thinking, that that it seems seems like I don't have enough pressure in the ducts at all and maybe should get a booster fan or something.

But something else just occurred to me, maybe that gap was supposed to be there.

There are supply vents in ever room in the house, there are returns in all the hallways bedrooms and living room spaces. There is no return in the kitchen and breakfast room which can be separated from the living room via sliding doors nor in the garage nor the bathrooms.

I had thought that gap was open it would always suck air from there which was relatively warm rather than through the returns but now I'm thinking that if the rooms are reasonably well sealed to the outside, having the supply vents pumping air into the room would displace air into returns via positive displacement. It would with the doors to the rest of the house closed help the kitchen extractor remove cooking smells. It would also seem to be necessary to condition the garage since displaced air in there goes out of the house and can't circulate back through the returns.

Am I on the right track here?
Old 11-12-15, 06:05 AM
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Is there a duct connected to the other side of air handler?? From the pic it looks like you don't have any return air.
Old 11-12-15, 07:15 PM
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Can you post a picture of the other side ?
Old 11-19-15, 08:54 AM
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Sorry for the late reply, I got caught up in some other things.

Here a side view of the air handler from before the opening was blocked off.

The elevated floor it sits on is the return air duct.

Here is a photo taken from underneath the air handler.

That is the supply air in the insulated duct.

I had thought if I hadn't closed the gap then there was no way the air handler could would suck anything from the rest of the house but what I'm thinking now is that that wouldn't matter since the aire handler is inside the house. Air pushed in to the rooms has to go somewhere, some will leak around the windows sure but the path of least resistance would push it into the returns or up the stairs to where the air handler is and into the gap.

It seems like this would explain why the return space could be open to the outside. Positive pressure would only let in air equal to air lost..

Sort of thinking aloud here.
Old 11-19-15, 10:02 AM
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So it draws the return air thru what looks like a damp crawlspace.
Old 11-19-15, 12:19 PM
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Its not really a crawl space, that's about only about 8 inches high or so. It might have been damp before but not since we've had it running.

To give a brief idea of what the lay out is like. The back wall along the picture is the side of the house. it shares a back wall with a open space running down to the semi basement which I've shown pictures of in other threads.

This picture is looking up at the supply duct from 40ft or so below the air handler.

From down in the basement this is open to the outside via a relatively long path through the ceiling.

Here's my attempt at an explication I sent to a friend who is a contractor asking for his opinion. Warning its a bit long.

Garage/Semi basment: The only official plans of my house I've managed to find are the plumbing ones so ignore all the colored lines this is just to give an idea of the lay out.

To understand this. The space where the arrow is on the top left is the exterior to the right of which is the garage which extends to the end of the plan. On the bottom half space on the right with a staircase and the number 7 is also an exterior patio. There is a bedroom at one end of the hallway and a laundry room at the other. Against the lower edge is the machine room, 2 bathrooms and a storage closet.


Location: As in the rest of the house there supply ducts are located in the drop ceiling.
Supply 9 registers in the garage
1 in the laundry room
1 in the bedroom
1 i each of the bathrooms.
1 register in the hallway located above the door to the garage also vertically positioned.
1-damper located in the hallway between door to the garage and the laundry room.

ReturnsNo return vents installed but in each of the two bathrooms there are 4x10 cutouts framed in the drop ceiling complete with taped off but live wires hanging down through them from junction boxes, presumably to install extractors. The drop ceiling is not connected between the two bathrooms nor to adjacent rooms (wet walls are plastered over to the floor above) but are connected to the drop ceiling above the adjacent hallway. One thing I realized just now as I was writing this post is that those holes in the drop ceiling are identified by green circles on the plumbing plan which indicates 'ventiation'.

Outside Air

Like most residences in Argentina, every window is fitted with roll up security shutters on the outside of the windows. Since they're rolling up from the outside of the glass, the space they roll up into is inevitably open to the outside air. Here is a picture so you can get a general idea.

In the basement level there are two of these windows. 1 in the laundry room and one in the bedroom.

In the laundry room this space is closed off in the drop with the wall above the door to the hallway as well as the door to the ‘machine room’ plastered off to the ceiling as you can see in the picture taken from the window looking towards the hallway, it is deliberately closed off

But in the bedroom it is not. It is open all the way down the hallway.

The hallway drop ceiling is open to the machine room.

This layout would seem to provide for outside to reach the gas water heaters in the machine room which did not have any other noticeable source for supply air since the space is closed off from the laundry room.
I thought I'd copy and paste that.

After the basement I'm a little less sure about everything but here is what I've come up with.

Ground floor

3 supply registers in kitchen/breakfast room (2 placed directly over doors rudimentary air curtain?). No returns
4 registers in the living room, 3 large ones plus a smaller one over front door. 2 large return vents (2x the size of the return vents in bedrooms with the same size supply vents.)
1 smaller supply register in the hallway with small return by the floor.
1 supply register in the half bath and again a framed cut out in the drop ceiling.

All three return vents are ducted up through the drop ceiling and to the floor above.

The ceiling is weird on this floor, the kitchen and breakfast room drop ceiling is open to the outside air and there is generally a noticeable air current. Especially on windy days however it is blocked off from the open spaces where the supply air comes in, the living room ceiling and hallway ceilings with one exception.


This appears to be deliberately opened and while I can't tell for sure just by sticking my phone through wholes in the ceiling I believe that duct continues to the bathroom and from there branches to the hallway, living room and the remaining duct in the breakfast room (all of which sealed with plaster).

In the bathroom there is more cause for gusswork above my head.

Looking at this picture you the duct going to the hallway at the bottom the duct going to the living room on the right and two holes in the ceiling the 1 with the large tape wrapped pipe is filled in with concrete and that pipe is fresh water for the toilet. The little bit of material in front of that pipe corresponds with the back wall of the bathroom.

The other hole is more intriguing since it appears not to be completely closed off which would mean that the bathroom would be vented up through the ceiling.

On to the upper floor.
Supply registers in each room plus the hallway. Return air from farthest bedroom is ducted through 1 space ceiling into another. Return from other bedrooms are ducted up to the ceiling where they run unducted back to that vertical passage and presumably the air handler

My conjecture is that the air from the bathroom on the main floor joins the ceiling space on this floor into which the bathrooms vent which itself is vented on the roof next to the AC unit (would explain previously unknown vent) while an independent plenum in the rest of the ceiling joins the returns from the ground floor plus the returns from the bedrooms and heads to the conduit.

Sorry for the extremely long and tedious post. The only way any of this seems like it might make sense is on a positive pressure design. That would explain what would seem to be an impossibility of accurately sizing returns. Also would explain how you could open the damper to the garage which is completely isolated from the house and separately ventilated without creating a negative pressure inside the house.

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