Can a recirculating fan deliver the indoor air quality we should demand?
Years ago I called the kitchen exhaust the most screwed up, badly designed, inappropriately used appliance in your home. Since then I have written a number of posts on MNN and TreeHugger and not much has changed. People are still debating the merits of recirculating vs exterior exhaust; I tend to agree with John Straube, who says that a recirculating exhaust hood makes as much sense as a recirculating toilet.
The issue becomes particularly important in super-insulated and sealed buildings like those built to the Passivhaus standard. That's why I was excited to see the guidelines released by the Passivhaus Institute (PHI), Kitchen exhaust systems for residential kitchens in Passive Houses (PDF Here). After plodding through it (I am trained as an architect, not a mechanical engineer, and am a few years out of practice), I asked for help on Twitter, so comments are interspersed throughout. Some, like Engineer Alan, think it is all very reasonable.
Recommendations seem sensible:— Alan Clarke (@AR_Clarke) August 27, 2019
if you want direct exhaust you need route for make-up air,
both exhaust and intake need airtight seal when not in use,
heat loss in use is manageble but significant so operation time should be controllable and limited.
Of the three types of exhaust systems – wall-mounted, island or downdraft – they note:
Wall-mounted hoods should be preferred over island extractor hoods because the fume capture is more steady and effective. With the same capture capacity, the volume flow rate of wall-mounted hoods can be approximately 40% lower than island extractor hoods.
I wish they had been more definitive. If you look at the Green Building Advisor post, Does My Vent Hood Need Makeup Air? a test was done on Alex Wilson's kitchen range and hood (seen in TreeHugger here), where Alex has an island and a hood hanging 3 feet above the range. The smoke just blows right by the hood, which seems almost useless.
There is no mention of the shape of the range hood sump shape -- bowl-shaped ones enable range hoods to achieve higher capture efficiency at lower flows, meaning they're quieter, meaning people will be more likely to use them.— Chris Stratton (@christratton) August 27, 2019
PHI suggests that hoods should 50-60 cm (about 2 feet) from the cooktop. They are almost always installed higher, but rapidly lose their effectiveness. I would add that GBA and others like Engineer Robert Bean suggest that it be 6 inches (15 cm) wider than the range, 3 inches on each side.
But it is also totally clear that wall mounted hoods are far more effective than island hoods. They should just come out and say it.
PHI then looks at recirculating hoods. They are not as dismissive as John Straube or Dr. Brett Singer, who calls them "forehead greasers." Or Robert Bean, who says ventilation to the exterior is a must:
It is now apparent to researchers that the smorgasbord of pollutants we sense as aromas, heat and moisture from indoor cooking are reaching concentration levels, which if measured outdoors would have environmental protection agencies shutting down kitchens and issuing fines.
PHI simply notes that "no moisture loads are removed with recirculation operation," so other ventilation is required; and "in order to ensure proper functioning of the recirculation air system and limit the pressure losses, the air filter must be cleaned and/or replaced at regular intervals." They almost never are.
PHI notes that systems that exhaust to the outside can cause issues.
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. The increased heating energy demand is not only due to the ventilation heat losses incurred during operation of the kitchen exhaust system, but also those losses that are possible at the exhaust air and intake air vents, where significant infiltration losses can occur if installation is not executed airtight.
Because of that they conclude: Preference should be given to recirculation hood systems. They acknowledge that exterior exhaust systems can be done, but they get complicated. It gets even worse for small apartments under 900 square feet:
In small apartments the heating demand, and also the heating load, are significantly increased due to the additional ventilation heat losses. Kitchen exhaust systems, which operate by exhausting the fumes externally, should not be used if the average size of the apartment is less than 90 m2.
Also I don't see anything about the pollutant source. Maybe I missed it. Preference should be given to induction ranges as they produce fewer emissions owing to the much lower cooking surface temperature and absence of combustion.— Chris Stratton (@christratton) August 27, 2019
PHI never mention that there is a far worse ventilation problem with gas ranges, which absolutely should have ventilation to the exterior. We have shown piles of research collected by Professor Shelley Miller showing how bad cooking with gas is for your health.
PHI never alludes or mentions interior air quality issues, discussed at length at the last Passivhaus conference by Gabriel Rojas, who found that recirculating hoods didn't do much at all, and that you shouldn't cook hamburgers inside. Meanwhile, Shelley Miller did her own testing of ventilation in Passive Houses in Colorado, and found:
The passive house concept can be an effective design approach to reduce energy use and improve thermal comfort, but it should not be assumed that this type of building has inherently good indoor air quality. Severe, but not atypical, cooking events drastically reduced the indoor air quality for many hours, and the temporary boost mode that many of the mechanical ventilators feature was ineffective at reducing PM emissions from the cooking activities.
Recirculating range hoods should not be sold. And should be forbidden for sale by OSH & all other relevant agencies.— Danny S. Parker (@dannysparker) August 27, 2019
After reading Gabriel Rojas' research, I came up with my own list of recommendations:
- Kitchen hoods should exhaust to the outside.
- Just stop putting gas into homes; induction cooktops work really well now. Without gas and a conventional 4 burner induction range, you can probably get by with a 250 CFM fan. That doesn't take a lot of makeup air.
- 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.
Recirc not ideal, but at least with Passive House there's a continuous HRV exhaust vent nearby to pick up the slack.— Brady Faught (@Bradyfaught) August 27, 2019
Should always be wall mounted, deep catchment. Combustion systems should not be permitted.
But again, I am not an engineer. I recognize that Passivhaus has strict energy limits, and that they are all engineers. I am being doctrinaire about air quality and they are being doctrinaire about energy consumption. Somewhere, there has to be a happy compromise.
I would love to hear other views in comments. In the meantime, I am exhausted. I am picking up pizzas for dinner; that may be the only way to truly solve this problem.
See also on MNN: Worrying about kitchen fans is exhausting