By Toby Davine
Creating a passive house kitchen is a balancing act. Designers must maintain airtightness, while not compromising air quality; keep energy down, while ensuring proper ventilation. There are different ways to build passive house kitchens and strong opinions on what is the best way to do it. Rather than offering a prescription, this article offers an overview of the different ways architects and designers have troubleshooted their passive house kitchens.
To put it simply, according to architect and passive house consultant Adam Cohen: “you have to McGyver it.”
Recirculating Fans or Extracting Range Hoods?
There are two main kinds of range hoods: extracting range hoods and recirculating fans.
The conventional (vented) extracting range hood expels air from the kitchen outside through an opening in the building envelope and make-up air from the outside is pulled through a passive vent by the exhaust fan on the hood to avoid depressurization of the building.
“If you want to have a vented kitchen hood in a residential building, you have to do it right. You must have robust methodology that won’t be ignored by residents,” says Cohen.
This is important, as some studies have shown that the majority of residents use their range hoods infrequently when cooking. Of course, these vent openings have an impact on the building’s overall airtightness. In cooler climates, like those in Canada, one might need to heat incoming make-up air with a small resistance coil duct heater.
Though one takes a hit on energy and airtightness through this method, some prefer the extracting range hood system. As Alex De Gagné, VP Sales & Marketing and Co-owner of Minotair Inc., explains, “One has to break some eggs to make an omelette. It will cost some electricity to make it work, but it will provide exactly what is needed.” He suggests using an extracting range hood connected to an air make-up system with preheating, like this model from Fantech, which has an automatically adjusting CFM rate up to 750 CFM.
De Gagné adds that this is in keeping with most provincial buildings codes. “ASHRAE Standard 62.2 [which is used by most building codes in North America], requires that local exhaust ventilation be provided for kitchens and must provide 100 CFM or more if intermittently operated or 5 air changes per hour (ACH) of kitchen volume if continuously operated,” explains De Gagné. “But there’s an understanding in the ventilation business that the effective bare minimum to do a decent job is to go with at least 200-250 CFM. This brings us to use a dedicated range hood fan, as regular heat recovery ventilation (HRV) and energy recovery ventilation (ERV) won’t be able to provide that amount unless you have a really big oversized unit.”
De Gagné believes that those that will simply rely on the home’s general ventilation system will not get the best results. “HRVs and ERVs are not made to process air with some grease in it. Plus, insurance companies probably won’t pay if a grease fire happens either in an HRV/ERV and/or its related ductwork as it’s not made to do that, even with the presence of a filter on the exhaust grill in the kitchen.”
Still, recirculating fans are often the hood of choice for passive house residential kitchens in Canada, as they are more energy efficient. Instead of expelling air outside, they typically filter air through a grease screen and an activated carbon filter and release it back into the kitchen.
A 2017 review from the Lawrence Berkeley National Laboratory revealed that there hasn’t been enough research to test the effectiveness of recirculating fans in highly energy efficient homes. In fact the study states that “no scientific study investigating the performance of recirculating range hoods was found”.
Many in the Passive Buildings Canada community, including our President, Terrell Wong, suggest that the quality of the recirculating fan you use is key. For example, Wong typically uses the Vent-a-Hood system, which has been most effective in her experience.
Adam Cohen was the first designer to complete a commercial kitchen to passive house standards. He explains that he didn’t know that commercial kitchens had not been built to passive house standards before he completed his design. He assumed, incorrectly, that the buildings he saw in Europe had included their commercial kitchens in the building envelope. When he had completed the kitchen, architects and engineers from Germany and Austria were calling him for the plans.
“For commercial kitchens, you need to deal with the energy use of that kitchen. The calculation for energy use is difficult. You need to try to understand if you need heat recovery on your hood, and the issue with hoods is, the more efficient systems don’t get along with grease from the kitchens, so you tend to employ a water jacket around the exhaust duct rather than have the heat exchanger in the air stream, which gets messy with grease and takes a lot more maintenance,” says Cohen. “There’s a lot of balancing in the calculations between energy use and energy lost, energy recovery, operational and maintenance cost...there’s a lot of nerdy detail that one gets into when making a commercial kitchen.”
As for the nerdy detail, for this project, Cohen built “two commercial extracting range hoods with UL listed dampers to control airtightness, as the majority of the time the hood is off. When the hood is on it has a balanced 2030 m3/hr make up and exhaust air system.” He did not utilize any heat recovery here because the cost is high, it is used infrequently, and it is not compatible with grease-laden fumes.
Cohen added, “Although the kitchen hood has a make up air component, we took the precaution of interlocking the ERV in the adjoining area with the hood controls. This interlock energizes the ERV to full on position when the hood is in use, thus reducing the chances of building depressurization.”
Whichever range hood system is used, it’s clear there is no one-size-fits-all solution. Various factors, like climate, cooking type and frequency, and energy load will determine what system works best in each building. Technology for passive house kitchens continues to evolve and until then, we’ll have to rely on our collective experience and the McGyver in all of us.
EDITOR'S NOTE: For more on range hoods see guidelines from Reducing Outdoor Contaminants in Indoor Spaces