Outlet Only · Spring or Gravity Close · No Power
BDH Back Draft
Hood
Exhaust Termination at the Building Wall · Spring or Gravity Closed · 4"–12"
- Closes the exhaust path at the wall the moment the fan stops — no power, no wiring
- Spring closed — any mounting orientation; end switch available in 4"–6" for fan proving circuits
- Gravity closed — horizontal discharge only, quieter operation, painted white default
- Both styles available in regular metal or painted white — specify finish at order
- Includes adjustable extension sleeve — 7½" standard, extends to 14"
- Sizes 4"–12" standard
- 10-year mechanical warranty
Is this the right product?
✔ Right product if
You need an outlet-only hood damper for an exhaust duct termination at the building exterior — dryer, bathroom fan, kitchen exhaust, HRV/ERV exhaust, or any exhaust duct at the wall.
✗ Not this product if
Fresh air inlet application → use OOH All-Weather Hood
Powered open/close control at the wall → use MOH Motorized Hood
Back draft damper inside the duct run → use BDD Back Draft Damper
Powered open/close control at the wall → use MOH Motorized Hood
Back draft damper inside the duct run → use BDD Back Draft Damper
What It Does
Every exhaust duct termination without a back-draft damper is an open path for outdoor air, pests, and cold the moment the fan stops. The BDH closes it at the wall.
A back draft hood does one thing: it lets exhaust air out and keeps outdoor air from coming back in. When the fan runs, the hood opens. When the fan stops, it closes. No wiring. No power. No maintenance. The BDH is the outlet-only companion to Hoyme's full hood and damper lineup — available in spring closed and gravity closed styles to suit the discharge orientation and aesthetic requirements of the installation.
In Canadian and northern US climates, an exhaust termination without a closing damper is an open path between the building interior and the outdoor environment whenever the fan is off. Cold outdoor air enters through the duct when indoor pressure drops — cooking exhaust fans, central vacuum systems, and forced-air heating all create brief negative pressure events. Pests follow the open duct path year-round. In dryer exhaust applications, lint accumulation inside an open duct without a closing damper is a documented fire risk.
Spring Closed vs. Gravity Closed — The Primary Selection Decision
The spring and gravity models are mechanically different. Mounting orientation and whether fan proving is required determines the right choice.
Spring Closed — Use When…
- Mounting orientation is anything other than horizontal
- Dryer exhaust — always spring closed
- Fan proving end switch is required (4"–6" only)
- Any non-horizontal discharge including vertical and angled
- Default finish: regular metal (white available at order)
Gravity Closed — Use When…
- Horizontal discharge only — gravity close will not function on vertical or angled runs
- Quieter operation is a priority
- Aesthetic match to white siding or trim
- No fan proving end switch required
- Default finish: painted white (regular metal available at order)
Gravity closed — horizontal discharge only. The gravity closed model relies on hood weight to close against backflow. On vertical or angled discharge it may not close reliably. If there is any doubt about discharge orientation, specify the spring closed model. For dryer exhaust, always use spring closed — dryer exhaust ducts are typically routed vertically or at an angle where gravity close will not function reliably.
Style Comparison
| Feature | Spring Closed | Gravity Closed |
|---|---|---|
| Mounting orientation | Any — horizontal, vertical, angled | Horizontal discharge only |
| End switch (fan proving) | Available in 4"–6" — optional | Not available |
| Default finish | Regular metal | Painted white |
| Alternate finish available? | Yes — white available at order | Yes — regular metal available at order |
| Operation noise | Standard | Quieter |
| Dryer exhaust | Correct product | Not recommended |
| Extension sleeve | Included — 7½" standard, extends to 14" | |
| Sizes | 4"–12" | 4"–12" |
| Warranty | 10-yr mechanical | 10-yr mechanical |
End switch for fan proving — spring closed 4"–6" only: When the exhaust fan runs and the hood opens, the end switch closes — sending a signal to the connected control that the hood has physically opened. Used in ventilation control sequences needing confirmed exhaust path, and in dryer interlock systems requiring proof of open exhaust before the dryer runs. The end switch is a passive contact — it does not require a power supply from the BDH. Standard end switch SKUs are available in 4", 5", and 6" spring closed only. For 7"–12" applications requiring an end switch, contact the factory. For precise airflow confirmation in the duct run rather than hood position, use the PFS Positive Flow Switch.
Key Specs
Application
Outlet only — exhaust termination
Styles
Spring closed / Gravity closed
Sizes
4", 5", 6", 7", 8", 9", 10", 12"
End Switch
Spring closed 4"–6" only — optional
Finish
Regular metal or painted white — specify at order
Extension Sleeve
Included — 7½" extends to 14"
Power Required
None
Mounting
Spring: any orientation | Gravity: horizontal only
Not Suitable For
Inlet air applications — outlet only
Warranty
10-year mechanical
Common Questions from the Counter
Answers to what comes up most when specifying the BDH.
Can I use the BDH for a fresh air intake?
No — the BDH is outlet only. On an intake application, outdoor air pressure would push the BDH open and hold it open, with no control over when the building receives fresh air. For a passive fresh air intake termination, use the OOH All-Weather Hood. For a motorized fresh air intake with powered open/close control, use the MOH Motorized All-Weather Hood.
Why is the spring closed model always specified for dryer exhaust?
Two reasons. First, dryer exhaust ducts are often routed vertically or at an angle through the wall — the gravity closed model requires horizontal discharge to close reliably, and most dryer installations don't have a horizontal discharge path. Second, the spring closed model is available with an end switch in 4"–6" for proving circuits if required by local code or the dryer manufacturer. The gravity closed model has neither of these advantages for dryer applications.
Can I get the spring closed model in painted white?
Yes. The spring closed model defaults to regular metal and the gravity closed model defaults to painted white — but both styles are available in either finish. If you need a spring closed BDH in painted white to match white siding or trim, specify the alternate finish at order.
Does the end switch require a separate power supply, and what sizes is it available on?
No — the end switch is a passive contact, normally open, closes when the hood opens. It doesn't require a power supply from the BDH itself. Wire the contact into your control circuit, ventilation controller, or interlock relay. Confirm the contact rating is compatible with your control system before ordering. Standard end switch SKUs are available on spring closed models in 4", 5", and 6" only. For 7"–12" spring closed with an end switch, contact the factory.
What size BDH do I need for a dryer?
Standard residential dryer exhaust is 4". Most dryer manufacturers specify a 4" exhaust duct and termination. If the duct transitions to a larger size before the wall, match the BDH to the duct diameter at the wall penetration. Do not upsize the termination hood without confirming the duct transition — a mismatched hood diameter creates a dead air zone at the wall that reduces exhaust velocity and increases lint accumulation risk.
How does the BDH compare to a standard through-wall vent cap without a damper?
A standard through-wall vent cap with a fixed screen has no closing mechanism — the duct is open to outdoor air, pests, and cold whenever the fan is off. A BDH closes the path. In Canadian and northern US climates, the difference is measurable in heating load over a season, and the pest exclusion benefit alone justifies the BDH on every exhaust termination. The spring closed model with a 10-year mechanical warranty is a one-time specification that doesn't need to be revisited.
Ready to Order
BDH — Back Draft Hood
Select your size, style (spring or gravity closed), finish (regular metal or painted white), and end switch if required (4"–6" spring closed only) — then add to cart. Dryer exhaust: always spring closed.
Configure & Order →
Size · Style · Finish · End Switch
Need help specifying the BDH?
Available through your Hoyme wholesaler. Contact Hoyme direct for sizes outside the standard range or end switch on 7"–12".