Most Massachusetts homes cool with something other than a central system. Of homes here, 87% use air-conditioning equipment, 32% use a central unit, and 58% use an individual one: a ductless mini-split, a window or wall unit, or a portable. Air conditioning in Massachusetts is largely a room-by-room affair, in a state where the Boston reference station averages about ten days a year at 90°F or higher.
What homes in Massachusetts actually cool with
The numbers come from the U.S. Energy Information Administration’s Residential Energy Consumption Survey (RECS) 2020, state table on air conditioning. For Massachusetts, it reports that 87% of homes use air-conditioning equipment of any kind. Central units show up in 32% of homes. Individual units show up in 58%. Ceiling fans turn in 50% of homes.
Read those three together and the pattern is plain. The distance between “has air conditioning” and “has a central unit” is the share running on individual equipment, and here that group is larger than the central one. A typical Massachusetts household is more likely to own a window unit, a wall unit, a portable or a mini-split than a ducted system with an outdoor condenser.
That is not true everywhere. The same survey puts Connecticut at 90% of homes with air conditioning but only 34% on central. Alabama sits at 94% and 81%. In Alabama, “has air conditioning” and “has a central unit” almost mean the same thing. In Massachusetts they do not. Other states in this series show their own mixes, such as What Do Homes in Michigan Cool With? and What Do Homes in Virginia Cool With?.
How much cooling the year actually asks for
The heat-load figure is 10.2. That is the mean number of days a year reaching 90°F or more at a single reference station, Boston, from NOAA’s 1991-2020 climate normals. It is a count of days. It is not an average summer temperature, and it is not a statewide measure.
Ten days a year is a little more than a week and a half of real heat, usually scattered across July and August in a few clusters. Most of the cooling season is spent below that line. A bedroom unit that runs on the hot nights and a living room unit that runs through the afternoons can cover a good deal of a typical year.
Equipment mix and heat load do not always line up. A hot state can run mostly on window units because its housing was built before ductwork was standard. A mild state can be nearly all central because new construction there came with ducts. Whether a house already has ductwork usually follows its heating system, which is why Heating a Home in Massachusetts is worth reading alongside this page.
Wet heat or dry heat, and what it changes
Summer rainfall at the same Boston reference station is about 10.4 inches across June, July and August, against 43.6 inches over the full year. Month by month it runs 3.89 inches in June, 3.27 in July and 3.23 in August, the driest of the three. These are 1991-2020 normals, not a forecast. No summer month drops below 3 inches, so summers here lean wet rather than dry.
That figure measures precipitation, in inches. It does not measure relative humidity or dew point, and it cannot tell you how a given afternoon will feel. What it does say is that summers here are not a long dry spell.
Why does that matter for equipment? An air conditioner does two jobs. It lowers the air temperature, and it pulls moisture out of the air as warm air crosses the cold coil. Removing moisture takes run time, because water only comes out while the machine is actually running. In a dry summer the machine’s work is nearly all temperature. In a wet one, a large share of the work is drying.
Oversizing hurts in both cases but in different ways. In a dry summer an oversized unit mostly wastes energy by cycling on and off. In a wet one it also leaves the room cold and clammy, because it hits the thermostat setting fast and shuts off before it has dried anything. For Massachusetts, the practical priorities look like this:
- Run time first. Let the unit finish long cycles instead of chasing a very low setpoint that it reaches in minutes.
- Fan on auto. A continuously running fan can push moisture from the wet coil back into the room between cycles, so auto is the usual choice when drying matters.
- Size down to the chart. Pick the capacity that matches the room, not the next one up “to be safe.”
- Use a dehumidify or dry mode if the unit has one. Many room units and mini-splits include it for exactly the damp, mild days.
- Ceiling fans as support. Half of homes here already have them. Fans move air across skin but do not remove moisture, so they help comfort without replacing the machine.
Sizing a room unit, from the published chart
With 58% of homes on individual equipment, the sizing question is the one people type most. ENERGY STAR publishes a cooling capacity chart that pairs the area to be cooled with the capacity needed, in BTU per hour:
| Area to be cooled (square feet) | Capacity needed (BTU per hour) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
Treat it as a starting point for a room unit, nothing more. The chart is not a load calculation, and it does not size a central system. A ducted system is sized by a load calculation that looks at the whole house. Since 32% of homes here are on central units, that distinction matters for anyone replacing one. The chart also does not convert into a figure per square foot, because the steps are not linear. Notice how the jump from 12,000 to 14,000 spans a wider band of floor area than the step from 5,000 to 6,000.
Resist the urge to round up. An oversized unit is a fault, not a margin. It cools the room fast, shuts off, and never runs long enough to pull moisture out of the air, which leaves the space cold and clammy.
What actually needs looking after here
Maintenance should follow the equipment mix. In Massachusetts that means starting with individual units, then covering the central systems that 32% of homes have.
Window, wall, portable and mini-split units
A house cooled by window units has no ducts, so duct leakage does not apply to it. The care is simpler and more hands-on. Clean the filter or screen according to the manufacturer’s manual, keep the coil and the outside vents clear, and make sure the unit drains properly. Seal the gaps around a window unit so cooled air stays in the room. A mini-split has no ductwork either, but it has indoor filters and an outdoor unit that both need to be kept clear.
Central systems
Here the filter comes first. ENERGY STAR advises: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.” That one sentence covers three kinds of equipment, so a heat pump owner is included. It says once a month, not every few months. The reason is airflow. ENERGY STAR states: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A system can be in perfect working order and still underperform if the filter is clogged.
Ducts are the other half, and this caveat applies only to central systems. ENERGY STAR notes: “In a typical house, however, about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.” A room that never cools may be losing air before it reaches the vent. ENERGY STAR’s page on duct sealing covers the fix, and its maintenance checklist covers the rest.
A seasonal pass that works for either kind of home:
- Early May: inspect every unit, window, wall, portable or mini-split, and clean or replace filters before the first warm spell.
- Start of the cooling season: run each unit for a full cycle, listen for odd noises, and check that condensate drains and the outdoor vents are clear.
- Every month through summer: inspect central-system filters, per ENERGY STAR’s once-a-month guidance.
- Midsummer: look at exposed ductwork in attics or basements for gaps or disconnected joints, if the home has central air.
- October: clean and cover or remove window units, and clean the filters one last time.
Common questions
What share of Massachusetts homes use air conditioning?
The RECS 2020 state table reports 87% of homes using air-conditioning equipment. Within that, 32% use a central unit and 58% use an individual one. The survey also reports ceiling fans in 50% of homes.
How many 90°F days does Massachusetts get?
At the Boston reference station, the 1991-2020 normal is 10.2 days a year reaching 90°F or more. It is a count of days at one station, not a statewide average or a summer temperature.
What size window unit does a 200-square-foot room need?
ENERGY STAR’s chart lists 150 up to 250 square feet at 6,000 BTU per hour. It is a starting point for a room unit, not a load calculation, and an oversized unit will cool the room quickly without ever drying the air.
How often should I check the filter?
For central air conditioners, furnaces and heat pumps, ENERGY STAR says once a month. Room units have their own instructions in the owner’s manual, so follow those for window, wall, portable and mini-split equipment.