Mobile Home Air Conditioning: HUD Code, Ducts and Wind

What the HUD code is, and why it changes the AC question

HUD defines a manufactured home as a structure built on a permanent chassis that is transportable in one or more sections. HUD's definition says the structure "includes the plumbing, heating, air-conditioning, and electrical systems contained in the structure". So the cooling system is part of what the federal standard regulates, not an add-on chosen only by the buyer. Florida's own statutes and rules often use the older words mobile home, and this page uses the two names for the same kind of home.

The standard is Part 3280 of Title 24 of the Code of Federal Regulations. Its scope covers "all equipment and installations in the design, construction, transportation, fire safety, plumbing, heat-producing and electrical systems" of these homes. The data plate says the home "is designed to comply with the Federal Manufactured Home Construction and Safety Standards in force at the time of manufacture". Our reading: the rules that shaped your home's ducts and cooling preparation are the ones from its build year, so two homes of different ages can differ.

A house built on site is a different case. Its ducts are covered in ductwork in Florida, and its system types in central air conditioning and split systems.

Two labels to find first: the data plate and the comfort cooling certificate

HUD requires a data plate "near the main electrical panel or other readily accessible and visible location". Among other things it lists "major factory-installed equipment, including the manufacturer's name and the model designation of each appliance". It also carries the roof load zone and wind load zone maps. A photo of the plate is the quickest way to tell a contractor what the factory installed.

A separate Comfort Cooling Certificate must be permanently affixed "to an interior surface of each dwelling unit that is readily visible to the occupant". HUD lets the maker choose one of three alternatives. In the first, the factory supplied a central air conditioner, and the certificate gives its certified capacity in Btu per hour. A Btu is a unit of heat, and a ton of cooling is 12,000 Btu per hour. In the second, the certificate says the air distribution system "is suitable for the installation of central air conditioning" and gives a maximum rated capacity. In the third, it says the system "has not been designed in anticipation of its use with a central air conditioning system".

This matters at replacement time. HUD's installation standards say the rated capacity of site-installed air conditioning equipment "must not exceed the air distribution system's rated BTU/hr. capacity as shown on the home's compliance certificate". They also say site-installed central air must be sized to the home's heat gain, using a 1997 ASHRAE handbook chapter or ACCA Manual J, 8th Edition. Our reading: the certificate sets a ceiling, and a quote for a larger unit than the certificate allows needs an explanation. Sizing itself is covered in the sizing and tonnage guide. If the third statement is on your certificate, equipment that needs no ducts is covered in systems without ductwork.

The ducts: supply runs, crossover ducts and the space under the floor

HUD defines crossovers as "Utility interconnections in multi-section homes" where the sections join, and says they include "heating and cooling ducts". In a double-wide, a crossover duct links the duct system in one section to the other. HUD describes ducts of this kind "under-chassis," meaning below the home's chassis, its supporting frame. Ducts "exposed directly to outside air" must be insulated to at least R-8, with a continuous vapor retarder rated at not more than 1 perm, and listed for exterior use. R-value measures resistance to heat flow, and a perm measures how much water vapor a material lets through.

The rules also say what a crossover duct must not do. The manufacturer must give instructions for supporting, fastening, sealing and insulating it, and "no portion of the crossover duct is to be in contact with the ground". On site, HUD's installation standards require every duct connection to be sealed against air leakage with listed tapes or mastics, and duct supports to be spaced no more than 4 feet apart.

For cooling capacity, the supply duct system must be "capable of handling at least 300 cfm per 10,000 btuh with a static pressure no greater than 0.3 inches of water". Cfm is cubic feet of air per minute. Static pressure is the resistance the blower pushes against. For returns, HUD requires "provisions" to return air from all rooms and living spaces except toilet rooms, and where a room is closed off it calls for permanent uncloseable openings.

A Department of Energy project page, describing work led by Slipstream, a research group in Madison, Wisconsin, describes the other side. It says the systems in manufactured homes deliver conditioned air "through ductwork prone to leakage," and that duct leaks "generally represent a direct loss of conditioned air to the outside". Our reading: these rules say how ducts must be built and joined, not what shape yours are in today. Whether to seal, repair or replace is covered in sealing versus repairing ducts, and a person should do any work under the home.

Package unit or split system on a manufactured home

A split system has an outdoor unit, with the compressor and outdoor coil, and an indoor coil that sits with the furnace or air handler. A package unit puts those parts in one outdoor cabinet and connects to the home by duct. HUD's text covers both. It speaks of "external self contained comfort cooling appliances" and says the connecting duct must be part of the listed appliance or meet the rules for the supply duct system. Ducts on external appliances must resist "ultraviolet rays, cold weather, or moisture" and be "resistant to insects and rodents".

For a split system, HUD says indoor coils and outdoor sections "shall be certified, listed and rated in combination for capacity and efficiency". If a coil is installed with a furnace, HUD says the furnace and coil "shall be tested and listed in combination". The installation standards add that an A-coil, a coil shaped like the letter A, must be "compatible and listed for use with the furnace in the home". Our reading: a mismatched coil, furnace or outdoor unit is a code question as well as a performance one, so ask for the listing or matched-set information.

A heat pump, which cools and heats with one outdoor unit, is covered in heat pumps in Florida. On a manufactured home, HUD says a heat pump with backup electric resistance heat must supply at least 60 percent of the calculated annual heating need by compression. HUD also requires a control that blocks the backup heat above 40 degrees Fahrenheit, except in defrost. HUD also says every installed appliance must be accessible "without removing permanent construction".

Wind, anchoring and the outdoor unit in Tampa Bay

HUD's wind map splits the country into three zones. Zone II is designed for 100 mph and Zone III for 110 mph. HUD's text lists Florida's Zone III counties, and Pinellas is among them. Hillsborough and Pasco are not in that list. HUD places Florida's other counties in Zone II. The zone is printed on the data plate, so you can confirm it for your home.

The data plate's wind statement also says the home "should not be located within 1500' of the coastline in Wind Zones II and III". The statement makes an exception for a home and anchoring designed to the higher Exposure D requirements. Ask whether that applies to your lot.

The home's anchoring is a separate system from the air conditioner. The Florida Department of Highway Safety and Motor Vehicles says tie-downs are "an essential part of the foundation system". It adds that Chapter 15C-1 of the Florida Administrative Code requires a licensed mobile home installer. It suggests having the tie-down system checked, and replacing straps or ground anchors that show corrosion or damage. That check is for the home, not for the condenser. Rule 15C-1.0102 says anchors, piers and tie-down components must be "tested, listed and approved" by that department.

We found no HUD or Florida text in this research that sets an anchoring method for the outdoor unit itself. HUD ties each appliance to "the terms of its listing", and to the manufacturer's installation instructions. Stands, flood height and anchors for outdoor units are covered in heat pumps in Florida, storm preparation in preparing the system for a storm, and salt air in coastal coil protection.

One more rule affects the space under the home. HUD says "all condensation must be directed beyond the perimeter of the home," and that runoff from air conditioning drains must be directed away from under the home. A clogged drain is covered in why an AC leaks water.

Questions to put in writing before you hire

1. Can you photograph my data plate and comfort cooling certificate and put the equipment model and the certified capacity in the proposal?
2. Which of the three certificate statements does my home carry, and is the proposed unit's rated capacity within the duct system's rating?
3. Was the size set by a load calculation for this home, and what does it say?
4. Will the ducts, including the crossover and any under-floor runs, be inspected, and what measured result will I get at the end?
5. Is the proposal for a split system or a package unit, and are the coil and outdoor unit a listed matched set?
6. Where will the condensate drain go, and how will the outdoor unit be supported, according to the maker's installation instructions?
7. Who checks the home's tie-downs, and is that person a licensed mobile home installer?

For permits, see HVAC permits in Florida. This research did not find a manufactured-home-specific HVAC permit rule, so ask your building department.