Single Wide Mobile Home Ductwork Layouts You Actually Need
Most people drawing ductwork diagrams for single wide mobile homes are working with about 12 feet of ceiling height and roughly 750 to 800 square feet of floor space. That constrains everything. The undersides of these homes leave very little room to maneuver, and the joist bays underneath are typically spaced 16 inches on center with roughly 14 to 16 inches of vertical clearance between the bottom of the floor joists and the ground or skirting. You work in that gap or you don't work at all. I spent three years retrofitting HVAC systems inmanufactured housing before I stopped trying to force things that wouldn't fit. The diagram below shows a layout that actually works for a standard single wide, roughly 14 by 70 feet, with the return positioned mid-house and supply runs split to the forward and rear sections.Underside Single Wide Mobile Home Ductwork Diagram
The core problem nobody mentions upfront is that single wide mobile homes usually have a central furnace or a heat pump located near the midpoint of the home, often directly beneath the living space where the ceiling joists run perpendicular to the length of the unit. When you're going underneath, your duct runs need to follow the joist bays, which means your supply trunks run parallel to the long axis of the home, branching off at right angles toward the perimeter walls. That branching is where most diagrams go wrong. Here is what a functional layout looks like in practice. The main supply trunk runs down the center bay, roughly 24 inches wide, with 10 inch flexible ducts branching left and right every 8 to 10 feet to hit each room. The return duct, usually 14 by 6 inches rectangular or 8 inch round flexible, pulls from a central location between the bedroom and living area and runs back to the furnace inlet. Keep the return path as short as possible. Every extra foot of return duct adds roughly 0.05 inches water column of static pressure, which matters when you are pushing air through a system that was designed for exactly this much capacity.Typical single wide layout: Furnace or air handler centered. Supply trunk 24 by 6 inches running front to back. Branch ducts 10 inch flexible at each room entry. Return 14 by 6 inches central. Total linear footage usually between 40 and 60 feet for a 14 by 70 foot unit.
I ran into a specific problem on a 1984 single wide in Oklahoma where the original duct diagram called for a 16 inch trunk running the full length of the home, but the joist bays were only 14 inches deep due to insulation sag and some previous contractor stuffing fiberglass between the bands. The 16 inch rigid duct simply would not pass through. I cut the trunk down to 14 by 4 inches rectangular and added a second branch loop on the opposite side of the house instead of one long run. The pressure drop increased by maybe 0.1 inches w.c., but the system balanced within three minutes and the homeowner stopped complaining about the bedroom being hot while the living room stayed cold. The counter-intuitive thing about undersided ductwork in single wides is that going with slightly smaller ducts in the main trunk and more branches actually performs better than one oversized run. Oversized trunk ducts create dead air zones where air sits stagnant, especially at the far ends of the run. Smaller trunks maintain higher velocity, which keeps the air moving and prevents that stagnation. The trade-off is more fitting work and more joints, but joints are where leaks happen anyway, so you might as well optimize for airflow. Another detail people miss: the insulation on the underside ducts matters more than the duct material itself. Flexible duct with foil facing and R-6 insulation will lose less conditioned air to the crawl space than bare rigid metal duct, even if the metal duct is larger. The crawl air underneath these homes can hit 120 degrees in summer and drop below freezing in winter, depending on your climate zone. That temperature swing will condense moisture on un-insulated ducts, which leads to insulation degradation and eventual mold growth inside the duct lining. I have pulled flexible duct out of single wides where the inner liner had delaminated from thermal cycling because nobody insulated the underside runs. That job cost twice as much as it should have.Insulation requirement: Minimum R-6 on all underside ducts in climate zones 2 through 5. Use foil-faced flexible duct with sealed joints. Rigid metal duct requires separate insulation wrap. Seal all seams with mastic, not duct tape.
Static pressure limits for a single wide HVAC system are typically around 0.5 inches water column at the fan coil. If your duct diagram pushes more than that, the blower motor will hunt for airflow, the system will short cycle, and you will hear the whining sound that indicates the motor is struggling. Measure it with a manometer at the supply plenum before you close anything up. If the reading is above 0.5, you have too much restriction, which usually means a trunk that is too small, too many bends, or a filter that is clogged. The downside of undersided ductwork is exactly what you would expect. You are working in a confined space with limited visibility, often on your back, with joists and wiring and plumbing all competing for the same gap. The typical work time for a full duct replacement on a single wide is about 6 to 8 hours for two people, versus 3 to 4 hours if the unit has a basement or crawl space with full standing room. You lose productivity on every bend you have to make in tight quarters, and every fitting you drag through a 14 inch joist bay takes twice as long as the same fitting in open space. If your single wide has skirting that is sealed tight, you also have a ventilation problem underneath that affects the duct insulation longevity. Moisture gets trapped, insulation compresses, and the duct surface corrodes if it is metal. I recommend leaving at least one vent per 8 feet of skirting, or using vented skirting panels, to keep the underside air moving. That simple step extends the life of both the duct insulation and the structural components around it.Work time estimate: Full duct replacement on single wide, undersided, two people, 6 to 8 hours. With standing room, 3 to 4 hours. Add 1 hour for insulation and sealing.
Get the Full Details
