Spring Return · Any Orientation · No Power

BDD Back Draft
Damper

Spring Return · Any Mounting Orientation · No Power Required · 4"–24"

✓Spring closes the blade regardless of mounting orientation — horizontal, vertical, inverted, any angle
✓No power, no wiring, no control sequence required
✓Standard inline style 4"–24" — inset butterfly (−B) for retrofit in 4"–6"
✓Galvanized steel construction — 10-year mechanical warranty
✓Closes against reverse airflow the moment forward flow stops
✓No end switch, no serviceable components

Is this the right product?

✔ Right product if

You need a passive spring-return back draft damper inside the duct run — inline or butterfly style, any mounting orientation, no power required.

✗ Not this product if

Back draft damper at the building wall → use BDH Back Draft Hood

Adjustable back pressure relief on a zone system → use HAC-BPD

What It Does

A back draft damper that only works in one mounting orientation isn't a solution — it's a constraint. The BDD closes in any orientation because the spring closes it, not gravity.

The BDD is a spring-return back draft damper for inline duct applications. It allows airflow in the forward direction and closes against reverse airflow — no power, no wiring, no control sequence required. The spring mechanism closes the blade regardless of how the damper is mounted: horizontal, vertical, inverted, or any angle in between.

A gravity-close back draft damper relies on the weight of the blade to close. That works correctly in exactly one mounting orientation: horizontal duct with the blade hanging vertically downward. Tilt the duct, mount it vertically, or invert it — and a gravity damper either closes partially, closes slowly, or doesn't close at all. In residential and light commercial mechanical rooms, duct runs rarely follow ideal geometry. The BDD's spring closes the blade regardless. Install it in the right location in the duct run and it works — that's the entire value proposition for a passive mechanical product with no serviceable components.

Secondary seal on fresh air runs: For HRV and ERV fresh air duct runs in Canadian and northern US installations, a BDD downstream of a motorized HAC damper is standard practice. When the HAC closes on a power loss or system shutdown, the BDD provides a secondary passive seal against reverse airflow — two layers of protection with no additional wiring or control complexity.

Standard Inline vs. Butterfly (−B) — Which Style to Order

Two styles for two different installation situations. If it's a new installation, use the standard inline.

Style	Size Range	Part Number Format	Best For
Standard Inline	4"–24"	BDD-[size]00-000	New installations and duct modifications — full size range, standard inline fitting, more reliable seal
Inset Butterfly (−B)	4"–6" only	BDD-[size]00-000-B	Retrofit into an existing duct opening where adding a full inline body is impractical — blade insets into existing duct, no full body required

Butterfly style is 4"–6" only. For retrofit applications in 7" and above, install a short inline section and use the standard BDD. The butterfly blade relies on a close fit to the existing duct to seal correctly — confirm the duct diameter matches exactly before ordering.

Part Number Examples

Part Number	Description
BDD-0400-000	4" standard inline back draft damper
BDD-0600-000	6" standard inline back draft damper
BDD-0600-000-B	6" inset butterfly back draft damper
BDD-1200-000	12" standard inline back draft damper
BDD-2400-000	24" standard inline back draft damper — largest standard size

Key Specs

Styles

Standard inline / Butterfly (−B)

Sizes — Inline

4"–24" standard

Sizes — Butterfly

4"–6" only

Operation

Spring return — no power

Mounting

Any orientation

Construction

Galvanized steel

Power Required

None

End Switch

Not available

Warranty

10-year mechanical

Common Questions from the Counter

Answers to what comes up most when specifying the BDD.

What's the difference between the BDD and the BDH Back Draft Hood?

The BDD is an inline duct product — it installs inside the duct run, between duct sections, at any point where reverse airflow needs to be stopped. The BDH is a wall termination hood — it installs at the building exterior where the exhaust duct exits through the wall. They solve the same problem at different points in the system. On a fresh air intake run, you'd use a BDH or OOH at the wall and may install a BDD in the duct run downstream of the motorized HAC damper if a secondary seal is required.

Can I use the BDD in any mounting orientation?

Yes — any orientation. The spring closes the blade regardless of whether the damper is mounted horizontally, vertically, inverted, or at an angle. This is the primary advantage of the spring-return design over gravity-close alternatives. Gravity dampers require near-horizontal mounting to close correctly. The BDD does not.

Should I use a BDD with a motorized HAC damper on a fresh air run?

It depends on the application. A motorized HAC damper provides powered open/close control and closes reliably on command. A BDD downstream provides a passive secondary seal that closes immediately on any reverse airflow event — including during the brief period between fan shutdown and HAC closure, and on power loss events where the HAC may be mid-travel. On HRV and ERV fresh air runs in cold climates, the BDD is commonly specified as a secondary seal. On standard fresh air intake runs with a well-maintained HAC damper, the BDD is optional but adds a layer of passive protection at low cost.

Does the butterfly style work the same way as the standard inline?

Same spring-return mechanism, same any-orientation capability, same no-power operation. The difference is installation method and available sizes. The standard inline installs as a full duct fitting with upstream and downstream duct connections. The butterfly insets into an existing duct opening without a full body — suited for retrofit applications where adding a full inline section is impractical. Butterfly is available in 4"–6" only. For retrofit applications above 6", install a short inline section and use the standard BDD.

Is there an end switch option on the BDD?

No. The BDD is a passive mechanical product — no electrical components, no wiring, no end switch. If airflow confirmation is required in the duct run, the PFS Positive Flow Switch confirms actual airflow rather than damper blade position. For motorized open/close control with position confirmation, use a motorized HAC damper with the end switch option.

Ready to Order

BDD — Back Draft Damper

Select your size and style (standard inline or butterfly −B retrofit) — then add to cart. Contact Hoyme direct for sizes above 24".

Configure & Order →

Size · Style

Need help specifying the BDD?

Available through your Hoyme wholesaler. Contact Hoyme direct for sizes above 24".

1-800-661-7382

BDD Back DraftDamper