Insist on professional HVAC calculations instead of guesswork. The biggest problem with sizing your furnace and air conditioner is that so few people know how to do it accurately. Even many contractors rely on a simplified rule of thumb (which is, incidentally, illegal) that translates square footage into a one-size-fits-all answer. The result is usually oversized units that cost more, use more energy and fail to maintain the temperature you desire inside your home. Professional calculation factors in many of your home's unique characteristics to obtain the correct HVAC sizing. Online calculators can often get you close. So will calculating some of these measurements yourself. It may not be as precise as an engineer's, but it will ensure you can spot any gross inaccuracies. Find your home's total square footage if you will be heating and cooling the entire area. Multiply the length by the width to obtain the figure if you don't have the exact amount from the house plans or sales contract.

Include home additions and multiple stories in the square footage. Exclude areas of your home that will not be heated nor cooled. Calculate the British thermal units required to heat and cool your house, based on square footage alone. Multiply the square feet by 25. The result is the base BTUs needed. Adjust the base BTUs if you have unusually high ceilings (more thab 8 feet). Multiply the BTUs by 25 percent. Add the result to the base BTUs. Factor in the people who are usually in the house as each will affect the heat generated inside the home. Multiply the normal number of occupants by 400 and add that result to the base BTU calculation. Alternatively, for high levels of activity, boost the BTUs per person to 600, if desired. Divide the total BTUs by 12,000 to calculate the equivalent air conditioning tonnage. Alternatively, use the BTUs to size the furnace required. With heating, the BTUs multiplied by the efficiency rating equals the output BTUs, which is the number your estimates require.

For example, if you need 30,000 BTUs and the furnace is 85-percent efficient, you need a furnace with 36,000 BTUs to match an output of 30,000 BTUs. Things You Will Need Measuring tape Calculator Tip References Ask Weldin: How Much Cooling Do I Need?: BTU Calculator Photo Credits Jupiterimages/liquidlibrary/Getty Images Suggest a Correction*Disclaimer - This calculator is intended for energy usage comparison purposes only. Do not use this calculator to size a system for purchase; ac/heat control unita licensed HVAC contractor or engineer should be consulted when designing a new system. how to charge outside ac unitThis calculator should not be used to determine the size of a new HVAC unit, it is intended for cost comparison purposes only.liquid line on ac unit

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In order to know exactly how big (or small) it should be, the contractor needs to calculate how much cooling and heating capacity it must have to keep the occupants comfortable. This is called the load of the home. The load of the home is partly dependant upon the home’s square footage. However, a true load calculation goes beyond that. An accurate load determination includes building construction; orientation to the sun; “R” value of the insulation; number, size, and placement of rooms; number, size, and placement of windows and doors; types of windows and doors (thermal efficiency); number and arrangement of floors; Residential load calculations make use of mathematical formulas that take all these variables into consideration. They have been computerized, so they’re not as time-consuming as they were in the past. Determining the load by using rules of thumb almost always leads to an over-sized heating and cooling system, resulting in an increased initial cost, increased monthly utility bills, increased maintenance, and shortened equipment life because the equipment cycles off and on too frequently.

Who Does the Load Calculation? The contractor you choose to install your new system should be able to perform these calculations. You may ask him for this service. If he doesn’t want to do it, or says it isn’t necessary, you may want to look for a different contractor. In some areas, the local utility will perform a load calculation – check with your local utility. How Big Is Big? Cooling size is given in tons of cooling; 1 ton = 12,000 Btu/h. 'SEER' (Seasonal Energy Efficiency Ratio) indicates the efficiency of the air conditioning unit, not its capacity. Why it’s Important to Have the Right Sized System Having the wrong-size heating-cooling system can lead to multiple problems. Over-sizing your cooling system is not a good thing. Without getting into too much detail, your cooling load consists of two parts: We’ve all heard the saying, “It’s not the heat; A cooling system that is too big cools down the temperature very quickly, but it does not run long enough to remove sufficient moisture or “humidity” from the air.

The result is you feel cool but clammy. Many people then turn down the thermostat to make the air conditioner run longer, increasing the utility bill. In some cases, the insufficient moisture removal resulting from over-sized cooling equipment can lead to mold growth and other types of moisture-related damage. A correctly sized cooling system runs long enough to “wring” moisture out of the air. This allows you to be comfortable at a slightly higher thermostat setting and allows you to save money on your utility bill. It also takes care of the “mugginess” without making you feel too cold. Air conditioning systems are sized in “tons,” which is a measure of the rate at which they deliver cooling, not how much they weigh. One ton of cooling is equal to 12,000 Btu/h (British Thermal Units per hour), the rate of cooling required to freeze one ton of ice at 32°F in one day. It is better to have a system that supplies a little less than the required tonnage of cooling, rather than a system that supplies more than the required tonnage of cooling.