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Moving air from warm to cold room

Myrtleboone | Posted in General Questions on

Hello. I would like to move some of the warm air that builds in one southern exposure room on my second floor to my north end 1st floor. I am thinking of a simple 4″ duct with inline fan that is piped from the source to the cold area. I was told to actually send the cold air upwards into the warm room which then would aid in drawing warm air downward. Any thoughts on how I should go about this? Thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    Matthew,
    Most attempts to do as you propose underestimate that volume of air needed. Let's assume that the air in the warm room is at 78°F or 80°F, while the air in the cool room is at 65°F or 67°F. That's a small delta-T -- only 13 F°. That's different from a hot air furnace delivering 160°F air.

    Since we know the specific heat of air (0.0182 Btu/cf/°F), we can calculate how many BTU it takes to raise the temperature of one cubic foot of air from 65°F to 72°F:
    7 x 0.0182 = 0.124 BTU
    So, for 60 cubic feet of air: 0.124 BTU x 60 = 7.44 BTU
    If the 4-inch inline fan is delivering 60 cfm, that means it is delivering 7.44 BTU/minute, or 446 BTU/h, which is about 130 watts. So the inline fan will deliver a bit more heat that a 100-watt lightbulb.

    (And by the way, the fan will probably draw 30 watts -- so you'll be using 30 watts of electricity to deliver 130 watts of heat. That's either good or bad, depending on your perspective.)

  2. exeric | | #2

    Martin,
    You gave this advice some time ago when I was trying to do something very similar to what Mathew is trying do. After much thought and (some internal resistance) I decided to alter my plans according to your math. What I've ended up doing is installing a large inline fan of 330 cfm above the ceiling in the bathroom. I then have installed periscope type thru ducts between the bathroom walls to both the bedrooms on either side of the bathroom. The periscope type ducts eliminate clear line of sight. So the plan is to intermittently run that fan to redistribute heat from the only heat source in the living room which isn't ducted. It only works with the bathroom door closed because an open door would short circuit the airflow path. It hasn't been commissioned yet but I hope it works.

    I should add that the periscope type thru ducts are made so that if there is any stratified air in the bedrooms that they pick up the cold air at floor level in the bedrooms.

  3. exeric | | #3

    There isn't a free lunch here because that big fan will be evacuating to the outside. So my idea isn't really a replacement for a ductless system. To actually recirculate the air I would have to run a return duct in the attic to the other side of the house. I may actually get around to it someday.

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