Pro Tip: Round up – If you’re more than halfway to the next larger size, round up to it. You would need a blower that can move 1200 CFM. ![]() Divided by 12,000 is 3 – you need a 3-ton system. Let’s say you need 36,000 BTUs of air conditioning. ![]() That means that for each “ton” of AC, it can move 12K BTUs per hour out of (AC) or into (heating) your space. Key calculation & example: Divide the total number of BTUs needed by 12,000, since 1 ton of air conditioning is equal to 12,000 BTUs. Humid summer air is heavier than dry winter air, so the blower needs more power to push and pull it around your ductwork.Īs a result, it is better to use the 400 CFM number to be sure the blower you select will do the job all year. The general guideline is that you need 300 to 350 CFM per ton (12,000 BTUs) for proper heating and closer to 400 CFM per ton of AC. You might want to open the AC BTU Calculator in a separate window for convenience, so you can easily return to this Ductwork Sizing page. It uses the most critical information about your home or building to determine the load requirement – how big the HVAC must be to do the job. It uses a long list of inputs such as square footage, layout, insulation, window quality, climate and much more to determine proper HVAC system size for the space.Ī quick way to get a close estimate is to use a Central AC BTU Calculator like this one. The best method for determining system size is to use a Manual J load calculation for the entire home or building. The total number is determined by the unit size of the AC, heat pump or air conditioner in tons/BTUs. How many cubic feet per minute, CFM, does your system’s blower need to move to provide sufficient air flow and circulation in your home and to each room? This will result in significant temperature imbalances that nobody will like. Ductwork that is too large won’t have enough pressure, and the velocity of air won’t carry it to the furthest reaches of the home or building. This is vitally important, something we’ll belabor a bit, because ducts that are too small or narrow will wreck efficiency and make the system work too hard, leading to early mechanical failure. Getting these data right means you’ll get the ductwork size right. Note: Maximum Friction Rate: 0.1 inWg/100ft Ductwork Sizing Calculator – Step by StepĬFM, Maximum Friction Rate and Maximum Velocity – what are these? How are they determined? You have two or three options for rectangular ducts that give you the same or nearly the same square footage per duct. ![]() One size is given for round ducts, its diameter. If you decide to get professional guidance and ductwork estimates, use the Free Local Quotes option or the toll-free number on this page for convenient, no-obligation advice and cost estimates.ĭo your own calculations and compare them to our chart – or use the chart for quick, accurate calculations for duct size based on CFM needed for a room. Planning to call a pro? The information here will make you a savvy homeowner that can discuss duct sizing with an HVAC professional, and you’ll have a good sense of whether their calculations are accurate. As explained below, you must know the CFM needed for your entire home and for each room you are sizing ductwork to serve.īelow the calculator, each step of the process is explained in detail, so you can be sure that you’ve got the right duct sizing information for a home that is properly heated and air conditioned Here is the calculator for homeowners with significant HVAC experience and for HVAC pros who understand what these boxes are asking for.
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