Chimney height and duct length: the installation details that decide suction

Two kitchens can have the same chimney model and get very different results. In one, the smoke from a tadka disappears; in the other, it curls around the hood and fills the room. When we look closely, the difference is rarely the motor. It is how high the chimney hangs, how wide it is compared to the hob, and the duct behind it: how long, how many bends, and what kind of pipe. This article explains the installation details that decide suction, so you can check yours or get a new one fitted right.

Height above the hob

Every chimney’s manual gives a recommended height above the cooking surface. For gas hobs and stoves, most manufacturers give a range of roughly 65 to 75 cm from the burners to the bottom of the chimney. Follow your own manual; the numbers differ a little by model.

• Too high and the rising smoke spreads out before it reaches the hood. Suction that is perfectly good on paper cannot catch it.
• Too low and the chimney sits in the heat and grease of the flame. Filters clog faster, the hood heats up, and you hit your head on it.

Height also depends on the cook. In a kitchen where the main cook is short, a chimney at the top of the range may feel high; at the bottom of the range, a tall cook may find it in the way. Find the middle that suits the person who cooks most.

Width compared to the hob

The chimney should be at least as wide as the hob or stove below it. A 60 cm chimney over a 75 cm or 90 cm hob leaves the outer burners uncovered: smoke from the side burner escapes past the hood. If you cook on all burners at once, a chimney a size wider than the hob catches more.

The duct: shorter, straighter, smoother

The motor pushes air through the duct. Every metre of pipe and every bend adds resistance, and the motor’s suction falls with it. Three things matter:

1. Length. The shorter the better. A duct that runs across the kitchen to reach a far wall can take away a large part of the suction you paid for. Your manual may give a maximum length.
2. Bends. Each sharp 90° bend costs a lot of flow. Use as few bends as possible, and gentle curves rather than sharp elbows.
3. Diameter. Keep the duct the same size as the chimney’s outlet all the way. A duct narrowed to fit through a small wall hole chokes the chimney.

The kind of pipe

• Rigid or semi-rigid smooth pipe lets air flow freely and is easier to clean.
• Flexible aluminium (the corrugated silver pipe) is quick to fit and is used widely. Its ridges slow the air and collect grease. If used, it should be pulled fairly straight, not left slack and saggy, and kept short.
• PVC pipe is sometimes used. It must be of a suitable size and not near the heat of the burners.

The outside end

Where the duct leaves the building, it should end in a cowl or louvre with flaps that open when air is pushed out and close against wind, rain and birds. An outlet facing straight into the prevailing wind, or ending under a balcony slab, pushes air back down the duct. If smoke seems to come back out of the hood, read about smoke coming back into the kitchen.

Sealing at the wall

The gap between the duct and the hole in the wall should be sealed on both sides. An unsealed gap lets air leak around the pipe, lets in dust and insects, and in the monsoon can let rainwater track inside.

Signs that the installation, not the chimney, is the problem

• Suction is weak even with freshly cleaned filters. See suction still weak after cleaning.
• The motor is loud at high speed but the smoke still escapes.
• The duct is long, has several bends, or is made of slack flexible pipe.
• Oil or water drips back from the duct into the chimney.
• Smoke from the outer burners escapes past the sides of the hood.

Ducted or ductless?

If there is no way to run a duct to the outside, a ductless chimney with charcoal filters is the alternative. It removes some grease and smell but does not take heat and moisture out of the kitchen. We compare them in ducted or ductless.

Questions people ask

Chimney gas stove se kitni upar lagani chahiye?

Follow the chimney’s manual. For gas cooking, most manuals give a range of about 65 to 75 cm from the burners to the bottom of the hood. Within that range, pick a height that suits the person who cooks most.

Chimney ka pipe lamba hai, kya suction kam hoga?

Yes. A longer duct and more bends reduce suction. Shortening the duct, removing a bend or replacing slack flexible pipe with a straighter run often helps more than a stronger motor would.

Can an existing chimney be moved higher or lower?

Usually yes, within the length of its chimney cover. The duct may need adjusting too, and the wall fixings must hold the chimney’s weight. It is a job for a technician, not a quick DIY.

If you are installing a new chimney, or yours never seems to pull the smoke well, book a chimney visit. The technician checks the height, the duct and the outlet along with the filters and motor. A visiting charge applies, and you are told the cost of any part before it is fitted.