If you are installing a furnace, boiler, fireplace, or atmospheric water heater in new construction or a major renovation, there is a good chance the authority having jurisdiction expects a controlled combustion air intake. Whether a motorized damper is required, and how it needs to be interlocked, comes up regularly at the counter and on inspections.
This guide covers the code basis, how interlock is supposed to work, and what to watch for when selecting a damper and relay adaptor.
The Code Basis: CAN/CSA B149
Combustion air requirements for fuel-burning appliances in Canada fall under CAN/CSA B149, the Natural Gas and Propane Installation Code. The relevant provisions require that appliances have an adequate supply of air for combustion, ventilation, and draft. On tighter modern construction, that means a designated controlled opening rather than relying on infiltration through the building envelope.
When that opening is motorized and can be closed while the appliance is off, the code requires it to be interlocked. The appliance cannot fire until the damper is confirmed open. A sealed building with a closed combustion air intake and a firing furnace will draw air from wherever it can find it: plumbing penetrations, exhaust fans, any available gap. The result is negative building pressure, combustion quality problems, and carbon monoxide risk. Interlock is the protection against that.
Requirements vary by edition of B149 and by jurisdiction. Some municipalities have adopted amendments. Confirm local requirements with the authority having jurisdiction before finalizing a design.
When a Motorized Damper Is Typically Required
A motorized combustion air damper is generally required when the combustion air opening is equipped with a closeable control device, which is exactly what a motorized damper is. It also applies when the building is tight enough that natural infiltration cannot reliably supply combustion air, when the mechanical engineer's design specifies a controlled intake, or when the appliance manufacturer's instructions call for a controlled combustion air source.
For permanently open passive openings, such as a fixed louvred intake or open grille, a motorized damper is not required. The requirement comes in once you introduce something that can close the opening.
How Interlock Is Supposed to Work
The interlock sequence has a specific order and each step matters.
- The appliance receives a call for heat.
- The relay adaptor opens the combustion air damper before the appliance fires.
- The end switch on the damper confirms the blade has physically reached full open.
- The relay adaptor receives that confirmation and releases the appliance to fire.
- When the call ends, the appliance shuts down and the damper closes.
Without the end switch, step 3 does not exist. The relay adaptor has no position confirmation and the interlock is not truly closed-loop. On a cold morning when the blade is partially frozen against ice or debris, the damper could be stuck at a partial open position and the appliance would still fire. The end switch is what makes the interlock reliable, not just present.
Selecting the Right Variant
The Hoyme HOM is available in two action configurations. Selecting the wrong one is a safety issue, not just a product mismatch.
SO (Spring Open / Power Close) is the correct choice for combustion air intake on furnaces, boilers, fireplaces, and atmospheric water heaters. The damper springs open on power loss, so if electrical power is interrupted while the appliance is firing, the combustion air path stays open. That is the fail-safe direction for a combustion air intake application.
SF (Spring Fail / Power Open) is used for exhaust or controlled makeup air applications where the damper needs to seal on power loss. It is not the correct action for combustion air intake.
For combustion air interlock with an ADP relay adaptor, the SO2 variant is required. It includes both the relay and the end switch. SO1 (no relay, no end switch) is used only in applications where an external controller handles position confirmation separately. Confirm the required action with the mechanical engineer or the appliance manufacturer's installation instructions. SO and SF cannot be changed after installation.
Matching the ADP Relay Adaptor
The ADP Interface Relay Adaptor handles the interlock wiring between the appliance safety circuit and the HOM damper. The right model depends on appliance type and system voltage.
- Gas furnace with millivolt safety system: ADP-0MV2-S5A
- Gas furnace with 24 Vac system: ADP-0241-05A
- Tankless water heater or boiler at 110 Vac: ADP-1102-S5A or ADP-1102-10A
The ADP wires into the appliance safety circuit, not the thermostat wire. It controls the 24 Vac supply to the HOM and uses the end switch signal to hold the appliance off until the damper is confirmed open. Wiring the HOM directly to the thermostat bypasses the safety circuit and the interlock does not function correctly.
Installation Notes Worth Knowing
- Size the HOM to the combustion air duct diameter, not the appliance inlet collar.
- The adjustable extension sleeve covers wall thicknesses from 7.5" to 14". Measure before ordering on unusual construction.
- The 1/4" intake screen ships installed. Remove it only for exhaust applications.
- Flashing for siding or stucco installation is included in the box.
- The HOM is not rated for dryer exhaust. Lint accumulation in the motor assembly causes premature failure. Use the Hoyme BDH Back Draft Hood for dryer exhaust terminations.
The HOM at a Glance
The Hoyme HOM Motorized Combustion Air Damper is CSA certified for Canadian and US installations, available in 4" through 9" diameters, and built on Hoyme's patented spring motor drive with a stall-protected Synchron motor and nylon bearing assembly rated for outdoor temperature exposure and freeze-thaw cycling. It carries a 2-year electrical warranty and a 10-year mechanical warranty.
View the HOM Combustion Air Damper
Hoyme Manufacturing is a CSA-certified manufacturer of motorized dampers and airflow controls based in Camrose, Alberta. Products are available through HVAC wholesalers across Western Canada.