Saturday, August 18, 2012

Does my house suck?

We are not talking in a slang term when we ask this question. What we want to know is if your home is under negative pressure or is it sucking. This important to know and understand when we look a a house as a system. A house under negative pressure will" suck" air in from a host of unwanted areas increasing the chance infiltration and all the negative attributes that come with it.
This is different than the stack effect but the two interact.
We are most concerned about negative pressure in a tight house. By nature a home that is not sealed well we have less of a risk of being in negative pressure. This is because make up air will be drawn in from whatever penetrations are available (infiltration) there and balance the pressure difference. Our homes can be under negative pressure for a number of reasons.  Wind can push on one side of our home and cause a pressure anomaly on the leeward side.
We are always worried about negative pressure in a tight home. This can cause a negative pressure boundary inside the shell of the home. This can cause back drafting and spillage in natural drafting combustion appliances and could bring dangerous CO gases into living space. These gases can then be trapped having nowhere to go in a well sealed home that remains under negative pressure. Moisture build up is also enhanced and the risk of mold, mildew and durability issues is increased. Sick house syndrome is often attributed to a home under negative pressure. Finding the homes minimum ventilation rate is essential information on a tight home.
We are also worried about mechanical systems in all homes. The average home owner might have the ability to put their home into a negative pressure by running appliances. These include but are not limited to clothes dryer at 200 CFM, bathroom fans at 50 to 150 cfm, central vacuums at 100 to 200 cfm, downdraft ranges, microwave and kitchen exhaust vents of 100 cfm or more. Whole house fans can put our homes under severe negative pressure. A leaky ducting system can cause a home to be under negative pressure. Fireplaces can also put our homes under negative pressure.
So what can we do about it? There are several strategies. First we identify through testing. This would be conducting a CAZ test. 
Our goal is to have the home neutral pressure or ideally just slightly positive. This will keep unwanted infiltration and all the problems that come with it entering our homes. If the home is under too much positive pressure we will waste conditioned air through exfiltration and possibly push contaminates back into our living space.
Understanding our pressure boundaries and establishing them where we want them is crucial for indoor air quality and energy efficiency.

1 comment: