Should Your Kitchen Have a Recirculating or a Direct-Exhaust Hood? I Am Exhausted Just Thinking About It

It is a real problem when designing energy efficient homes, and it seems that there is no good solution except ordering in.

I have called the kitchen exhaust hood the most screwed up, badly designed, inappropriately used appliance in your home. As houses and apartments get built better and leak less, exhaust hoods become even more problematic. I quoted an expert earlier:

Frying, grilling or toasting foods with gas and electric appliances creates particulate matter, nitrogen dioxide, carbon monoxide and carbon dioxide, and volatile organic compounds....Emissions of nitrogen dioxide in homes with gas stoves exceed the Environmental Protection Agency’s definition of clean air in an estimated 55 percent to 70 percent of those homes, according to one model; a quarter of them have air quality worse than the worst recorded smog (nitrogen dioxide) event in London.

For designers and occupants of super-insulated houses like those designed to Passivhaus or Passive House standards, it is a serious problem. There are two main kinds of fans -- the recirculating fan that can have different kinds of filters which clean the air as it passes through; they are disparaged by many as “forehead greasers”. Then there are the ones that exhaust directly to the outside; they get rid of all that stinky, greasy air, which then has to be replaced. That’s easy in a draughty old house like mine, but in a tightly sealed Passivhaus design it is a real problem. How much of a problem?
At the recent 22nd International Passivhaus Conference in Munich, Monte Paulsen and James Montgomery of RDH did the math and found that in San Francisco, a recirculating fan would save up to 2.2 kWh/m2a. In much colder Edmonton, Alberta, it would save as much as 8.6 kWh/m2a. Given that the total energy consumption of the building cannot exceed 60 kWh/m2a, that is a big chunk of lost energy. It’s no wonder that most Passive House designers use recirculating hoods.

Gabriel Rojas/ Photo Lloyd Alter/CC BY 2.0

But do they actually work? Later in the conference, Gabriel Rojas presented his work at the Lawrence Berkeley National Laboratory. We all got hungry as he fried onions (which reduced the particle concentration by 50 percent compared to not filtering). Then came salmon cooked in canola oil, which reduced particulates by 45 percent. A couple of slices of toast? 45 percent. Finally, a burger got fried up, which pumped out a lot of particulates.

Gabriel Rojas with food/ Photo Lloyd Alter/CC BY 2.0

After digesting all of this (figuratively, not literally, I assume), Rojas concludes:

The recirculating filter tested in this limited study reduced particle concentration in the room to some extent. For the onion, fish and toasting events, the total particle count as measured by the FMPS [Fast Mobility Particle Sizer Spectrometer] was reduced by roughly 50 percent. Experiments with the burger found a reduction of roughly 20 percent.

And where do the other half of the particles go? Probably stuck to the walls or those fuzzy objects in the room, or sucked into the HRV system where they probably coat the filters. As John Straube told me:

There is a fair bit of experiential evidence that recirculating fan hoods do not remove enough of the pollutants.... Also, many of the contaminants other than coarse grease particles don't get captured — all kinds of gases and particles are released that cannot or are not effectively be captured by grease filters.

What’s a designer or a homeowner concerned about air quality to do? We can’t recommend ordering in from a properly vented commercial kitchen, and giving up burgers is a stretch. But Monte had a few suggestions for recirculating hoods:

• Require deep range hoods with better capture efficiency.
• Interlock the cook top, hood fan and Heat Recovery Ventilator booster switch.
• Require both grease and charcoal filters.
• Identify reasonable noise limits for range hoods.

Wolf/ There is no way that range hood is going to anything. /Promo image

However, I read Gabriel Rojas’ research and wonder if that doesn’t demonstrate that we really should exhaust to the outside, even in a Passivhaus. In which case I would add:

• Just stop putting gas into homes; induction cooktops work really well now.
• Put ranges against a wall. This is a no-brainer but won’t stop people from putting little hoods over big ranges on islands.
• Engineer Robert Bean recommends that it be wider than the range, not more than 30 inches from the top, and against a wall. Oh, and the duct runs should be short and straight.

This is a difficult issue, and as I noted in a previous discussion of this, it's frustrating. It seems that there really is no clear answer, that more research is needed, it's complicated and confusing. But then I'm just venting.