Whenever we propose a variable speed drive retrofit on a commercial kitchen exhaust fan, the smart operators ask the same question: "If I slow the fan down, am I still compliant with AS 1668?"

It is exactly the right question to ask. A VSD is an easy energy win on paper, but if the retrofit compromises the airflow your kitchen was designed around, you can save money on electricity and then lose your occupancy certificate. We have seen both outcomes.

Here is the straight answer: a properly commissioned VSD retrofit does not break AS 1668 compliance, and done right, it actually gives you more compliance headroom, not less. But there are three specific things that have to be true for that to hold.

What AS 1668 Actually Requires from Your Exhaust Fan

AS 1668.2 defines the minimum extract airflow required at the canopy during cooking. That number is calculated from the cooking surface area, the type of appliance (a char-grill needs more extract than a combi oven), and the canopy geometry.

The important word is during cooking. The standard does not require your fan to run at maximum capacity while the kitchen is closed and nothing is on the stove. It requires adequate capture while cooking is happening.

That distinction is what makes VSDs compatible with the standard. You are not cutting corners on the design extract rate, you are cutting runtime at speeds you never needed in the first place.

The Three Conditions That Have to Be True

For a VSD retrofit to preserve AS 1668 compliance, the install has to satisfy three things:

1. Full speed must still deliver the design extract rate

When a cooking line is fired up and the fan is commanded to 100%, it has to pull at least the airflow the kitchen was originally designed for. This is non-negotiable.

On older systems we often find the opposite situation, the existing fan was undersized and has been running flat out for years without ever actually meeting AS 1668.2 extract rates. In that case, the VSD retrofit is also a chance to correct the baseline, either by tuning the fan curve or upgrading the motor at the same time.

How we verify it: airflow measurement at the duct, not at the canopy face. We take a traverse measurement with a vane anemometer or pitot tube and compare against the original design spec. If the original design spec is missing, we calculate it from the canopy geometry per AS 1668.2.

2. The minimum speed must still achieve capture at the canopy

The more interesting constraint is the lower bound. You cannot drop the fan to 30% during service just because the kitchen feels quiet, the canopy still needs to capture the plume from whatever is cooking.

This is usually not a problem during controlled reduction. Most kitchens we retrofit end up with a service-mode speed of 80-90% and an off-peak/clean-down speed of 40-60%. The lower bound is set by the specific kitchen, not by a rule of thumb, we measure it during commissioning with smoke tests or tracer visualisation.

Where it gets interesting is on kitchens that cook intermittently across an extended period, cafés with breakfast-then-lunch service, or kitchens that handle room service overnight. A smart control strategy will ramp the fan based on actual activity rather than the clock.

3. Makeup air has to track with exhaust

AS 1668.2 treats the ventilation system as a whole, not just the exhaust side. When the fan ramps up, the makeup air supply has to ramp with it, otherwise the kitchen goes into negative pressure and the canopy stops capturing properly.

In practice, there are three clean approaches:

The option you pick depends on what is already installed. Retrofits on older kitchens almost always end up with option 1 or 2; newer installs sometimes justify option 3.

What Should Appear on the Commissioning Report

Whenever we finish a VSD retrofit, the compliance paperwork should include, and this is what a certifier or insurer will want to see:

If a contractor installs a VSD and hands you back only a keypad and a grin, you have an energy saving but no evidence trail. That is the failure mode we see most often when reviewing other installers' work.

The Side Benefit: More Diagnostics Than You Had Before

One thing that often gets lost in the compliance conversation is that a VSD actually makes your system more auditable, not less.

The drive itself measures motor current, output frequency, and runtime hours. If you pair it with a simple monitoring module (wired or wireless), you get a live record of:

When an inspector asks "can you show me that this fan was running at full speed on the night of the incident?" you have data, not a shrug. That is a compliance position a fixed-speed fan cannot offer.

The Short Version

A VSD retrofit does not compromise AS 1668 compliance as long as:

  1. The system still hits its design extract rate at 100%.
  2. The minimum operating speed still captures the cooking plume.
  3. Makeup air tracks with the exhaust.

Get those three right, with measurement, not assumption, and you end up with a system that is more compliant, more efficient, and more diagnosable than the one you started with.

Thinking about a VSD retrofit but worried about the paperwork? We handle both sides, the drive install and the AS 1668 commissioning report, so you end up with a compliant system and the evidence to prove it. Call 0423 063 061 or request a quote through the contact page.

Related reading: AS 1668 and Your Commercial Kitchen: A Plain-English Guide and Why a VSD Retrofit is the Cheapest Energy Win in Your Kitchen.