Positive-Pressure Air: The 6-Inch Build (About 25 ft and Up)

Who This Build Fits

Long travel trailers, fifth wheels, toy haulers, Class A and larger Class C motorhomes — units holding roughly 2,000 to 3,000 cubic feet of air, usually with two to four slides. In this class the problem is not volume. A 6-inch fan could pressurize a much bigger sealed box. The problem is perimeter: every slide adds forty or fifty feet of wiper seal and bulb seal that were never airtight when new, and the basement bays, pass-throughs and utility hatches add penetrations a small trailer does not have. The build is sized to overcome leakage, and commissioning is mostly leak-hunting.

If your unit is under 25 feet with no slides, the 4-inch build is cheaper and quieter. If it is 25 to 30 feet, this build at low speed is the safer choice. Either way, read the intake-location box on the concept page before choosing where anything goes — on a large coach, the number of exhausts and vents to stay clear of is considerable.

Why 6-Inch, Not 8-Inch

The instinct on a big coach is to go big on the fan. It is usually wrong. A modern 6-inch mixed-flow EC fan delivers on the order of 350–425 CFM in free air and several hundred Pascals of static-pressure capability. That is three to four times the airflow a typical 40-foot unit needs at its pressure target, with reserve to push through a loaded carbon canister and a HEPA. In service the fan runs at a fraction of its rating — quieter, cooler and longer-lived than an 8-inch fan idling at its minimum.

Use 8-inch only when a capable 6-inch fan at or near maximum still cannot hold +3 Pa after the leaks have been sealed. At that point the problem is the trailer, not the fan, and the money is usually better spent on slide seals.

Shopping List

Print this. Check the "what matters" column against each listing before ordering.

Approximate cost, parts only

Carbon at This Scale

A 6-inch refillable canister holds roughly 8 lb of granular carbon. That is still a shallow bed by the standards of a house-scale system, but at 100 CFM it gives the air twice the contact time of the 4-inch canister at the same flow, and it loads about half as fast. For a coach that spends smoke season parked, or a full-time occupant who reacts to fragrance, that difference is the whole reason to choose the refillable unit: bulk carbon is cheap, the canister is opened from the equipment bay, and nothing has to be shipped.

At the September 2026 check, standard 6-inch canisters ran about $50–90 and the refillable 6-inch kit about $140. The rule against thin carbon sheets and token pellet layers applies exactly as it does on a small trailer — more so, because there is more coach to protect.

Where the Equipment Goes

Large units usually have the one thing small ones lack: a basement bay. A pass-through or utility bay with 120-V power is the natural home for the train, with two conditions. The outdoor intake is ducted up to a clean location on the side wall or roof — never drawn from the bay itself, which shares air with the generator, the propane compartment and the road — and the bay floor is kept clear so the carbon canister can be changed without dust entering the living area above.

A weather-protected exterior box on a rear rack or the hitch platform is the alternative, and on a coach it has the advantage of putting the noise and the carbon dust as far from the bedroom as the unit allows. An interior closet is a last resort on a large unit and rarely necessary.

Chemically sensitive occupants: prefer a metal or known low-odour enclosure over painted or adhesive-lined boxes, and air new equipment outdoors before service if it has any smell.

Installation in Five Steps

1. Map every exhaust before choosing the intake. On a 40-foot unit that list is long: generator, furnace, water heater, refrigerator, holding-tank vents, the tow vehicle or the coach's own engine, and the neighbour's generator slot in a typical campground. Then pick the cleanest high point and fit the hood. If it is on the far side from the bay, plan the duct run rather than compromise the location.
2. Mount the train in order. Hood, MERV 8, carbon canister, fan, H13 HEPA. Gasket and tape every joint. On a coach the duct run is longer, so the seal discipline matters more — a leak halfway along the dirty side pulls bay air into the clean stream.
3. Bring clean air into the living space. One sealed floor or wall penetration, or an existing service route, discharging into the main living area. On a long unit a second diffuser near the bedroom end is optional and helps even out the space; it does not change the pressure. Do not connect to the coach's factory ducting — a basement-ducted furnace system on a Class A often leaks to the outside in several places.
4. Power up at low speed, close everything. Doors, windows, roof vents, and — importantly — all slides fully out or fully in, in the position the coach will be lived in. Slide position changes the leak area; commission in the real position.
5. Measure, seal, re-measure, then set. Indoor port inside, reference port outside and shielded from wind. If the meter shows less than +3 Pa at moderate speed, start with the slides: run a hand along the wiper seals for airflow, then the bay doors, then the entry door. Only when the obvious leaks are addressed raise the speed. If a capable 6-inch fan near maximum still falls short, the leak is large enough to find, and finding it is cheaper than an 8-inch fan.

Where the Leaks Usually Are

Commissioning on a large unit is mostly leak-finding, and the same handful of places account for most of it.

• Slide seals. The largest single item. Wiper seals that have taken a set, bulb seals with flattened spots, and the corners where the two meet. Conditioning the rubber helps; replacement is the real fix and is worth doing for reasons beyond air.
• Bay and compartment doors. Especially the ones that open into a space connected to the living area — pass-throughs under a bedroom are a common path.
• Roof vents and skylights. Lids left cracked, and frames whose sealant has failed.
• Plumbing and wiring penetrations. Under sinks, behind the water heater, around the shower drain.
• The entry door. Less than people expect on a coach; more on an older trailer.

With the system running, the meter tells you whether a change helped; a tissue at a suspected gap tells you where air is moving. Work from largest to smallest and stop when the fan is holding pressure at a comfortable speed. The goal is not an airtight coach — that would need its own relief path — but one whose leakage the fan can comfortably out-supply.

Do Not Add a Relief Vent

The trailer is the relief vent. Slide seals, bay doors, window frames, roof vents, the range-hood flap and the penetrations behind the water heater leak far more than a 6-inch fan supplies, and that leakage is what keeps the pressure at +3 to +5 Pa instead of climbing. A coach that needs a deliberate relief path is rare: it reaches +5 Pa at the lowest fan speed and the door pushes back. Turn the fan down first; if that is not enough, a one-way backdraft damper in the bathroom-fan opening is the fix — an existing hole, not a new one. Never cut a new vent into the wall for this. Full reasoning, including whether the range-hood flap will crack open under pressure, is on the concept page.

The Pressure Check

Commissioning: a differential gauge that reads Pascals and resolves about 1 Pa, with the reference port outside and out of the wind. Read with everything closed, slides in their lived-in position and every exhaust fan off.

Everyday: a tissue or ribbon at a slightly cracked door, drifting outward, confirms direction. On a coach with slides, re-check the gauge after every slide movement and every re-park; it is the reason a permanent gauge earns its place on a large unit.

Not this: smoke pencils or scented test smoke around a chemically sensitive occupant.

Troubleshooting

Maintenance

Carbon life is set by what is outside, not by the calendar. An 8-lb bed in continuous service through heavy wildfire smoke or beside a busy road can load far faster than the schedule above. Keep a spare, change conservatively, and treat the first faint smell indoors as the signal.

Example Components

Illustrations of the class of hardware meant. Prices and availability checked September 9, 2026; none is an endorsement and no manufacturer has any relationship with this site.

• 6-inch mixed-flow EC fan — e.g. AC Infinity Cloudline S6: 425 CFM free-air, 503 Pa static pressure, about $99.
• Refillable 6-inch carbon canister — e.g. AC Infinity 6-inch refillable kit, about 8 lb per fill, about $139.
• Standard 6-inch carbon canister — sealed granular-carbon grow-room units from several makers, about $50–90.
• 12×12×6 H13 HEPA — e.g. Spall Labs 12×12×6 H13, 99.97% at 0.3 µm, rated about 206 CFM, about $60; a 24×12×6 in the same series if you want lower resistance.
• 0–30 Pa gauge — e.g. Dwyer Magnehelic 2000-30PA, 1-Pa divisions, about $90.