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Community and Q&A

Room to Room Fan

Remodelpro | Posted in Mechanicals on

I need to design a room to room fan system that will circulate cold or warm air from a large room that is conditioned by a mini split to a smaller room that is approximately  56 square feet.  The mini split was sized using Manual J to handle the heating/cooling loads of both rooms. 
– How large should the room to room fan be? (The HVAC installer suggested using 1CFM per square foot of floor space ) 
– Should the fan push cool air into or suck it out of the small room?
– Should the cold air supply grill be placed high or low on the wall?
– Should the return air grill be placed low or high on the wall.?
– If I use a 4″x 8″ grill on the room to room fan how large should the return grill be?

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  1. brian_wiley | | #1

    It might be helpful to know what the small, 56 sq ft room is used for (mechanical closet, bathroom, etc) and the proximity to the larger room (does it share a direct wall, is it across a hall, etc).

    In any case, 1cfm per square foot feels a bit oversized to me unless it’s on some sort of timer or thermostat control. Most of that is discussed here though:

  2. Jon_R | | #4

    To answer the questions remaining after the well written #3, the best duct design in such a case is equal sized, high and low vents with a fan in each. The directions depend on the season.

    A ducted mini-split would be much better.

    1. Expert Member
      Dana Dorsett | | #16

      >"A ducted mini-split would be much better."


      Size the ducted cassette correctly for the combined room loads (do NOT oversize by more than 1.25x from a Manual-J if you can help it) and it'll be a far better solution than a noisy room-to room fan that does very little other than create wind chills & noise.

  3. Expert Member


    I'll leave it to others to decide whether a fan can effectively mitigate the temperature difference between the rooms, but you still face another problem.

    Whether you push or pull the air, the equivalent amount must make its way from the bathroom to the larger one. That is air which is usually exhausted due to moisture or odors, so it's not at all a desirable situation.

    For the small energy penalty supplemental electric resistance heat for such a small space represents, that's the direction I'd go.

  4. Remodelpro | | #2

    Thank you for your prompt reply.
    The small room is a bathroom and it is shares a wall with the larger room.

  5. Expert Member
    NICK KEENAN | | #3

    I know you said you wanted to design this, but I'm going to try and explain why this is a design challenge.

    Go back to your Manual J. It will show heat losses and gains for each rooms. The Manual J assumes that the interior of the house is at a constant temperature, so there are no heat transfers between rooms. What you want to model is what happens if all of the heat is dumped into one room and then travels from that room into the other. In order for heat to flow there has to be a temperature difference -- if you try to model how much airflow it takes to heat the smaller room when both rooms are at the same temperature, you'll come up with infinity cfm. That might be objectionably loud!

    So then the question becomes, what temperature difference between the two rooms is acceptable? I would start off by trying to model what happens when there is no air exchange. Even with no air exchange, some heat will flow from the big room to the small room through the shared wall. Drywall is quoted at R-1 per inch, so given the area of the wall and the temperature difference you can calculate the heat flow. In the smaller room, with the Manual J you can calculate the heat flow at the outdoor design temperature for different interior temperatures. Through trial and error you can figure out the interior temperature where those two flows are equal. My suspicion is it will be pretty cold.

    Then pick an interior temperature and see how much air flow you need to reach that temperature. The amount of heat carried by air, in BTU/hr is equal to 1.08*CFM*temperature delta. So let's say you decide it's OK to have the big room at 75 and the small room at 65, that's a ten degree delta. At 56 CFM you get 1.08*56*10=604 BTU/hr. With your Manual J info, is 604 BTU/hr plus what flows through the wall from the big room enough to keep the small room at 65F at the outdoor design temperature?

    If you share the Manual J estimates for both rooms, the outdoor and indoor design temperature, and the area of the shared wall, I can help with the math.

    I think what this will show is that room-to-room ventilation is not a great solution, even with a big fan you get pretty big variations in room temperature. I would say this is the greatest flaw of minisplits, along with the fact that it's hard to get small enough heads in an efficient house.

    1. charlie_sullivan | | #7

      Great explanation and a useful example calculation showing why this isn't often all that effective.

      One way this could work better than this calculation is if the vent from the bigger room into the bathroom is located very close to the minisplit head, such that the air it takes in is mostly the output of the miniplit, before it has mixed with the room air, at which point it would be well above the room temperature.

      1. Expert Member
        NICK KEENAN | | #8

        Taken to its logical conclusion what you'd have is a ducted minisplit.

    2. Remodelpro | | #17

      Thank you for your reply.
      This building is in Florida so heating is not really an issue.
      The large room is heated by the 7500 BTU Mini Split and the small room is heated by a 1500 watt electrical resistance heater.
      According to the manual J calculations the large room needs 5699 BTUs of cooling and 188 CFM and the small room needs 2442 BTUs of cooling and 80 CFM.
      This was calculated using a 75 degree indoor temperature and a 95 degree outdoor temperature.
      The shared wall measures approximately 7' x 9' and is insulated with R-15 Rockwool for sound control.
      The small room can be 5 degrees warmer than the large room since it is a bathroom.
      My largest concern is sizing the return air transfer grill to maximize the air movement between the two rooms.

      1. Expert Member
        NICK KEENAN | | #18

        At a 5F delta, a 7x9 wall with R15 passes 21 BTU/hr. If the small room needs 2442 BTU/hr of cooling to maintain 75F at 95F outdoor, it needs 1832 BTU/hr to maintain 80F.

        To move 1811 BTU/hr with a 5F delta you need 335 CFM.

        1. Jon_R | | #19

          335 CFM from a pair of fans is doable (but not recommended). Doing it while maintaining that sound control is quite hard.

          An open door with a 5F delta-T is good for about 1700 btu/hr (close enough).

  6. Expert Member
    Michael Maines | | #6

    I'd put a small electric resistance heater in the bathroom. If you don't like how electric baseboard looks, you might like Envi or Convectair wall-mounted convection heaters, no fan needed. The mini-split will help keep the room warm so the electric resistance won't have to work too hard.

    1. Expert Member
      MALCOLM TAYLOR | | #9

      You could even remove it for most of the year.

  7. Jon_R | | #11

    Note that if the rooms are at the same temp, the bathroom heater will supply 100% of the bathroom heat load. It's not just a little supplemental heat.

    It's likely that your water heater is more cost efficient than resistance heat. So consider a hydronic towel radiator for the bathroom. Instant radiant heat (heat lamp) is also partially effective.

    1. Expert Member
      Michael Maines | | #12

      The mini-split will keep the bathroom's interior walls warm, leaving heat loss only at the exterior walls, and depending on the situation, the ceiling and floor. Many of my clients turn on the electric resistance (often in-floor) only for an hour or two, when needed; the rest of the time the door is open and the mini-split heats the room. I agree that hydronic heating would likely be more efficient, but the installed cost is likely higher as well.

      1. Expert Member
        NICK KEENAN | | #13

        Jon's point is that heat only flows if there is a temperature difference. If the bathroom is at the same temperature as the adjoining rooms, there is no heat flow through the walls or floor. In that case all of the heat to make up for the loss through the exterior walls is coming from the resistance heater.

    2. Expert Member
      MALCOLM TAYLOR | | #14


      Which for a bathroom can be a problem, as usually you want it warmer than adjoining spaces.

      1. Jon_R | | #15

        Agreed, "warmer bathroom" is a component of what I consider a comfortable house.

  8. gusfhb | | #10

    The bathroom will have a fan no?

    That may figure into your calculations

    I don't think I want big vents into the next room from a bathroom for the noise alone

  9. Remodelpro | | #20


    You were correct that this was a challenging design.
    After a great deal of trial and error, here is what worked:
    One 4" x 8" supply grill set high in the wall fed by a 4" ECM blower delivering a measured 80CFM.
    The fan is ducted so that it pucks up conditioned air directly from the outlet of the mini split and delivers it to the bathroom.
    A 10" x 6" transfer grill set low in the wall allows airflow back to the mini split and prevents the bathroom from becoming pressurized.
    With this setup, the bathroom stays within 1 degree of the adjoining room that is cooled by the ductless mini split.
    Thanks to everyone who replied to my question and provided suggestions

    1. Jon_R | | #21

      > transfer grill ... prevents the bathroom from becoming pressurized.

      Transfer grills reduce pressurization but never prevent it. 60 in² (51 free) for 80 CFM is about 2.2 pascals, which is OK but not great.

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