Duct Length, Elbows and Why the New Hood Underperforms

A hood that never worked properly is almost always a duct that was never measured. The appliance gets blamed because it is the visible part, it is the part with a badge on it, and it is the only part anybody paid for as a single line item. The airway behind it was routed on site by whoever could reach the joist bay, and nobody wrote it down.

This page is about reading a duct run the way the hood experiences it, which is not as a length in feet but as a total resistance.

A fan works against what the duct resists

Every blower has a performance curve. It moves the most air when it has nothing to push against, and as resistance rises the volume it moves falls away. The installation decides where on that curve the hood actually sits, and the hood has no say in the matter.

Resistance comes from four places, and all four are decided during construction: how far the air has to travel, how many times it has to change direction, how small the duct gets at its tightest point, and what it has to push open at the end. A hood at the top of its curve and the same hood halfway down it are physically the same appliance behaving like two different products.

That is the whole mechanism. Everything below is a way of finding out where on the curve a particular kitchen has landed.

What a turn costs

Air does not like corners. Made to change direction, it separates from the inside of the bend, sets up turbulence downstream and loses energy doing it. A gentle radius bend costs relatively little. A tight mitred elbow costs considerably more, and two elbows stacked close together cost more than the same two spaced apart, because the second one receives air that is already disturbed.

The trade expresses this as equivalent length: a fitting is rated as though it were so many feet of straight duct. Published tables exist, they are specific to the fitting geometry and the duct size, and they belong to the mechanical contractor who is designing the run. The number is not something to take from memory, and it is not printed here for that reason.

What a designer can do without a table is count. Every turn on the drawing is a cost. A route with one bend to an outside wall and a route with four bends to reach a roof are not the same job, even when the straight footage is similar, and the four-bend route deserves a larger duct or a frank conversation about expectations.

The reduction where the duct met a joist

This is the failure we see most often, and it is invisible once the ceiling is closed. A hood ships with a collar of a particular size. The route crosses a joist bay, or a beam, or a run of plumbing that got there first, and the only way through is a duct one size down. So a transition goes in, the duct necks down for a few feet, and the whole system is throttled to the smallest opening in it.

Reducing the diameter does not reduce airflow in proportion; it reduces it far more sharply than the change in size suggests, because the cross-sectional area falls with the square of the dimension and the friction rises as the air is forced to move faster through what remains. A single short reduction can cost more than the rest of the run put together.

In older Denver houses, where framing was never drawn with a professional hood in mind and a pop-top has often added a floor above the kitchen, the reduction frequently happens in the first few feet, where the duct leaves the canopy and meets a joist running the wrong way. The fix is a route that respects the collar size, decided while the ceiling is open. There is no later.

Measuring the run you actually have

If the hood is already in, the run can still be read. Start at the collar and note its size. Follow what is visible through a cabinet, a soffit, an attic or a basement, and note every change of size and every turn. Look at the outside face of the building and find the termination: the cap should be a real exterior fitting with a damper that swings and closes, not a screen, not a mesh insert packed with lint, and not a grille with the damper blade wired open or painted shut.

Two limits on that survey, and they matter. Nothing on the list involves going onto a roof, and nothing on it involves opening a duct that is fixed in place. A roof termination is looked at from the ground or by somebody equipped for roofs, and a concealed run is inspected by the trade that installed it.

With the run described, the diagnosis is usually short. If the duct is generous, straight and terminated properly, the fault is in the appliance and we can fix it. If the duct is a long throttled path with four elbows into a stuck damper, no blower we fit will ever satisfy the person cooking under it, and saying so early is worth more than a part.

Spec check

Confirm each of these before anything is cut, ordered or delivered.

1. 1 The duct diameter or rectangular size, taken at the smallest point in the run rather than at the hood collar
2. 2 The straight length from the blower outlet to the outside face of the building
3. 3 The number of turns, and whether each one is a smooth bend or a tight mitred elbow
4. 4 Every transition where the size changes, and what forced the change at that point
5. 5 What the run terminates in, and whether that damper still swings freely

Questions on site? Viking Appliance Repair Denver, (720) 770-4189.

Follow-ups we get asked

Why does a hood that tested fine at the factory disappoint in the house?

Because a rating is measured under a resistance the laboratory chooses and your duct imposes its own. Length, turns, size reductions and the termination each take something off the top. Two identical hoods in two houses will move different amounts of air for exactly that reason.

Is flexible duct really that much worse?

Corrugated flexible duct has a rough inner wall and rarely runs perfectly straight, so it resists airflow considerably more than smooth rigid duct of the same nominal size. Where a short flexible section is unavoidable, keep it short, keep it straight and keep it pulled taut rather than concertinaed.

Can you tell whether my duct is the problem?

Usually, yes. Viking Appliance Repair Denver measures the run, counts what is in it and checks the termination, then tells you whether the fault is the blower or the airway behind it. Call (720) 770-4189, and the honest answer is sometimes that the repair belongs to a mechanical contractor.

ventilationductworkremodel