I recently finished my basement and I have about 1000-1100 ft2 that I would like add HVAC to. My question is:

Would I run into any major issues with adding two registers (and an equal sized return) for the basement off of my 3 ton AC/ 55,000 BTU/hr furnace, that currently services the 1st floor at around 1300 ft2 ?

It has two way trunk lines and I'd add a branch to each trunk for each register. The branches would be very close (if not 1st branches) off of the trunk lines and run for maybe 4 or 5 ft, max. I also keep two registers closed on the 1st floor. Also, tips on branch size would be helpful as well. I was thinking 4 or 6 in?

The temperature in the basement is very stable, just stays a bit too hot in the summer (now) at 73 - 74 F. Current AC/heat runs fine on the 1st and 2nd floors. I was hoping to steal a bit from the first floor to knock the temp down a degree. Perhaps I could install the registers and if it overworks my current set up, disconnect them and seal the ducts and go minisplit?

  • What is the existing duct made of?
    – JACK
    Aug 16, 2023 at 20:23
  • @JACK it is metal ducting
    – mrWizard
    Aug 16, 2023 at 20:36

2 Answers 2


This is one of the very rare instances where I'd say to try it and see what happens. It would require little work, just the ducts which should remain metal to reduce resistance to flow and registers that can be opened and closed. As you've stated, the basement remains fairly cool as has been my experience and you have closed two registers already. If it works, fine. If not, no harm done. It might be good enough to last until you have to replace your AC unit and then get a 4 ton. I wouldn't just jump in and spend a few grand or a lot more until I was sure it was needed.

  • The ducts can always be blocked off after if it does not work well, without hurting the system.
    – crip659
    Aug 16, 2023 at 21:49

For new space now totaling 2400 sqft

Based simply on the sqft you would need new 4 Ton AC assuming you have the required insulation.

Better look it up in the tables for your location and insulation.

Rather than overloading the current AC and no-one gets enough, consider a mini split add on to the basement.

A Mini Split needs no ducts, and can cool and heat.

It is also more budget friendly then new 4 Ton HVAC.

It is a DIY instalation


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