Soundproofing the Furnace Room
Plan the soundproofing project. Soundproofing materials are not usually attached directly to the furnace. Instead, the furnace room or closet is soundproofed, isolating the noises produced by the fan to that area, which is usually not part of the living quarters of the home.
Install commercial soundproofing materials to the walls and ceiling of the furnace room according to the instructions. Soundproofing material can be a blanket-type material that can be cut to size and stapled to the walls and ceiling.
Other soundproofing material is rigid and applied similar to drywall. In either case, make sure necessary clearances around furnace are maintained for adequate ventilation and fire protection. Consult the installation manual with the furnace for these clearances.
Install a solid-core door on the furnace room or closet. Make sure there is no gap between the door and floor.
Soundproofing the Heating System
Determine whether the duct work is rigid at the point it attaches to the furnace. The heating system duct work, the metal tubes and boxes that carry the air heated by the furnace around the home, can also transmit the noise of the furnace around the home. Noise is vibration. The vibrations of the furnace blower fan causes the duct work to also vibrate, spreading the noise through the home. Both the outbound hot air ducts and the cold air returns should be isolated.
Replace a portion of the rigid duct work near the furnace with something flexible to break the connection that allow the vibrations and sound to spread through the home. Use commercial flexible duct materials if possible.
Other options include disconnecting two pieces of duct work and installing some sort of cloth or rubber gasket between them.
Check local building codes before making any modifications to the heating ducts in the home.