Can Mini Split Use Existing Ductwork?

Many people want to switch from a central air conditioner to a mini split system. However, they wonder if they can use ducted mini splits and connect them to the existing ductwork. So, I did some research.

It turns out that mini splits can use existing ductwork. Existing ductwork from the central air conditioning system can be connected to ducted mini splits for air distribution given that the size and capacity of the ductwork are adequate.

Most mini splits don’t have high fan static pressure. Hence, ductwork for mini splits must not be too long. Understanding the principle behind ducted mini splits can provide more insights.

How to Connect Mini Splits to Existing Ductwork?

Single-family houses often have an attic and the central air conditioner is usually located in the attic. Given that there is enough space, a few ducted mini splits can be placed in the attic to provide zoned heating and cooling.

Below is a typical single-family home layout plan with an adequate attic space:

The first step is to identify the capacity needed (BTU) and airflow (CFM) in each room. For a quick calculation, use 30 BTU per square foot and 1 CFM per square foot to estimate.

With that said, below is the summary of the BTU and CFM required for the above example:

If the above home has a central air conditioning system, the ductwork layout may look something like the below drawing:

Most central air conditioners use flexible ducts extensively. Flexible ducts are easier to install but more prone to high static loss and airflow reduction. Nonetheless, well-installed flexible ducts perform just as well as rigid ducts.

To make it simple, I’ll use the full flexible ductwork design as an example to show you the possible way to switch to ducted mini splits while still using the existing ductwork.

The most important thing to understand about the switch from a central air conditioning system to ducted mini splits is the sizing of ducts and grilles.

From the BTU and CFM summary earlier, several ducted mini splits can be deployed as follow:

For maximum independent controls, the above single-family house can use 4 ducted mini splits where 1 unit is for the living room, another 1 unit is for both the dining room & the kitchen, another 1 unit is for the master bedroom and the last 1 unit serves both the bedroom 2 & 3.

However, since you don’t use the living room, the dining room and the kitchen when you sleep, you can reduce the number of mini splits by increasing the capacity.

For instance, you may use 1 unit of the ducted mini split with a capacity of 18000 BTU @ 500 CFM for the living room, the dining room and the kitchen while all the bedrooms can be served by another unit with a capacity of 15000 BTU @ 400 CFM.

When you have 4 ducted mini splits, the ductwork may look something like this:

All 4 ducted mini splits can be conveniently placed in the attic. However, each ducted mini split has a condensing unit that needs to be placed outside the house. So, if you have more mini splits, you’ll have to deal with more condensing units and the pipework.

Central air conditioners often have a single return air. However, if you want to use ducted mini splits, you need to install one return air for each unit as shown in the above drawing.

To use the existing ductwork, ducted mini splits must have a rectangular-to-round duct adapter. Such an adapter allows the existing round flexible duct to be securely connected to the supply outlet of the ducted mini split.

For the return air, it is similar to the central air conditioner which is using a return air plenum box.

Existing rectangular ducts also can be connected to a ducted mini split given that the duct size is enough which it usually does. In that case, a rectangular duct reducer is needed instead.

When switching from a central air conditioner to ducted mini splits, you’ll mostly end up getting rid of the main duct and connecting the mini splits to the branch duct.

How to Tap a Mini Split into an Existing Flexible Ductwork?

To tap a ducted mini split into existing flexible ductwork, you need to install a rectangular-to-round duct adapter on the supply outlet of the ducted mini split. Then, chop off the existing flexible duct at the appropriate length and glue it onto the round duct adapter. Finally, secure the connection with a steel band.

Alternatively, you can use a cable tie and duct tapes for the connection as shown in the video below:

However, if you have multiple flexible duct outlets from a single mini split unit, you may need a custom-made supply air plenum box that has as many round adapters as you need. In that case, make sure you’re able to control the airflow for each flexible duct so that you’ll have a balanced heating/cooling.

Ducted Mini Split Fan Static Pressure

The fan of an air conditioner must have enough static pressure to overcome the airflow resistance in ductwork in order to deliver the required airflow.

Central air conditioners usually have a much higher fan static pressure than ducted mini splits. In context, most ducted mini splits have a fan static pressure up to 0.2 in.wg only while central air conditioners usually can go up to 0.8 in.wg.

So, a ducted mini split can’t be connected to ductwork that has a design like a central air conditioner where there are long flexible ducts and sharp bends; the ductwork connection for a ducted mini split cannot be like the below photo:

Your existing central air conditioner is strong enough to push through long and sagging flexible ducts but if you don’t rectify those saggy flexible ducts, your new ducted mini splits may not be able to push through and deliver the airflow as needed.

Nonetheless, some ducted mini splits have comparable fan static pressure but they are usually high in capacity and airflow, much like a central air conditioner but it defies the purpose of using ducted mini splits for zoned heating and cooling.

Ducted Mini Split Temperature Control

Similar to a central air conditioner, ducted mini splits are controlled by a wired thermostat. Each ducted mini split has one thermostat and the thermostat is located in the room it serves. So, everyone can have their own temperature control, scheduling and more.

To learn more about ducted mini splits, see my post Ducted Mini Split Guide: Basics, Comparison & Installation.

Conclusion

Mini split can use existing ductwork given that the existing duct size is adequate. So, the cut-off point of the existing ductwork is crucial to the success of the new ducted mini split system. In addition, ducted mini splits have low fan static pressure and thus, saggy flexible ducts are not encouraged.

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