Split air conditioner efficiency

Are split air conditioning systems more efficient than central AC?

Which is more efficient, a split air conditioning system or a forced air central AC unit? We need to replace our central air conditioner and I’m wondering if a minisplit air conditioner is a better option.

Answer from Green Energy Efficient Homes

Both central air conditioners and ductless minisplit air conditioners have a Seasonal Energy Efficiency Rating (SEER, or now SEER2, an updated version of the same general idea), and if you look at the range of systems available in both classes, you’ll find that the best ductless minisplits have a significant edge on efficiency over the most efficient central air conditioning systems. For example, the best SEER ratings on central air conditioners for some of the major brands are 25.8 (Lennox Signature Series), 24.0 (Lennox SL28XCV, Carrier Infinity and Bryant Evolution) and 22.5 (Amana, Daikin and Goodman), while the best ductless minisplit ratings are 35.0 (Mitsubishi M-Series), and 33.1 (Fujitsu H Series, Rheem Classic Plus and Ruud Classic Plus). That’s a huge difference, and clearly ductless minisplits are in the lead in terms of efficiency.

Ductless minisplit systems are a great way to go if you don’t already have central ductwork. With the help of a qualified electrician you can easily install one yourself, and if you have a traditional two- or three-storey home you really only need the indoor units on the top floor if you’re only using them to cool your home, since (A) heat tends to build up most on the top floor where the ceiling touches the attic, and (B) cold air falls.

A ductless minisplit system has one or more evaporator units that are installed in particular rooms, and a single condenser unit located on an outside wall or on a flat roof. The refrigerant circulates between the evaporator units and condenser unit in pipe less than an inch thick, so you just need a few small holes in your walls to install a system. Compare that to installing HVAC ducts throughout your home and you can see why ductless minisplit systems are popular as a first air conditioning system in homes without ductwork.

One of the main efficiency benefits of ductless minisplits is that, because you can install multiple indoor units, you can control each unit separately, even though there is just one condenser unit doing the actual cooling work. For example, you can have a unit installed in each bedroom and one in the main living area, and keep the bedrooms cool at night when you’re asleep, but leave the units off during the day when the bedrooms are empty; meanwhile keep the living area cool when you’re up and about, and turn the living area unit off when you’re in bed. For a bedroom especially, a ductless minisplit system can cool very quickly, so as long as you turn it on a quarter hour before you’re ready to head to bed, you’ll be comfortable without spending a lot of money keeping an empty bedroom cool.

Most ductless minisplits can operate in fan-only mode as well, which I recommend for warm but not insufferably hot nights; a gentle (or powerful) breeze from your bedroom unit can provide enough air circulation to keep you cool, at a much lower cost than the actual cooling. As long as your attic hasn’t built up a huge amount of heat through the day, this is often all you need to stay comfortable overnight.

Before you invest in any large air conditioning system, though, you’d be well advised to find less energy intensive techniques for staying cool. Making sure you have high efficiency windows, good wall and attic insulation, and adequate attic ventilation, all make a huge difference; and if you’re careful about keeping the windows closed and the blinds drawn during the day, and running window fans at night when the temperature drops a little, you can often get by even on the hottest days with no AC at all.

My parents live in a century-old home heated by radiators, and there are a few days each summer when the upstairs gets really uncomfortably hot. They installed a split air conditioner in the stairwell to the second floor, and it does a great job of keeping the upstairs cool. As an added benefit, this ductless minisplit doubles as a heat pump, meaning it can run the refrigeration cycle in the other direction – extracting heat from the outdoors in cold weather and pumping it into the house. Many ductless minisplits sold in the 2020s have a heat pump component built in. If you live in an area with a heating season, a ductless minisplit with heat pump is a great way to go because you get peak efficiency on the cooling, highly efficient heating, and in both cases fine-grained control over which rooms are keept cool or warm at what times of day. The only thing to watch for is that hot and cold air behave differently. Hot air rises, while cold air falls, which means an air conditioner at the top of the stairs does a decent job cooling the top fllor and the ground floor of my parents’ house (some cool air stays upstairs and some falls down to the main floor), but it does nothing to heat the downstairs in winter – all that hot air stays on the top floor. This is why people generally install ductles minisplits that are intended for both heating and cooling, with at least one unit on each floor, so that you can count on rising heat from lower levels in cold spells and falling cool air from upper levels during heatwaves.

For more information…

Here are some pages where I cover related topics that can help you keep cool in summer:

• Energy saving air conditioners
• Summer energy saving tips
• Attic ceiling insulation