Every Hoyme motorized damper installation needs an interlock circuit. If you are installing a combustion air damper, read that guide first before selecting your ADP model.
The ADP Interface Relay Adaptor is the pre-engineered way to build that circuit correctly — the relay ratings, input voltage, output logic, and sequencing are already matched to the application. You wire it in, follow the colour wiring diagram in the box, and the interlock works the way it is supposed to.
There are eight models. Picking the right one comes down to two questions: what is the appliance, and what is the system voltage.
Why the ADP Exists
Sourcing relays and building an interlock circuit in the field is possible, but there are ways it goes wrong. Relay coil selection for a millivolt or 24 Vac safety circuit requires matching the coil voltage and current draw to the source — getting that wrong loads the safety circuit and can cause nuisance lockouts or damage the safety system. The sequencing logic on fresh air applications, where the damper has to prove open before the fan energizes, is non-trivial to get right in a junction box. And on permitted jobs, the inspector wants to see documented wiring, not a field-assembled relay circuit.
The ADP handles all of that in advance. It also means the A/C circuit is not affected by the interlock logic on any model — a common concern that the ADP addresses by design.
Model Selection: Match to Your Application and Voltage
Start with the appliance type, then confirm the system voltage.
- Gas furnace with millivolt safety system (standing pilot, thermopile or thermocouple): ADP-0MV2-S5A. This is the only model in the series rated for millivolt voltage levels (250mVdc to 100Vdc). Standard 24 Vac models will not work on these systems.
- Gas furnace combustion air, 24 Vac system: ADP-0240-S5A. Damper proves open before the furnace fires.
- Fresh air damper and furnace fan sequencing, 24 Vac: ADP-0241-S5A. Opens the HAC damper first, waits for end switch confirmation, then energizes the furnace fan. See the note on cold air dump below.
- Boiler or tankless water heater combustion air, 110 Vac, up to 5A load: ADP-1102-S5A.
- Boiler or tankless water heater combustion air, 110 Vac, up to 10A load: ADP-1102-10A. Same application as the S5A but for larger appliances with higher electrical load at the safety circuit.
- Gas dryer interlock, 24 Vac gas valve circuit: RADP-0240. Includes a manual red indicator light and switch for visual confirmation and service bypass.
- Gas dryer interlock, 110 Vac: RADP-0110. Same as RADP-0240 at line voltage.
- Two-zone furnace fan control, 24 Vac: ADP-0244-05A. Coordinates fan sequencing for two independent zone damper circuits.
If you are not certain which voltage the appliance safety circuit operates at, check the electrical specification label on the appliance before ordering.
The End Switch Is Not Optional
This is the most common installation error on ADP applications. The interlock sequence relies on the end switch signal from the damper to confirm the blade has physically reached its open position. Without it, the ADP has no position confirmation and cannot hold the appliance off until the damper is fully open.
When ordering the HOM combustion air damper for use with an ADP, specify the SO2 variant — it includes the relay and the end switch. The SO1 variant has neither and cannot be used in an ADP interlock circuit.
When ordering the HAC inline damper for fresh air sequencing with the ADP-0241-S5A, specify the SPO or SPC variant — both include the end switch.
If the damper is already on site without an end switch, the ADP interlock circuit will not function correctly. The end switch has to be on the damper.
The Cold Air Dump Problem and the ADP-0241-S5A
On fresh air ventilation systems, one of the most frequent complaints from contractors and homeowners is a blast of cold air at the supply registers when the ventilation cycle starts. This happens when the furnace fan energizes before the fresh air damper has had time to open — outdoor air below freezing enters the duct and goes straight through the system before anything can temper it.
The ADP-0241-S5A is built specifically to prevent this. On a ventilation call, it opens the HAC damper first, waits for the end switch confirmation that the blade is fully open, and only then energizes the furnace fan. When the ventilation cycle ends, it reverses the sequence — fan off first, then damper closes. Wiring the fan and damper in parallel, which is the most common approach in the field, skips this sequencing entirely and is the direct cause of cold air discharge complaints.
If an existing fresh air system is producing cold discharge at the registers on ventilation calls, the ADP-0241-S5A is the correct fix. It is also the correct specification for any new fresh air installation to Canadian or US ventilation code.
What Is in the Box
Every ADP ships with the relay enclosure pre-wired and a colour wiring diagram specific to the model. No relay design is required in the field. The relays are field-replaceable — a relay failure does not require replacing the complete assembly.
You supply all field wiring between the ADP, the damper end switch, the damper motor, and the appliance safety circuit. Wire gauge and routing per local electrical code.
A Note on BAS and DDC Systems
The ADP is the right solution for residential and light commercial installations where the interlock is not handled by a building automation system. If the job has a BAS or DDC controller with its own relay outputs, wire the damper end switch directly to the controller input and use the controller's relay output to power the damper motor. The ADP is not required in that configuration.
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.