A blower does not have an output. It has a curve. Give it open air and it moves everything the label claims. Put a real run in front of it — pipe, a transition, a couple of elbows, a backdraft damper and a cap on the outside wall — and it moves whatever is left once the run has taken its share. The label never changes. The kitchen does.
You do not perceive that share as less air. You perceive it as noise. A strong blower on a restrictive run is louder than a modest blower on a good one, and a loud hood gets run on its lowest speed or not at all. Same as not having bought it.
Sizing a range hood covers what happens above the pan. This is about the pipe behind it — built in at framing, by whoever is holding the snips, and never looked at again.
Resistance, and why you hear it first
Every foot of duct, every change of direction, every change of shape and the cap at the far end push back against the fan. Static pressure, in the trade. A blower has a fixed amount of push, and what it spends on resistance is not available to move air with.
Two things follow. Less grease and steam leave the room, so more of both land on the cabinet doors either side of the range. And the air that does move travels faster to get through the restriction, which is where the sound comes from. Velocity noise across a seam, a damper blade or a hard turn has nothing to do with the motor. So the useful question is not what the hood is rated at. It is what route the house will give you.
Diameter is the one you cannot revisit
The installation manual states a duct size. That is not an opening position for a negotiation on site — it is the size the blower was tested against. Match it. Go up a size on a long run. Never go down.
Going down happens for reasons that sound sensible at the time. There is old pipe in the ceiling and it is smaller. The cabinet above the range will not clear the larger transition. In goes a reducer, and the system is throttled at one fitting inches from the collar, where a restriction costs most. If the manual asks for a diameter the route will not take, change the route or change the hood.
Round loses less than flat duct of the same capacity — a circle puts the least wall against the air and no corners to trip over. Smooth is the other half of that, and so is metal. Flex is a ribbed surface: it costs airflow at every ridge, it sags between hangers so the ridges deepen, and in a grease exhaust it is a shelf. We will not run it, and where we find it in a ceiling it leaves with the demolition.
Rectangular duct exists because walls are thin. A flat profile that slides into a stud bay solves a real problem and charges for the favor: more surface against the airflow, plus two transitions, round to rectangular and back. Manufacturers publish the round equivalent so the two can be compared honestly. Seal every joint with mastic or foil tape, not cloth duct tape, which dries out and lets go.
Every turn is a length of pipe you cannot see
Fittings get measured in equivalent length: the run of straight duct that would cost the same pressure the fitting does. It is the model to carry into a design conversation, because it puts unlike choices in the same units. A hard right-angle elbow is worth a long stretch of straight pipe. A long-radius elbow doing the same work is worth much less. A pair of shallow bends around an obstruction often beats one square turn, even though the pipe is longer.
Placement matters more than the raw count:
- An elbow at the blower collar costs more than the same elbow further along, because air leaving the fan has not organized itself yet.
- Elbows stacked close together compound. Air still tumbling out of the first arrives at the second badly, and the pair costs more than either would alone.
- The termination is a fitting. A wall or roof cap is often the most restrictive component in the assembly, and the one nobody prices.
- Mesh screens clog. An insect screen over a grease outlet greases over and chokes the run. A cap with a damper that swings fully clear is the better buy.
None of that argues for a heroic run. It argues for the shortest sensible route, the gentlest turns, and money spent at the cap rather than saved there.
Where the run can actually go
A range on an exterior wall is the easy case: back and out, short run, and the argument is mostly about which cap. A range on an interior wall has to rise into the floor above and travel, and joists run one direction only. Along a bay is cheap. Across them means boring within the limits allowed for holes in a joist — set by where the hole sits on the span and how deep the joist is, not by what fits — or dropping a bulkhead to carry the pipe where the framing will not.
The space above the wall cabinets is the other unknown. A soffit may be a hollow box closing a gap, or it may already be carrying the things that make the house work — which is exactly what is inside the soffit above your kitchen cabinets, and why it is never assumed empty.
An island hood is its own project. Nothing sits behind it, so the duct goes up — into the bay above and out through a flashed roof cap, or inside a chase where a room is in the way. Downdraft inverts the problem, dropping through the floor and out the rim joist, which means a ceiling opened below if the basement is finished. All of it is part of what moving a sink or a range drags along.
It ends outside. That is the whole rule.
Not in an attic, not in a soffit, not in a floor cavity, not in a vented crawl space, and not in a stub of pipe behind a cabinet connected to nothing. Those all turn up when a ceiling comes open. Keep the outlet clear of soffit intake vents and operable windows too: air pulled straight back in has not been exhausted, only circulated.
Cold pipe, and a damper that seats
Warm, wet, greasy air meeting cold metal in an Indiana January condenses, and the water runs back to the lowest point, which is the hood. Insulate anything crossing unconditioned space and pitch the run toward the outside — ordinary heating-season behavior here, not a defect. If a wall is open for the cap anyway, that is also the cheapest moment to handle what the energy rules ask for.
Every ducted hood needs a backdraft damper, usually a gravity flap at the cap. That flap is the only thing between your kitchen and the weather. A cheap one that does not seat gives you cold air falling over the range all winter and a rattle when the wind turns. Small upgrade, loud result.
Makeup air, and the house as one system
Every cubic foot the hood throws outside gets replaced from somewhere. In a leaky old house it arrives through the gaps. In a tight one — and much of what has gone up around Fishers, Westfield and Geist is tighter than what it replaced — there are not enough gaps, so the house goes negative and the air comes in wherever it finds a hole.
Wherever it can includes the flue of an atmospherically vented water heater. That is backdrafting: combustion products traveling into the house rather than out of it. Milder symptoms arrive first. An exterior door that pulls hard with the hood on. A whistle at a window. A hood that never clears the room whatever speed it runs at.
Makeup air answers that with a duct to outside, a motorized damper interlocked with the blower so it opens when the hood starts, and tempering if you would rather not admit a slug of February air every time you sear something. Above a certain exhaust rate the mechanical code requires it. We are not printing the threshold, because the figure that governs your house comes from the code your jurisdiction has adopted and the department that enforces it.
Ask early, because it is a duct, a damper and a control, not a fitting bought at the end. A new penetration through an exterior wall or roof is mechanical work, and in Fishers the permit trigger is structural, electrical, plumbing or HVAC change. The city says a complete residential submittal is typically handled in no more than 15 working days, and work has to commence within 12 months of issuance and be complete within 24, absent a granted extension. Permitting and Inspections is on 317-595-3120. Carmel, Noblesville and Westfield each have their own authority, so do not carry a Fishers answer across a city line — the longer version is worth the read.
So blower capacity is not free. Buying more of it than the cooking surface needs can push a job over that line and add a system nobody budgeted for.
What the estimate should say
Hood ducting is a short line with a wide range hiding behind it. Straight out through an exterior wall is pipe, a transition and a cap. The same hood over an island above a finished basement is a floor penetration, a bulkhead, drywall repair and a painter coming back. Same appliance, two scopes, and a line reading only "vent range hood" does not say which one you bought. Ask in writing: diameter and material, the route in plain words, the termination, and who owns the makeup air if it is required.
We do not publish our own prices, for the reason we give everywhere on this site: a number written before anyone has seen your kitchen is a guess. What we can point at is what independent research says this market spends. Angi, Homeyou and Block Renovation's published Indianapolis kitchen research puts a kitchen project here at roughly $18,000 to $24,000, across a full spread of about $8,000 to $40,000.
Ducting is a rounding error inside a number like that, right up until it is the difference between a hood you use and a hood you resent. Which is why moving services sits on the kitchen remodeling page among the handful of things that genuinely move a quote.
The short version
- Use the diameter the manual asks for, or one size larger on a long run. Never smaller, and never at the collar.
- Rigid smooth-wall metal, the fewest and gentlest turns the framing allows, and count the cap as a fitting.
- End outside, behind a damper that seats — never an attic, a soffit, a floor cavity or a crawl space.
- Insulate anything crossing cold space and pitch it outward.
- Settle the makeup air question before you choose the blower.
The duct is decided at framing and lived with for as long as the kitchen stands. The hood in front of it can be swapped out on a Saturday. Get the order of those two right and the appliance choice gets easier.
If you want somebody to look at the route your house actually offers, ask for an estimate. We usually start under the range, in the basement, working out which way the joists run.
