HVAC Heating and Air Conditioning

Safety is the primary focus of code provisions governing heating and air conditioning systems.

Improper appliance installation can cause the appliance to malfunction, creating a fire hazard as well as the potential for carbon monoxide poisoning.

Fuel-burning appliances, such as gas and oil furnaces, must be installed in safe locations and must be provided with enough outside air to support fuel combustion. Vents for these appliances must be installed so that they do not ignite combustible materials and so that combustion gases are not circulated back into the building.

In this post, you will learn about the safe installation of heating and air conditioning appliances and their associated venting equipment. You will also learn about how to identify improperly installed forced-air ducts that can increase your energy costs and decrease your appliance’s useful life. focus and keep reading.

HVAC Appliances

Heating, ventilation, and air conditioning systems are often referred to as a group by the term HVAC.

This is because they share a common purpose: to keep the air in your home comfortable. All HVAC systems—even hot-water-based systems— rely on the flow of air to heat, cool, and ventilate your home.

Most homes today include two separate major appliances in the HVAC system: a furnace and a central air conditioning unit. In most cases, even when the AC has been retrofitted, the appliances share ductwork.

The information in this chapter is intended mostly for general education purposes and to assist with diagnosing system problems. Unless you are a very experienced DIYer, installing and servicing HVAC equipment is a job best left to professionals.

Central air conditioner parts include: (A) Power shutoff for service personnel, (B) Condenser unit, (C) Fan, (D) Condenser coil, (E) Compressor, (F) Plenum, (G) Evaporator coil, (H) Blower motor, (I) Filter, (J) Air return.
Gas furnace parts include (A) Heat exchanger, (B) Circulating fan/blower motor, (C) Vent damper, (D) Vent connector/flue, (E) Combustion chamber, (F) Burners, (G) Blower motor.

Prohibited Locations for Appliance Installation

  1. Do not locate gas or other fuel-burning appliances in bedrooms, bathrooms, toilet rooms, or storage closets. Do not draw combustion air for any fuel-burning appliance from these rooms regardless of where the appliance is located.
  2. Do not locate appliances anywhere that is not approved by the manufacturer’s instructions.
  3. Do not apply this provision to appliances powered by electricity.

Exceptions: Prohibited Locations for Appliance Installation

  1. You may install direct-vent appliances in prohibited locations if the appliance draws all combustion air directly from the outdoors.
  2. You may install vented room heaters, vented wall furnaces, vented decorative appliances, and decorative appliances listed for installation in vented, solid fuel-burning fireplaces (such as gas logs) in prohibited locations if the room satisfies combustion air volume requirements.
  3. You may install one listed, wall-mounted, unvented room heater in a bathroom if the appliance has an oxygen depletion safety shutoff system, if the appliance input rating is not more than 6,000 BTU/hour, and if the bathroom satisfies combustion air volume requirements.
  4. You may install one listed, wall-mounted, unvented room heater in a bedroom if the appliance has an oxygen depletion safety shutoff system, if the appliance input rating is not more than 10,000 BTU/hour, and if the bedroom satisfies combustion air volume requirements.
  5. You may install appliances in an enclosure accessible from the prohibited locations if all combustion air is drawn directly from the outdoors and if the enclosure is equipped with a self-closing door that is weather-stripped.
  6. The standard combustion air volume requirement is at least 50 cubic feet per 1,000 BTU/hour appliance input rating. This standard volume requirement does not apply if the home is tightly sealed with a known air infiltration rate of less than 0.40 air changes per hour. Tightly sealed homes are rare.

A typical forced air heating system delivers heated air to a room through registers while drawing cool air through return ducts. The cool air is reheated in the furnace and recirculated for maximum efficiency. A fresh air intake provides a constant supply of combustion air.
An air intake vent that draws combustion air into the system allows you to install appliances in areas that lack adequate combustion air. The vent shown is for a high-efficiency gas furnace.

electric baseboard heaters often are used to provide supplementary heat. Models (such as the one above) that do not produce external temperatures greater than 125° F may be mounted directly to drywall walls, but a clear space should be maintained between the appliance and the floor.

Electric Radiant (baseboard) Heating Systems

Electric radiant heating systems provide heat to a single room. They usually have no fan and provide heat by the natural movement of the heated air and by thermal radiation.

They are common in small, seasonally occupied buildings, some rural homes, and in buildings without ducts for forced-air heating and cooling. These elements become very hot, and proper installation is important to prevent fires and electrical problems.

Installation Requirements

  1. Install electric radiant heating panels according to the manufacturer’s installation instructions and applicable provisions of general and local codes.
  2. Install radiant panels parallel to wood framing members and fasten the panels to the surface of the framing members or mount the panels between framing members.
  3. Install fasteners only in areas of the appliance’s radiant panel designed for fasteners. Install fasteners at least ¼ inch away from a heating element.
  4. Install radiant panels as complete units, unless listed and labeled for field modifications.
  5. Do not install radiant panels on drywall unless the panel’s maximum operating temperature is not more than 125 degrees Fahrenheit.

Estimate Your Heater Needs

  1. measure the area of the room in square feet (length × width):
  2. Divide the area by 10 to get the baseline minimum wattage:
  3. Add 5% for each newer window or 10% for each older window:
  4. Add 10% for each exterior wall in the room:
  5. Add 10% for each exterior door:
  6. Add 10% if space below is not insulated:
  7. Add 20% if space above is not well insulated:
  8. Add 10% if the ceiling is more than 8 ft. high:
  9. Total of the baseline wattage plus all additions:
  10. Divide this number by 250 (the wattage produced per foot of standard baseboard heater):
  11. Round up to a whole number. This is the minimum number of feet of heater you need.

HVAC Appliance & Duct Sizing

Appliance Sizing

Size heating and cooling appliances according to The Air Conditioning Contractors of America (ACCA) Manual S or a similar approved method.

Manual S and Manual J account for conditions such as the direction the structure faces, the size and type of windows and doors, local temperature conditions, and insulation.

Contractors should provide a copy of the sizing calculations for all new construction and when replacing existing appliances. Improper appliance sizing may cause inefficient appliance operation or may allow excessive moisture to remain in the home. Excessive moisture can damage the home and provide moisture for mold growth.

Air Conditioner Condensate Disposal

The process of removing heat from the air is often called air conditioning. Water is a byproduct of air conditioning because water vapor condenses out from the air when the air temperature is reduced.

In areas with high humidity, air conditioning can produce significant amounts of water. The water removed from the air during air conditioning is called condensate.

Condensate Disposal Location Requirements

Do not discharge condensate on to a street, alley, or any other place that would create a nuisance.

Some jurisdictions, particularly in warm moist areas, require that you discharge condensate away from the foundation. Verify discharge location requirements with the local building official.

Condensate Discharge Pipe Requirements

Condensate from air conditioner units should be directed to a drain through discharge pipes.

  1. Use at least 3⁄4-inch diameter pipe for primary and auxiliary condensate discharge pipes. Do not decrease pipe size between the collection and discharge point.
  2. Install horizontal pipe sections with a uniform slope in the direction of the discharge point of at least 1⁄8 inch in 12 inches.
  3. Use fittings, primers, cement, hangers, and other components that are compatible with the pipe material. Install the pipe according to the provisions in general codes.
  4. Use pipe material and use a pipe size that will accommodate the condensate temperature, pressure, and flow rate produced by the air conditioning system.
  5. You may use most water supply and drain pipes for condensate discharge pipes. The most common condensate discharge pipes are PVC, CPVC, and ABS.
  6. Connect a condensate pump to the appliance it serves so that the appliance will not operate if the pump will not operate. This applies to pumps located in spaces such as attics and crawlspaces.

Condensate Auxiliary (backup) System Requirements

  1. Install an auxiliary condensate system when the air conditioner evaporator coil is located where building damage may occur if the primary condensate discharge system malfunctions. This usually applies to evaporator coils installed in or above finished space. Many jurisdictions require auxiliary condensate systems for all air conditioning systems unless all of the equipment is located outside the building.
  2. Install one of the following auxiliary condensate systems when an auxiliary condensate system is required. (a) Condensate auxiliary drain pan with discharge pipe.
  3. Install an auxiliary drain pan under the evaporator coil. Use a pan that is at least 1½ inches deep and at least three inches larger than the evaporator coil in both length and width.
  4. Construct the pan using either at least 0.0276-inch galvanized sheet metal or at least a 0.0625-inch, nonmetallic pan.
  5. Slope the pan toward the discharge pipe connection. Install the auxiliary discharge pipe using the same materials and methods as the primary discharge pipe.
  6. Terminate the auxiliary discharge pipe at a conspicuous point so that the occupants can see that the primary condensate discharge system is not functioning properly (a conspicuous point often means above a window). (b) Water level cutoff switch.
  7. Install a water level cutoff switch above the primary condensate discharge pipe where it connects to the evaporator coil and below the evaporator coil interior condensate pan overflow rim, or you may install the switch in the primary or secondary discharge pipes. The switch location should allow the switch to shut off the air conditioner before the water overflows into the building.

Maintenance Tip: Keep Discharge Tubes Clear Condensate discharge tubes that run from your air conditioner evaporator have a strong tendency to become clogged with mildew, algae, bacteria, and other unappealing nuisances. To prevent clogging, flush the discharge tube every couple of months with a solution of 2 tablespoons household chlorine bleach dissolved into a cup of hot water.

HVAC Duct Installation

Improper duct installation and duct damage are common problems. Ducts that leak, have sharp bends, or sag reduce the volume of air that moves through the duct and increases the friction between the air and the duct walls. This causes the HVAC system to work harder and longer than necessary, wasting energy and money.

Flexible HVAC Duct Installation

General Installation Requirements

  1. Install flexible ducts according to the manufacturer’s installation instructions. The installation instructions that follow are based on material from the Air Diffusion Council. The full, original version may be downloaded from their website.
  2. Use flexible ducts that are labeled at least every 36 inches with information such as the manufacturer’s name and the R-value of the duct insulation.
  3. Do not expose flexible ducts to direct sunlight such as may occur under roof vents. Direct sunlight can damage the duct outer cover.
  4. Extend flexible duct to its full length. Do not leave excess duct material in a duct run and do not compress the duct.

Duct Support

  1. Support flexible ducts using material at least 1½ inches wide. You may support flexible ducts on 1½-inch-wide framing.
  2. Support flexible ducts at not more than 4-foot intervals. You may use a fitting or distribution plenum to provide initial flexible duct support.
  3. Do not allow the flexible duct to sag between supports more than ½ inch per foot.
  4. Support bends in long horizontal runs of the flexible duct at not more than one duct diameter on both sides of the bend.
  5. Support bends in a flexible duct that occurs near the plenum connection. Allow flexible ducts to run at least several inches beyond a plenum connection before making a bend.
  6. Provide independent support for duct fittings and distribution plenums. Support duct fittings at not more than one foot from the fitting.
  7. Support vertical runs of the flexible duct at more than six-foot intervals.

Duct Bends

  1. Do not bend flexible ducts at sharp angles across obstructions such as framing lumber and pipes. Such bends reduce the duct area and restrict air flow.
  2. Do not bend flexible ducts so that the bend radius at the centerline is less than one duct diameter.
  3. Avoid changing the shape of the duct. The area of a round duct is greater than the area of the same duct compressed into an ellipse.

flexible ductwork may not be bent at an overly tight angle. The radius of any curved bend should be no less than the diameter of the duct.

intervals between supports (hangers) for flexible HVAC ductwork can be no more than 4 ft. apart.

Duct Connections & Splices

  1. Connect and splice ducts according to the manufacturer’s instructions.
  2. Connect flexible ducts to metal collars that are at least two inches long.
  3. Splice two ducts together using a metal sleeve at least four inches long.
  4. Use approved clamps and tape to secure nonmetallic flexible ducts to metal collars and sleeves.
  5. Use approved tape and mastic to seal duct collars, plenums, and other fittings to ensure minimum air leakage.

Duct Insulation And Sealing

  1. Insulate HVAC supply ducts located in attics to at least R-8. Insulate all other ducts to at least R-6.
  2. You are not required to insulate ducts that are completely within conditioned space.
  3. Seal ducts, furnaces, air handler, filter boxes, junction boxes, and fittings.
  4. Verify duct and air handler sealing with a duct pressure test. You may perform this test during rough-in or after final installation of the HVAC system. Refer to the IRC for test requirements.
  5. Do not use framing cavities such as stud walls and floor framing as HVAC ducts or plenums.

Prohibited Sources for Return & Outdoor Air

Prohibited Sources of Return Air

  • 1. Do not locate a return air register in a closet, bathroom, toilet room, kitchen, garage, mechanical room, furnace room or closet.
  • Do not locate a return register closer than 10 feet in any direction to an open combustion chamber or to a draft hood-equipped appliance. Common examples of these prohibited sources include a fireplace and a water heater.
  • Do not take more return air from a room or space than is supplied to that room or space

Prohibited Sources of Outdoor Air

  1. Locate outdoor air intake openings connected to forced-air heating and cooling systems at least 10 feet horizontally from contaminant sources including gas equipment vents, chimneys, plumbing vents, or the discharge outlet of an exhaust fan.
  2. You may locate outdoor air intake openings closer than 10 feet horizontally to a contaminant source if the outdoor air intake opening is at least three feet below the contaminant source.
  3. Do not locate outdoor air intake openings where objectionable odors, fumes, or flammable vapors may be present.
  4. Locate outdoor air intake openings at least 10 feet above a public walkway or driveway.
  5. Do not locate outdoor air intake openings at grade level next to a sidewalk, street, alley, or driveway.
  6. Provide an automatic or gravity-operated damper for air intake systems that will close the damper when the system is not operating.


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