Summer Weatherization Tips: Part Five

On the hottest days in summer, a house cooled to the low 80’s may just not be cool enough, especially with high humidity conditions and if you have underlying health concerns. If central air conditioning is a need, and you are stressing about the cost and logistics of installation within your historic home, fear not – modern central air systems do not require large duct work as it did in previous decades, and can even be installed without any duct work at all! As a bonus, these systems can also shift seasonally to assist or replace your existing heating systems.

A through-the-wall ductless HVAC system costs about $500 (plus installation) for up to 12,000 BTUs of cooling on a 110V service. These systems work especially well with traditional closed floor plans – each air handler can be controlled independently, so you can target the cooling to the spaces you are using, saving on energy costs.

A high-velocity HVAC system uses mini-ducts, which are usually about 3” in diameter, and often are snaked between floor joists or through walls. These use small vents, which you can match to the aesthetics of your home interior design.

The first step to deciding a system is to determine how much cooling your home will require. First determine the floor square footage- multiply the length by the width; if the space to be cooled is not a square or a rectangle, adjust with the appropriate square footage equations for the shape. Multiply this amount by 25 to determine how many BTUs of cooling will be required or refer to this table from (click for larger version)

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A one-ton central AC system can remove 12,000 BTUs in an hour – after you have the number of BTUs needed for your home, divide this amount by 12,000 to determine the tonnage needed for the HVAC unit. A 1000 square foot house will require a 25,000 BTUs of cooling, or a 2.5-ton system.

(SQ FT X 25) = AC unit tonnage required

Be sure to also check the unit’s efficiency rating, or Seasonal Energy Efficiency Ration (SEER) to see how long it will take before the investment in this system pays off. This will likely be on the yellow EnergyGuide sticker on the unit.


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