Finally: A Recirculating Range Hood That Is More Than a 'Forehead Greaser'

Conventional range hoods either exhaust all your conditioned air or just move the grease around.

Andrea Rugg / Getty Images

I thought I had pretty much exhausted the subject of what I have called the most screwed up, badly designed, inappropriately used appliance in your home: the kitchen exhaust hood. The problem with almost all of them, including the one shown above, is they are on islands, or they are too far away from the stove, or that the ductwork is too long, or that they have flat bottoms or all of the above.

Also, as houses have become more airtight and energy-efficient, a hood venting to the outside becomes a problem as so much conditioned air is pumped out and has to be replaced.

But recirculating hoods — or "forehead greasers" as the engineer John Straube called them — don't do the job either. Physicist and ventilation expert Allison Bailes says recirculating range hoods are as effective as recirculating toilets. It is such a difficult problem that I have complained it seems there is no good solution except ordering in.

Aaron Woods

Aaron Woods has been struggling with this too. He builds commercial ventilation systems by day but has also been working on a new kind of recirculating exhaust hood for residential uses, to address the needs of Passivhaus builders in British Columbia, Canada. He demonstrated it over Zoom for me, and since I am an architect rather than a physicist, I asked Bailes to join me in watching it again. (This is all a work in progress so we apologize for the quality of the photography and the zoom screen-grabs.)

Aaron Woods

The ActiveAQ unit doesn't look like much. In fact, you can't even see it because it is built into the cabinetry, it is only 12.5 inches deep. This is important because most of the money in traditional hoods is in all the fancy glass and stainless steel to make them pretty. With the Active AQ, the money goes into the actual hardware and the controls.

Instead of a stainless steel prow sticking out, the Active AQ has an air curtain coming out of little holes on the front of the hood that diverts the fumes into the unit itself. This has the added benefit that people don't bump their heads on it and their tops don't get covered in grease and dust.

Aaron Woods

Behind the kitchen cabinetry, the unit is a large box made of relatively heavy-gauge steel with a series of compartments. In the bottom compartments there is a wool filter, which Woods says is very effective at capturing grease, and being natural is biodegradable.

Aaron Woods

The problem with it is if you are generating a lot of moisture: When it gets wet, Woods says "it smells like a barn." He is looking for alternatives, possibly polypropylene. Above that, there is a one-inch panel containing a lot of heavy-duty activated charcoal.

The middle compartment contains two fans that pull from below but also push air around the outside to make the air curtains. They run at a relatively low 150 CFM; any higher and you get turbulence that sends the fumes everywhere.

Then in the top compartment, there is another activated charcoal filter and a MERV-13 or even a HEPA filter, fine enough to take out particulates and even viruses. It won't take out gases like carbon dioxide (CO2) or nitrogen oxide, so forget about using this over a gas stove. But it will get most of what comes out of food when cooking over electric or induction, and a lot still does.

Zoom Screen Capture by Lloyd Alter

All of this is controlled by electronics that turn the fan on automatically when the stove is turned on, with other sensors and detectors that can be connected to crank up the Heat Recovery Ventilator or to an air quality monitor like the Awair. The unit can even be left on at very low power when people are not cooking to act as an air cleaner; with all that charcoal and the HEPA filter, it can clean the air in the unit all day and night long.

Aaron Wood

It is all heavy gauge metal so it doesn't bend under the air pressure, and none of the components or controls are cheap, but it still will probably cost more than a high-end hood. There's lots more work to do as well, including testing to measure exactly what does get through. Woods is also working on a budget model for all the multifamily Passivhaus buildings that are going up in Vancouver, which need something like this.

I asked Bailes to watch the demonstration of the ActiveAQ and hhe tells me: "BTW, you’re right to be excited about this hood." He will be writing about it, probably in a little more technical detail on his website, Energy Vanguard.

But he is right, I am excited, this has been a problem for a while. It's silly to pay for heating and cooling air in a very efficient building where ventilation is carefully controlled and balanced, and then pump it out the wall and have to reheat or recool its replacement.

As the Passive House Institute noted in their look at the problem:

"In buildings with a very low heating demand, such as Passive House buildings, the use of a kitchen exhaust air system may increase the heating energy demand of the dwelling significantly."

But just pumping it around in circles without removing the grease and the VOCs or the particulates doesn't do anything either. While some in the Passivhaus community say that the recirculating fan takes out the big stuff and lets the main ventilation system do the rest, others are not so sure that it does enough.

Aaron Wood

The ActiveAQ should make everybody happier; it reduces that energy demand but will catch the PM. The boost mode will help with humidity and other gases. It may finally be the solution to what has seemed an intractable problem.

We will keep readers informed of progress, and link to Bailes' post when it is done. For more information, contact aaron (at) dynamichvac.ca