Multiple mini-split condensers mounted on a red exterior wall

Ductless mini-splits provide heating and cooling for your home and are a great alternative to central air systems, which lose a lot of energy from the ductwork that distributes conditioned air around the house.

Because mini-splits don’t need ductwork, they require less maintenance than central AC and are sometimes the only viable way to install heating and cooling in parts of a home where ductwork would be too difficult to install.

In addition, mini-splits don’t suffer from the air quality issues caused by the ducts used in central AC. A common question about these units is whether they bring in fresh air from outside the home or recirculate air already in the room. You might have concerns about the air quality in your home and are looking for reassurance about how ductless mini-splits affect this.

If you’re interested in how ductless mini-splits can improve the air quality in your home, continue reading this article, which explains in detail how these systems work and how they safeguard the air you and your family breathe every day.

For more tips on improving indoor air quality in a green building, why not read our article covering eight ways homeowners can achieve this?

Do Ductless Mini-Splits Bring In Outside Air or Recirculate?

Ductless mini-splits draw air from the room and blow it across the coil in the air handler unit before returning the conditioned air to the room at the desired temperature. The air is taken from and recirculated to the same room by the air handler, so no outside air is brought in.

What Is a Mini-Split Heat Pump?

Mini-split heat pumps are comprised of two units, one outside and one inside.

The outside unit is where the refrigerant pump (known as the compressor or condenser) is housed, along with heat transfer coils and control hardware that ensures the system is running correctly.

The indoor unit, called the air handler or head unit, is typically installed high up on an inside wall. The head unit is responsible for blowing conditioned air into the house and can precisely control a room’s temperature.

A ductless mini-split in the corner of a room below the ceiling

It is possible to have multiple head units running off a single outdoor unit, with four usually being the maximum. Homeowners can control each head unit individually to set the room’s temperature or zone at the desired level.

This feature gives mini-split systems flexibility and is part of their appeal. For example, if you aren’t using a room, there is no need to control its temperature, and you can stop running the head unit in that room, which will save you money.

How a Mini-Split Recirculates Air

Mini-splits operate by circulating refrigerant around the system, along the refrigeration lines. The refrigerant collects heat energy from the cold side of the system and moves it to the warm side.

When in cooling mode, heat is collected from inside the home and moved to the outside condenser unit, where it is emitted to the outside air.

Two mini-split compressor units outside a home in snowy conditions

In heating mode, the system runs in reverse, with heat being collected from outside and moved inside, where it is transferred to the air blowing across the head unit coil, creating a warm flow of air into the room.

The heat pump’s job is to maintain a comfortable temperature in your home, and it does this without drawing in air from outside.

There is a caveat to this, though, mainly if you live in an older house that is less airtight. Many older homes allow air to enter through the fabric of the building, such as windows that don’t make a tight seal against the frame when shut, gaps around soffits, and cracks in walls.

These minor imperfections allow the house to “breathe,” which lets fresh air in and prevents the buildup of contaminants and humidity that could cause health problems for the occupants and cosmetic or even structural issues for the building.

Do I Need Mechanical Ventilation for My Home?

Given that a mini-split doesn’t bring in outside air but simply recirculates the existing air, you might be wondering what the best way to provide ventilation for your home is.

Over recent years, we have been building houses in the US with increased insulation that are much more airtight than ever before. This improvement is great for energy efficiency but not so good for ventilation.

In modern, airtight houses, we need to provide mechanical ventilation to prevent such issues. How much ventilation is required depends on the size of your home and how many people live there.

According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the recommended ventilation standard for residential homes is 7.5 cfm (cubic feet per minute) per person living in the house, plus another three cfm for every 100 square feet of floor area in your home.

A modern airtight home will not passively provide through imperfections and cracks in its structure, so we need to provide this ventilation using mechanical means. In other words, a fan.

A circulating ceiling fan used to supplement a mini-split system

How to Fix Poor Ventilation in a House

All mini-splits come fitted with filters, which will help remove particles of dust, pollen, and other things that can cause health problems such as allergies.

As mentioned, we also need to provide adequate ventilation to avoid the buildup of moisture from humid air and assist in flushing out dust and other pollutants that cause health problems.

One way to do this is to open your windows, but if you’ve spent lots of money insulating and sealing your home to make it as airtight as possible, opening windows all the time to let in fresh air undermines your home efficiency efforts.

A better, more energy-efficient way to provide the necessary ventilation is using a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV).

A closeup of the exchange mechanism of an ERV
The exchange mechanism of an ERV

These clever devices allow fresh air to enter the building, but on its way in, they use exhaust air from the house to bring the temperature of the incoming air closer to the indoor temperature. This process means your AC or heating needs to work less and saves energy and money.

For more information about ERVs and HRVs, including their benefits and when you should consider installing one, look at our articles on those subjects. Click on the respective links to be taken to the ERV article or the HRV article.

Summary

Ductless mini-splits are an energy-efficient way to heat and cool your home comprised of external and internal units. The indoor air handler draws in air from the room, heats or cools it depending on what mode it’s running in, and then returns the conditioned air into the same room.

Because mini-splits recirculate the air in the house without bringing in outside air, sometimes additional ventilation can be a good idea.

If you need to provide additional mechanical ventilation, a good option is to install an HRV or ERV, which will help bring the outside air closer to the indoor temperature of your home, saving you energy and money.

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