I am finishing my basement and need some insight from someone who understand HVAC design. I would like to know if my main trunk line (the 34' one in the diagram) can handle another four 6" ducts off of it that would heat the basement? HVAC Diagram

I do plan to add air returns. I just need to identify the venting strategy first.

  • How much static pressure can your HVAC blower push against? – ThreePhaseEel Dec 27 '17 at 4:41
  • How would I find out how much static pressure the blower can push against? A couple of google searches indicates this may take a manometer. Furnace is a Carrier 59SC2A utcccs-cdn.com/hvac/docs/1009/Public/02/59SC2A-02PD.pdf – Steve Oaks Dec 27 '17 at 11:25
  • Should be in the furnace specs, although 1/2" WC is typical if it's not. – ThreePhaseEel Dec 27 '17 at 12:32
  • 0.15 Heating external static pressure (in. W.C.) . – Steve Oaks Dec 27 '17 at 16:04
  • Why were the resources removed from this question? In its current state it's not clear or helpful to the community. Please delete if you don't intend to leave it up as an archive. – isherwood Jan 5 '18 at 15:13

I admire the drawing of your existing duct-work. Using a .1" external static resistance for the duct-work and guessing that you have a 3 ton A/C system that requires 1200 CFM of air flow, I would estimate that the 8"X20" duct is almost fully utilized as is the 8"X8" duct. The 8"X14" duct could handle all the airflow required for the basement heating. You did say that you wanted to utilize the 8"X20" duct for the basement heating. You can do it but it may unbalance the existing airflow. You may have to increase the blower speed to achieve the required air flow in that duct and add dampers to the other 2 ducts to help control the increased air flow. Adding 4 heating registers would require approximately 320 CFM more air flow you may or may not have available. I would add the ducts you require and balance the resulting system as best I could and live with the results. If the blower that is installed in your present heating system is sized for more air flow and a higher static head than I allowed for say you could easily add the 4 registers you will need. If on the other hand your blower is maxed out the system will still heat but the 2nd. floor heating may suffer. This is the best I can do since I am not at you residence and I am only guessing as to the existing set-up. good luck with your project. I think that you will be OK. I compared your home to mine 2000 sq. ft rqnch and finished basement. Mine heats great.

  • I forgot an item, what is the size of your A/C in btu's or tons and what are the last digits of the furnace's model number which tells us the furnace input? If you have more than 3 tons of A/C adding ducts to the 8X20 duct may not be feasible. – d.george Dec 27 '17 at 23:21
  • Full furnace model #: 59SC2B080S1712-16 – Steve Oaks Dec 28 '17 at 12:12
  • The AC unit is 3-tons. – Steve Oaks Dec 28 '17 at 12:23
  • You should be OK on adding 4 ducts to the 8X20 duct and if you have A/C they would probably be closed or mostly closed when the A/C is running. – d.george Dec 28 '17 at 12:23

You can certainly add as many ducts as you like, the more the merrier in fact as far as your equipment is concerned. The only problem you may run into is balancing the entire system. Assuming the ducts were sized correctly in the first place you will be taking air from the other ducts on that trunk line. The furnace is still putting out the same amount of air for the most part only the distribution will change. The only way to assure proper distribution is to first do a Manual J load calculation and then a Manual D duct sizing calculation (or equivalent) Anything else is guessing. Short of that you could install dampers in all the branch lines and close them as necessary to try and balance the system. Total air flow should then be measured to make sure you haven’t closed too many dampers.

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