I have a number of clients who remove the standard hood fan in the kitchen, to install a designer unit, usually with a capacity of about 1,200 cfm (cubic feet per minute) in lieu of the standard 180 cfm hood fan. I have been told that in winter, designer fans can completely exhaust all the inside warm air in less than an hour. If this is so, what can my clients do to maintain comfort levels and still run these monster fans in their kitchens?

Monster fans can be scary things in ways that people don't even consider. I would have a tendency to worry about something else other that heat loss and comfort. Let me explain. Let's say that the average new house contains 25,000 cubic feet of air and the fan is certified to exhaust 1,200 cubic feet per minute. This would mean that all the house air would be exhausted in 20.83 minutes. It is reasonable to assume that one cooking session, with the fan on, would be at least 21 minutes, so you would have to put your winter parka on to cook dinner. This scenario is unlikely, so let me explain further. I have completed many air tightness tests on homes of all ages. New homes have a tendency to be 'tight', meaning that there are few areas in the building's envelope where outside air can make its way into the house through small cracks and holes. If, collectively, there are not enough cracks and holes in the envelope of the house to supply the fan with 1,200 cfm, the fan will pull air from other sources like flues/chimneys that exhaust combustion byproducts from your furnace, water heater, and gas or masonry fireplaces. This will lead to carbon monoxide back drafting into the house and, if inhaled by the occupants, can lead to sickness and possible death. To compound this problem, add on the other appliances in the house that exhaust air out of the building's envelope, like central vacuum systems (if the motor is mounted in the garage) bathroom fans, and dryers. Ventilation requirements for new homes, in my opinion, are inadequate and should be upgraded. I would suggest anyone who wishes to place monster exhaust fans in their home, whether the home is old or new, should have an air tightness test and a back drafting test performed on the house before using them. Perhaps another suggestion for all your clients is to offer to install heat recovery ventilators in their homes, and have these devices ducted to all the rooms with high humidity sources. A properly installed and maintained heat recovery ventilator will be effective in exhausting stale air from the house and recovering heat from the stale air before it leaves the house, so that comfort and energy savings are maintained.