CSA Certified · Hot Air Zone Control · 10% Bypass

HAC-H
Heat Bypass
Inline Damper

24 Vac  ·  Factory-Machined 10% Bypass  ·  CSA Certified  ·  2"–9"

  • ✓10% bypass machined into the blade at the factory — permanent, not field-adjustable
  • ✓Maintains minimum airflow through closed zones — prevents high-limit trips on single-speed furnaces
  • ✓Same patented Hoyme 24 Vac spring return drive as the standard HAC
  • ✓CSA certified for Canadian and US installations
  • ✓PO or PC — end switch and relay options available
  • ✓Relay not available on 2" and 3" sizes
  • ✓Motor, spring, and microswitch kits stocked — serviceable in place
Hoyme HAC-H Heat Bypass Inline Damper — 10% bypass blade
Is this the right product?
✔ Right product if
You need zone control on a hot air supply duct and full shutoff causes heat buildup, high-limit trips, or thermal stress in the closed zone — single-speed forced-air furnace systems.
✗ Not this product if
Fresh air intake, make-up air, outdoor air, or any ventilation application → use the standard HAC. The bypass is permanent — outdoor air will infiltrate continuously through it when the damper is commanded closed.

120 Vac site → HAC-12H

What It Does

For hot air duct zone control where full shutoff causes heat buildup. 10% bypass — factory built in, not field-modified.

The HAC-H is a CSA-certified motorized inline damper for hot air duct zone control applications where a continuous minimum airflow must be maintained even when the zone is not calling for heat. A 10% bypass is machined directly into the damper blade at the factory — not a field modification, not an adjustable insert, not removable. When the damper is commanded to close, 90% of the blade seals and 10% remains permanently open, maintaining a controlled minimum flow path through the closed zone.

On a single-speed forced-air furnace with zone dampers, closing all zones simultaneously while the furnace fires pushes the entire supply air volume against a blocked duct system. Static pressure spikes, airflow through the heat exchanger drops, and discharge air temperature climbs. Without a bypass, sustained full shutoff creates the conditions for high-limit switch trips — the furnace shuts off on over-temperature, the zone calling for heat gets nothing, and the cycle repeats until a service call. The 10% bypass in the HAC-H blade maintains a minimum flow path through every closed zone, keeping static pressure and heat exchanger airflow within normal operating range.

Do Not Install on Fresh Air, Make-Up Air, or Ventilation Applications The 10% bypass is always present and cannot be disabled. A HAC-H on a fresh air application means outdoor air infiltrates the building continuously through the bypass every hour the damper is commanded closed — every day, every season, winter and summer. The control system shows the damper as closed, but air is flowing through the bypass regardless of what the controller reads. If a HAC-H has been installed on a fresh air application in error, it must be replaced with a standard HAC.

HAC-H vs. Standard HAC — Application Determines the Product

One question settles the specification: does the application require full shutoff, or minimum continuous airflow?

Application Right Product Why
Hot air supply duct zone control — single-speed furnace HAC-H Full shutoff causes heat buildup and high-limit trips — bypass required
Hot air supply duct zone control — variable speed furnace HAC or HAC-H Variable speed modulates down — bypass may still be preferred; confirm with system designer
Fresh air intake HAC (standard) Full shutoff required — bypass allows continuous outdoor air infiltration
Make-up air supply HAC (standard) Full shutoff required — bypass defeats the purpose of controlled intake
HRV / ERV duct control HAC (standard) Full shutoff required — bypass allows reverse infiltration when unit is off
Return air zone control HAC (standard) Bypass not required on return side
Part number structure: HAC-[size][relay]-[action]-H. Example: HAC-0611-SPO-H = 6" diameter, relay included, Power Open, end switch included, heat bypass. Example: HAC-0410-OPC-H = 4" diameter, no relay, Power Close, no end switch, heat bypass. The −H suffix confirms the heat bypass variant on every order.

Key Specs

Sizes
2", 3", 4", 5", 6", 7", 8", 9"
Voltage
24 Vac
Bypass
10% — factory-machined, permanent
Drive
Patented Hoyme spring motor — stall-protected
Action
PO or PC — specify at order
End Switch
Optional (S prefix) — all sizes
Relay
Optional (11 motor) — 4"–9" only
Certification
CSA certified — Canadian & US
Not Suitable For
Fresh air, make-up air, ventilation, outdoor air
Warranty
2-yr electrical / 10-yr mechanical

Common Questions from the Counter

Answers to what comes up most when specifying the HAC-H.

What exactly is the 10% bypass and where is it in the damper blade?
The bypass is a precision-machined opening in the damper blade itself — not a separate spring-loaded vent, not an adjustable insert, not a hole added in the field. When the blade rotates to the closed position, 90% of the blade area seals against the duct liner and 10% remains as a permanent open flow path. The geometry is set at the factory and is consistent on every unit. You will not see it as a visible opening when the damper is in the fully open position — the bypass only becomes the active flow path when the blade closes.
Is the 10% bypass adjustable? Can I get a tighter seal for a specific application?
No. The bypass is machined into the blade at the factory and cannot be adjusted, reduced, or eliminated. If the application requires full shutoff — fresh air intake, make-up air, ventilation — you need the standard HAC, not the HAC-H. The HAC-H is a specific product for hot air zone control where the bypass is a required design feature, not a limitation.
What happens if a HAC-H is accidentally installed on a fresh air intake?
Outdoor air continuously enters the building through the 10% bypass whenever the damper is commanded closed — every hour, every day, year-round. In winter that means uncontrolled cold air infiltration raising heating loads and causing comfort complaints. The control system shows the damper as closed because the motor is in the closed position, but air is flowing through the bypass regardless of what the controller reads. Replace with a standard HAC.
Can the HAC-H be used on a variable speed furnace system?
On a variable speed system, the furnace fan modulates down when zones close, reducing the heat buildup risk that makes the bypass necessary on single-speed systems. Whether the HAC-H is required on a variable speed installation depends on the system design — some designers still specify the bypass as a safeguard, particularly on larger zone systems where pressure imbalances are more pronounced. Confirm with the mechanical engineer or system designer.
Is there a 120 Vac version of the HAC-H?
Yes — the HAC-12H combines the 120 Vac built-in transformer with the same 10% bypass blade. Use the HAC-12H when only 120 Vac is available at the damper location.
Related Products
HAC
Standard inline damper — full shutoff. For fresh air, make-up air, ventilation, and return air zone control.
HAC-12H
120 Vac version — same 10% bypass blade with built-in transformer for 120 Vac sites.
HAC-BPD
Back pressure damper — duct static pressure relief for zoned systems. Complements zone damper control.
ADP Series
Interface relay adaptors — for fresh air damper and furnace fan sequencing. Use with standard HAC on fresh air applications, not HAC-H.
Ready to Order
HAC-H — Heat Bypass Inline Damper
For hot air supply duct zone control only — not for fresh air, make-up air, or ventilation. Select your size, action (PO/PC), end switch, and relay option — then add to cart.
Configure & Order →
Size · Action · End Switch · Relay
Need help specifying the HAC-H?
Technical support available through your Hoyme wholesaler or direct.
1-800-661-7382