Commercial Kitchen Hood Design: Save Costs with DW/172 vs AS 1668.2

Let's be honest for a moment: if you're involved in designing, constructing, or financing a commercial kitchen in New Zealand, you'll eventually encounter challenges with the mechanical ventilation. In particular, you'll face issues with large stainless steel ductwork that doesn't fit in your ceiling space, driven by a roof fan that roars like a jet engine and consumes a lot of electricity.

For decades, the Australian/New Zealand standard AS 1668.2 has been the default compliance pathway for kitchen exhaust hoods. But here is the dirty little secret of the building services industry: AS 1668.2 is a blunt instrument. It forces you to over-engineer, over-size, and over-pay.

Thankfully, there is a smarter, leaner alternative gaining serious traction across Aotearoa: the UK’s BESA DW/172 specification. Here at Reshape, we use the DW/172 for over 90% of our designs, saving our clients thousands of dollars.

If you are an architect trying to save ceiling height, or a restaurant owner sick of paying astronomical power bills, this article will explain why clinging to AS 1668.2 is a mistake—and how switching to DW/172 can save you money.

The Math: Blunt Force vs. Precision Engineering

AS 1668.2 calculates exhaust airflow primarily by looking at the physical size of the hood and mandating a minimum capture velocity across the face of it. It’s a geometry-first approach. According to this standard, whether you are boiling a pot of pasta or searing 50 steaks an hour, if the hood is 4 meters long, it demands a hurricane of air to satisfy the code.

DW/172, on the other hand, is an appliance-first approach. It calculates the airflow based on the actual thermal plume generated by the specific equipment underneath the canopy. A combi-oven does not generate the same volatile, grease-laden plume as a high-temperature wok burner, so DW/172 doesn’t treat them the same.

The result? Designing to DW/172 can safely and compliantly reduce total exhaust airflows by 30% to 40% compared to AS 1668.2.

See the Savings: Interactive Ventilation Calculator

Don't just take our word for it. Use the comparison tool below to visualise the shift from AS 1668.2 to DW/172 based on your hood length.

The Domino Effect: Why High Airflow Bleeds the Budget

If you think a 40% reduction in airflow is just a nice metric for an engineer’s spreadsheet, wait until you see what it does to the physical build. High airflows trigger a costly domino effect across the entire project:

• The "Anaconda" Ductwork: Air needs space. If you are pushing 3,000 L/s under AS 1668.2 instead of 1,800 L/s under DW/172, your ductwork size balloons. In modern high-density developments, ceiling space is premium real estate. Oversized ducts lead to ceiling height drops, clashes with structural beams, and painful architectural compromises.

• The Monster Fans (CAPEX): To move that sheer volume of air, you need a physically larger, heavier centrifugal fan. This means a heavier roof structure to support it, a larger crane to lift it, and a significantly higher upfront equipment cost.

• The Running Costs (OPEX): This is where AS 1668.2 truly hurts operations. Thanks to the Fan Affinity Laws, fan power is proportional to the cube of the airflow. If you increase the airflow by 50%, your fan uses over three times more energy. Furthermore, every liter of air you extract must be replaced. If you are pulling thousands of liters of air out of a building, you have to bring that same volume of cold outdoor air back in—and pay to heat it so the kitchen staff don't freeze.

Commercial Kitchen Hood Design: AS 1668.2 Vs. BESA DW/172 Breakdown

Here is the exact breakdown of how these two standards stack up against each other in the real world.

Feature / Metric	AS 1668.2 (Standard NZ Pathway)	BESA DW/172 (Alternative Solution)	The Project Impact
Calculation Basis	Hood Dimensions. Blunt volumetric approach based on the canopy size.	Appliance Plumes. Precise airflow mapped to heat/grease emissions.	DW/172 focuses on exactly what you are cooking, not just the size of the box.
Exhaust Airflow	High. Requires a massive volume of air movement.	Low. Highly optimised (Up to 40% reduction).	Lower air volume means smaller mechanical plant requirements.
Physical Canopy	Standard. Tight overhang limits.	Slightly Larger. Increased overhang for a greater catchment area.	DW/172 catches smoke passively via containment before exhausting it.
Ductwork Size	Massive. Crowds ceiling voids and service risers.	Compact. Easily clears structural beams and tight spaces.	Architects win back valuable ceiling height and reduce clashes.
Capital Cost	Expensive. High-pressure, heavy-duty fans and structural steel required.	Cost-effective. Smaller fans requiring less roof support and craneage.	Drastically slashes upfront equipment and installation costs.

Can I Use DW/172 in New Zealand?

Yes. While AS 1668.2 is an Acceptable Solution, DW/172 is highly recognised and commonly accepted across New Zealand as an Alternative Solution under the NZ Building Code (Clause G4).

Getting it through council simply requires a mechanical engineer to provide a Producer Statement (PS1) backed by rigorous design calculations, proving that the DW/172 design meets or exceeds the performance requirements of the building code. Find out more about this service.

The Verdict for Commercial Kitchen Hood Design in New Zealand

AS 1668.2 will always keep you safe and compliant, but it is an incredibly expensive safety blanket.

By utilising DW/172, you stop paying to vent empty air. It requires a bit more brainpower and design documentation upfront, but the payoff is smaller ducts, cheaper fans, and a drastically lower energy bill for the lifespan of the restaurant.

Tired of oversized ductwork ruining your architectural vision, or watching your capital bleed out into the roof plant? Stop settling for lazy engineering. Contact the team at Reshape today. We specialise in high-performance HVAC design across New Zealand, and we know exactly how to guide DW/172 designs through council to save you space, energy, and money.