Tips

Make Up Air

Another thing to make you go hmmm.

Back when humans lived in caves, we could burn a wood fire in the cave without any chimney, and things were OK. There was enough draft to vent the smoke and still allow us to breathe. Wood was easy to come by. The smoke kept the saber tooth lions away. Life was good.

Things have progressed some. Now we have walls, windows, doors, insulation, and natural gas. Does that mean we don’t need chimneys? Nope. People would never run a gas grill in the house, but they do run gas stoves and ovens. Is that logical? Nope.

We in the building trades can build a nearly airtight house. If someone burns a fish in there, it will smell for days. Most people find that unacceptable.

To control these problems, Architects are designing with exhaust fans. Point specific exhaust ventilation is not effective if the house cannot draw in make up air. When you design a big mansion type house with a gourmet kitchen, Viking Range, downdraft cook top, and 1200 cfm exhaust fan, you MUST make a provision for make up air. You must have some way for air to be brought into the house to replenish the air those fans pull out. If you do not, one of two things will happen. Either the fan will make lots of noise and move no air, or the fan will find a place that is convenient to pull in make up air that you didn’t plan on. Usually the make up air is drawn from a wood fireplace or gas fireplace chimney. Pity the novice who installs an 80% furnace and natural draft water heater in a home with a downdraft grill. At some inopportune time the cook top will pull air through the furnace or water heater while one of them is firing. That creates a blow torch effect in the basement. Cool.

Seriously though, think about the specification on your houses. There is an interrelationship between air going out of the home and air coming in. The better you plan the routes out AND in, the better you will sleep and the more your customers will respect and brag about you.

In the houses of today with tight building envelopes and multiple exhaust appliances (down draft cook top, central vacuum, range hoods at 600+cfm, multiple laundry dryers, multiple bathroom fans, fireplaces, whole house vent fans, and God knows what I may have missed here), there comes a time when you need to install make up air.

By the way, the devices known as Heat Recovery Ventilators (HRV) or Energy Recovery Ventilators (ERV) are NOT make up air devices. They are balanced ventilation devices. Their whole purpose is to remove and bring in a balanced air flow. They do not supply air. If you have a situation where a range hood sucks ashes from the fireplace onto the carpet, the HRV will filter out the dust and smells, but it will not prevent or solve the problem.

One more thing, when you specify a bathroom exhaust fan, always put the fan on a timer switch. There is a switch called a fan delay timer available from EFI (800 876 0660) that makes this easy when you have a fan light combination and only space for a single gang switch. If you put the fan on the light switch, then the fan is not doing the job. When people use the bathroom they turn on the light and fan, do their thing, and when they leave they turn off the light. (Well, the parents do anyway. My kid can’t seem to locate ‘off’ on a light switch. ) If the fan goes off with the light, then the moisture or smells in the bathroom probably have not been evacuated because the fan could not pull make up air. When the door opens, the fan has great make up air, but the person leaving the room turns it off. Oops. With a timer switch in the mix, the fan runs for another 10 minutes and you don’t have to go back to turn it off.

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