Makeup Air Surprise

On today’s show we are taking a look at how some changes to the building code are driving significant new costs.

Buildings are becoming healthier than they once were. It’s desirable to insulate a property in order to maximize efficiency. However, these highly sealed homes and offices can also build up toxins or behave in unexpected ways.

If you like to cook, then chances are you want to have an effective exhaust fan above your stove in order to prevent both grease and smells from permeating all over your home. A high capacity range hood is key. But if the range hood is going to pull kitchen smells effectively out of your home, you need a high capacity fan and a large duct to the outside. In the old days, homes were leaky enough that the range hood did not pull enough air to cause a fall in air pressure inside the home. The numerous gaps and leaks around windows, doors, electrical outlets and so on enabled fresh air to seep into the house without creating a problem.

Where problems do arise is when a home is very tightly sealed. At that moment, there are only a few remaining openings for air to enter the house.

There are the bathroom exhaust fans which have a gravity damper that lets only a small amount of air to backdraft into the home. There is the dryer vent which would allow air to be sucked back into the house.

But the problem is that many houses have appliances that burn fuel. Specifically, a natural gas furnace, a natural gas water heater, and a fireplace or wood stove. The exhaust vent for each of these fuel burning systems is another hole in the house. These perforations are designed to exhaust the fumes from the combustion process. But the laws of physics says that air will flow from the location of highest pressure to a lower pressure area.

The problem is that if the fuel burning appliance is having air sucked through the chimney back into the house, you could have carbon dioxide and carbon monoxide being sucked into the house. The larger the fan, and the more sealed your house, the greater the risk.

As a result, the building code is being amended in many communities to introduce an active solution to this problem.

Back drafting of fuel burning appliances like furnaces water heaters and fireplaces can be amplified by the wind created areas of high pressure or low pressure on one side of the house or another depending on wind direction.

In many parts of the US, if an exhaust fan has a capacity of more than 400 CFM, then a makeup air system is required. Some building codes require a makeup air system regardless of the exhaust fan capacity. This requires the addition of a blower of equivalent strength as the exhaust blower to restore balanced air pressure in the house. A passive system is not enough to meet the new code. The makeup air system must have several components that are all interlocked with the kitchen exhaust fan. It must have a damper, a blower, a temperature sensor and a heater. This heater is going to be pretty strong. We’re talking a 10kw heater to warm the air.

You might be thinking that you’re going to go out and spend a few hundred dollars on a nice shiny stainless steel range hood to put above your brand new stove. Then surprise you’re now facing a bill of an additional $3,000 for the makeup air system to balance the air pressure for the range hood.

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Host: Victor Menasce

email: podcast@victorjm.com