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How To Split Flex Duct [7 Step-By-Step Guide]

There are many types of HVAC systems. They include central air, heat pump or heat pumps, gas furnace, and central heater. Also how to channel air from one part of the room to another part.

You might be wondering – how to split flex duct?

The steps are quite easy.  At first, you have to identify the point from where you want to split the duct. Then install another duct from that point. It’s better to use straight runs. You also need to tighten it up. Finally, you must conduct a final examination..

This is just a scratch on the surface. This article contains detailed steps on how you can split your flex duct.

Without any further ado, let’s find out what we have to do.

Splitting Flex Ducts: Is It A Good Idea?

Splitting flex ducts can bring you advantages as well as obstacles. Here are the pros and cons of splitting flex ducts you need to know before doing it.


If the ducts are over 8 feet long, splitting the flex ducts can save you money. You can be equally benefited if you have two or more 90-degree elbows in the system.

You can also enjoy the facility in two rooms from a space of one flex duct.


Splitting a duct reduces the efficiency of the space’s air conditioning. At first, you’ll need to know if the HVAC system has enough CFM Flow for the addition. Ideally, a line should be routed directly from the HVAC unit’s main box.

In this post, we’ll talk about how to avoid these problems. Don’t worry! Surely, this is easier than attaching a duct board plenum and an air handler.

Split Flex Ducts: 7 Easy Steps

We have prepared a step-by-step guide to carry on along the process easily. But before that, let’s take a look at the requirements-

Required Tools

There are some tools that you’re gonna need while carrying along with the procedure. We have listed down the tools-

  • A long Knife
  • A pair of wire cutters
  • Panduit gun
  • Male and female slots
  • Panduit straps
  • Duct tape

All these tools are the basic ones that you should have at your home. The Panduit gun is the one thing that you might be wondering where to find. That’s why we have chosen a few brands that are the most suitable for you.

Product 1
Product 2

The products are durable and good performing. Although, it is up to you which one you want to buy. Now, let’s get to the steps of splitting your flex duct.

Step 1: Primary Calculations

At the very first, you need to perform a manual J load calculation. You also need to do a manual D calculator for duct design. And a manual T calculation to determine the grille sizes. 

In most cases, the calculations will show you that it’s possible to install the duct. In very few cases, the results don’t allow you to install the ductwork. Keep in mind that if you are unable to install it, you should not force it. Otherwise, your whole HVAC system might be affected.

Step 2: Identifying The Point Of Splitting

The next step is to identify the point from where you’re going to split the duct. It’s very important to know where you want to split. You can try setting up a whole new duct channel from the register canon. Or, you can set up a channel by cutting the duct.

Step 3: Cutting The Duct 

If you want to set up a channel from the register canon, you’ve to drill another hole. Install another canon that is similar to the previous one beside it. In this way, you are splitting the channel from the same point. So you are not setting up an entire directory for the duct.

If you want to cut it from within the duct, you have to use a knife. You can also cut the duct with a jigsaw. Then cut to the length of your duct. Next, you’ll have to join the two ducts together.

Step 4: Joining The Ducts

Attach a female slot into the pierced part of the duct. Attach it into the slot with tape. 

Next, attach a male slot into the duct if it’s not already there. Squeeze the port a little bit and try to insert it through the female port. Once you’re done, seal the surroundings with duct tape.

Step 5: Use Straight Runs

Straight runs almost eliminate the flex duct’s tendency to underperform owing to drooping and kinking. On a straight course, the duct can still sag. However, if your contractor correctly supports it, it will not sag.

Make use of a hard pipe! Bends, elbows, and anything coming directly from the plenum is all included. Air will not flow properly through a bent flex duct. But it flows through a hard pipe duct.

Step 6: Pulling It Tight

When your contractor installs a flex duct for straight runs, it’s vital that they pull it tight. Otherwise, the duct will be rough and squeezed on the inside. It won’t be able to breathe correctly, and it’ll be more prone to sagging with time.

Step 7: Test & Balance

It’s critical to verify the static pressure and air movement through the system after installation. These measurements can be taken with the equipment provided by your HVAC contractor. If your system isn’t moving enough air the contractor will need to make adjustments until it is.

There you go! These were all that you needed to know to split flex ducts. 

How To Keep Your Split Flex Ducts In Good Condition

The main concern when you install a new duct is what duct board you should use. You might be wondering if it’s best to go with a rigid board or a flexible board

Flexible boards are often easier to install, while rigid boards are more durable. In some cases, you may need both.If you want something long-lasting, then you need to go with rigid. Keep that in mind before choosing any duct-work options for your residence.

We’ve compiled a list of tips to keep your flex duct in good condition:

  • Don’t consolidate or stretch the joints excessively. They can grow apart and not seal properly if they’re stretched too much. This can cause leaks and other issues. 
  • Use high-quality polyurethane pipe jointing compound (we recommend 3M’s, JB Weld).

This is all that you have to keep in mind. Otherwise, you’ll have to replace the ductwork which might cost you a fortune.


Question: Is it okay if I use dryer sheets in my vents?

Answer: Absolutely! You can use a dryer sheet on top of the filter if you have an air conditioner. The same result can be obtained by attaching a dryer sheet to any heating vent.

Question: Can you squeeze the flex duct?

Answer: Flexible ducts shouldn’t be squeezed into any cavities smaller than the duct’s outer diameter. Also including duct insulation. When installed in unconditioned portions, they shouldn’t be installed in cavities smaller than the duct’s inner diameter.

Question: how much does it cost to replace ductwork?

Answer: Ductwork replacement might cost between 5000 and 6000 dollars. It might cost you up to $10,000 in extreme situations. But it won’t set you back any more than that.


Now you know how to split flex ductwork.  We hope you can carry along the process pretty smoothly. One last tip, perform a cleanup for your entire duct system once in one or two years.

If you have any issues while following these steps, feel free to share them with us.

Good luck!