Two years ago I converted the attic of my 1.5 story house into a livable space. It had no HVAC ducts going up, so I ran some coming from the basement where the furnace is.

Air flow to the attic via the ducts is really low. I would say it's at least 10 times weaker than the first floor air flow. This isn't so bad in the winter because there's computers and a projector that emit heat (as well as radiant heat coming from the first floor).

I had a new central air system installed a few months ago, and since the flow is so bad upstairs, I'm forced to put the old window unit back in, which really bums me out.

I installed two duct fans in the attic to try and improve the air flow, but it only marginally improved things. Not enough to make any difference.

Is there anything else I can do?

  • How big are the ducts that you ran from the basement, how are they connected, and did you also run a return? Do any of your ducts include baffles to adjust the flow to different parts of the house?
    – BMitch
    Jun 10, 2012 at 18:15
  • 1
    You need an HVAC person out there to a) make sure your unit is sized properly and b) the duct work calculations are correct.
    – DA01
    Jun 10, 2012 at 19:21
  • I actually had an hvac guy install the ductwork to the attic. There's three lines going up, one for the reutnr and two for the exhaust. The return is 3" all the way up, and the exhaust looks like 4"x2" so they could fit in the closet going up to the attic. I also had a company install the central air compressor, and they said it was the right size for the house but again, even the heat is weak. I do have some baffles, but they didn't seem to do too much. I'll try playing with them some more.
    – zombor
    Jun 10, 2012 at 20:28

3 Answers 3


First, realize that with a single zone system and multiple floors, it's difficult to get the temperature correct everywhere. Also, since you mentioned this was an attic space, you're likely against the roof and getting heat from every wall plus from the computers and people in the space. In short, you're fighting a losing battle.

I've got a similar challenge, though not to your extreme, and here's what I have done. First, learn how to adjust the baffles on the ducts. In the summer, you want as much as possible going to the high floors, and the reverse in the winter. Get someone to feel the air coming out while you're pushing the baffle all the way to one side or another. For me, it made little difference until I pushed it all the way.

Next, exclude any rooms you don't need to heat/cool, e.g. a guest room or storage space in the basement. Shut the door and close the vents to reduce the effort you're putting on the HVAC.

Finally, I swapped out my return grille with models that accept a filter (they are hinged and open after releasing two small clips). These return grilles are special orders from the big box stores, but they carry them. I still leave a thin blue "rock catcher" filter directly on my HVAC blower itself. And I put a filter in every return except the upstairs in the summer and downstairs in the winter. The idea being to suck out as much air as possible from location that needs it.

This means that switching between heat and cool is a process, flipping the baffles and swapping in and out filters. But a little effort is better than a lot of sweating.

One last suggestion, since it sounds like you're running a small server farm in your attic. Either move that down to the basement, or consider replacing the window unit with a ductless mini-split system. You get to see out the window, and these systems have configurable thermostats.

  • All of my vents can close, so I closed them all except the attic ones as a little experiment. I'll let it go for a while and see how the whole house feels. I am against the roof, but when I did the construction, I insulated it as much as I could (R-38 against the roof and side walls). Of course, I realize heat will still get in. Not quite a server farm, just a htpc along with the other media stuff. I keep it on 24/7, but the projector spits out at least twice the heat, which isn't on all the time.
    – zombor
    Jun 10, 2012 at 21:43
  • It's also even worse than you think, because none of the first floor walls are insulated at all. It's on my list of things to get done :(
    – zombor
    Jun 10, 2012 at 21:47

Even a correctly done duct size calculation will not be completely accurate but in this case, it's very clear the attic duct sizes are wrong and are much too small.

The ideal solution would be to make the attic ducts much bigger and put dampers on every duct so that you can rebalance your airflow when you switch from cooling to heating and back.

Having said that, running new and bigger ducts might be a major project. Another option is to put booster fans near the source rather than in the attic.

In a restricted flow condition, you can get more airflow by pushing than by pulling. Consider a vacuum cleaner. If you plug its intake, it doesn't matter how many horsepower it has. The maximum pressure you can have by pulling is 15 PSI. But there is no practical limit to the pressure you get by pushing. 30, 45, 60 PSI are all possible with enough horsepower.

Here's a quick intro to High velocity air distribution systems:


Maybe something like this 530 CFM fan:



I didn't see the reply posts, but to improve air flow and control airflow from room to room for a balanced system install manual dampers on the supply runs throughout your home. You can then close dampers and adjust airflow room to room, which will push more to other areas. It will fix rooms that are too cold or too hot. You could also close off registers, and do the same thing if the ductwork is sized correctly and that's a big if. Most ductwork in homes isn't sized correctly and too much pressure is created through undersized ducts, and your airflow can get loud as you close off grills. Try that or install manual dampers, which is better to do anyway because you can control the amount of air to your rooms and push more air upstairs. I see this problem all the time, even in new houses today which is why we always install dampers on supply runs just so we can control airflow, and hopefully you ran a big enough supply duct to your upstairs as well. I'll guess that it should be a 10-12" duct based on 2-3 rooms upstairs.

  • 1
    Please see the faq for the rules on self-promotion on this site.
    – Niall C.
    Oct 7, 2012 at 5:29

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.