PRE-ENGINEERED DAMPER INTERLOCK · 24 MODELS · MILLIVOLT · 24 VAC · 110 VAC
ADP Interface Relay Adaptors
Where a Hoyme damper has to interlock with a circuit it cannot connect to directly, the ADP series does that work in advance: each model is pre-engineered for a specific damper and appliance sequence, with matched relay ratings, wiring logic, and color diagrams in every box.
Interlock handled by a BAS or DDC controller? Wire the damper end switch directly to the controller input: the ADP is for non-BAS residential and light commercial installations.
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Not sure it's the right part? Call 1-800-661-7382 for free technical support.
WHY IT MATTERS
What Happens Without Proper Interlock
A combustion air damper that is not interlocked with the appliance it serves is a code violation and a safety risk on every permitted residential and commercial job in Canada and the U.S.
Without interlock, the appliance fires regardless of damper position.
In a tight modern home, that means the appliance is operating under negative pressure, pulling air past backdraft hazards and potentially down unsealed chimneys and other appliance vents. Combustion quality drops, carbon monoxide production rises, and the risk of backdrafting combustion products into the living space increases with every firing cycle.
An interlock prevents this: the appliance cannot fire until the damper end switch confirms the combustion air path is open. If the damper fails to open (frozen blade, motor fault, wiring problem) the appliance stays off and the fault is detectable rather than invisible.
Pre-matched relay ratings
Getting relay coil selection wrong on a millivolt or 24 Vac safety circuit loads the circuit and causes nuisance lockouts or damage. The ADP is pre-matched to the system voltage, no relay selection required in the field.
Documented for inspection
A field-assembled relay circuit in a junction box does not satisfy the documentation requirement inspectors look for on permitted installations. The ADP enclosure and color wiring diagram do.
Field-replaceable relays
A relay failure does not require replacing the complete ADP assembly. Relays are field-replaceable. Order the relay, swap it in place. Up to 4 relays in a single enclosure depending on model.
ADP SERIES vs. FIELD-ASSEMBLED RELAY CIRCUIT
Field-Assembled Relay Circuit
Hoyme ADP Series
MODEL SELECTOR
Find Your ADP Model
All 24 models across three voltage groups. Find your system voltage, then match your application: each row is a unique ADP model verified against installation instructions and the 2026 price list.
The end switch on the damper is required on proving applications.
Proving adaptors (the TPX models and the RADP gas dryer models) wait for the damper end switch to confirm the blade has opened. Without it, the sequence cannot complete. For these, order the HAC as SPO or SPC. The HOM (SF1 and SF2) always includes its end switch. The ADP-0241-05A and ADP-1101-05A do not use an end switch: order HAC OPO or OPC. This is the most common installation error on ADP applications.
Not sure which model fits your system?
Call technical support with your appliance type, system voltage, and damper model: we'll confirm the right ADP before you order.
MODEL DETAILS
What Each Model Does, and Why It's Different
Each ADP model is engineered for a specific application. Selecting the wrong model for the system voltage will not function correctly and may damage the safety circuit.
Covering the nine primary decision points. For all 24 models including dual-fan, zone, and special adaptors, see the complete selector above.
Combustion Air Interlock for Standing Pilot Furnaces
Older gas furnaces with a standing pilot use a thermopile or thermocouple safety system that operates at millivolt voltage levels, 250mVdc to 100Vdc. That voltage is too low and carries too little current to drive a standard relay without loading the safety circuit and causing nuisance lockouts or damage.
The ADP-0MV2-S5A is designed specifically for this voltage range. Two relays handle the interlock logic: the first accepts the millivolt signal from the safety system, the second provides the 24 Vac output to the damper motor and the proved-open return signal. A test/run switch allows commissioning without cycling the appliance.
Use when: the furnace has a standing pilot, thermopile, or thermocouple. If the furnace predates 24 Vac electronic ignition controls, this is the correct model.
Fresh Air Damper + 120 Vac Fan, 10A Contacts
Same application as the ADP-0241-05A, with a second relay rated 10A for switching a 120 Vac fan. It is not a combustion air adaptor: on a 24 Vac furnace or boiler, the HOM wires directly into the thermostat circuit and needs no ADP.
Use when: a 24 Vac ventilation switch has to start a 120 Vac fan along with the fresh air damper. For HOM combustion air on a 24 Vac furnace, no adaptor is needed. For a standing pilot furnace, use ADP-0MV2-S5A.
Fresh Air Damper + Furnace Fan: Ventilation Code Compliance
The standard ADP model for 24 Vac fresh air ventilation. A single relay (DPDT, 24 Vac coil) activates when the ventilation switch or exhaust fan switch closes: simultaneously opening the 24 Vac HAC fresh air damper and turning on the furnace circulation fan. One switching action handles both functions together.
In the standard configuration, the fresh air damper does not open during furnace firing: ventilation calls only. With a Power Close damper and relay, the damper can optionally also open during furnace firing.
Need damper to prove open BEFORE the exhaust fan is allowed to start? That requires end-switch proving logic: use RADP-1102-TPX (plug-in fan) or ADP-1103-TPX (hardwired fan).
Combustion Air Interlock for Boilers and Tankless Water Heaters
Both models interlock the HOM combustion air damper with 110 Vac boiler or tankless water heater control circuits. The damper proves open before the appliance is allowed to fire. The only difference is relay output rating: confirmed by the appliance's electrical specification label.
ADP-1102-S5A
5A relay: most residential boilers and standard tankless water heaters
110 Vac · up to 5A load
ADP-1102-10A
10A relay: larger commercial-grade boilers and higher-load tankless units
110 Vac · up to 10A load
⚠ Confirm the appliance load rating before selecting. The correct relay rating is on the appliance's electrical specification label. Wrong relay output for the load can cause relay failure.
110 Vac Exhaust Fan → Fresh Air Damper + Furnace Fan
The 110 Vac equivalent of the ADP-0241-05A and the standard 110 Vac ventilation adaptor. A single relay (DPDT, 120 Vac coil) activates when the 120 Vac exhaust fan switch closes: simultaneously opening the 24 Vac fresh air inlet damper and turning on the furnace circulation fan.
Designed specifically to not interfere with the combustion air damper circuit or the A/C circuit. The combustion air damper opens only when the furnace fires: the ADP-1101-05A ventilation circuit operates independently. In standard wiring, the fresh air damper does not open during furnace firing.
Need the damper to prove open before the exhaust fan starts? Use RADP-1102-TPX or ADP-1103-TPX: the proving models. ADP-1101-05A is simultaneous operation only.
Damper Proves Open Before Exhaust Fan Starts
Both models interlock the exhaust fan to a fresh air damper with end switch using a proving sequence: the ventilation switch opens the damper, the damper end switch must confirm the blade has physically reached its open position, and only then does the exhaust fan start. The furnace circulation fan starts when the damper opens. The ADP-1101-05A runs everything simultaneously; these proving models hold the exhaust fan off until the damper confirms open.
RADP-1102-TPX
Plug-in exhaust fan via split duplex receptacle. Damper does NOT open during furnace firing. 2 relays (C120V DPDT + C24V SPDT 10A).
Diagram #12A · plug-in fan
ADP-1103-TPX
Hardwired fan. Damper ALSO opens during furnace firing. 3 relays (C120V DPDT + 2× C24V SPDT 10A).
Diagrams #12B1, #12B4 · hardwired fan
⚠ End switch on the damper is required. Without the end switch, the adaptor has no position confirmation signal and the proving sequence cannot function. Order HAC with SPO or SPC variant.
Automatic Timed Ventilation: Grasslin 24 Hr Timer
When a ventilation code requires automatic, scheduled ventilation without manual intervention, the TMADP-1102-TPX handles both the scheduling and the circuit control. The built-in Grasslin 24 hr interval timer activates the ventilation sequence at set intervals: simultaneously starting the principal exhaust fan, turning on the furnace circulation fan, and opening the fresh air inlet damper.
A central ventilation switch also provides manual override at any time. In the standard configuration, the fresh air damper does not open during furnace firing. The enclosure is larger than standard ADP models (8×4×2.5" vs 5×4×2.5") to accommodate the timer mechanism. Refer to the Grasslin Timer Centre Pin Settings guide for programming.
Use when: ventilation code requires automatic scheduled ventilation rather than occupant-controlled ventilation. If damper must also open during furnace firing, use TMADP-1103-TWP. Wiring diagrams #9, #9A, #9A1, #9A2, #9B, #9B1, #9C, #9C1, #10.
Gas Dryer Interlock with Red Indicator Switch
Gas dryers installed with a motorized fresh air or combustion air damper need an interlock: the dryer cannot operate until the damper proves open. The RADP-0241-S10A includes a manual red light on/off switch: turning it on opens the damper and proves its position, then the dryer receptacle becomes live. Visual confirmation the circuit is active is provided by the indicator light remaining on while the dryer runs.
The dryer plugs directly into the GAS DRYER side of the split duplex receptacle, no hardwiring of the dryer connection required. The included 5 ft plug-in cord mounts near the dryer for easy switch access.
110 Vac gas dryer applications: contact factory. The current product lineup does not include a direct 110 Vac equivalent with the same red indicator proving circuit.
Two-Zone Furnace Fan Control
On two-zone systems with independent zone damper circuits, the furnace fan must respond correctly to zone calls from both circuits. Where the thermostat or zone controller does not directly manage fan sequencing, the ADP-0244-05A coordinates fan operation: preventing the fan from running when neither zone is calling and handling correct fan start/stop logic when one or both zones are active.
Use when: two-zone HAC or HAC-H system where the thermostat or zone controller does not directly manage furnace fan sequencing for both zones.
BEFORE YOU ORDER
The End Switch on the Damper Is Not Optional
Several ADP models rely on the end switch signal from the damper to complete the interlock sequence. Without it, those models have no position confirmation and the interlock cannot function. This is the most common installation error on ADP applications.
What the end switch does, and which models need it
A motor receiving 24 Vac does not confirm the blade has actually moved. The end switch is a physical microswitch mounted on the damper body that closes when the blade reaches its open position: providing the proved-open signal that confirms the damper has physically arrived at open, not just that power was applied.
Proving models, those with "TPX" in the model number and the RADP-0241-S10A, use this signal to hold the appliance or exhaust fan off until proved-open is received. Simultaneous models (ADP-0241-05A, ADP-1101-05A) activate all outputs at once and do not require an end switch. The compatibility table below shows which applies to each model.
On proving models: what goes wrong without end switch:
Order the damper with end switch: where required:
SF1 or SF2, end switch built in
Relay and end switch included on every HOM. On 24 Vac appliances the HOM wires directly into the thermostat circuit with no ADP. Use ADP-0MV2-S5A for millivolt systems or ADP-1102-S5A for 120 Vac safety controls.
Specify SPO or SPC variant, for proving models only
S prefix = end switch included: required for RADP-1102-TPX and ADP-1103-TPX proving sequences. ADP-0241-05A and ADP-1101-05A use OPO or OPC (no end switch).
Order with end switch option
Required for RADP-0241-S10A gas dryer interlock proving circuit
The end switch: what to order
The HAC damper shown here is the SPO variant: S prefix confirms end switch included. When the blade reaches fully open, the switch closes and sends the proved-open signal to the ADP relay adaptor. Order HAC with S prefix for proving model applications, or the HOM (SF1 or SF2) for combustion air applications. For ADP-0241-05A and ADP-1101-05A (simultaneous models), use OPO or OPC, no end switch needed.
SHOP ADP SERIES
Order the Right ADP Model
All 24 models ship from the same product page. Select your model at checkout. The 8 primary models are shown below. For all 24 models including timer, dual-fan, and proving adaptors, see the complete selector above.
ADP Interface Relay Adaptors, 24 Models
ADP-0MV2-S5A · ADP-0241-10A · ADP-0241-05A · ADP-1102-S5A · ADP-1102-10A · ADP-1101-05A · RADP-0241-S10A · ADP-0244-05A · and 16 more. Select at checkout
QUICK REFERENCE: PRIMARY MODELS BY APPLICATION
Gas furnace: millivolt
Standing pilot · thermopile/thermocouple · only model rated for millivolt safety circuits
24 Vac ventilation switch, 120 Vac fan
Damper and 120 Vac fan up to 10A switch together · not needed for HOM combustion air on 24 Vac
Fresh air damper + furnace fan, 24 Vac
Ventilation switch simultaneously opens HAC fresh air damper and activates furnace fan · standard ventilation code model
Boiler / tankless, 110 Vac · up to 5A
Most residential boilers and standard tankless water heaters · confirm load rating before ordering
Boiler / tankless, 110 Vac · up to 10A
Larger commercial-grade boilers and tankless units · confirm load rating on appliance spec label
Fresh air damper + furnace fan, 110 Vac
120 Vac exhaust fan switch simultaneously opens fresh air damper and activates furnace fan · the standard 110 Vac ventilation adaptor
Gas dryer interlock, 24 Vac
Red indicator switch proves damper open before dryer operates · split receptacle · dryer plugs in directly · 5 ft cord included
Two-zone furnace fan control, 24 Vac
Two independent zone circuits · HAC or HAC-H · where zone controller doesn't manage fan sequencing
COMPANION DAMPERS: WHAT TO ORDER
For combustion air
HOM Combustion Air Damper
Order SF1 or SF2, end switch built in · wires direct on 24 Vac · ADP-0MV2-S5A (millivolt) or ADP-1102-S5A (120 Vac)
For ADP-0241-05A and ADP-1101-05A
HAC Inline Damper: OPO or OPC variant
O prefix = no end switch · simultaneous models do not require an end switch
For RADP-1102-TPX and ADP-1103-TPX
HAC Inline Damper: SPO or SPC variant
S prefix = end switch included · proving models require end switch to complete interlock sequence
Not sure which ADP model fits your system?
Call technical support with your appliance type, system voltage, and damper model: we'll confirm the correct ADP and damper variant before you order.
COMMON QUESTIONS
ADP Relay Adaptor FAQs
Questions contractors and counter staff ask most often: answered directly from the product.
READY TO ORDER
The Right ADP, the Right Damper, the First Time.
Confirm your ADP model and end switch variant before ordering. Call technical support with your appliance type and system voltage.