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Potential Problems With Controlling Air Flow In The Kitchen

Posted by John Sims on Tue, Mar, 22, 2016 @ 13:03 PM

Many modern kitchens are outfitted with large range hoods. Some are almost theAir Flow Problems size of a commercial installation. If this describes your current home or perhaps the one you're designing, this is information is important to know!

In a prior article called Controlling Air Flow Is Important For Comfort I explained the reality of One 'Unit' Out = One 'Unit' In. If air is leaking out, it must also be leaking in; a vacuum can’t be created under natural conditions.

It's not just air leaking in that you have to be aware of; homes have many fans inside of them. They are there to exhaust smelly air, humid air and the combustion by-products of heating appliances. (Furnace and  water heater)

Let’s look at one more area of air flow: The Range Hood/Kitchen Exhaust Fan

Air volume is measured in cubic feet. So a fan is rated in cubic feet per minute or "cfm". As you can see by this chart, the range hood can operate at 1,500 cfm in a 30 minute cycle which equals 45,000 cubic feet out. Your kitchen fans could be pulling dangerous fumes through your fireplace in the living room. 


Cubic Feet Per Minute (CFM)

Clothes Dryers:

150 – 250

Bath Exhaust Fans:

50 – 100

Kitchen Exhaust Fan:

100 – 1,500+

Whole-House Fans:

~2,500 – 5,000+

Central Vacuums:


Fireplaces (pull in air for combustion):

up to 400

Stack Affect (convection loop):

~15 - 30

Every time an exhaust fan removes air from your house, an equal volume of air must enter. The air that enters cracks in a home’s envelope to replace air that is exhausted is called “makeup air.” Two trends affecting makeup air are causing increasing problems for homeowners: homes are getting tighter, and range-hood fans are getting more powerful.

Here's the problem.

If the house doesn't have enough random air leaks around windows, doors, and mudsills, the makeup air is often pulled backwards through water-heater flues or down wood-burning chimneys — a phenomenon called backdrafting.

Since the flue gases of some combustion appliances can include carbon monoxide, backdrafting is dangerous. In some cases, it can be life-threatening.

Larger range hoods have at least one thing in common with combustion appliances: they also require makeup air. Unfortunately, installation instructions didn't refer to the need for providing makeup air like furnaces and water heaters.

So, here’s some great advice I found from a building science expert:

Hope this helps!

Topics: IAQ, Indoor Air Problems, Air Flow, Air Balance

Controlling Air Flow Is Important For Comfort

Posted by John Sims on Fri, Mar, 11, 2016 @ 11:03 AM

We all want a home that is comfortable. You need to know this information to have one! Understand air flow in your home is very important. Controlling it is the challenge!

This is the foundation for achieving the greatest level of comfort. 



For your home to do its job, it must separate the inside from the outside! It's just not that simple. Air will take path of least resistance through largest hole.



One 'Unit' Out = One 'Unit' In If air is leaking out, it must also be leaking in; a vacuum can’t be created under natural conditions.


It's not just air leaking in that you have to be aware of; homes have many fans inside of them. They are there to exhaust smelly air, humid air and the combustion by-products of heating appliances. (Furnace, clothes dryer, water heater)

Air volume is measured in cubic feet. So a fan is rated in cubic feet per minute or "cfm".

Let’s look at one that is often ignored: the exhaust fan for clothes dryers. Air Out-- from your laundry room into the dryer à exhaust outdoors. Air In-- from the holes with least resistance.

Average size (2,000sq.ft.) home = ~18,000 cubic feet. As a result, just running the clothes dryer for a 60 minute cycle(12,000 cubic feet) will replace approximately 2/3 of all the air in the home.

Homes often have their laundry rooms situated next to or near the garage. Dangerous fumes like carbon monoxide could be pulled into your home with unwanted air flow.

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Topics: Air Leakage, Comfort, Air Flow, Air Balance

Problems With Air Duct Systems.

Posted by John Sims on Fri, Aug, 22, 2014 @ 12:08 PM

We do our best to bring homeowners the most relative and helpful information we can find on a wide variety of home performance topics. We were on the topic of 'sucking' dust in your home through your air duct system and we were discussing air pressure imbalance. 

So we researched what some of the best building science experts say about topics we happen to be covering. That way, you don't have to take just our word for it. Some of this information may be a little more than you want to know, but there is some really good stuff. Hey, if it helps you solve a problem, then it's worth it. 

Here's some information from Greenbuildingadvisor.com, a pretty good collection of experts.

ABOUT DUCT SEALING - Leaky ducts do more than waste energy.

Supply and return ducts are assembled on site from many individual pieces, and each connection is a potential air leak. Leaky ducts create a number of potentially serious problems in addition to wasting energy dollars. Unhealthy air from an attic or crawl space can be sucked into the system and distributed around the house. Leaks also can contribute to the growth of mold and mildew.

Mastic. The best material to use for sealing ducts is mastic

ABOUT BALANCED SYSTEMS - Provide a pathway for replacement air.

Many forced-air distribution systems create unwanted pressure imbalances within the building envelope — higher air pressure in some parts of the house, low air pressure in others. This problem can be minimized by providing adequate return ducts to move air back to the air handler or furnace.

There are two general types of return systems: a central return, which serves a limited number of rooms, or individual room returns, which are designed to return air from each conditioned room.

Individual air returns are usually quieter and do a better job of minimizing pressure imbalances. But they are also more expensive to install, and they may require a larger blower motor in the air handler to overcome the increased friction of air moving through a larger number of ducts.

Unbalanced HVAC systems make a house uncomfortable.

Rooms with undersized returns become pressurized, forcing air into wall and ceiling cavities. During the winter, warm interior air can carry moisture into the walls where it condenses on cooler surfaces. This may lead to the growth of mold. A room with a large return-air grille but an inadequate supply-air register can become depressurized, drawing outside air into building cavities.

These problems can be avoided when the volume of air supplied by the furnace is balanced by an equal volume of air being drawn into the return ductwork.

Air Ducts: The goal is a balanced system.

Dr. Michael Busby, a former engineer who worked with a firm that supported NASA says, a well designed HVAC system, supply and return ducts deliver an equal amount of air. The system inhales as much as it exhales. In a poorly designed duct system, the air imbalance is made up through your walls, floor, and roof.

Dr. Busby's engineering firm has engineered 1000's of homes all over the country. He goes on to say:

Leaky supply ducts waste energy. Suppose the supply side of the system is leaky but the return ducts are well sealed. One obvious outcome is that some of the heated or cooled air isn’t getting where it’s supposed to go. That’s an energy loss. Another consequence is negative pressure inside the living space. The imbalance can draw air into the house from the attic, crawl space or around doors or windows. This replacement air could contain mold, dust, or other contaminants.

Leaky return ducts lower air quality. When there are leaks in the return side of the system, another kind of problem develops. The furnace fan can draw in air from a basement or crawl space, with the potential of introducing cold air (an energy loss), radon, dust, flue gases or other pollutants.

Do you think you have these problems or you'd like to find out, call us or schedule an appointment.

Topics: Indoor Air Problems, Dust Problems, Air Balance, Duct Leakage

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