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Musings of an Energy Nerd

Using a Bath Fan to Equalize Room Temperatures

A little math clarifies why this approach usually doesn’t work

If you want to use a bath fan to reduce room-to-room temperature imbalances, your house needs a high-performance envelope, and you'll need a powerful fan. Carl Seville, a consultant in Decatur, Ga., uses a 190-cfm exhaust fan to send conditioned air from a room with a ductless minisplit to an adjacent room with no minisplit. This approach only works in a home with a low rate of air leakage, above average insulation details, and high-performance windows.
Image Credit: Carl Seville

On Green Building Advisor, readers regularly ask questions about room-to-room temperature imbalances — the type of imbalances that may occur when a home has a point-source heater like a ductless minisplit or a wood stove. Here’s a typical question: “I’d like to install a bathroom fan to move air from a warm room to a cool room. Will this approach be enough to equalize the temperatures between the two rooms?”

Scott Gibson did a good job summarizing one such discussion in his Q&A Spotlight article, “Can Bathroom Fans Be Used to Distribute Heat?” But there’s more to say on the issue. In this article, I hope to thoroughly address this topic, once and for all.

We can approach this question by breaking it down into parts:

  • First, some math. We need to know how much heat is lost through the exterior walls and the windows in the room that’s cold. We also need to know how much heat flows through the partition walls (and in some cases, the floor assembly or ceiling assembly) separating the cold room from adjacent warmer rooms.
  • Next, we need to use math to figure out how much heat a bathroom fan can move.
  • Then we can apply what we’ve learned about space heating in winter to the question of summertime cooling.
  • Finally, we can look at different options for solving temperature imbalances between rooms, to determine which approaches make the most sense.

Wall and window specifications matter

How much heat is lost through the exterior walls and windows of any particular room on a cold night? To answer the question for a specific room, we need to know the area of the exterior walls, the area of the windows, and the relevant R-values (or U-factors).

The exterior wall area…

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  1. _Stephen_ | | #1

    So, why does leaving the door open work?
    I really wouldn't expect leaving the door open to have higher air / heat movement than a dedicated fan, but this does seem to make a significant difference. Why?

    Nevermind. I see discussion above, but it seems crazy to me that an open door pushes 300 CFM...

  2. Jon_R | | #2

    open doors and infiltration
    Nice article.

    Here is a source for open door flow calculation. I get 322 CFM at 5F.

    A 20" box fan assisting air movement through an open doorway can efficiently move 1000+ CFM.

    If a ducted fan blows into a closed door room and slightly pressurizes it (say 3 pascals), in most conditions, there is no heat loss due to outdoor air in-filtration. I assume the values used reflect fairly windy design conditions.

    Be careful with "supplemental" heat - if the bedroom is kept 1 degree warmer than the rest of the house, it will always supply more than 100% of the bedroom's heat.

    Using Nick Welch's nice program and simply changing to 0F outside, it yields 62F, 66F and 69F at 0, 100 and 300 CFM.

  3. GBA Editor
    Martin Holladay | | #3

    Response to Stephen G
    Q. "Why does leaving the door open work?"

    A. On the molecular level, the molecules in the air -- mostly nitrogen, but also oxygen and other molecules -- are always bouncing around, as long as they are warmer than absolute zero. The molecules near the open doorway are capable of bouncing through the open doorway. This is the mechanism that allows the molecules rising from a smelly pair of tennis shoes to reach your nose 10 feet away.

    Air also moves by convection. Air tends to stratify in a still room, with warmer air near the ceiling and cooler air near the floor. (Small air leaks in the ceiling and floor in winter accentuate this stratification.) If the hallway is 4 F° warmer than the bedroom, then strata at different temperatures are encountering each other at the open doorway. At any given stratum, the warmer hallway air will tend to move upward diagonally through the doorway to the cooler bedroom. As air moves through the doorway into the bedroom, the cool air on the bedroom floor moves horizontally through the open doorway into the hallway, setting up a convection current.

  4. GBA Editor
    Martin Holladay | | #4

    Response to Jon R
    Jon R,
    You wrote, "If a ducted fan blows into a closed door room and slightly pressurizes it (say 3 pascals), in most conditions, there is no heat loss due to outdoor air infiltration."

    But you can't have infiltration without the same amount of exfiltration. Pressurizing a room increases exfiltration; the makeup air would be the air pushed into the room by the pressurizing fan, of course. Is that good or bad? You decide.

    Clearly, if the pressurizing fan is pushing indoor air from the hallway or living room into a bedroom, then outdoor air is entering cracks in the thermal envelope -- perhaps cracks in the hallway or living room -- to replace the exfiltrating air leaving the bedroom due to pressurization.

  5. Reid Baldwin | | #5

    Another strategy (maybe)
    I was thinking about the issue of heating bedrooms with closed doors and it occurred to me that a Minotair, CERV, or other heat-pump based ventilator would have sufficient capacity to handle these rooms in many houses. Of course, there is more to the issue than having enough capacity. I haven't figured out exactly how it would work, but I will share my half-baked ideas here in case others are inclined to help finish baking them.

    Obviously, the fresh air supplies would need to be in these bedrooms. The Minotair controls sense temperature of the stale air (I don't know about the CERV). If the stale air returns are in bathrooms, the unit wouldn't call for heat until the bathrooms got cold and then wouldn't stop producing heat when the bedrooms get to the setpoint. To make sure the unit provides heating and cooling at the right times, you either need to put the stale air returns in the bedrooms or modify the unit to use a remote temperature sensor. If the returns and supplies are both in the bedrooms, does the rest of the house get ventilated properly? Is it practical to modify the unit to use a remote temperature sensor?

  6. GBA Editor
    Martin Holladay | | #6

    Response to Reid Baldwin
    If the living room is heated by a ductless minisplit, you are basically suggesting that the homeowner could purchase and install an additional heat pump -- in this case, a heat pump manufactured by CERV or Minotair -- to heat the cooler bedroom or bedrooms.

    That approach would work, of course, with the right ducts, controls, and thermostat settings. But your suggestion still means that the house needs a second heat pump to handle the heating load of the bedrooms. I fail to see why this suggestion is better than simply installing a ducted minisplit to handle both the living room and the bedrooms.

  7. Jon_R | | #7

    "If a ducted fan blows into a
    "If a ducted fan blows into a closed door room and slightly pressurizes it (say 3 pascals), in most conditions, there is no heat loss due to outdoor air infiltration."

    Martin is correct that this would be more precisely worded as " closed door room heat loss...". There is makeup infiltration elsewhere, but it isn't relevant to the subject of this article.

    A non-thermostatically-zoned ducted system isn't going to regulate well as loads change non-proportionally (eg, solar gain in one room and not the other).

  8. Reid Baldwin | | #8

    Response to Martin
    I labelled this a half-baked idea because I hadn't yet convinced myself it is a good idea nor convinced myself it is a bad idea. I agree with you that when the bedrooms are close to the family room, a single ducted mini-split would be preferable. For the original article, close proximity is a fair assumption.

    What about a situation where the bedrooms are on a second floor? In that case, the ducted mini-split for the bedrooms would also be an incremental unit and probably more costly than the incremental cost of a heat-pump ventilator over a conventional ERV. (I am presuming that a fully ducted, balanced ventilation system is already in the budget.) In that case, would having most of the fresh air supplies in the bedrooms make the ventilation system bad at its primary job?

  9. Jon_R | | #9

    The advantage of a HRV
    The advantage of a HRV connected heat pump is avoiding additional ducts. But I doubt they are designed to be able to independently control the amount of fresh air and heat going to a specific room. For example, someone closing the register to create a cool bedroom would get no ventilation air.

  10. GBA Editor
    Martin Holladay | | #10

    Response to Reid Baldwin
    I'm siding with Jon on this one. The Minotair and CERV controls basically aim to achieve good ventilation performance -- the heat recovery function (via the air source heat pump) is a benefit, but these appliances aren't controlled by a thermostat, and they aren't intended to provide space heating as a primary function.

  11. AndyKosick | | #11

    Magic Box
    First, Bravo for answering a controversial question with a lesson in room by room load calculation.

    Second, while recognizing that the Minotair and CERV are designed to provide ventilation, I've spent a lot of time thinking about these things lately. When I consider the fact that my local Habitat affiliate is building a house that only requires about 400cfm or less to heat and cool, I hope somebody somewhere is trying to design a single box with a single set of ducts that can heat, cool, and ventilate a high performance home. It would seem a shame if someone is not, because it's entirely possible, and, I think, essential to scaling affordable high performing housing.

  12. Jon_R | | #12

    I agree - it is possible to
    I agree - it is possible to build something that would produce a tailored mix of heat/cool, ventilation and humidity for each room and deliver it via a dedicated duct. It would provide more comfort than anything being discussed.

    A Chiltrix heat pump with a fan coil in each room + HRV ducts is the closest thing I can think of. And it isn't crazy expensive.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Andy Kosick (Comment #11)
    It's possible to design a "magic box" that provides space heating, cooling, ventilation, and even domestic hot water. Several companies have worked on developing such a device, and a few have even hit the market, in Canada, Europe, and the U.S., over the past 20 years.

    It's a little like selling a TV with a built-in video tape player, DVD player, and intercom. You can do it. But when one of the items in the device breaks, you either have to take it in to the repair shop, or throw the whole thing away.

    Sometimes separate devices make more sense.

  14. user-6504396 | | #14

    Great Information Here
    It seems to me that this may also speak to the importance of getting the ducting right and using a room by room heat loss analysis for ducted heat pump systems. We are looking at using ducted Mitsubishi setup in our house using a multi-position and horizontal ducted indoor units. I don't have a lot of experience with what came before, but it appears that the temperature and the volume of the heated air is much less with these systems than say with a ducted gas furnace. It seems like that then may require more precision with getting the right air flow to each room when using a ducted multi-split system with a single thermostat for multiple rooms.

  15. GBA Editor
    Martin Holladay | | #15

    Response to Kevin Camfield
    If you are designing a heating system -- regardless of the type of heating system -- the first step is to perform a room-by-room heat loss calculation.

    Whether the rooms have ducts connected to a furnace, or ducts connected to a ductless minisplit, or baseboard radiation connect by tubing to a boiler, you can't design the system without an accurate understanding of each room's design heat load.

    If you do a bad job with the room-by-room heat load calculations, occupants can be uncomfortable -- whether the heat comes from a furnace, boiler, or heat pump.

  16. user-6504396 | | #16

    Response to Martin

    Thanks. I'm working through that now with a local energy consultant. I haven't found an HVAC company that willing to go to that detail. I agree with you that without the analysis one is just guessing.

  17. GBA Editor
    Martin Holladay | | #17

    Response to Kevin Camfield
    Your report -- "I haven't found an HVAC company that willing to go to that detail" -- is common but infuriating. Building codes require that a room-by-room Manual J calculation be performed before installing a heating system, but the provision is widely ignored and rarely enforced.

    For more information, see Saving Energy With Manual J and Manual D.

  18. Harr1z | | #18

    Not second guessing the science, but just crazy to think that a fan wont help transfer heat/cool to an adjacent room, yet all the articles here and elsewhere ensuring we plug up the holes around light fixtures in the ceiling or wires running through studs. I know, different math entirely, just mind blowing. No pun intended.

    1. Expert Member
      MALCOLM TAYLOR | | #19


      The advice to seal holes in ceilings and walls is to stop the movement of moist warm air into exterior assemblies where it may condense when hits a cold surface. Moving air between rooms is an attempt to try and change the temperature of the whole space. The two are completely unrelated.

  19. Harr1z | | #20

    Completely? lol. The big difference is the purpose and the Delta T. Otherwise, it is warm air moving through a hole.

    1. Expert Member
      MALCOLM TAYLOR | | #21


  20. jmidway22 | | #22

    Great Articles as usual!

    Question: would this approach be useful for smaller rooms, like 40-70sf bathrooms?

    Our scenario: I am remodeling an old cape cod with a shed dormer in virginia and we are using Mitsubishi SLZ ceiling cassettes in the two upstairs bedrooms. Both have attached bathrooms (40 and 70sf) and walk in closets with no supplies (trunk lines or horizontal ducted pizza boxes just wouldn't fit). We are insulating the underside of the ceiling deck with closed cell so all (slz and fan/jump duct) will be within the envelope and overall the house will be fairly tight. Looking to ensure there bathrooms are comfortable and as near the bedroom temps as possible.

    One of our ideas was to use a inline fan (bath fan or ac infinity fan, etc) with a jump duct to move conditioned air from the bedroom to the bathroom pressurizing the bathroom and pushing air back into the bedroom for the SLZ to condition. Will also probably augment the temperature slightly with the temp delta but like you guys have noted, few degrees warmer air doesn't do much. I might have not caught all your calculations but I didn't see a calculation for CFM exiting under say the door to return back to the bedroom and be reconditioned by the minisplit. Seems if this was exchanging the air at the proper rate it would keep it close to target temp. Maybe I am completely off the mark

    Other ideas were to use an auto-circulating bath fan to exhaust on intervals to pull in enough air from the bedroom to condition the bathroom (not liking the idea of dumping conditioned air outside). Or to just rely on the passive heat/air exchange between the rooms when the bathroom doors are open.

    Any help or input would be greatly appreciated!

    1. GBA Editor
      Martin Holladay | | #23

      As the article makes clear, you can't use a bathroom fan to move much heat from a 72 degree room to a 68 degree (or 70 degree) room. The amount of heat isn't much.

      Just leave the bathroom door open when the bathroom isn't in use.

      1. jmidway22 | | #24

        Sorry to beat a dead horse but just trying to understand it better

        Does the formula "BTUh = cfm of the fan • delta-T • 1.08" account for the air that is being pushed out of the bathroom? That air would not be heated/cooled to overcome the delta but would be drawn back in the main room's cassette. Maybe that's accounted for in the equation somehow that I don't understand. If I put a 100cfm inline fan to a 5x9x8' bathroom (360cf) I would be approx. exchanging the air every 3.6mins or 16 times per hour.

        Also another thought was that depending on the placement of the jump duct entry in the bedroom, it could be near the cassette's supply vane direction and potentially have higher/lower heat/cool temp than the target/average temp. Might be 4 degrees warmer than target temp giving it a 8 degree delta to the bathroom.

        In my mind it seems like this approach is just trying to make the two rooms operate more like one open room by keeping the ACH high enough. Using the fan power jump duct to nullify the wall division. Or this might all be extreme overkill for a 40sf bathroom right beside a 9k BTU cassette.

        1. GBA Editor
          Martin Holladay | | #25

          The volume of air in the bathroom stays the same, so:
          cubic feet of air out = cubic feet of air in

          If we are adding 80 cfm to the room via a fan that transfers air to the bathroom, then 80 cfm of air is simultaneously leaving the bathroom. Most of that air will probably be transferred to the adjacent bedroom, but some of that air will be forced out of the bathr00m's exterior wall and ceiling through small cracks. The heat transfer equations assume that the air leaving the bathroom is at the same temperature as the average of the air temperature in the bathroom. So if the bathroom is at 68 degrees, the air leaving the bathroom is also at 68 degrees. If you are introducing 72 degree air to the bathroom from the bedroom, the only heat added to the bathroom is the heat contained in that 72 degree air.

          1. jmidway22 | | #26

            Ahh that does make sense, sorry about that. I think I was verbalizing a brain fart earlier.

            Any idea on how common it is now for homes with mini-splits/cassettes in each bedroom, to not have a supply in small attached bathrooms like these? (40 and 70sf).

          2. Expert Member
            MALCOLM TAYLOR | | #27


            Whether to have supplemental heat in small rooms not serviced by the mini-splits is a topic that comes up here quite often. My own preference would be to include an electric resistance heater as insurance. If it turned out to only be necessary for a short time each year, maybe use one that could be removed, like this:

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