Friday, 30 September 2016

Kitchen extractor fans, and their fans

What about ventilation in the kitchen then? 

The manufacturers of Best cooker hoods recommend you exchange something like ten times the volume of your kitchen per hour, which for a regular kitchen is between three and four hundred cubic metres per hour. Passive House recommends you need to ventilate the whole house around 35 cubic metres per person per hour. So these kitchen hoods need two or three times more ventilation than the whole house. Obviously this is a peak load, and the kitchen is not going to be ventilated the whole time.

Japanese cooker hoods typically have three settings: Strong, medium and weak. It may be my Japanese web-searching inability, but numbers don't seem to jump out when I try to find them. Instead there are rather a lot of sites asking why their extractor fans don't seem to be extracting very well.

This site of notes about making your own home (in Japanese) provides a lot of data about different fuels, and some calculations giving a figure of 1,166 cubic metres per hour (assuming three gas rings), or 551 if there are only two rings. Other sites and some of Mitsubishi's extractor fans go into the thousands. 

There are probably differences in peak cooking intensity between Japan and the UK. The former has a culinary history based on wood as a fuel, which gives a strong high heat, ideal for steaming and stir frying. The latter, on the other hand, used peat or coal, which give a lower, longer heat more suited to baking or roasting. Hence the Japanese language has one work—yaku—which can translate into English bake, roast or grill, while there are various different Japanese words for fry, whether itameru (stir fry) or ageru (deep fry). But I'm supposed to be writing about ventilation here, not deep culinary thermodynamics.

The choice in Japan is typically between extractor only fans and those that will bring air into the house at the same time as they are expelling air. The latter are often recommended for houses that are airtight. In our house we got an extractor only fan, with the reasoning that it would only have one hole in the wall rather than two, and therefore make the house less leaky since extractor fan ducts are major causes of reduced airtightness. I'm not sure whether we made the right choice; w hen we switch the kitchen fan on, the front door becomes difficult to open.

In other countries, at least for passive house, another choice is recirculating hoods that will send the air through a charcoal filter to remove the oil and kitchen crap before releasing the scrubbed air back into the kitchen. The heat will then stay in the thermal envelope, and the air will sooner or later pass through the heat recovery ventilation. 

Lloyd Alter on Mother Nature Network has interesting insights into the lack of clarity on the subject. And some very nice pictures of dream kitchens. He discusses how some people rail against recirculating kitchen ducts, while the people wanting low-energy buildings see them as essential. He points out that in Ireland you're allowed to connect a kitchen duct to a heat exchanger but in Canada it's illegal. 

So what should you do? Recently I've been thinking about making a pizza oven in the garden. I know that doesn't entirely answer the question, but if you want to have a low energy house, do you want high-energy cooking inside? We then get into the question of whether low energy buildings should be enforcing low-energy lifestyles, or whether they should allow people do whatever they want while reducing the energy use. 

By the way, if you were wondering why there are so many people complaining that their extractor fans don't work well, it's because they need cleaning.