A contractor picks up a Hoyme HAC-F at the counter, wires it up, and calls for the appliance to fire. The damper closes. That seems wrong. A flue damper should open when the appliance fires, not close.
It is not wrong. It is the only correct design for a motorized flue damper on a fuel-burning appliance, and understanding why changes how you think about flue damper selection entirely. This guide covers how the HAC-F works, why the interlock is a code and safety requirement, and how to wire it correctly for every common appliance type.
Why the Flue Damper Closes When Powered
The HAC-F uses Power Close / Spring Open action, designated SPC-F. When 24 Vac is applied, the damper closes. When power is removed or lost, the spring opens the damper and the flue is clear.
That seems backwards until you understand what the fail-safe direction needs to be. A motorized flue damper on a fuel-burning appliance has one non-negotiable requirement: if power is lost while the appliance is firing, the flue path must stay open. Combustion gases including carbon monoxide must be able to vent regardless of what happens to the electrical system.
A flue damper that fails closed on power loss traps combustion gases in the appliance and heat exchanger. On a gas furnace or boiler the result is carbon monoxide accumulation and potential backdrafting into the building. The spring is the safety mechanism. It works without electricity, without a control signal, and without any part of the control system functioning. When everything else fails, the flue opens.
So in normal operation, the HAC-F sits spring-open when the appliance is off — the flue is clear before the appliance ever fires. When the appliance calls to fire, the control system energizes the HAC-F, the damper closes to reduce standby heat loss through the flue, and the interlock sequence runs. If power is lost at any point, the spring immediately opens the damper. The fail-safe is always in the direction of a clear flue.
How the Interlock Sequence Works
The interlock sequence on a properly installed HAC-F with an ADP relay adaptor works like this:
- The appliance calls to fire.
- The ADP relay adaptor energizes the HAC-F. The damper closes (Power Close action).
- The end switch on the HAC-F confirms the damper has physically reached the closed position.
- The ADP receives that confirmation and releases the appliance safety circuit.
- The appliance fires.
- When the call ends, the appliance shuts down and 24 Vac is removed from the HAC-F.
- The spring opens the damper. Flue is clear.
If the HAC-F fails to reach the closed position — motor fault, mechanical obstruction, frozen blade — the end switch does not confirm, the ADP does not release the safety circuit, and the appliance does not fire. That is the correct fail-safe behaviour. The appliance is held off until the system is confirmed ready.
Without ADP interlock, the appliance fires regardless of damper position. That is the unsafe installation and a code violation on permitted work in most Canadian and US jurisdictions.
The Interlock Is a Requirement, Not a Recommendation
Every HAC-F ships with the relay and end switch included because a motorized flue damper without appliance interlock is an incomplete safety installation. The end switch confirms the damper position. The relay provides the switched output that feeds the appliance safety circuit or ADP relay adaptor. Without both, the interlock cannot be wired correctly.
Always interlock the HAC-F with the appliance controls so the damper proves its position before the appliance fires. On permitted residential and commercial installations in Canada and the US, operating a motorized flue damper without appliance interlock is a safety risk and a code violation.
Selecting the Right ADP Relay Adaptor
The ADP model depends on the appliance type and system voltage. Three models cover the common residential and light commercial applications:
- Gas furnace with millivolt safety system (standing pilot, thermopile or thermocouple): ADP-0MV2-S5A. This is the only model rated for millivolt voltage levels. Standard 24 Vac models will not work on millivolt systems.
- Gas furnace with 24 Vac system: ADP-0240-S5A. Confirms the HAC-F end switch before releasing the furnace safety circuit.
- Boiler or tankless water heater at 110 Vac: ADP-1102-S5A for loads up to 5 amps, ADP-1102-10A for larger appliances.
Confirm the appliance system voltage before ordering the ADP. Check the electrical specification label on the appliance if the voltage is not clear from the installation instructions.
HAC-F vs HAC-FS — Certification Matters on Permitted Jobs
The HAC-F is CSA certified. The HAC-FS uses the same damper body and actuator without CSA certification. It exists for non-permitted applications where the authority having jurisdiction genuinely does not require certification.
On any permitted residential or commercial job in Canada or the US where the AHJ will verify equipment, only the HAC-F satisfies the requirement. The difference between a certified and non-certified flue damper is a documented product qualification that an inspector can and will check. Do not substitute the HAC-FS on a code-required job as a cost-saving measure. If the AHJ verifies and the wrong product is installed, it comes out.
Can I Use a Standard HAC on a Flue Application?
No. A standard HAC with Power Open / Spring Close action fails closed on power loss. On a flue application that means the flue seals if power is interrupted while the appliance is firing. On a gas or oil appliance a closed flue during operation creates carbon monoxide accumulation and backdraft risk. The HAC-F SPC-F actuator is selected specifically for flue applications because it fails open. Do not substitute a standard HAC on a flue application regardless of size or voltage match.
Size Is the Only Selection
Every HAC-F in the 4" to 9" range ships identically configured — relay included, end switch included, Power Close / Spring Open action on every unit. Select size only. Match the HAC-F size to the flue or vent pipe diameter exactly.
- 4" flue: HAC-0411-SPC-F
- 5" flue: HAC-0511-SPC-F
- 6" flue: HAC-0611-SPC-F
- 7" flue: HAC-0711-SPC-F
- 8" flue: HAC-0811-SPC-F
- 9" flue: HAC-0911-SPC-F
For flue runs 10" to 30", see the HAC-F Large Diameter series which uses a Belimo TFB24 actuator with the same SPC-F spring-open fail-safe.
Installation Planning Notes
- Confirm the flue or vent pipe diameter before ordering. The HAC-F installs inline in the round flue or vent run serving the appliance.
- Plan interlock wiring at rough-in. The ADP relay adaptor wires to the HAC-F motor, the HAC-F end switch output wires back to the ADP input, and the ADP connects to the appliance safety circuit.
- Confirm clearances around the actuator housing. Flue runs are often tight. Service access to the actuator needs to be planned before the wall is closed.
- Confirm the appliance manufacturer's installation instructions specify or accept a motorized flue damper before installation.
View the HAC-F Motorized Flue 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.