In my 100+ year old 2-story house, it's quite warm upstairs. One bedroom has a duct blowing in up from the air handler in the basement, and a return on the opposite side of the room. The second bedroom has a duct blowing in, but no return. The upstairs bathroom across the hall has a return, but no duct blowing in. There is a third room some 15 ft. down the hall from the bedrooms, 10 ft. down the hall from the bathroom, which is open on one side as a loft above a downstairs room.

Neither bedroom upstairs has sufficient airflow. We've tried closing all the vents downstairs, but this didn't help much. I don't believe it's reasonably possible to run additional duct work. The second bedroom I'm assuming is a lost cause. Fortunately, it's the guest room and isn't regularly occupied. The first bedroom however, we need to fix.

As sort of a bandaid on the situation, I'm considering building some small wooden boxes with fans which will slide into the duct work, replacing the existing vents. I plan to suck more air upstairs this way. If this isn't sufficient, I plan to build some into the return vents as well, blowing back down into the duct.

Other than that, "this may not work", do you see any issues with trying? I'm also open to other crafty ideas that don't involve ripping out walls at this time. :-) Thanks.

  • 3
    They make specific fans for ducts. Closing vents or registers does little, but sometimes there is a balancing control to close vents where the lateral splits from the main duct if you have access to those place (i.e. unfinished basement). The real problem is likely heat gain from lack of adequate insulation, to counter that you might be able to install one or more mini-split type air conditions to supplement.
    – Tyson
    Commented Jul 6, 2018 at 15:25
  • 1
    Check for balancing dampers first. Possible to get get more air out of supply vent with a fan pulling air out, but don't try to force air into the return with a fan. Isn't necessary, won't work, and not a good idea to pressurize the return. Commented Jul 6, 2018 at 16:23
  • Try posting the model and serial numbers of the furnace and the A/C unit and also the dimension of the supply and return ducts. Pictures of the duct work if you can would be helpful.
    – d.george
    Commented Jul 6, 2018 at 17:44

4 Answers 4


The fan idea would increase airflow, but the fans would have to be small to fit inside the duct. Small fans trying to move a lot of air means lots of noise, so be prepared to deal with that. There are commercial products available that do what you're describing. I tried one (http://a.co/8beWhs0) and it helped, but it wasn't worth the noise to me. It was especially annoying when the temperature was very close to the set point of the fan's thermostat because the fan would turn on and off every few seconds.

I suggest that you replace your HVAC system's thermostat with one that lets you schedule the fan to run periodically throughout the day. (The Nest thermostat has this feature.) It helps to even out the temperature around the house even when AC and heat aren't being used.

An attic fan and/or additional attic insulation could also help.

  • You want well built fans too. Bearings rather than bushings. You'll likely have to take them out to clean every few years. I put in a large attic fan myself. It helped a lot. Commented Jul 6, 2018 at 17:44
  • @WayfaringStranger An attic fan is a good idea. I'll add that to my answer.
    – mrog
    Commented Jul 6, 2018 at 18:23

Just remember you were the one who brought up "band-aid". To me crawling into unconditioned space and installing fan boxes is not a Band-aid.

Basically what you are talking about is a small VAV system (variable air volume), and it does work at least commercially. Smaller systems have been shopped around for residential work for 20 years, but I don't think with much success.

Your biggest problem would be the size and location of your duct work. Point being air is volume (CFM) and it must fit in a duct CSA in sq/in x length. To do that you need pressure (Static). You believe that the fan coil unit does note have enough pressure to force the conditioned air through the duct at this time so you want to put in fans to up the pressure. Eventually air will not go through the duct fast enough (sensible volume), no matter how much pressure you try to use (thus the noise). There is much more involved but this is just a simple explanation.

So if you are trying to cool down an area with these fans, you might be able to drop the temp 1 or 2 degrees F, but probably not much more than that. If that is all you need then OK, but no guarantees it will even be that.

If you are still dead set on fans, you might check with a trusted and knowledgeable HVACR contractor and see if the unit you have comes with a higher speed motor (some do) and that would probably increase your duct work to maximum volume.

If you want to still think in terms of a band-aid and all you really only need one bedroom cooled right now. Why don't you consider a window unit for auxiliary cooling in that one room. Probably an 8000 BTU unit would cool better than what you are suggesting it's about the same price and it's a lot easier to install.

Then when you want to rip the band-aid off, you can repair it properly by installing two or even three zones instead of the one you have.

  • Thanks, yeah, a bit of back story... we have one of those air conditioners that sits in the room with the hoses out to the window that we planned on using in this room, but our wonderful movers lost it. So, we just moved in and are trying to put together some quick solution. I happen to have a whole bunch of larger PC case fans (~8" 12V), some wood, and a power supply, so I figured I'd give this a go. I appreciate the input though on the effectiveness. I think I'll just go buy a new air conditioner for the room at this point.
    – Brad
    Commented Jul 6, 2018 at 21:31

I would not waste my time installing duct fans since most will not be worth your time spent installing them. The computer fans are too small to increase the volume of air in any duct. Find out what you have installed including the size and capacity of the equipment, the capacity of the duct work, the location of the supply and return registers. As the "retired master electrician" stated you need to find a good HVAC contractor to show you what your needs are and stop with the time consuming ideas that have no real gain. my 2 cents


From your descriptions of Ducts and multiple return ducts in the same locations seems to me as an odd arrangement. Having multiple ducts to feed those rooms is fine - but your duct distribution box needs to be configured properly to do this.

Your issue seems to be two fold your ducts feeding the upper rooms may not be the proper sizing - depending on where you live and how the sun hits those rooms could be a big determinant in what you will need to do (for example oversize the ducts 6" instead of 4"). You do not state if the ducts are on the ceiling or the floor - if on the floor not so good .. better from the ceiling when cooling ..

What I would do is have one main feed from my distribution box for the upstairs & one for downstairs and then split those feeds for each floor accordingly with a secondary distribution box. This way I can control the flow to each floor much better; further, I can control the flow on each level with that secondary distribution box - so I can get proper airflow to the rooms that need it and I can control that flow via the placement and baffles in my secondary distribution boxes. No need for any fans, no need for any motorized vents to turn on or off to control that.

I would then place my return air duct in a location furthest from the supply vents - so the air from those areas is drawn through the living areas and back to the Air Handler - (Currently you feed one room and draw that back at the same time - very inefficient when trying to cool the whole upstairs that is like making a bypass).

  • Make sure your Circulation Fan has enough CFM - that it is operating correctly and at proper speed.
  • Change your Baffle distribution.
  • Size the ducts appropriately.
  • Install the Ducting into the distribution / baffles appropriately.
  • Use one well placed return air duct.

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