Installing new heating or air conditioning duct. Heating System Changes:

Adding A New Duct To A Heating Or Cooling System

In This Article:

A hole is cut into an existing main heat duct and small 4 inch round duct is routed to a point between the floor joists where a register can be installed. A duct damper is installed to control the air flow.

Related Articles:
Skill Level: 2-3 (Basic to Moderate) Time Taken: About 4 Hours

By , Editor


In our old house remodeling project we made some minor changes to the interior wall layout. The previous owner had turned an attic into a master bedroom, but the only way to reach that bedroom was through another bedroom. If you talk to real estate professionals, they will tell you that such a weird layout will disqualify one of those rooms from being considered truly a bedroom. That's a problem, because it means the house can only be marketed as having 3 bedrooms instead of 4.

So we framed a partition to form an entry hall to the master bedroom. Of course, this meant robbing the other bedroom of a 3-foot strip of floor area. There is enough room in this entry hall for some shelving and maybe even a small built-in desk. I thought it would be nice to have a little bit of heat too, since the small space would be cut off from other sources of heat.

This is the fireplace enclosure that will allow us enough room to add another duct.


The lower part.


The entry area to the master bedroom. A little more heat here would be a good idea, because once the walls are covered this small hallway will have no heat source.

Since this area is at the center of the building it may not need any heat, but the room it leads into lacks sufficient heat, so I'm betting that installing a small duct might help and won't hurt. I can always shut off the air flow.


The Big Deal: Drilling a 4½" hole.

We needed to make a big hole in the sub-floor so the 4" round duct could extend from the basement to just below the second-story floor.

This is easy enough when you have a big drill (such as this right-angle drill) and a hole saw. Otherwise, this can be done with a jig saw or a reciprocating saw.

Cutting large hole in floor for new ductwork.

I bought a 4½" hole saw because it can be used for many projects, 4" metal ducting for dryer vents, 4" PVC pipe for plumbing drains, etc.


The hole viewed from the basement. The rectangular duct on the right will be supplying the warm air.


We temporarily connected several pieces of ductwork to lay out the best location for the take-off.

We used a permanent marker to draw a line around the outside of the take-off.

Laying out location for new take-off duct component.

A "take-off" is a sheet metal fitting that is used to create a branch duct in the heating or cooling ductwork.

5 inch duct take-off for a heating system branch. This is the take-off we used. There are several other types of take-offs commonly available. Round take-offs are common.


We drilled some 3/8" holes at the corners of the 6½" x 6½" square cut-out.


Cutting a square hole in large main duct plenum. Using tin snips I cut out the opening in the rectangular duct.


The take-off grabs onto the big duct using these little metal tabs or fingers.

Here I'm demonstrating this on a piece of sheet metal, not the actual duct.

Securing take-off to larger duct.


Installing new duct branch "take-off". I set the take-off in the hole. This can be challenging because ductwork components are not made very precisely.


I reached inside the take-off to fold over the metal fingers. Reaching inside take-off to fold over metal tabs.


Duct with 5-to-4 inch reducer fitting. Next I installed a 5" to 4" reducer fitting.


Then I installed an elbow.


Installing new straight duct into elbow fitting. I dropped a 2 foot long piece of 4" duct down through the hole and connected it to the elbow.


The assembly so far.

But... before the duct leaves the basement it needs to have a damper installed so the air flow can be regulated.


Metal foil duct tape around joints, UL-181A tape. I applied some metal foil duct tape to the joints. For heating systems, code requires tape that carries the UL-181 rating.

Regular cloth duct tape is absolutely useless for holding heating ducts together because within a year or two the adhesive dries out.


Installing A Duct Damper:

This is a 4" duct damper. There are also dampers for 5" and 6" round ducts.

This is just a little flap that rotates and blocks the flow of air.

Duct damper to control flow of air.


I drilled two 5/16" holes, about 4 inches from the end of the straight duct segment.

The hardest part of this entire procedure is getting two holes marked and drilled so they are exactly opposite each other. If not, the damper may hit the side of the duct and not close properly.


On one end of the damper is a spring-loaded threaded rod. The handle and wing-nut fasten to this.


Installing duct damper in straight duct. I slipped the "dead" pin into one of the holes, and compressed the "live" spring-loaded pin as I pushed the damper into the duct.


With the damper in place the "live" pin popped out through the other hole, letting the damper rotate.


When open, the damper looks kinda like this.


I attached the handle and wing nut.

With most of these dampers, the handle positioned perpendicular to the duct means that the duct is shut off.


The corrected duct with damper.

The damper needs to be located where you can reach it, such as in a basement, crawl space, or attic. There is no point in putting dampers in a wall cavity where they will be covered up.


The duct coming out of the floor needs to jog to the side a couple of inches, so it will leave room for a bookcase I plan to build on the sides of the fireplace enclosure.


I installed a pair of elbows, each adjusted for just a slight bend, to make the duct jog to the side.

Then I connected a 5 foot length of duct to the elbows.


This shot is looking almost straight up.

I continued the duct upward, then added a couple of elbows to jog around the fireplace chimney enclosure.

Note that at this point the duct is running just below the level of the floor joists.

New branch duct running up beside fireplace chimney.


Jog in duct to go around obstacle. Here is yet another jog to go above the top plate and into the final joist bay.

All these twists and turns cause extra resistance to air flow, so it's best to avoid them. By making only a slight turn with some of these elbows I can minimize the reduction in air flow.


The Final Cut:

Since I couldn't find any sort of register boot fitting or adapter for a 4" duct, I decided to go the easiest route: I bought a 6" to 4x10 right angle register boot fitting, and a 4" to 6" reducer. Register boot for ductwork.

It's poor practice to increase the size of the duct, because it makes the air velocity slow down, which might mean that the air won't circulate well. Since this is a supplemental duct (I could probably do without it) I'm not really concerned about perfect warm air distribution.

I laid out a location for the register.

I cut a 4" x 10" piece of plywood for a template, just for kicks.


I drilled a hole in the floor, so I could go downstairs and see where this point was located relative to the floor joists.


Hole in floor for register boot. I cut out the 4" x 10" opening with a jig saw.


To connect the 6" x 4x10 register boot to the 6" to 4" adapter, I needed to make a piece of duct with both ends crimped.

I used a duct crimping tool to make a series of "bites" in the smooth end of the duct.

Crimping end of round duct with crimper tool.


Before I bought a crimping tool I had made mediocre crimped ends with a pair of needle-nose pliers.

I would just grab a bit of metal and give it a twist, then move over and do it again. It's not pretty, it doesn't fit tightly, but it works in a pinch.

If you are going to any significant amount of ductwork, it's worth investing in a few tools, like the usual 3 types of tin snips (straight, left turn, and right turn cutting) and the crimping tool.

This is the creature I built.

It's kinda stupid, but it worked. If there is a register boot available for 4" duct, it's not carried at my local Home Depot.

I could have made my own register boot, but that would take far more time than it's worth.


I squeezed the "creature" into the 4x10 register hole. It actually stayed there by itself because the fit was so tight.


I cut the last piece of 4" duct and connected it between the end of the duct and the 4-6" reducer. Final section of round duct.


I drove in 4 sheet metal screws to hold the register boot to the sub-floor.

I also need to add some supporting straps to keep the duct from falling apart under its own weight.


Supporting The Duct:

According to the book Code Check® HVAC (published by Taunton Press and sold at Home Depot and others), metal ductwork needs to be supported at least every 10 feet. There are many methods available, such as telescoping metal brackets that are simply hammered into the floor joists. I often just cut a scrap of 2x4 to fit between the floor joists and secure it with deck screws.

The duct at least needs something to support its weight, but it may be acceptable if the duct can move slightly from side-to-side.

I have seen lots of older ductwork that was supported by steel wire that was strung between two nails driven into opposite floor joists. I'm not sure if that is acceptable any more. Perforated steel strap (plumber's tape) is often used to support ductwork.


Code Requirements:

There are many mechanical code requirements that affect duct installations, such as:

Duct openings are not allowed in an attached garage. The duct can pass through the garage, but you cannot run a new household heating or cooling duct to serve the garage. I suppose this could be very tempting for some handy-people who want to use their attached garage as a workshop, but it's a code violation. A garage needs its own separate heat source.

There must be fireblocking around a duct that passes between floors. This can simply be a tight-fitting piece of wood. If you hack a big oblong hole in the floor, the gaps around the duct need to be filled. There is a special intumescent caulk (which expands during a fire) for this purpose, and it's required in some construction projects (like apartment complexes) but this stuff is expensive. Regular Alex plus caulk probably does a decent job of fireblocking, as would a piece of sheet metal.



Tools Used:

  • Cordless Drill/Driver
  • Heavy-Duty Drill with 4½" Hole Saw
  • Aviation-Style Tin Snips
  • Duct Crimper
  • Jig Saw
  • Basic Hand Tools

Materials Used:

  • 4" Galvanized Duct, 2' Long, 5' Long
  • 4" Galvanized Elbows
  • 4" Duct Damper
  • 5" Take-off
  • 5" to 4" Reducer
  • Plumber's Tape, Sheet Metal Screws

Back To Top Of Page 


 Read our Disclaimer.

Search Page

Home  What's New  Project Archives  H.I. World

 Rants  Contact Us



Copyright © 2005

Written April 15, 2005