When sizing your furnace, you need to count all the square footage of rooms you want to heat. If you want to heat your basement, count it when sizing your furnace.
Regardless of how frequently you use the basement, it is necessary to include it in the total square footage of your property. Depending on the size of your basement, it is preferable to install a forced-air furnace in the attic rather than the basement.
A basic rule of thumb for furnace size calculation is that it takes 30 BTUs for every square foot of house. So, if you have a 1,000 square foot house, you need a furnace that has a 30,000 BTU output.
As a general rule of thumb, listing agents and appraisers don't count a finished basement toward the overall square footage, especially if the basement is completely below grade—a term that means below ground level. Whether an appraiser includes basement living space ultimately depends on which state you live in.
Determining the Needed HVAC System Size
A professional HVAC technician can help you evaluate your equipment and the extra demands it will face, based on your basement's proposed layout, duct dimensions, and insulation, as well as your area's climate.
One of the most used methods is the HVAC duct sizing rule of thumb. In fact, even professional AC contractors might use this method to size the AC ducts in your home. According to this rule, you need to install an air conditioner of one-tonne capacity for every 500 square feet of the floor area of your residence.
If you have an unfinished basement, including it depends on whether you plan to finish it. If you've already got a finished basement, you should include that square footage in your size estimate.
Usually, at least from what I have seen, they include the basement finished square footage in their finished square footage estimates.
A basement or other level below grade designed for human occupancy (i.e., equipped with lighting, ventilation, and means of egress) counts as a story. Basements or other levels not containing any space designed for human occupancy are not considered stories under the Standards.
Even a slightly undersized furnace will accomplish the job the majority of the time. Taking things down a few sizes will more than likely solve some of your common heating problems like too high of a utility bill while half of your house is blazing and the other half is an icebox.
How many square feet will an 80000 BTU furnace heat? A furnace with 80,000 BTU capacity can effectively warm up a typical home measuring 2,000 square feet.
In many new builds, the garage is designed with a special “alcove” that holds the furnace without sacrificing space for parking vehicles. Utility Rooms: Some homes—especially those without basements or garages—have a specialized room in the center of the house that holds the furnace and water heater.
Unlike a partial basement, a full basement foundation is always the same size as the main floor of the house and shares that footprint.
It can only "count" in the actual liveable square footage if: 1) it is finished in a like-manner to the rest of the house, 2) it is conditioned (heated) and, 3) it has an egress (a large window or another door where you can exit). Otherwise, it's just "extra" but doesn't "officially" count.
In many places, whether a basement has an ingress or egress — or a way in or out of the home — determines if you can count the basement as square footage. If the space lacks legal ingress or egress, it won't be counted as part of the home's square footage.
Finished basements mirror the aesthetics of the rest of the house and have fully finished walls complete with drywall, insulation, and paint or wallpaper. That means no exposed studs, pipes, or electrical systems, which are visible in most unfinished basements.
Your Home's Square Footage — This includes all the rooms that require heating, including finished basements and attics. Rooms vary in size and shape, of course, all of which will be taken into account.
Heating Guide
For this example, using an 80% efficient furnace, the 1900 square foot home above would require a 90,000 Btu input furnace that produces 72,000 Btu's of heating, which is close enough to the 76,000 Btu's required using the climate heating factor.
My colleague John Puryear teaches the simple “2 Foot Rule” for takeoff placement in his classes. When there is a change in air direction in a duct, it takes roughly 24 inches for airflow to restore its pattern. Because of this, John recommends keeping takeoffs 24 inches away from any turns, transitions, or the end cap.
A good rule of thumb to follow is that you'll need approximately 20 BTU for every square foot of floor area. However, there are also other considerations to take into account before calculating the capacity, including the height of your ceiling as well as how many windows and doors are in a specific room.
Just as oversized ducting can wreak havoc on your heating and cooling systems, so can undersized ductwork. When the ducting is too small for the specific HVAC system that you have, air flowing through it will start to back up. The air pressure within the ductwork will become higher.