No Power · Spring Return · Any Orientation · Adjustable Threshold
HAC-BPD
HAC-BPD
Back Pressure Damper
Automatic Duct Pressure Relief · No Wiring · 4"–20" (see size table)
- Opens automatically when duct static pressure exceeds the set threshold
- Closes as pressure drops — dynamic response, not fixed restriction
- Adjustable counterweight sets opening threshold at commissioning
- Spring return — any mounting orientation, no gravity constraint
- Foam rubber sealed for tighter leakage than unsealed bypass dampers
- Sizes 4"–20" — 10-year mechanical warranty
Is this the right product?
✔ Right product if
You need automatic static pressure relief in a duct system — grille noise when zones close, VFD hunting, HRV airflow instability — and want a mechanical solution with no wiring.
✗ Not this product if
Fixed airflow restriction that stays set → use HAC-MBD Manual Balance Damper
What It Does
The HAC-BPD is a spring-return mechanical back pressure damper. It opens automatically when duct static pressure exceeds the threshold set by the adjustable counterweight, and closes as pressure drops. No wiring, no control signal, no power supply. On a zone damper system without a back pressure damper, every zone that closes adds to the static pressure seen by the remaining open zones and the air handler fan — grilles noise, VFD hunts, HRV airflow destabilizes. The HAC-BPD installed in a bypass branch takes the excess pressure off the main duct automatically.
Commission on the job — no factory default setting. The correct counterweight position is application-specific. Set it with the system running under representative conditions. Start at a moderate position and adjust until the bypass activates only when excess pressure is present and closes cleanly during normal operation.
HAC-BPD vs. HAC-MBD — Two Different Tools
| HAC-BPD (Back Pressure) | HAC-MBD (Manual Balance) |
|---|---|
| Opens/closes automatically on duct pressure | Set manually at commissioning — stays there |
| Responds dynamically to changing conditions | Fixed position — does not respond to pressure |
| System protection — pressure relief | Airflow balancing — trims a specific flow rate |
| Install in bypass branch off main duct | Install in the main duct run at balance point |
Part Numbers — Diameter Is the Only Selection
| Size | Part Number | Size | Part Number |
|---|---|---|---|
| 4" | HAC-0400-BPD | 10" | HAC-1000-BPD |
| 5" | HAC-0500-BPD | 12" | HAC-1200-BPD |
| 6" | HAC-0600-BPD | 14" | HAC-1400-BPD |
| 7" | HAC-0700-BPD | 16" | HAC-1600-BPD |
| 8" | HAC-0800-BPD | 18" | HAC-1800-BPD |
| — | — | 20" | HAC-2000-BPD |
Note on sizes: 9" is not a stocked size — the range steps from 8" directly to 10". For square or rectangular duct back pressure relief applications, contact the factory — custom configurations are available. Call 1-800-661-7382 with duct dimensions.
Key Specs
Sizes
4"–20" (see table)
Operation
Spring return — automatic
Threshold
Adjustable counterweight
Mounting
Any orientation
Seal
Foam rubber sealed
Power
None required
Install In
Bypass branch — NOT main duct
Warranty
10-year mechanical
Can the HAC-BPD be installed vertically in a bypass riser?
Yes. The spring return means the damper can be installed in any orientation — vertical, horizontal, inverted, or at any angle. Gravity-close bypass dampers only work reliably on horizontal duct. The HAC-BPD works reliably regardless of orientation because the closing force is mechanical rather than gravitational.
What's the difference between the HAC-BPD and a manual bypass damper?
A manual bypass damper is set to a fixed open position and stays there — always leaking a fixed amount of air regardless of how many zones are open. The HAC-BPD opens and closes dynamically based on actual duct pressure — closed when pressure is normal, open when pressure rises, closed again when pressure drops. The dynamic response means the HAC-BPD provides relief only when needed.
Is the HAC-BPD right for a system that's noisy with all zones open?
Not directly. If the system is noisy with all zones open, the problem is likely system static pressure at the design condition, not pressure spikes from zone closure. The HAC-BPD is for pressure that builds when zones close. For a system that's noisy at all zones open, the issue is duct design, fan sizing, or grille selection — upstream of the BPD.
Ready to Order
HAC-BPD — Back Pressure Damper
Select diameter (4"–20", see table) — install in bypass branch, not main duct. Commission counterweight threshold on the job with system running.
Configure & Order →
Diameter only
Need help specifying the HAC-BPD?
Available through your Hoyme wholesaler or direct. Square/rectangular configurations — call factory.