I have a 2 story house, 4000 sq. ft, that has two furnaces in the basement.

The first floor heats and cools okay.

The second floors, 2/3's of the upstairs nearest to the furnace gets sufficient airflow and heats and cools like you would expect a 2nd floor (less efficient than first floor due to heat loss in the ducts, but still acceptable).

There is one very isolated branch which runs to the remaining 1/3'rd of the upstairs which is the master bedroom and bath. This of course gets no airflow as it's the furthest branch from the furnace. There's almost nothing coming out of the supply lines.

I've checked all inline dampers on the 2nd floor system and played with balancing, and I think the furnace fan is just straight up too weak to run the system out to this last 1/3'rd of the house.

There's no other magic levers I can come up with too pull, and I've disassembled the HVAC duct run up until the point it goes up exterior walls. There's cold air as far as I can follow the duct, but once it has to go up two stories there just isn't enough push.

In the master bedroom, I've kind of just accepted to run a portable AC / electric heater for the seasons and just move on. But, I'm wondering if a 260 CFM booster fan could help on the main duct which supplies this last 1/3rd?

There's a really good point (I'm in Chicago, everything is piped in... electric box is real close) about 20' from the 2nd furnace into this lone run that supplies that upstairs area where I could put in a booster fan.

Has anyone ever installed one of these rectangular duct fans and thinks you could get a meaningful boost on a main branch? There's probably 3 or 4 supply runs that are another 35' (7" to 4" off this branch which run to the master bedroom and bath), and if this could help boost them maybe it'd be worth trying (but if it would require 4 boost fans on each supply [which the manual seems to allude too], I don't see the economic value versus the spot heating / cooling and getting a bigger furnace blower when the current one dies).

Anyone ever driven a few supply lines off of a blower like this with positive results?


[Overall, cheap builder house -- no optimization whatsoever HVAC wise. I'm sure the HVAC contractor was not paid to crank out 100 houses in this subdivision and calculate anything].

  • Do you have problems with both heating and cooling (that's what I thought you wrote)? Usually, with 2 story houses, the problem is with cooling because it's harder to push the cool, denser air up two stories than it is to push the warmer air up, which wants to rise on its own anyway.
    – SteveSh
    Commented Jun 13, 2021 at 21:05
  • Also, where are your vents located on the 2nd floor? Are they in the floor or the ceiling?
    – SteveSh
    Commented Jun 13, 2021 at 21:06
  • On phone: both heating and cooling, same issue, no push in last 1/3 section. Supply vents in floor of 2nd story.
    – Leroy105
    Commented Jun 14, 2021 at 2:10
  • Do you have ample return vents in the affected areas? If there is a lot of positive pressure in the last 1/3 then air will simply avoid it and travel to where there is less resistance. Have you tried leaving the doors open? Did your house receive an addition at some point and the furnace was not upgraded to handle the additional CFM?
    – MonkeyZeus
    Commented Jun 14, 2021 at 14:19
  • I am not a fan of duct boosters. you may have better luck purchasing a 20$ anemometer and adjusting the flows. Air handlers have a specific output throwing another motor in the mix usually is not the best choice if the ductwork is the limiting factor the booster will cause problems in other areas. Adding return vents or other ways for air to return is usually a better choice and then adjusting the registers so the rooms have the appropriate flow for each room.
    – Ed Beal
    Commented Jun 14, 2021 at 15:38

1 Answer 1


I would increase the fan speed and reduce the air delivery from the registers that get sufficient air flow. I have used fans similar to the one you are thinking of installing. They work fairly well but are not very quiet. The ones I used were made by Tjernlund but look to be of similar construction. Another thing you could do is have an HVAC company check the duct work to see if it has been properly sized and make sure that the furnace fan is running in the right direction. Once in a while they are installed backward. They will blow air but at a reduced velocity and strength.

@leroy105; I doubt that the fan is only a single speed. Single speed motors were installed in furnaces way back when I got into HVAC in the late 1960's in both belt drive and direct drive blowers but were switched to multi-speed, mostly direct drives to add versatility. If your furnace has a belt drive blower motor, which it probably does not there are still options. You probably have a multi-speed direct drive motor where you change the blower speed by rewiring the motor using a different color wire to determine the speed. My advice is the same as some of the other guys in that it is time to call in a professional since correcting your problem is "above your pay grade". They could also check the electronic air filters to make sure that they are installed correctly. Also, these electronic air filters need a regular cleaning schedule depending upon the size of the house and the number of people and pets in that house. I clean mine in the dish washer every 3 months. So, before you add booster fans or do any other dumb things call in a professional for help. My 2 cents.

  • I took a quick look at than and furnace control board instruction manual, and I think the fan is just single speed from the furnace control board, there's no board logic to change speed (cause that what my first thought is to try to boost the fan as well). I think by defintion of no air flowing to other 1/3'rd we can all agree the duct work isn't sized right. ;) -- am I going to rip apart two stories of drywall to fix it, definently not.... I'm considering booster fans really as the only modification I'd consider since I can get at the duct work in the basement easy enough.
    – Leroy105
    Commented Jun 14, 2021 at 18:34
  • You know what is really interesting...? There are electric air filters in the basement, and I had one open on the 2nd story furnace (this weekend) and they put in the electric air filter in the wrong orientation on the return. Both furnaces share a huge return duct. Now you have my wondering as well if the furnace fan is backwards (I think the company that put in the electric filter just didn't consider rotating it to match the filter inflow correctly). The 2nd floor feels "normalish" in the closer runs, so I feel like the furnace fan is the right orientation, but I am going to check!!!
    – Leroy105
    Commented Jun 14, 2021 at 18:43
  • Leroy maybe you should get a pro to teach you how your system is supposed to work. The electronic filters should be on the return all the debris they take out of the airstream won’t end up in your heat exchanger. Your air handler can only pump so much air and that amount of air is sized to the system, dampers are used to adjust for seasonal differences we pump more cold up in the summer and less heat in the winter. Go ahead with your plans as it sounds like you really don’t want our assistance.
    – Ed Beal
    Commented Jun 14, 2021 at 19:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.