forced air question

robert griggs

I am bidding on a job where they are fininshing the basement. There is forced air heat and central AC in the house. The additional load for the basement is only about 35,000 BTUs. I was wondering if I can increase the size of the furnace and AC, yet use the same duct work, and tap in the feeds and returns for the basement. I was not sure if this would mess up the balance of the system that is existing. Installing new trunk lines is not going to be too easy the way the architect has designed the basement.

The other option, since most of the existing trunk duct is being moved over, is to alter the size of this duct work.

Any thoughts would be appreciated.

Thanks,

R. Griggs

Mike T., Swampeast MO

Around here at least

people have just tapped into the existing main supply plenum--no returns--when finishing basements. Have seen this done innumerable times and people don't seem to complain. I know it sounds crappy, but it seems to work OK. Sizing seems by the "this should work" method.

Don_2

force air

Robert, I can't see it from here but i would say,that there
not a chance that your existing ductwork can support additional 1200 cfm.... And if you try then you can look forward to equipment failure and as we call it jet noise...
If you have no chance to installed a bigger distribution system to support the new load then you need to look for a better alternative.And this might even be a blessing,when you think of your comfort level with one system on two story envelope. If your basement is going to be wide open with no partition may I suggest something like a ductless split system for the basement area. You will be better off if you zone the basement from the main house anyway...

Bill Riordan_3

You must have a huge basement to have a heat loss so high. Typically, basements have very low heat loss and gain.My experience is to leave the ductwork alone. Adding vents to basement area will result in subcooled enviroment when the A/C is running. Cool dense air falls to the basement naturally and vent in the ceiling will only make it more uncomfortable. Use heating only, preferably radaint floor heating since your feet are on the cold slab no matter how much hot air is blown down from the ceiling.Second choice is baseboard or electric baseboard.

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Unknown

I would not suggest doing that

as you increase the BTU input and change the cooling load the static duct pressure is going to be affected. You may find that when recalculating the load and resizing the ducts that an increased return duct size may be required. Using the existing duct system may hamper efficency and comfort. It is also perhaps required in your area that when remodeling that the duct work be brought up to code. I would also be concerned about air for combustion, pressurization of the appliance areas and other factors that come into play when you increases the size of equipment. It might be a good idea to have some heating and cooling engineers come in and resize it for you. The architect should actually know that.