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Community and Q&A

Moving Conditioned Air Between Rooms – Science of Temperature Delta?

1869farmhouse | Posted in General Questions on

I know it’s nearly impossible to balance the temperature between two rooms by moving air with fans.  Despite being told this, I even tried it myself once in a house that had a mini split in one bedroom with nothing to condition in the adjacent room – two ~150 CFM fans did almost nothing. 

My understanding is that this boils down to the temperature delta and the fact that it takes a ton of ~71 degree air to bring the temperature of a ~78 degree room down anywhere near that.  But I’m curious to learn the specifics well enough to hypothetically calculate how much air it WOULD take.  I don’t expect it to be a practical solution – it’s more just about understanding the science behind it.  Could anyone point me in the right direction?

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  1. Expert Member
    BILL WICHERS | | #1

    It's actually quite possible to balance the temperature by moving air around with fans, it's just that it takes a LOT of airflow to do it. Your two 150 CFM fans, for example, provide a combined 300 CFM. We'll assume that there is nothing going on to reduce the actual airflow provided too, although in reality they probably move less than rated airflow due to things like dirty fan blades, dusty grilles, obstructions along the path they're blowing air, etc.

    So we have 300 CFM moving around. If we have two rooms, 12 feet square with 8 foot ceilings, we have 1,152 cubic feet in each room. To fully "move" that volume of air with your fans would take about 4 minutes. Not too bad, but you're probably not set up for ideal air exchange, with cross flow ventilation, so you're probably only "forcefully" circulating a fraction of the air in the room, with the remainder of the air volume turning over very slowly. You can usually walk into a room and notice that the air is moving in the path from the fan, but other parts of the room have little, if any, noticeable air movement. Those areas of non-moving air are like insulation, slowing the energy transfer from that area of the room to the rest.

    You basically have several problems to deal with here: 1- inefficient air exchange between the two rooms, meaning that your fans aren't really mixing the air between the two rooms very well, 2- heat coming into the hotter room (assuming cooling season here) that effectively "cancels out" some of the air movement from the other room, 3- a very small temperature differential, requiring that large air volumes be exchanged to make a noticeable improvement. If you can move enough air (3) efficiently (1) to cancel out the heat coming into the room (2), you WILL be able to balance out temeratures, but you'd probably need some ducting for cross flow to address (1), or at least you'd need careful placement of multiple fans, you'd need high volumes of airflow to deal with (2) and (3).

    For a practical example, we DO do this sort of thing in datacenter facilities, because we want even temperatures in conditioned spaces. We do two big things to achieve this that are not easily done in normal structures like a house: 1- we partition the spaces to optimize airflow, which means baffles and barriers/doors WITHIN the occupiable space, with those baffles there solely for airflow control. We use fencing in place of solid walls where we want the air to move. We don't use small grates and vents, we have entire open sections of "wall" that are wire mesh. We also use very high air volumes. A typical air handler is usually around 10,500+ CFM (often a 10HP blower motor). There are two very noticeable results from all this: you get NOTICEABLE air movement EVERYWHERE. The funny example I usually use, since we typically have cold air under the floor coming up from floor vents, is the famous picture of Marilyn Monroe with the sidewalk grate blowing her dress up. We also partition hot and cold areas, so you can walk through a sliding door and go from cool wind of 70ish degrees into a hot blast of 100+ degrees. Avoiding mixing improves cooling efficiency.

    For your home, try using a fan in each room to 'stir' the air (blow air in a big circle around the room), and another fan or two to 'transfer' air between the two rooms. This helps ensure you are evening out temperatures as much as possible. You'll still probably not get things perfectly balanced, but this might get you closer.


  2. Expert Member
    Akos | | #2

    Air moves 1.08BTU per CFM per degF.

    Say you have a room with a 2000BTU heat gain, to cool it to 73F with 71F air it would take:

    2000BTU / (1.08*(73F-71f))=925CFM.

    for 75F that would be half that or 410CFM.

    Either case, it is pretty hard without a lot of air flow.

    The one that can work is if you pick up air from near the outlet of an AC unit which is much colder. In that case even something like a 150CFM bath fan can provide a fair bit of cooling as long as the air it picks up is around 50F to 60F.

    1. Expert Member
      DCcontrarian | | #3

      And the smaller the temperature differential, the more air. To get to 72F you need 1850 CFM. To get to 71.5 you need 3700 CFM. To get to 71.25 you need 7400 CFM. You can never actually get to 71.

      1. 1869farmhouse | | #4

        This is perfect. Exactly the formula I was looking for. Although I'm not sure why my own experiment in a rental house was such a bust. The fans are rated for 200cfm, but I ventured to guess they only move ~150. I have one return located on the floor below the mini split exhausting air up the wall and near the ceiling of the adjacent room. And another return on the ceiling of the unconditioned room exhausting air near the floor of the conditioned room. The change in temperature in practice is basically negligible - but they continue to run.

        1. Tim_O | | #5

          Often times fans get rated at a low/unrealistic pressure. Depends how honest the manufacturer wants to be vs how much they want to put a big number on a spec sheet.

          Temperature is harder to move between rooms with a fan, but humidity will move better. If one room is still too hot, maybe dropping the humidity in the conditioned room will help the neighboring room feel a bit more comfortable if it's at an elevated temperature, but slightly lower humidity than before.

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