2-Story Rooftop HVAC Package Unit Installation / Curb Replacement

Notice the duct moved from the back to the bottom of the HVAC unit

See the rusted metal stand below the HVAC unit. That stand was from 1978. The HVAC unit was replaced in 2025, but the HVAC contractor was cheap and didn’t replace the stand with a modern curb. Instead, since the new HVAC unit was too large for the stand, he strapped two pressure-treated 2x4s to the stand using a strip of thin metal.

The old stand was replaced with a “curb”, which is an enclosed stand. Also, the elbow was removed and the air inlet/outlet on the back of the HVAC unit was closed off with sheet metal because the air now flowed into the building from below the HVAC unit. You’ll also notice the electrical wires (power and thermostat) come out of the roof below the HVAC unit rather than next to it.

Order a curb that is designed for the HVAC unit you have.

Connect the 4 rope hooks to the 4 holes in the metal base of the HVAC unit. Instruct the crane operator to life the HVAC unit and place it somewhere else.

With the HVAC unit out of the way, we can see the old elbow and ducting that goes into the attic. The left duct is for air to return to the HVAC unit and the right duct is for conditioned air to flow into the building.

Remove the elbow. You may need a pry bar to lift some shingles and a utility knife to cut some sealant connecting the elbow to the shingles.

After dry-fitting the curb on the roof where you want it to go, mark along the inside perimeter and then cut the shingles along the left and right edges of the perimeter. If using a circular saw, adjust the blade cutting depth to avoid cutting deeper than the shingles.

Using a pry bar, a row or two of shingles at the top edge and a bit of shingles on the left and right edges.

Drill a hole in the roof inside the curb so that the electrical cables can come up through the roof from below the HVAC unit rather than next to the unit.

Turn off the circuit breaker to the HVAC unit and check for any voltage. My Lennox 2.5 ton package HVAC unit had two hot wires, red and black, each at 120V, for a total of 240V. The green wire was for ground.

Remove the line cable, which supplies electrical power from the circuit breaker box, from the HVAC electrical box.

Since the Romex line cable will be moved to come up through the roof from under the HVAC unit, we removed the conduit.

We then pushed the Romex back into the attic so we could push it through the hole in the roof we made earlier.

Go into the attic so you can push the Romex cable through the new hole in the roof.

You can now see the electrical cable is inside the curb

Slide the top half of each side of the curb under the shingles and move the curb into its final position.

Place a magnetic torpedo level on one side of the curb, then lift the front of the curb up until the level shows it is level. This is so the HVAC unit will sit level on the curb rather than angled following the slope of the roof.

When it’s level, drill a self-tapping screw for metal from the outer side of the curb through the inner side of the curb to hold the outer side in place.

Install the 4 brackets that came with the curb – 2 on the front and one on each side.

Drill a 3/4″ diameter hole in the front of the curb for the electrical Romex cable and the t-stat (thermostat) cable to go through.

We needed some sheet metal to connect some ducts and cover some openings. It’s much cheaper to buy a roll at a dedicated HVAC store than the big-box stores like Home Depot, which only sell small sheets. However, some HVAC stores only allow licensed contractors to buy from them.

When going up and down the ladder, make sure to strap the ladder to the building, like an eye hook in the eaves, to keep the ladder from moving. Some city inspectors require this when they come for an inspection.

Cut pieces of sheet metal to connect the duct in the attic to the curb.

Use a flashing folding tool to fold some edges of the sheet metal.

Then screw the cut sheet metal to the curb. Repeat until you’ve connected the duct in the attic to the curb.

Apply HVAC foil sealer duct tape at the edges of the sheet metal connections.

If it helps, you can reuse old duct parts to connect the duct in the attic to the curb. In the following photo, the left opening is where air flows into the building. The right opening is where air flows from the building back into the HVAC unit.

Connecting the duct to the curb can be tricky because there may not be enough slack in flexible ducts and you are sometimes working in tight spaces.

In this case, the contractor had to create a short circular duct to extend the flexible duct that was in the attic.

Hscrewed in the extension piece using self-tapping sheet metal screws.

He applied HVAC tape to all duct and curb connections.

Apply HVAC sealant (Pookie) to all seams and connections. When dry, this sealant becomes very hard.

Stick the electrical line cable and load wires (red, black, and green) through the hole previously made in the curb. The load wires connect from the wires in the line cable to the terminals in the HVAC unit.

Stick the wires through the hole in the electrical box. Screw the box to the curb.

Since both the black and white wires in the line cable are hot (120V), the white wire must be colored black to indicate it is hot. You can cover a portion of the white cable sleeve with black electrical tape.

Connect the wires to the appropriate terminals in the electrical junction box.

Tape the ends of the load wires and the thermostat wires together to make it easier to access them when the HVAC unit is lowered onto the curb.

Drill a 3/4″ hole at the bottom of the HVAC unit near where the power and thermostat wires will connect to the unit.

The contractor unscrewed and removed the two panels at the bottom of the HVAC unit since airflow will flow through the bottom rather than the back.

He then measured the openings so he could create some custom panels using sheet metal.

The contractor pulled the load and thermostat wire through the hole he made in the base of the HVAC unit.

The unit was then lowered onto the curb such that the base would sit on the 4 brackets around the top of the curb.

He connected the load wires (black, red, and green) to the appropriate terminals.

He connected the thermostat wires to the corresponding color-coded wires in the HVAC unit.

Put HVAC tape to close the gap between the openings at the bottom of the unit and the duct openings in the roof.

Fold the 4 corners of the sheet to fit over the lip of the openings at the back of the unit.

Put HVAC tape around the panels, but he didn’t apply HVAC sealant (Pookie) over the tape so that maintenance and inspection can easily be done in the future, if needed. (Pookie dries really hard and is hard to remove)

Put new shingles over the area where he had to remove some old shingles.

Since the old electrical line cable was moved from outside the HVAV unit to under the unit, he covered the old hole in the roof with HVAC tape, which is very sticky tape.

Apply silicone sealant over screws that could result in roof leaks

Apply silicone sealant around the electrical box

Apply sealant in the holes beneath the top layer of shingles where old holes were