What is there to a range hood? Well, it depends on how deep you want to dig.
In simple terms: You want a functioning range hood over your stove to dump cooking odors and moisture to the outside. It helps a long way with maintaining good indoor air quality (IAQ). Who really enjoys smelling the cooking from a few days ago across the house or wiping the cooking grease film off of the living room furniture?
But simple is boring! The nuances that surround the range hood are much more interesting.
Nuance heaven
I mentioned it, and I’ll say it again: Be picky about the ventilation rate!
Logic suggests that a hood with a high ventilation capacity would be more effective in exhausting cooking odors. But it wouldn’t necessarily be efficient. How much airflow would it really take? Any additional flow beyond that represents wasted electricity.
Widely referenced residential ventilation rates can be found in the ASHREA Standard 62.2. The minimum required exhaust rate for user operated range hoods is listed at 100 cubic feet per minute (cfm). The standard was published back in 2003. I expect that it will be updated considering the increasing emphasis on reducing air leakage in buildings.
Our freshly installed hood has two ventilation settings – 100 cfm on low and 250 cfm on high. There should be a balance between the amount of air exhausted to the outside and the makeup air.
High capacity range hoods (starting at 600 cfm) may throw that balance off and begin to depressurize the building, which can cause a number of problems.
Take open combustion appliances, such as a water heater for instance. With the high capacity range hood cranking at full speed, the exhaust fumes from the water heater may backdraft into the building.
Or – during the summer months – a high capacity range hood may be the culprit for hot and moisture laden air leaking into the building. If the building is air conditioned, interstitial condensation forms on the cooler interior surfaces, which can lead to mold problems.
Options, options, options…
Have you seen those range hoods that ventilate back into the building? They are the preferred option in some particularly airtight Passiv Haus buildings. A charcoal filter in the hood should eliminating the cooking odors – as long as it is regularly replaced.
Suppose we are on top of the filter maintenance. We still haven’t addressed the additional moisture load from the cooking.
Let’s ventilate to the outside instead and think about managing the makeup air. Opening the kitchen window a crack while the range hood is operating would be an option. Other folks borrow from commercial kitchen design, and install a fresh air supply duct that terminates near the stove.
Either option carries an energy penalty: Introducing unconditioned air into the conditioned building interior. This may be less of a problem in the winter when heat generated from the cooking may somewhat lessen the energy loss. During summer, however, the make up air would be hot and humid and one can start to count the energy penalty in sweat beads.
Why hasn’t anyone come up with a heat exchanger for range hoods, similar to the Heat or Energy Recovery Ventilators (HRV’s or ERV’s)? The engineering challenge lies in the cooking grease.
Read ERV or HRV installation manuals, and you are instructed not to install an exhaust in the kitchen. The cooking grease will ruin the heat exchanger, or at a minimum shorten its lifespan.
Should we grease up some sharp minds and try to solve this fat engineering challenge? Even if we do, the grease removal or management would likely require additional energy input. That flies in the face of the energy reduction goal.
Going back in history to a time where energy or electricity was precious can be enlightening. Jeremy Spates, who studies historic preservation at the School of the Art Institute of Chicago, pointed out that “especially in Chicago, cooking was probably done with coal or wood in an iron stove, but with a smaller blaze than one might use in winter”.
Larger houses had the kitchens in the back or the basement, such as the Clark House. I only can speculate to what extent this may have reduced the heat gain from summer cooking.
Jeremy also pointed out that outside the urban environment, some houses (or mansions) in the eighteenth and nineteenth centuries had outdoor kitchens where the summer cooking took place, thus keeping the extra heat out of the main building.
Now here is a good idea – simple yet effective, and may be more suitable for energy efficient buildings than we realize.
The concept
Kitchens are difficult to manage in airtight homes. Why shouldn’t we place the kitchen outside the airtight envelope – at least on a seasonal basis. It would address the excessive heat gain, moisture load and makeup air issue. It would solve a big headache.
This idea has been lingering for a while and our back porch has come up as a potential space where we could conduct our summer cooking. The deeper we get into the summer, the more I warm up to the idea and I am increasingly glad that we extended the gas line into the back porch – just in case we decide one day that summer cooking on the porch is the cool thing to do.
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