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Community and Q&A

Air flow oddities

tekjunkie28 | Posted in General Questions on

6 Months ago I had a 2.5 ton Daikin Fit HP installed and they replaced the square trunk line with round duct and added new and better trunk collars. (its the ones that have the rubber adhesive that seal up tight) They added another return grille for me so I now have 2 20x25x1 with honeywell 3″ deep MERV 13 filters in them.  IDK what size the return duct or main supply trunk is or exactly how its ran as I have not been up in the attic since it was done but I plan to get up there soon and take measurements. Previous unit was a 3 ton single stage HP from 2000.

Now for the problems, I took a nap earlier today and I had the door closed to the bedroom. The temperature in the room went from a comfortable 73 to about 77 within an hour.  Of course I woke up sweating but the rest of the house was fine at around 73-74  I have a small door undercut in that room but I also have the Tamarack Door vent cut into the bottom of the door.  It seems to do nothing though. 
Another problem I have is that as long as all the bedroom doors are open the house is a sort of comfortable but once one bedroom door is closed it seems to cause all kinds of problems.  For example, when the heat was on shortly after the unit was installed the living room, kitchen and dining room (main floor of ranch style home so bedrooms on one end and living, dining and kitchen on other end, typical mid 60s-70s ranch home construction AFAIK) would get to 75-78 degrees while the bedrooms would be 68, which is comfortable to sleep. Thermostat would be set to 68-69. It seems like the inverter unit can not pressurize a room like the old unit could and overcome the closed doors.  The same problem happens during cooling… When I get up in the morning the bedrooms are 70-71 but the living room is 66… the thermostat is reading 70ish but the air just seems to go into the living area with all the bedroom doors closed OR the thermostat cannot pick up the air leaving under the doors for the bedrooms.

During the winter living room is too hot typically.. during the summer the living room is still too hot.

Also the thermostat is at one end of the hall and the return grilles are at the other end of the hall into the kitchen area.  Is that too far? Return vent to thermostat is 12 feet.

So do these new ECM variable speed motors have any characteristics that exaggerate imbalance issues?
Do I need jumper ducts along with door vents?
Is it possible my static pressure is too low when the unit is on low speed and the air just flows into the living area instead of the sleeping area? They tested my static at 0.2,  unit can do up to 0.9 but I think 0.5 is where they like it

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  1. user-5946022 | | #1

    Jumper ducts may definitely help with airflow.
    Dampers may also help with air balancing which it sounds like what you need

    If your current ducts are in your attic, it is entirely possible that your contractor failed to seal the duct system properly when they replaced portions of it, and your conditioned air is leaking into the attic. Did they test the seal on your ducts? You need to look at this to see if they look sealed. They should have used a white mastic at every joint that you can see and if you put your hand near a joint when the system is on you should NOT feel any air.

    1. tekjunkie28 | | #2

      No, they did not test. I have mastic and I can seal it up.

      How much heat gain or loss would I have with jumper ducts, I always have opted not to put in jumper ducts because of that. If its minimal then I'll do it.

      As for the dampers, I guess that would work but Id have to climb in the attic every spring and fall and move them.

  2. Chris_in_NC | | #3

    It's not that your blower static is too low, it's that the static pressure in the closed rooms is too high compared to the non-closed areas with more parallel supply flow and return flow. I've leave that to the smarter people to elaborate on.

    Jumper ducts won't cause heat gain/loss unless they are in an area that can cause them to lose/gain heat. No different than any other ductwork; location and insulation matter with jumper ducts as well.

    1. tekjunkie28 | | #4

      I think they would have to go in the attic. I guess they could go in the wall or the floor butI have no clue how beneficial that would be. The duct would have to be smaller to fit in the wall.

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