2.2 Heat Pumps and Air Conditioning Systems

2.2 Heat Pumps and Air Conditioning Systems (IRC Section M1401, refrigeration systems)

Component Locations and Airstream Sequence

Residential cooling and heat pump systems consist of an indoor evaporator coil, an outdoor compressor and condenser coil, expansion devices, and refrigerant piping. The placement of these components in relation to heating equipment is strictly regulated:

• Evaporator Coil Location (IRC M1411.8): Evaporator coils must be installed on the downstream (leaving-air) side of fuel-fired furnace heat exchangers. The cooling process naturally dehumidifies the air, creating a wet environment on the evaporator fins and inside the drain pan. If the coil were located upstream of a gas, oil, or wood furnace, the cold, moisture-saturated air would blow across the steel heat exchanger, causing rapid rust, corrosion, and structural cracks. The only exception is if the furnace is specifically listed and labeled for use with an upstream cooling coil.
• Furnace Blower Capacity: The furnace blower must be rated to handle the added static pressure resistance of the evaporator coil without reducing airflow below the furnace's minimum heating requirements.

Condensate Disposal Systems

Moisture condensed on the evaporator coil must be collected and disposed of safely to prevent structural damage and mold growth. The primary disposal system must meet these parameters under IRC M1411.9:

• Drain Pipe Size: The condensate drain line must have a minimum nominal diameter of 3/4-inch. It must not be smaller than the primary drain pan outlet connection.
• Piping Slope: Drains must maintain a minimum slope of 1/8 inch per foot (1% slope) in the direction of flow. Slopes less than 1/8 inch per foot pool water, leading to algae clogs.
• Materials: Acceptable materials include PVC, CPVC, copper, and galvanized steel. Drains must run to an approved disposal point (such as a plumbing waste receptor, floor drain, or exterior splash block) and must not discharge into crawl spaces, attics, or onto public walkways.

Secondary and Auxiliary Overflow Protection

Where an evaporator coil or air handler is installed above a finished ceiling, in an attic, or in any location where water damage could occur if the primary drain clogs, secondary protection is required. Under IRC M1411.9.1, four options are permitted:

1. Auxiliary Drain Pan with Separate Drain: A corrosion-resistant pan is placed under the cooling coil or air handler. The pan must have an independent drain line that routes to a conspicuous location (e.g., over a window or door eave) so the homeowner will see the drip, alerting them to a primary drain failure.
2. Secondary Drain Line: A separate drain pipe is connected to the secondary (higher) outlet of the primary drain pan and routed to a conspicuous disposal point.
3. Auxiliary Pan with Water-Level Detection Switch: An auxiliary pan is installed under the unit, fitted with a water-level detection device (float switch). If water accumulates in the pan, the switch interrupts the 24V thermostat control circuit, shutting off the cooling system before an overflow occurs.
4. Water-Level Detection Switch on Primary Pan: A float switch is installed directly in the primary drain pan or the secondary drain port of the primary pan. If the main drain clogs, the rising water triggers the switch and shuts down the compressor.

• Auxiliary Pan Sizing: The auxiliary pan must be at least 1-1/2 inches deep and must extend at least 3 inches larger than the equipment or coil cabinet on all sides.

Refrigerant Piping Support and Protection

Connecting refrigerant lines (suction/vapor and liquid lines) carry high-pressure refrigerants and must be protected:

• Piping Support (IRC M1411.14): Refrigerant lines must be supported at regular intervals to prevent sagging, vibration fatigue, and joint leaks. Near the outdoor condensing unit, the piping must be securely fastened to a permanent support within 6 feet of the unit.
• Framing Penetrations: Refrigerant lines passing through wood or metal framing members must be protected against physical damage. If the piping is within 1-1/4 inches of the edge of the framing member, 16-gage steel shield plates must be installed to prevent drywall nails or screws from puncturing the line.
• Insulation: The suction (vapor) line must be fully insulated with closed-cell insulation (minimum 1/2-inch wall thickness) to prevent condensation and energy loss.
• Locking Access Port Caps (IRC M1411.15): Outdoor refrigerant circuit service ports must be fitted with locking, tamper-resistant caps. This prevents unauthorized access, refrigerant theft, and accidental inhalation (huffing) of toxic refrigerants.

Outdoor Coil and Heat Pump Elevation

Outdoor units (condensers and heat pumps) must be supported on a level, stable pad (concrete, masonry, or approved composite) raised at least 3 inches above the surrounding grade.

• Snow Elevation: In regions subject to snowfall, the outdoor unit of an air-source heat pump must be elevated above the local average design snow depth (using riser feet). This prevents snow and ice from burying the outdoor coil, which would restrict airflow, starve the system of heat, and cause the unit to freeze up. During defrost cycles, melted ice must drain freely away from the pad to prevent ice dams from building up under the fan blades.

Condensate Disposal & Secondary Overflow Methods