Blog Tag: negative air pressure

Positive and Negative. Two words that are often used to indicate how someone feels about a certain issue or to describe the particular temperament of an individual as in “he is always so negative.” These two words are also used in the math world to indicate whether a particular number is positive or negative. But, for the purposes of this blog, we will use these two words to discuss the physical relationship of positive and negative pressures as they relate to heating/cooling systems and mechanical air filters .

When people think about air filters they normally associate them with the one they have in their heating/cooling ( HVAC) system. They need to be replaced about every 3 months or so as they get dirty and can create some air flow problems in the HVAC system. Most people are unaware of the fact that air flow through the HVAC system is affected by a number of variables.

In order to move air through an HVAC system requires a motor powered fan system. The size of the motor, in horsepower, is dependent upon the size of the system. For the most part, there are four sizes; 1/3,  1/2, 3/4 and 1 HP. The fan/motor assembly has to have enough power to move hot or cold air ( and cold air is heavier so it requires more power to move the same amount of air ) through a series of supply and return air ducts and grills as well as through the HVAC filter. Most motors are now multi-speed or ECM digitally controlled motors that automatically adjust speed to fit the air temperature and pre-determined comfort levels for the home.

The fan/motor assemblies have to both pull the air from the return air ductwork, pull the air through the air filter and push the air through the burner or condensing coil and supply side ductwork. The pressure to do this is measured in inches of water. For example, a 1200 CFM 3 ton HVAC system would use a 1/2 HP motor and the total system static, less the air filter, would be .5 ” to .7″ of water. This total pressure does not include the use of a standard, throw-away fiberglass furnace filter. The pressure drop or resistance across a minimum duty filter clean is about .2″. For some of the more dense media filters like the 3M Filtrete filters the clean pressure drop can be as high as .4″ to .5″. Even clean these filters already have almost as much negative pressure as the entire HVAC system.

Once these filters or any air filter gets dirty the pressure drop across some filters can reach 1.0″. This means the motor/fan assembly will not be able to pull and/or push enough air through the HVAC unit to adequately heat or cool the home. As the pressure drop increases, unless you add more horsepower, the motor/fan assembly will actually slow down. This is why it is so important to understand the relationship between pressure ( positive and negative ) as it relates to the overall effectiveness of the HVAC unit.

For more information on understanding the adverse affects of filters on air moving devices please go to the “Pure Air University” tab of the following web site. www.pureairsystems.com

It is always interesting to view homes when driving. Whether its a county dirt road or a major highway you are always within close proximity to someones place of residence. I’m sure that the people that live close to the roads enjoy the quick access and associated short drives to their destination. But are they sacrificing health for a quick trip to the store?

When cars and trucks  travel down any road tiny bits of dirt, dust and tire debris are thrown into the air. In addition, the amount of NO2 and NOX that is released from the exhausts can exceed the OSHA and ASHRAE limits when you are in close proximity to these effluents.

On a hot, dry, windless day you can often see the layers of dirt or dust hanging in the air close to a highway or dirt road. These layers of “hanging” dust obviously include particles that you can see ( usually larger than 20 microns in size) but this layer of dust also includes almost twice as many sub-micron particles ( usually less than 10 microns) that you can’t see.

When you combine the small, respirable sized particles along with the exhaust gases you have a fairly toxic chemical soup. While most homes along major highways are usually 200 to 400 feet or more from the road, a large number of homes rest within 50 t0 100 feet from secondary and dirt roads.  The air currents from the traffic alone will push the airborne pollution away from the roads and toward the homes. Add some wind and you could  have a pollution cloud around your house 12 to 24 hours a day.

On mild days many people open their windows or doors and allow the “fresh” air to come into the homes. If your home is in a low traffic zone you will notice a little outside dust on the furniture and flat surfaces. However, for those people living close to or near roads that are heavily used, you may find that after only a few hours there is a layer of dust and dirt every where. In fact many of these “close to the road” homeowners may not be able to open their windows at all at anytime of the year.

This issue is not limited to just the summer months. Since most homes use forced air ducted heating/cooling systems they employ a series of supply and return air grills located throughout the house.  Often times the systems are not properly balanced and there is more return air than supply air pulled through the HVAC system. This creates a negative pressure in the house as the additional return air is pulled from the outside through small openings and communication points in the shell of the house.

Since the air outside has a large amount of this airborne particulate, some of that will be pulled into the house. This is why you can have a substantial amount of dust in the house even though the house is buttoned up.

Pure Air Systems offers a complete line of whole-house HEPA based air filtration systems that attach to the return air side of your HVAC systems. These unit have carbon media as well so they can effectively remove both the small airborne particles as well as the gaseous contaminants. In addition, the PAS units can assist in keeping the home pressurized to eliminate the negative air issue.

For more information go to their website at: www.pureairsystems.com.