What's your number? Air conditioning
By Dan Kolbert
MAINE SUMMERS ARE KNOWN FOR ocean and lakefront breezes and cool mountain air. We are ranked as the 4th coolest state in the nation in the summer, and tourists flock here from the rest of the country to escape the heat.
Thus, cooling homes has historically not been an issue. When I arrived in Maine three and a half decades ago, it was very rare to work on a house with even a portable window AC, much less a whole house system.
Unfortunately, like the rest of the planet, Maine is warming, and the summers are getting hotter. Ninety-degree days and above are no longer rarities; every summer seems to bring at least one week of stifling heat. Because they have been relatively rare in Maine, AC systems are not very familiar to many building or HVAC contractors, or homeowners. So it’s important to know what to look for when installing a system.
Let’s start with—what else?—the numbers. You may have heard AC systems described by how many tons of cooling they provide. This is not a description of the weight of the units; a ton of cooling indicates the amount of heat a ton of ice can absorb in 24 hours. In more specific heat measurement terms, it is 12,000 BTUs (British thermal units) per hour. Most residential units provide between one and five tons of cooling.
Seasonal Energy Efficiency Ratio (SEER)
Just like heating, cooling can be delivered more or less efficiently. SEER, which stands for Seasonal Energy Efficiency Ratio, is how we measure it. The “ratio” in SEER refers to the BTUs the AC unit provides in a “standard” year divided by how much energy it consumes. The higher the number, the more efficient the unit is. The U.S. Department of Energy has set a minimum SEER rating of 14 in the southern half of the continental U.S. and 13 in the northern half. But there are units with ratings in the low 30s, though it’s worth noting that a SEER rating, like EPA gas mileage, measures maximum efficiency under ideal conditions. Keep in mind, too, that cooling is a lot easier than heating. The difference between indoor and outdoor temperature (called the “delta-T,” with delta being the Greek letter used to denote change or difference) is rarely more than 30°F when you are cooling air, but when heating a home on a frigid Maine night, the difference could be 70–80°F or more.
Cooling options: window units, split units, and heat pumps
So what exactly is an air conditioner? If you know what a heat pump is, you know what an air conditioner is—they are the same thing. In Maine, heat pumps are mostly used for heating; an AC utilizes the same technology, but it runs in the opposite direction. To heat, the unit absorbs heat from outside and uses it to heat the interior. To cool, it absorbs heat from inside and dumps it to the exterior. A refrigerant with a very low boiling temperature (-40°F) is compressed and then allowed to expand, absorbing and releasing heat at different points in the cycle. Most refrigerants are potent greenhouse gases, although newer-generation chemicals are less so. But a leak can easily undo any environmental gains, so the quality of the installation is critical.
Different units use different mechanics to deliver the conditioned air. Many of us have simple window units that sit in the window frame in the summer and can cool a single room. They are typically not very efficient, but are relatively inexpensive, have no installation costs and can make sense if you just want to cool your bedroom on the hottest nights or office during the day. Split units have an outdoor unit that runs refrigerant to one or more indoor units (usually a wall cassette). Split units can also work with an air handler, which distributes conditioned air to multiple locations via ductwork. These units work well, typically last longer than ductless systems and are not as visible but are hard to retrofit into existing buildings.
Given that an air conditioner is a heat pump, does it make sense to install an AC system that doesn’t heat your home, too? Probably not, since the delivery system is the same, and the additional cost for a system that heats and cools rather than just cools is relatively small.
Reduce your cooling needs
Similar to planning for heating, when you plan for cooling, you should start by reducing how much cooling your house needs in the first place. Many of the same rules apply, if sometimes in reverse. For instance, in the winter you may want to make the most of the sun’s warmth that comes through your windows, but in the summer, it just adds to your cooling load. What can help? Curtains, awnings that block the high summer sun but allow in the lower-angled winter sun, avoiding large western-facing windows that let in the hot afternoon summer sun, and low-emissivity (low-e) coatings. And a good shell—airtight and well-insulated—is critical as well. Some of these strategies are obviously only possible with new construction, but shades and awnings are always possible, and many storm windows have low-e coatings that help keep out heat but let in light.
To get the opinion of an HVAC pro, I talked to Jim Godbout, owner of Jim Godbout Plumbing & Heating in Biddeford. He’s been my preferred mechanical contractor for a long time and has always taken a practical, experience-based approach to equipment choices and design. One of his frustrations is the frequent changes of refrigerant in manufacturing: he is concerned that disposing of old equipment will create more global warming than the newer generation of refrigerants will prevent. And some of the new ones are mildly flammable, unlike the non-flammable refrigerants used until now.
He’s clear on this point, too: “Our climate is changing, and cooling is definitely required now in Maine for health and comfort. Absolutely we should use heat pump technology when possible, to save energy when selecting cooling systems.” He recommends getting a unit with a SEER rating of 17–23 and argues that spending more to get beyond that is “splitting hairs.” Just as they do with heating, his company develops a room-by-room heat gain calculation to design their systems. And despite being in the business of selling and installing heating and cooling equipment, he agrees that “insulation and shades are the best investment whether heating or cooling.”
This article appeared in the Spring 2022 edition of Green & Healthy Maine HOMES. Subscribe today!
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