EXPLAINER

Kitchen Chimney Suction & Ducting Explained: Installation

Master kitchen chimney suction power and ducting systems. Learn about CFM ratings, duct types, installation requirements, and how to maximize efficiency.

Updated Jan 2026Installation-focused12 min read

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The short version

WHAT SUCTION POWER IS

CFM, cubic feet per minute

The volume of air the chimney extracts. Higher CFM removes smoke and odour faster.

WHY DUCTING DECIDES IT

It can cost you half the machine

Poor ducting cuts real-world performance by 40 to 60%, whatever the rating says.

That second point is the one this page exists for. A 1,200 CFM chimney on badly routed duct performs worse than a 700 CFM chimney on a short, straight run — and you paid for the difference.

Understanding suction power and CFM

Suction power decides how effectively the chimney clears smoke, grease and odour. Sizing it starts with the room, not the listing.

THE SIZING FORMULA

Required CFM = kitchen volume × 10 to 12. Measure length × width × height in feet. Use 10 for light cooking and 12 for heavy Indian cooking.

Worked CFM examples for a small, medium and large kitchen
Kitchen	Dimensions	Volume	Recommended CFM
Small	8' × 6' × 9'	432 cu ft	400–500 CFM
Medium	10' × 8' × 9'	720 cu ft	600–800 CFM
Large	12' × 10' × 9'	1,080 cu ft	900–1,200 CFM

Scroll the table sideways to see all columns.

The four CFM bands

300–500 CFM — LIGHT DUTY

Best for. Small kitchens
Cooking. Light Indian cooking
Usage. One or two burners, occasionally

Suits singles and light cooking.

600–800 CFM — MEDIUM DUTY

Best for. Medium kitchens
Cooking. Regular Indian cooking
Usage. Two or three burners daily

Suits families of two to four.

900–1,200 CFM — HEAVY DUTY

Best for. Large kitchens
Cooking. Heavy Indian cooking
Usage. Three or four burners, frying, grilling

Suits families of four or more.

1,300+ CFM — COMMERCIAL GRADE

Best for. Very large or open kitchens
Cooking. Restaurant-style volume
Usage. Multiple burners at high heat

Overkill in most homes, and noisy.

Ducted vs ductless systems

This choice affects installation complexity, running cost and how much smoke actually leaves the room.

DUCTED — RECOMMENDED

How it works. Pushes air outside through ducts
Efficiency. 85–95% effective removal
Installation. Needs external ducting
Maintenance. Fewer filter replacements

Complete smoke and odour removal, no recirculation, lower ongoing cost. Against it: complex installation, external wall access needed, higher fitting cost.

DUCTLESS (RECIRCULATING)

How it works. Filters air and returns it to the room
Efficiency. 60–75% effective removal
Installation. No external ducting
Maintenance. Regular filter cleaning and replacement

Easy to fit, no structural work, good where a building forbids external venting. Against it: weaker on heavy cooking, higher maintenance cost, and efficiency drops as filters saturate.

Ducting fundamentals

Poor ducting can cost 40 to 60% of the chimney's capability, which makes a high CFM rating meaningless.

DUCT DIAMETER

300–600 CFM. 6 inch
700–900 CFM. 8 inch
1,000+ CFM. 10 inch
The rule. Never narrower than the chimney outlet

MATERIALS

Aluminium. Light and rust-resistant — good
Galvanised steel. Durable and affordable — good
PVC pipe. Not heat resistant — do not use
Flexible duct. Use sparingly; it costs airflow

Layout, from best to worst

IDEAL

Shortest possible path
Two bends at most
Straight horizontal run
Gradual slope for condensation

ACCEPTABLE

One or two extra bends where unavoidable
Slightly longer run
Smooth bends rather than sharp
Adequately supported throughout

AVOID

Several sharp 90° turns
Very long horizontal runs
Vertical drops with no slope
Undersized duct diameter

What each compromise actually costs

Chimney power loss by ducting layout
Layout	Power loss
Straight 6ft run	0%
One 90° bend	10–15%
Two 90° bends	20–25%
Poor installation overall	40–60%

Scroll the table sideways to see all columns.

Installation, step by step

1

Planning and measurement

Measure the exact distance from chimney to exit point and plan a route with the fewest bends. Check for electrical wiring and gas lines along the path, and confirm you have external wall or window access. Mark the whole duct path before any cutting starts.
2

External exit point

Cut the hole one inch larger than the duct diameter. Fit a cowl or louver so rain cannot enter, seal around the opening with weather-resistant material, and give it a slight downward slope for drainage.
3

Duct installation

Clamp the duct to the chimney outlet and support it every four to six feet. Use smooth bends rather than sharp turns, and seal every joint with aluminium tape.
4

Testing and optimisation

Run the chimney at maximum speed and check for air leaks at every joint. Verify suction at cooktop level rather than at the hood, and adjust the damper if the model has one.

Three installation mistakes that cost real performance

UNDERSIZED DUCTS TO SAVE MONEY

Always use the manufacturer's recommended duct size or larger. A narrow duct creates back-pressure and cuts efficiency sharply — far more than the few hundred rupees it saved.

MULTIPLE SHARP 90-DEGREE BENDS

Use smooth radius bends and cap the run at two turns. Plan the layout to minimise direction changes rather than working around them afterwards.

POOR SEALING AT JOINTS AND EXIT

Use quality aluminium tape on every internal joint and weather-resistant sealant at the external connection. Leaked air is extraction you paid for and did not get.

How this changes what you buy

WHAT TO LOOK FOR

A CFM rating around 1.5× your calculated requirement
A duct outlet of 6 inches or more
Variable speed controls
Professional installation included
Baffle filters you can actually reach to clean

RED FLAGS

A CFM rating that looks too high for the price
No provision for external ducting
Cheap plastic components in the airflow path
Installation excluded, or priced absurdly
No warranty on the motor or the installation

Maintenance schedule

WEEKLY

Wipe external surfaces
Run every speed setting once
Listen for unusual noise

MONTHLY

Deep clean the baffle filters
Check duct connections for leaks
Inspect the external cowl or louver

ANNUALLY

Professional duct cleaning
Motor lubrication, if the model needs it
Full system inspection

Frequently asked questions

Is higher CFM always better?

No. Size CFM to your kitchen and your cooking. An oversized chimney wastes energy and is noticeably noisier. Work out the requirement with kitchen volume × 10 to 12, then buy around 1.5× that.

Can I install the ducting myself to save money?

You can, but professional installation is worth it here. Poor ducting costs 40 to 60% of performance, which wipes out the saving and then some. There are also safety considerations around gas lines and wiring.

How much power does each bend cost?

Roughly 10 to 15% per 90° bend. Two bends cost 20 to 25%. That is exactly why the shortest, straightest path matters more than the headline rating.

What is the maximum useful duct length?

Keep the total under about 20 feet in a home. Longer runs need a wider diameter and a higher CFM rating just to hold adequate suction at the cooktop.

Ducted or ductless in an apartment?

Ducted, whenever external venting is permitted — the performance gap is large. Where the building forbids it, buy a good ductless system and stay disciplined about the filters.

How often should ducts be professionally cleaned?

Every 12 to 18 months for heavy Indian cooking, and every two to three years for light cooking. Reduced suction and visible grease at the external exit are the signs it is overdue.

A note on scope

Suction and ducting requirements vary with kitchen layout, cooking habits and local building codes. Consult a qualified installer and follow the manufacturer's guidelines for your specific model. This page is educational, not an installation specification.