Every fresh air duct has two ends to think about: where the air comes in at the wall, and where the damper sits that decides when it flows. Most installs put those in two different places. There is a hood on the outside, a motorized damper somewhere in the duct run, and a stretch of duct in between that is open to outdoor air the whole time the system is off.
That works, and it is still the right answer for a lot of jobs. But there is a second option that a lot of contractors do not know Hoyme makes, and it is worth knowing when to reach for it.
Option 1: damper at the wall with the MOH
The MOH motorized hood damper puts the hood and the motorized damper in one unit at the wall penetration. When it closes, the air path is sealed at the outside of the building, so there is no column of outdoor air sitting in the duct between ventilation cycles.
What comes in the box matters on a retrofit. The MOH ships with flashing for siding or stucco and an adjustable extension sleeve that reaches from 7½" up to 14", so it covers a 2x4 wall, a 2x6 wall and most stucco assemblies without a special order. The wiring runs inside the duct, so there is nothing to route along the outside of the house.
The specs:
- 4" to 8" diameter
- 24 Vac, the same Hoyme spring return drive used in the HAC and HOM
- Power open (PO) or power close (PC), with an end switch option
- Canada/USA CSA certified
- Available in white by request
Option 2: damper in the duct with an HAC and an OOH hood
The classic layout is an OOH all-weather inlet hood on the wall and an HAC inline damper in the duct. The OOH is passive. It has a ¼" screen, either fixed or on a removable Velcro mount so it can come off for cleaning, and it also comes with the 7½" to 14" extension sleeve. The HAC does the opening and closing.
Splitting the job gives you more choices. The HAC runs from 2" to 9" and the OOH from 4" to 12", the HAC can be ordered with a relay as well as an end switch, and the damper sits indoors in the duct run where you can get at it.
Which one fits the job
Use the MOH when:
- The duct is 8" or smaller.
- You are replacing an old hood anyway, and one unit is cleaner than a hood plus a damper.
- There is no good spot in the duct run for an inline damper.
- Sealing at the outside wall is the priority, for example on a long or uninsulated duct run.
Use the HAC and OOH when:
- The duct is 9" or larger. The MOH stops at 8".
- You need a relay on the damper to switch something else.
- The damper should be indoors and reachable for service.
- The house already has a good inlet hood and only the control is missing.
If the HAC is going into unconditioned space or a cold section of duct, look at the insulated HAC-L. Condensation on the damper body is the usual winter complaint on an inline install, and the factory insulation is there to deal with it.
What it costs at list
At 2026 suggested list, a 6" MOH in power open is $260. A 6" HAC in power open is $195 and a 6" OOH is $96, so the two-piece layout comes to $291 before the second install. At 8" it is $268 for the MOH against $312 for the HAC and OOH. Your wholesaler's pricing will differ, but the gap usually holds, and the MOH is also one wall penetration and one install instead of two components.
A few things to get right
Pick PO or PC before you order. Both are set at the factory. For a plain fresh air intake, power open is the usual choice, because the damper seals whenever the system is off. Our power open vs power close guide walks through when power close makes more sense.
Combustion air needs the end switch. If the MOH is supplying air to a fuel-burning appliance, order it with the end switch (SPO or SPC), so the appliance only fires once the blade has proved open. Call technical support to confirm the interlock wiring for your appliance before you install.
Keep it off the dryer exhaust. Lint and moisture in a dryer exhaust will shorten the life of any motorized damper. Terminate dryer exhaust with a BDH back draft hood instead. If the dryer needs its own air supply, see our gas dryer make-up air guide.
The OOH is inlet only. If the duct is an exhaust, use a BDH, not an OOH.
The short version
- MOH: hood and motorized damper in one unit at the wall, 4" to 8", sleeve and flashing included, seals at the outside.
- HAC plus OOH: damper in the duct, hood on the wall, more sizes, relay available, damper reachable indoors.
- In cold or unconditioned space, use the insulated HAC-L for the inline option.
- At list, the MOH usually costs less than the two-piece layout.
Contractors can order the MOH, HAC and OOH through their HVAC wholesaler. Find a wholesaler near you. Not sure which layout suits the house? Call free technical support at 1-800-661-7382 with the duct size, the wall construction and what the air is for, and we will help you pick the part numbers.