2.6 Evaporative Cooling and Floor/Wall Furnaces

2.6 Evaporative Cooling and Floor/Wall Furnaces

Quick Answer: Evaporative cooling systems (IRC M1413) must connect to potable water with backflow protection (minimum 1-inch air gap or double check valve assembly). Outdoor air intakes must be located at least 10 feet horizontally from vents, chimneys, or other contaminant sources (IRC R303.5.1). Floor furnaces (IRC M1408) must not be installed in concrete slabs on grade or in traffic paths. They require a 6-inch wall clearance, a 12-inch door clearance, and at least 6 inches of clearance from the bottom of the casing to the ground (2 inches if sealed). Wall furnaces (IRC M1409) must not be installed in pockets unless listed, and must have clearances to prevent door swing interference.

This section covers the mechanical and plumbing requirements for evaporative cooling units, vented floor furnaces, and vented wall furnaces. These systems are common in older homes or specific climates (arid regions for evaporative coolers). The M1 exam tests their installation geometry, clearances, venting, and water-safety connections.

Evaporative Cooling Equipment (IRC M1413)

Evaporative coolers (also known as swamp coolers) use the latent heat of vaporization to cool outdoor air by passing it through water-saturated media. Because they utilize water directly in contact with air streams, water safety and contamination prevention are critical code focuses.

Potable Water Supply Protection

Evaporative coolers require a continuous makeup water supply connected to the dwelling's potable water system. Under Section P2902, this connection must be protected against backflow (back-siphonage) to prevent contaminated sump water from being drawn back into the household drinking supply.

• Air Gap: The most common protection is an air gap, which must be at least twice the effective opening diameter of the inlet pipe, but never less than 1 inch.
• Backflow Preventers: If an air gap is not used, a listed backflow preventer (such as a pressure vacuum breaker or double check valve assembly) must be installed.
• Bleed-Off Systems: Evaporative coolers require water recirculation. A bleed-off line is used to periodically discard mineral-rich water to prevent scale buildup. This line must discharge to an approved location, such as an indirect waste receptor or onto the ground in a manner that does not cause damage or a nuisance. It must not discharge directly into the sanitary sewer system without an air gap.

Outdoor Air Intake Placement

Evaporative coolers draw outdoor air into the house. To prevent pulling sewer gases, flue exhaust, or chemical fumes into the living spaces, the outdoor air intake must comply with strict separation distances:

• 10-Foot Rule: The air intake must be located at least 10 feet horizontally away from any plumbing vents, appliance chimneys, appliance vents, streets, alleys, or parking spots.
• Vertical Exception: The intake may be closer than 10 feet horizontally only if it is positioned at least 3 feet vertically below the level of the contaminant source.

Vented Floor Furnaces (IRC M1408)

Floor furnaces are self-contained heating appliances suspended from the floor joists, with a register grate flush with the floor surface. Because their registers reach high operating temperatures, they present significant burn and fire hazards.

Location Restrictions

• No Concrete Slabs: Floor furnaces are strictly prohibited in concrete floor construction built on grade. They are designed only for suspended wood floors over crawl spaces or basements.
• No Traffic Paths: Floor furnaces must not be installed in the floor of any aisle, hallway, entryway, doorway, or path of travel. They cannot be placed in a location where a door could swing within 12 inches of the register grille in any open or closed position.

Clearance to Combustible Materials

• Wall Clearance: Floor registers must be located at least 6 inches from any wall. Inside corners require a wall register to be at least 6 inches from the adjoining wall.
• Draperies and Doors: Registers must be at least 12 inches from draperies, curtains, doors, or other combustible objects.
• Overhead Clearance: The register must be at least 5 feet below any projecting combustible materials (such as wood beams, shelves, or ceiling joists).

Ground Clearance and Support

• Bottom Clearance: The bottom of the furnace casing must clear the ground in the crawl space by at least 6 inches.
• Sealed Casing Exception: This clearance can be reduced to 2 inches only if the lower 6 inches of the casing is sealed and waterproofed to prevent water intrusion.
• Support: Floor furnaces must be supported independently of the floor register. They must not rest on the ground. They are hung from the floor joist framing.
• Crawl Space Access (M1408.4): To allow servicing, a foundation opening of at least 18 inches by 24 inches or a floor trap door of at least 22 inches by 30 inches must be provided.

Vented Wall Furnaces (IRC M1409)

Wall furnaces are recessed or surface-mounted heaters designed to heat a room from a wall cavity. They can be gravity-type (relying on natural air convection) or fan-type (using a blower to move air).

Installation and Venting

• No Pocket Installations: Wall furnaces must not be installed in a wall pocket unless specifically listed and labeled for pocket installation.
• Door Interference: Wall furnaces must be located so that their air intakes and discharge grilles are not blocked by open doors, draperies, or furniture. They must not be installed in a location where a door could swing and strike the heater casing or draft hood.
• Venting Requirements: Gravity wall furnaces must be vented using a chimney or vent system complying with Chapter 18. Specifically, gravity wall furnaces require Type B-W vents, which are flat oval vents designed to fit within standard 2x4 stud walls.

Appliance Comparison Chart