Thin Air and Hood Capture at 5,280 Feet

Denver sits about a mile above sea level, and every appliance in the city is working in air that is thinner than the air it was designed and tested in. For a gas burner that is a well-documented problem with well-documented answers. For a ventilation hood it is subtler, frequently overstated by people selling larger blowers, and worth understanding properly before it becomes the excuse for everything.

The short version: altitude changes what a given volume of air is carrying. It does not change the volume the fan sweeps.

Volume moved is not mass moved

A blower is a volumetric machine. Its wheel sweeps a fixed space every turn, so at a given speed it moves broadly the same number of cubic feet per minute wherever it is installed. That is why hood ratings are quoted as a volume in the first place.

What changes with elevation is how much matter is in each of those cubic feet. Standard reference tables put the air at five thousand feet at roughly eighty-five per cent of sea-level density, and Denver sits a little above that mark. So the fan is shifting the same volume and carrying away appreciably less air by weight, along with less of the moisture and grease suspended in it.

The same reduction affects the pressure the fan can develop. Fan pressure is corrected for density, which means a blower turning at the same speed pushes less hard against the resistance of the duct than it would at sea level. That is the part that actually matters, and it points straight back at the airway rather than at the appliance.

What that changes in practice

Three things, in descending order of importance.

First, the duct deserves more respect here than a marketing chart implies. A fan with less pressure in hand loses more to a throttled run than the same fan would nearer sea level. A long route with several turns and a size reduction is a bad idea anywhere and a worse one at this elevation.

Second, design headroom is worth having. Specifying an exhaust rate right at the edge of what the appliance documentation recommends leaves nothing to pay for the installation, and the installation here has a slightly stiffer bill. That is an argument for a sensible margin, not for the largest blower in the catalogue.

Third, the make-up air question does not go away because the air is thin. A tight house is a tight house, the exhaust still has to be replaced from somewhere, and the threshold the jurisdiction enforces is written in terms of the volume the hood moves, which is the thing altitude did not change.

What it does not change

It is worth being equally clear about what elevation is not responsible for, because the excuse gets stretched a long way in showrooms.

Elevation does not put grease on the cabinet sides — a canopy hung too high or too shallow does that. It does not make a hood roar — a duct that is too small or too convoluted does that. It does not cause a hood to stop pulling after years of good service — a failing blower, a seized damper or a filter nobody has cleaned does that. And it does not turn a recirculating hood into a vented one.

If somebody is explaining a specific complaint in a specific kitchen by pointing at the mountains, ask what the duct diameter is and how many turns are in the run. The answer to that question explains far more kitchens than the elevation does.

Specifying with it in mind

The practical advice fits in a paragraph. Choose the canopy for the appliance and the room, with a reasonable margin rather than a heroic one. Spend the budget on the airway: the largest duct the structure will carry, the shortest route, the fewest turns, a real exterior termination with a damper that swings. Ask the hood manufacturer whether it publishes anything specific for high altitude installations, and treat silence as silence rather than as permission. Settle the make-up air provision on the same drawing.

Do that and the elevation becomes a footnote, which is what it should be. Get the duct wrong and the thin air will be blamed for a problem it did not cause, usually for as long as the kitchen lasts.

Spec check

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

1. 1 Whether the hood manufacturer publishes any guidance or derating for installations well above sea level
2. 2 Whether the blower and its control are specified for the elevation of the address, not for a national average
3. 3 The duct size, since a generous airway matters more where each cubic foot carries less
4. 4 Whether the make-up air provision was sized on the same assumptions as the exhaust
5. 5 That the exhaust rate chosen leaves some headroom rather than sitting at the edge of the appliance recommendation

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

Follow-ups we get asked

Does a hood move less air in Denver than at sea level?

It moves roughly the same volume, because a fan is a volumetric machine and its blades sweep the same space per revolution. What falls is the mass in that volume. So the hood is doing the same amount of moving and carrying away less matter with every cubic foot.

Should I specify a bigger hood because of the elevation?

Headroom is sensible here, but the bigger gain is usually in the duct rather than the blower. A generous, short, straight airway costs nothing extra to design and returns more than a step up in fan size, which mostly buys noise. Discuss both with your mechanical contractor.

Does altitude explain a hood that has always been poor?

Almost never on its own. A hood that has disappointed since the day it was commissioned usually has a duct, a height or a make-up air problem, and elevation is the thin edge on top of it. Viking Appliance Repair Denver can tell you which on (720) 770-4189.

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