Purchase a new HVAC unit from your local HVAC supplier. Make sure the unit size matches the one you have. The unit size is listed on its access panel. Units are sized in BTU (British Thermal Units) for natural gas units and kilowatts for electric units. Purchase your unit accordingly.
Disconnect power to the HVAC unit. Turn off the breaker in your electrical panel, and remove all wiring to the unit. Remove thermostat wiring from the unit, taking care to label the wires to which corresponding terminal they go to. Disconnect condensate drain piping from the HVAC unit. If the unit is natural gas, turn off the gas valve to the unit. The gas valve is located on the gas pipe before it enters the unit. Disconnect the gas piping to the unit. Do not disconnect anything at or above the valve.
Disconnect the sheet metal supply air plenum and return air duct from the unit. Depending on how it's connected, you may need to either unscrew or cut the duct with your aviation snips.
Slide the unit out from its place. It should come out easily now that it is all disconnected.
Slide your new unit into place. You will notice that most likely it will require some extensive reconnection work to hook everything back up to the new unit.
Connect the supply air plenum to the top of the HVAC unit. A sheet metal transition will most likely be required to make the connection from your old plenum to the top of the unit. Most new units are shorter than old ones. If yours is taller, you may need to cut your existing plenum back. A sheet metal shop may have to fabricate some duct for you. Connect the return air duct up to the side of the unit. You will have to cut a hole in the side of the unit to match the size of your return duct. It may require an extension piece or you many need to cut some off depending on your unit's configuration.
Reconnect the gas piping to your unit if it's natural gas. Connect the electrical wiring back to the unit. Consult local electrical codes to see how to make the connections. Connect the condensate piping back to the unit. Ensure it is trapped and vented per local plumbing code so it drains correctly. Reconnect the thermostat wires to the unit. Connect them according to how you labeled them in step 2. Remove the labels as you connect them.
Turn the gas valve and the electrical breaker back on to the unit. Your new HVAC unit should now be correctly installed. Turn the thermostat on to see if it is working correctly.
Things You Will Need
- HVAC hand tools
- Electrical tools
- Pipe wrenches
- Gas piping fittings
- Electrical wire and fittings
- Sheet metal ductwork fittings
- PVC drain pipe and fittings
- PVC glue and cleaner
- Pipe thread compound
- Cordless drill
- You may need to bleed the gas line out a little when you connect it to the furnace. Do this by undoing the cap on the drip leg. If it's not bled, the furnace may not ignite right away.
- Make sure you replace the HVAC filter when you change out the unit.