Frequently Asked Questions
Technical answers for HVAC contractors, mechanical engineers, and wholesalers. If you do not find what you need here, call 1-800-661-7382 or visit the Technical Library.
Combustion Air Dampers — HOM
When is a combustion air damper required in Canada?
A motorized combustion air damper is required under CAN/CSA B149 — the Natural Gas and Propane Installation Code — when the combustion air opening is equipped with a closeable control device. Once you install a motorized damper on a combustion air intake, the code requires it to be interlocked with the appliance so the appliance cannot fire until the damper proves open.
On tight modern construction where natural infiltration cannot reliably supply combustion air, a controlled intake is increasingly required regardless of appliance type. Requirements vary by edition of B149 and by jurisdiction — confirm with the authority having jurisdiction before finalizing a design.
Read the full guide: When Is a Combustion Air Damper Required in Canada? ›
What is the difference between SO and SF on the HOM — which one do I need?
SO (Spring Open / Power Close) springs open on power loss. The fail-safe direction is open, keeping the combustion air path clear if power is interrupted while the appliance is firing. SF (Spring Fail / Power Open) opens when powered and spring-closes on power loss — the damper seals if power is lost.
For combustion air intake on furnaces, boilers, fireplaces, and water heaters, SO is correct. If power is interrupted while the appliance is running, the spring opens the damper and keeps combustion air available. SF is for exhaust or controlled make-up air applications where the damper must seal on power loss.
For ADP relay adaptor combustion air interlock, specify the SO2 variant — it includes the relay and the end switch. SO and SF cannot be changed after installation. Confirm the required action with the mechanical engineer or the appliance manufacturer's installation instructions before ordering.
Does the HOM need to interlock with the furnace, or can I wire it to stay open continuously?
It needs to interlock. A combustion air damper wired to stay open continuously provides no benefit over a passive hood — you have installed a motorized product and are operating it as a passive one. On systems where code requires controlled intake, continuous open operation does not satisfy the requirement.
The correct installation has the HOM closed when the appliance is off and opening only when the appliance calls for heat. The ADP relay adaptor manages that interlock and holds the appliance off until the HOM end switch confirms the damper is fully open.
What happens if the damper does not open and the furnace tries to fire?
With proper ADP interlock and the end switch variant (SO2), the furnace cannot fire. The relay circuit holds the appliance off until the end switch confirms the HOM is fully open. If the blade is frozen or mechanically blocked, the appliance stays off.
Without interlock, the appliance fires regardless of damper position. On a cold morning with a frozen blade, combustion air is sourced from building infiltration — creating negative building pressure, combustion quality problems, and carbon monoxide risk. The interlock is the protection, and the end switch is what makes the interlock reliable rather than theoretical.
What size combustion air damper do I need?
Match the HOM to the combustion air duct diameter — not the appliance inlet collar. The HOM is available in 4", 5", 6", 7", 8", and 9" diameters. Duct sizing for combustion air is determined by the total BTU input of all appliances sharing the combustion air source and the duct run length, per CAN/CSA B149 Appendix D or the appliance manufacturer's instructions.
If the duct is already sized and installed, match the HOM diameter to the existing duct. For new installations, have the combustion air duct sized by the mechanical engineer or per code before selecting the damper size.
Can I use the HOM for dryer exhaust?
No. Lint from dryer exhaust accumulates in the motor assembly and causes premature failure. The HOM is not rated or suitable for dryer ventilation. Use the Hoyme BDH Back Draft Hood for dryer exhaust terminations — it is specifically designed for that application.
What is the difference between the HOM and the MOH?
The HOM installs inline in the duct run — the exterior termination is a separate hood (OOH or existing opening). The MOH is a motorized all-weather hood that replaces the exterior hood fitting and provides damper control at the wall penetration in one combined unit. Use the HOM when you want the control point in the duct run. Use the MOH when combining the hood and damper in one exterior unit is preferred.
Fresh Air Dampers — HAC
What is the difference between Power Open and Power Close on the HAC?
The action code describes the fail-safe direction — what the damper does when power is removed.
PO — Power Open. The damper opens when 24 Vac is applied. When power is removed, the spring closes it. The duct is sealed when the system is off. Power Open is correct for fresh air intake, make-up air, and HRV/ERV duct control on residential forced-air systems.
PC — Power Close. The damper closes when 24 Vac is applied. When power is removed, the spring opens it. The duct stays open when the system is off. Power Close is specified only when the system design requires the duct to remain open on power loss.
Neither is universally correct — each is the wrong choice in the wrong application. The Hoyme HAC actuator is accessible for field reconfiguration if requirements change after installation.
Why is there a cold air blast when my ventilation system starts?
This is the cold air dump problem. It happens when the furnace fan is wired to energize at the same moment as the fresh air damper — the fan starts pulling outdoor air through a partially open damper before the damper has reached full open.
The fix is the ADP-0241-S5A relay adaptor, which opens the HAC damper first, waits for the end switch to confirm the blade is fully open, and only then energizes the furnace fan. This sequencing also satisfies ventilation code proving requirements. The HAC must be ordered in the SPO configuration — Power Open with end switch — for this to work.
Can the HAC action be changed in the field?
Yes. The Hoyme HAC actuator is accessible for OPO to OPC field reconfiguration if requirements change after installation. This is a competitive advantage — some products in this category cannot be field-changed and require full damper replacement. Confirming the correct action before the order is placed is still the better outcome, but field reconfiguration is available on the Hoyme HAC if needed.
What is the HAC-H and when do I need it?
The HAC-H is the heat bypass version of the HAC, with a 10% heat bypass blade machined into the damper. It is required for supply air zone control on single-speed furnaces.
On a single-speed furnace, closing all supply zones simultaneously while the furnace fires pushes the full air volume against a blocked system. Static pressure spikes, heat exchanger airflow drops, and high-limit switches trip. The 10% bypass blade on the HAC-H maintains minimum airflow through each closed zone and prevents that failure mode. Use the standard HAC for fresh air, make-up air, and return air zone control. Use the HAC-H for supply zone control on single-speed systems.
What is the difference between the HAC and the HAC-L?
The HAC-L is the factory-insulated version of the HAC. The insulated body is appropriate where the fresh air duct passes through conditioned space and condensation on the damper body is a concern — typically in cold climate applications where the duct runs through a heated mechanical room before reaching the exterior wall. Both products use the same motor, same configurations, and same part number structure. The HAC-L part number includes "L" after the size code.
Motorized Flue Dampers — HAC-F
Why does a motorized flue damper close when it is powered?
The HAC-F uses Power Close / Spring Open (SPC-F) action — it closes when 24 Vac is applied and springs open on power loss. This is the only correct fail-safe direction for a flue damper on a fuel-burning appliance.
If power is lost while the appliance is firing, the spring immediately opens the damper and keeps the flue path clear. A flue damper that failed closed on power loss would trap combustion gases and create carbon monoxide risk. In normal operation the HAC-F sits spring-open when the appliance is off, confirming position before the appliance fires, then closing to reduce standby heat loss through the flue when the call ends.
Full guide: Motorized Flue Damper — Why It Closes When Powered ›
What is the difference between the HAC-F and the HAC-FS?
The HAC-F is CSA certified. The HAC-FS uses the same damper body and actuator without CSA certification. On any permitted residential or commercial job where the authority having jurisdiction verifies equipment, only the HAC-F satisfies the requirement.
Do not substitute the HAC-FS on a code-required job. If the AHJ verifies and the non-certified product is installed, it comes out. The HAC-FS exists for applications where certification is genuinely not required.
Can I use a standard HAC inline damper on a flue application?
No. A standard HAC with Power Open / Spring Close action fails closed on power loss. On a flue application, a closed flue during operation traps combustion gases and creates carbon monoxide risk. The HAC-F SPC-F actuator is specifically selected for flue applications because it fails open — the spring opens the damper on power loss, keeping the flue path clear. Do not substitute a standard HAC on a flue application regardless of size or voltage match.
Does the HAC-F need to interlock with the appliance?
Yes — and every HAC-F ships with a relay and end switch included for exactly that reason. A motorized flue damper without appliance interlock is an incomplete safety installation. The end switch confirms damper position. The relay provides the switched output for the appliance safety circuit. Without interlock, the appliance can fire regardless of damper position.
Operating a motorized flue damper on a gas appliance without appliance interlock is a safety risk and a code violation on permitted work in most Canadian and US jurisdictions.
ADP Interface Relay Adaptors
Which ADP relay adaptor do I need?
Selection comes down to two questions: what is the appliance, and what is the system voltage.
- Gas furnace with millivolt safety system (thermopile or thermocouple): ADP-0MV2-S5A — the only model rated for millivolt voltage levels
- Gas furnace combustion air, 24 Vac system: ADP-0240-S5A
- Fresh air damper and furnace fan sequencing, 24 Vac: ADP-0241-S5A
- Boiler or tankless water heater, 110 Vac, up to 5A: ADP-1102-S5A
- Boiler or tankless water heater, 110 Vac, larger appliances: ADP-1102-10A
- Gas dryer interlock, 24 Vac gas valve: RADP-0240
- Gas dryer interlock, 110 Vac: RADP-0110
- Two-zone furnace fan control, 24 Vac: ADP-0244-05A
Confirm the appliance system voltage from the electrical specification label before ordering. The ADP wires into the appliance safety circuit — not the thermostat wire.
Does the ADP relay adaptor affect the air conditioning circuit?
No. All ADP models are designed so the relay logic does not affect the air conditioning circuit. The A/C system operates independently of the damper interlock circuit on all ADP installations.
What is the difference between ADP-0240-S5A and ADP-0241-S5A?
These are two different applications on the same 24 Vac furnace system.
ADP-0240-S5A is for combustion air interlock — the HOM damper must prove open before the furnace fires. It holds the appliance safety circuit open until the damper end switch confirms the blade is fully open.
ADP-0241-S5A is for fresh air ventilation sequencing — the HAC damper must prove open before the furnace fan energizes. It is the fix for the cold air dump problem and satisfies ventilation code proving requirements on fresh air installations.
Do I need the end switch on the damper to use an ADP relay adaptor?
Yes. The ADP interlock sequence relies on the end switch signal from the damper to confirm the blade has physically reached its open position. Without the end switch, the ADP has no position confirmation and the interlock is not closed-loop.
For combustion air with ADP: order the HOM in SO2 variant (relay and end switch included). For fresh air sequencing with ADP-0241-S5A: order the HAC in SPO or SPC configuration (end switch included). If the damper on site does not have an end switch, the ADP interlock circuit will not function correctly.
Zone Control Dampers
Which HAC configuration do I need for zone control?
The correct action depends on the system and how the zone control panel handles the 24 Vac signal. For return air zone control, Power Open works in most configurations. For zone damper applications on variable speed systems, either action can be appropriate. Confirm with the zone control manufacturer's wiring documentation.
For supply air zone control on a single-speed furnace, use the HAC-H with the 10% heat bypass blade — not the standard HAC. Closing all supply zones simultaneously on a single-speed furnace creates static pressure problems and high-limit trips. The bypass blade maintains minimum airflow through each closed zone and prevents that failure mode.
What sizes are available for zone control applications?
The standard HAC series covers 2" through 9" round duct for fresh air and return air zone control applications. For larger supply zone control applications in commercial or multi-family construction, the HAC-50/51 quarter-turn series covers 6" through 14" round duct and uses a quarter-turn actuator rather than the spring-return motor drive. The HAC Large Diameter Belimo series covers applications up to 30" using Belimo actuators.
Match the damper diameter to the duct diameter at the installation point. For zone control sizing guidance, contact technical support at 1-800-661-7382.
Ordering and Availability
Where can I buy Hoyme products?
Hoyme products are stocked at HVAC wholesalers across Canada including ECCO Supply, EMCO, Andrew Sheret, Bartle and Gibson, Alggin, and City Electric Supply. Use the Find a Wholesaler page to locate your nearest stocking location by province.
Products can also be ordered directly at hoyme.com with shipping across Canada and the United States.
Are Hoyme products CSA certified?
Yes. Hoyme motorized dampers including the HAC, HOM, HAC-F, MOH, and ADP relay adaptors are CSA certified for Canadian and US installations. Hoyme Manufacturing has held CSA certification since 1984.
The HAC-FS is the only product in the Hoyme lineup that is not CSA certified — it is clearly identified as non-certified on the product listing. The HCR/HSI slip-in damper series is non-returnable and has specific application limitations noted on the product page.
What is the warranty on Hoyme products?
Hoyme motorized dampers carry a 2-year electrical warranty and a 10-year mechanical warranty. The warranty covers defects in materials and workmanship under normal installation and use conditions. For warranty service, contact Hoyme Manufacturing directly at 1-800-661-7382 or through the Contact page.
Where can I get installation instructions and wiring diagrams?
Installation instructions and colour wiring diagrams for every Hoyme product are available at no cost on the Technical Library page. All documents are downloadable PDFs. Each ADP relay adaptor ships with a colour wiring diagram specific to the model in the box.
What is the 500,000 cycle motor life claim based on?
Hoyme's patented spring motor drive uses a stall-protected Synchron motor with a nylon bearing assembly specifically engineered for the mechanical stresses of a combustion air intake: freeze-thaw cycling, outdoor temperature exposure, and the stall loads that occur when a blade is held against ice or elevated static pressure. Stall-protected means the motor holds against load without tripping or burning out.
The 500,000 cycle rating reflects the design intent of the motor and bearing assembly for long-service installation in HVAC applications. It is supported by Hoyme's 10-year mechanical warranty — the strongest in the category.
Still Have Questions?
Hoyme technical support has been answering installation and specification questions for over 40 years. Call us before the order ships — not after the damper is in the wall.