High efficiency air conditioners for zoned cooling

Ductless AC units are among the most efficient air conditioners on the market, and, if properly sized and installed, can provide even greater energy savings than their energy efficiency ratings suggest. This article covers the basics on what is a ductless AC unit, then describes some advantages and disadvantages of split AC units, as these units are also known.

All air conditioners consist of a condenser unit that diffuses heat to the outdoors, and an evaporator unit that cools the indoor air. In a window or portable air conditioner, the condenser and evaporator are housed together; in a central AC unit, the condenser is the large outdoor unit, while the evaporator is connected to a blower which pushes the cooled air through house ductwork.

In ductless AC units, the condenser unit is typically attached to an outdoor wall or roof, or placed at ground level, and has refrigerant line and wiring connections to one or more indoor units placed in the upper wall or ceiling of specific rooms. They are called ductless because no ducts are required.

Ductless AC units are also called split AC units or ductless mini splits because, unlike central air conditioners that have a single large evaporator attached to the house ductwork, the condenser on a ductless AC unit can be connected (by ‘splitting’ the refrigerant lines) to several indoor evaporator units.

Different condensers support different numbers of indoor evaporator units, with up to four indoor units supported by some condensers.

Ductless AC units are a great way to provide cool air to selected rooms or zones in your home, so that you don’t spend a fortune cooling the entire home when you’re only in part of it.

For example, at night there’s no reason to cool your living and dining room, kitchen and basement, if you’re sleeping in an upstairs bedroom. This selective cooling helps boost the energy savings from split AC units: since you can cool specific rooms, you’re not using energy to cool areas you’re not currently occupying.

An evaporator on a ductless AC unit can cool a single room very quickly. If you don’t use your bedrooms during the day, keep the doors closed and let the rooms get hot; shortly before you head to bed, turn on the room unit for each bedroom, and your room will be comfortably cool within minutes.

Advantages of ductless AC units

As I mentioned above, one advantage of ductless AC units is that you cool only the rooms you’re using, instead of your whole home; another is the higher SEER ratings available at the upper end. Let’s look at these and other advantages, grouping them into efficiency, effectiveness, cost savings, and flexibility.

Energy efficiency

Ductless AC units offer above average efficiency compared to both window air conditioners and whole house air conditioners.

Energy efficiency of ductless and whole house air conditioners is measured by SEER, or Seasonal Energy Efficiency Ratio. The higher the SEER rating, the better.

SEER ratings for split AC units run from 13 to as high as 24.5, while central air conditioners typically have efficiencies in the 13 to 19 range. The minimum SEER ratings required for an ENERGY STAR designation are also slightly higher for ductless AC systems – 14.5 SEER for ductless, compared to 14 SEER for central AC systems.

While the vast majority of both central and mini split systems are at the low end of their mandated ranges, there is a reasonably good selection of ultra high efficiency ductless units in the 23 and up SEER range.

The top rated unit, consisting of the B6VMAI24K-B indoor unit and the FS4BI-024K indoor unit, has an SEER of 24.5 and is packaged by leading brands including Broan, Frigidaire, Maytag, Nordyne, NuTone, Tappan, and Westinghouse.

Another key factor in the energy efficiency of ductless AC systems, which is not reflected in the SEER ratings, is their lack of reliance on often leaky or poorly insulated ductwork. On central AC systems, duct energy losses can add up to 30% reduction in efficiency, especially if the ducts are in an attic.

Losses can occur due to leaks in the ductwork, where cool air is pushed into areas that don’t require cooling (attics, crawl spaces etc.), or due to heat gain where the ductwork is in a hot space such as an attic and the ducts are not properly insulated.

Cooling effectiveness

Cold air falls, so having the cooled air enter your rooms near the ceiling is better than having it enter through floor vents. Although some ductless systems allow installation lower down, I don’t recommended installing close to the floor because it will keep the cool air at floor level, leaving upper parts of the room hot.

Unlike most window AC units, which have a simple on-off thermostat, ductless units have sophisticated thermostatic controls that keep the room at a constant temperature, adjusting the amount of cooling to keep that constant temperature.

Window units tend to cool until the low temperature is reached, then switch off the compressor until the high temperature is reached, so there is considerable cycling, which both reduces efficiency and increases noise levels in the room.

Many ductless mini splits have a variable speed blower, and electronic controls that adjust the blower speed based on the cooling needs. They also provide both horizontal and vertical axis vanes to direct the cooled air to where it’s needed most.

Overall cost savings

If you just want to make it through the hottest days of summer with a little cool air, ductless AC units are cheaper than central air conditioners, in terms of purchase price and installation cost. This is especially true where no ductwork is available, since central units require ductwork leading to every room that is to be cooled.

Note however that the cost per BTU of cooling tends to be higher for ductless AC units than for central AC systems, because most split AC systems have significantly lower BTU/hr ratings than typical central systems.

Flexibility

Some ductless AC units come with both cooling and heating capability. There are two ways a split unit can accomplish this: either a heat pump, where instead of pumping the heat from the indoors to the outdoors, it reverses direction and pumps heat form the outdoors to the indoors; or with a heat strip, which operates on the same principal as a baseboard or space heater.

A heat pump can produce 2-4 times as much heat per unit of energy input than a heat strip, particularly when the outdoor weather is moderately cool. However when temperatures drop closer to or below freezing, heat pump efficiency drops (because there is so little heat to pump out of the cold outdoor air) and a heat strip is a more efficient solution at those times.

I would recommend a heat pump version of a ductless mini split system if you already have some alternative heating system for the winter but want to take advantage of the energy savings of a heat pump in the shoulder season.

However, while heat pumps are 2-4 times more efficient than electric heating, even in moderately cool weather they are typically on a par with, or slightly more than, the cost of heating with natural gas.

Ductless AC units are more powerful than window air conditioners, and they don’t tie up a window or need to be installed or removed as seasons change. You can install them in rooms that don’t have the right type of window (such as bay windows with side casements, or windows too narrow for the window AC unit).

Window air conditioners provide a great opportunity for burglars to break into your home, since they are typically installed in a sash window with the sash unlocked. Ductless AC units offer better security because there is typically only a three-inch hole in the wall to allow ductwork from the outdoor unit to reach any particular indoor unit.

If you don’t have ductwork, a ductless AC system is a great option. For example, in tropical climates where there is no heating system, or in older homes in temperate climates where radiator heating is used, it’s much easier to install ductless than central air conditioning.

Even if you do have central forced air heating, if that forced air system was a retrofit in an older home, the ductwork was likely not designed by an HVAC specialist, and therefore not properly sized and configured to maximize airflow both to individual rooms, and back to the blower.

For example, my 100-year-old home has original ductwork with a single cold air return on the main floor, and none on the second floor. Contractors added a basement cold air return ten years ago when we renovated, but this doesn’t help draw return air back from the bedrooms!

If there is not enough return air flow through cold air returns to the central AC blower, even with the AC running on full blast you will tend to get hot and cold areas. Cold air tends to fall, which means you’ll get loads of it on the ground floor, and almost none on upper floors.

Most split AC units have a remote control that lets you turn the cooling level up, down, on, or off from anywhere in the room they are installed.

In a split AC system, the outdoor unit can be up to 50 feet away from the farthest indoor unit, which gives you some flexibility in placing the condenser. Note however that the longer the distance, the lower the efficiency, because the coolant returning to the evaporator units will absorb more outdoor heat if it has to travel a longer distance outside.

Mini splits can be at least partially installed by the homeowner, although you will void the warranty if the job is not overseen and finished by a professional installer, and there are steps such as testing the copper lines for a seal, and filling with refrigerant, that a pro needs to do. I don’t recommend DIY installation of a ductless minisplit but if you are very handy and willing to engage a professional for the final steps, you may want to consider it.

Note that many of the ductless systems available for purchase from major online retailers may claim to be energy efficient, but if you check their SEER ratings you’ll find they are actually at the low end of the efficiency spectrum. For example the Amvent 12000 BTU ductless mini split room air conditioner claims “High Efficiency Cooling” in the same line it mentions its SEER 13 rating, which is the lowest allowed efficiency for a currently manufactured model!

Disadvantages of split AC systems

There are three main disadvantages of ductless AC units, which is why, if you already have house ductwork, few HVAC companies will even attempt to sell you such a unit.

The first disadvantage is aesthetic: the evaporator units are bulky, and because they are best placed high on the wall, they are very visible and not terribly attractive. While this is no worse than the aesthetic impact of a window air conditioner, the window AC unit can be removed at the end of the cooling season, while the evaporator unit stays.

A second disadvantage is that a ductless minisplit system is not up to cooling a whole house. If an HVAC company is trying to sell you a ductless system, be clear about your needs. A ductless minisplit offers relief from super hot days in selected rooms, but won’t keep your whole house perfectly climate controlled all summer.

Bear in mind the heat sources in your home, and the air flows between the areas you want to cool, and the planned locations for evaporator units.

For example, the ductless minisplit in my sister’s 4-bedroom Toronto home had its single evaporator unit installed in a stairwell some distance from the bedrooms, and that distance combined with a poorly insulated and ventilated attic meant a lot of heat gain in the bedrooms but little cooling effect.

Finally, while a ductless mini split system installed to keep part of your home cool will be cheaper than a central AC system for your whole home, the cost per BTU is about 30% higher than for a central AC where ductwork is already installed.

Ductless AC units are a good investment where high efficiency and energy savings are key and you want to cool only certain zones; where the cooling season is shorter; or where installing ductwork is not a viable option.

You may want to get quotes from a number of qualified HVAC companies for both central and ductless, and ask each company for their opinion on the relative merits of each approach, for your home.

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