1. Base it on the home’s overall size and the HVAC SEER rating
2. Use detailed ‘Manual-J’ calculations, and take into account the home’s age, construction, floor plan location, size, number of windows, etc, family size, and their lifestyle
You want to keep your home comfortable, and you want your AC to be efficient, so it does not cost more than necessary. An HVAC system’s load is calculated in tons, and it has a SEER rating. A house in the Arlington area of between 2401 and 3000 square feet will be assumed to need a 5 ton unit. The SEER (Seasonal Energy Efficiency Ratio) will determine the system’s maximum efficiency. The SEER rating tells you the cooling output that is needed to maintain a constant indoor temperature over a typical north Texas season.
Modern AC units cycle on and off as needed. A system that is too small for a living area will have to stay on for longer to maintain the desired temperature, and one that is too large will cycle on and off too frequently. Both conditions will add to the electric bill.
Should a 3000 square foot house have a 5 ton unit or two smaller units? Should a 6000 square foot home have one 12 ton unit, two 6 ton units or four smaller units? Should any or all of the units be zoned? The answer depends on a large number of details.
Calculating the Number of Units
The ‘Manual-J’ calculation, mentioned above, works out the overall load requirement. The calculation is done on a room-by-room basis. It takes into account the building’s size, age and construction materials. A home built in the 1980s, for example, may have poor insulation compared to a modern home, so it may need a more powerful system.
If the house has an open floor plan, high ceilings, large windows facing the south, and a large family who spend most of their time in the kitchen, then it will need a certain HVAC unit configuration to maintain comfort levels and to keep bills low. It is worth noting that each of those family members will raise the kitchen’s temperature by about 250 BTUs. This home may benefit from one unit of a higher SEER for the ground floor, and another to control upper floor bedroom temperatures when the outside temperature is lower anyway. Zoning may not solve the high-load capacity, so a one-unit system may be more expensive to run than a two-unit system.
A modern home of the same size, but not open plan, with two occupants who spend their time in the media room will benefit from a completely different HVAC unit configuration. A detailed analysis is needed.