We have seen that even in a very leaky house it is easy to find weather conditions that result in no pressure difference from inside to out, and hence little to no incoming fresh air:  in fact this will occur any time there is little wind and the outside and inside temperature are similar.  We have also seen that the amount of air coming into a leaky house varies dramatically with weather, and that too much air leaks in during the extreme weather conditions (see energy section).    The demand for fresh air, on the other hand, has to do with the number of occupants and is fairly consistent all year long.  Each person needs not only fresh air to breath, but needs pollutants and excess water vapor removed.

Fresh Air Requirements
The most commonly agreed on standard fresh air requirement is that 1/3 of the volume of air in the house should be replaced with fresh air every hour (stated as 1/3 ACH or .35ACH, with a little rounding). Because this amount will vary with the volume of the house, an alternative statement is that each person needs 15CFM.  The difference between the two is significant: two people getting 15CFM results in 1800 cu ft per hour, which is only 1/10 of the volume of a 2000 sq ft house, or about 13% of a 1500 sq ft house.  In practice the amount of fresh air necessary will also likely vary significantly based on how many pollutants are likely to be introduced into the house, and how much spot ventilation will work to control them.   A home where shoes are left at the door, without pets, without hobbies that involve toxics, without toxics cleaners and pesticides and that avoids VOCs, will need much less fresh air than ones which do have some of these things.

All houses will have some natural ventilation, and that amount will be greatest when the wind is high and the temperature outside is cold.  This component of ventilation is usually ignored because it is not reliable, but may be great enough during the winter such that mechanical ventilation is either not needed or the need is much reduced.  The problem with this approach is the question of whether accumulated pollution and excess moisture is removed by natural ventilation, and in the case of moisture, whether it collects in walls or other undesirable places on the way out. The topic of mechanical ventilation is a source of much controversy, and will probably continue to be so for a while.

Where ventilation is needed
Some fresh air is needed everywhere, but pollutants and excess moisture are usually confined to specific areas.  As a rule of thumb, fresh air is generally supplied to rooms that are most frequently occupied (bedrooms, living rooms) and exhausted from rooms that generate the most pollution (kitchens, bathrooms, offices).

• Kitchens - cooking can create both excess moisture and odors, and so a range hood that vents to outside is normally used to eliminate these sources. Circulating fans (ie no duct to outside) are not effective at all. Any ventilation in the kitchen needs to have a grease filter in it to avoid gumming up the ducts and fan.  Downdraft cooktops work by sucking large quantities of air (to fight capture the warm waste air from cooking, which wants to rise), and because of that can remove all the air in a house in short order.  A 600CFM downdraft fan will remove all the air in a 2000 sq ft house in about 30 minutes.  It is not advisable to use these, but if it is used a window should be opened, or other source of make-up air used to prevent the house from de-pressurizing.  Since there are no sealed combustion gas ranges available yet, the use of a gas range will consume oxygen and produce carbon monoxide and polyaromatic hydrocarbons.

• Bathrooms - bathing produces significant excess moisture which generally requires 30-60 minutes of a fan running at 50CFM to remove it. Because this amount of fresh air is much greater than the amount of fresh air normally needed a separate exhaust fan should be installed and connected to a crank timer which allows the fan to automatically turn off when it is done.

• Laundry - dryers produce excess moisture and should be vented to outside.  Since most people hang some clothes to dry, a spot exhaust fan should be installed.

• Utility - Furnaces and hot water heaters should be sealed combustion units, which are self venting to outside.  

Understanding Air Flow and Air Pressure
Air moves from areas of high pressure to low pressure until the pressure difference is zero, unless there is a barrier to prevent it. Like water, air will go through all susceptible openings, and almost all barriers will fail to prevent it doing so.  Air pressure differences are created due to a natural conditions or by mechanical fans.  Areas in a house are often at different pressures, and the house can be at higher pressure than outside or lower.  Although these pressure difference can be undesirable, we do our best to prevent them and live with the rest.

There are two natural driving forces for natural air movement: temperature and wind.  Everyone has hear the old adage that heat rises, but in fact heat moves from warm to cold in all directions equally.  What does rise is warm air, because it is lighter than cold air.  This principle has been used in hot climates to provide natural cooling, but on cold days it causes increased pressure on whatever barriers exist near the top of the house.  It also tends to cause the upper floors of a house to be warmer than lower ones.  Wind causes areas of higher pressure on the side of the house it strikes and lower pressure on the other side, putting pressure on windows, doors and wall to let it through.  Wind moving over the top of a chimney creates a suction which pulls air out of the house.

One special case of natural ventilation is due to fireplaces and some wood stoves:  these devices not only consume oxygen, but the hot air that rises up the flue sucks the warm out of the house and out the chimney.  This is a case where an extreme temperature difference creates a very large air flow. 

(pictures of air movement!)

Types of Ventilation Systems
There are three types of mechanical ventilation systems, and a strategy for passive natural ventilation.  No matter how a house is built it is likely to have some degree of natural ventilation and during some of the year, that amount may be sufficient to meet the house's fresh air needs.  Mechanical ventilation has the advantage of easily removing contaminated air in addition to supplying fresh air, in addition to be able to provide filtration by circulating the air in the house.

Exhaust Only Ventilation
In the simplest form of this system a spot ventilation fan (typically the bathroom) is fitted with a timer that turns the fan on for part of the day every day.  This system forces air out of the house, de-pressurizing it so that outside air will leak in to return the pressure to zero.  If the house is sealed very tight, one or more vents are installed in the window or wall (typically in the bedroom) to allow makeup air to enter.  In the more complex version, a whole house fan is used with a duct system to remove air from multiple locations (bathroom, kitchen, office an closets are typical).  There are two disadvantages of this system: first is that if the pressure gets too low, backdrafting can occur (that is the exhaust from a furnace, hot water heater, etc will go in the house instead of us the chimney).  The second problem occurs when a basement or crawlspace has a higher pressure than the rest of the house, any radon or mold down there will then be drawn up into the house.  The degree that this is a problem depends somewhat on how tight the house is, as ventilation systems usually ran at fairly low volumes: 50-100 CFM, and so the pressure difference they create is often quite small.  In both exhaust and supply ventilation systems, the heat lost due to the air that leaves the building must be made up by the heating system.

Current Washington State code requires the simple exhaust ventilation system as the minimum ventilation requirement, but many people disable them.

Supply Only Ventilation
This system is the opposite of exhaust only: rather than pumping air out of the house, air is pumped in.  While this prevents backdrafting and drawing pollutants from a crawl space, positive pressure in a house increases the driving force of water vapor to move through walls, increasing the chance of excess moisture collecting in walls.  As with the exhaust only system, vents can be added to give the air a way out, but depending on weather they can also work backwards.

Balanced Ventilation
This is the system of choice as it (hopefully) has no effect on the air pressure in the house.  Its biggest drawback is that it has a higher initial installation cost.  In this system, air is pulled out of some rooms and put into others, with ducts distributed fairly evenly throughout the house so that the overall air pressure is balanced.  The most common kind of balanced ventilation is the Heat Recovery Ventilator (HRV), because it captures much of the heat from the exhaust air and uses it to heat the incoming air, resulting in very little energy penalty.  Balanced ventilation systems often share the same ductwork as a forced air furnace.

Duct layout
Whether or not ducts are used for delivering heat, they should be kept inside the building's envelope to prevent heat loss.  They also should be sealed tightly (mastic works best, duct tape often fails) to prevent dust from wall cavities from entering them, and so no wall cavity should be used as a duct without a metal liner.  Smooth metal ducts are preferred since they provide the least resistance to air flow, and so reduce the strain on the heating and ventilation system fan motors.  It is also important to size the ducts to allow for sufficient air flow as well as to avoid bends, especially sharp bends, whenever possible as they also increase the back pressure making the motors work harder.

In a forced air system the ducts are designed to deliver enough heat to each room, while in a ventilation system they have to deliver enough fresh air.  The air movement requirement for heat delivery is much greater than that for ventilation (5-10 times greater), so any duct system capable of delivering heat will deliver fresh air effectively.  In forced air systems, there is usually only one or two air returns, which if placed so that warm air is delivered evenly, will also deliver fresh air evenly.  The disadvantage of having limited returns is that closets and offices may not be ventilated enough.

In ventilation only systems, air is supplied to the rooms that are most occupied and removed from ones that cause the most contamination.  While this sounds good in theory, the two largest sources of water vapor (kitchens and bathrooms) will not be well served by a whole house ventilation system because they need a much higher volume of air, and so those locations will need spot ventilation fans anyhow.  Closets and offices could be well served by a whole house system.

Getting the right amount of air in each room is accomplished by installing dampers on the path to each duct and adjusting them so that the right amount of air comes out.

Filtration (types & efficiency rating)
The filter that is normally installed in furnaces is only good at keeping very large particles out of the furnace (to protect it), and does little to stop dust, and nothing to stop the fine duct particles that are of health concern.  There are unfortunately a number of different rating systems used for filters so when you buy a filter that is "90% efficient" you need to know what it is 90% efficient at removing, as they are rated by one of three methods: the arrestance method, the dustspot method and the D.O.P method.    A filter which is 97% efficient in the arrestance method, is only 50% efficient in the dust spot method, and only 10% efficient by the D.O.P. method.  Medium efficiency filters are rated by the dustspot method, and a filter of 30-40% should be the minimum standard, with a 60-70% filter being a better compromise.  HEPA filters are the cream of the crop, typically achieving 95% efficiency in the D.O.P. method.

As the efficiency of a filter goes up, so does it resistance to air flow, and so the efficiency of the filter is usually limited by how much it affects the furnace or ventilation fan.  One way to get higher efficiency without affecting the blower motor is to increase the diameter of the filter, which increases its surface area.

References

Understanding Ventilation, John Bower, Healthy House Institute, 1997

Builders Guide Series, Joe Lstiburek, EEBA, 2000.

The Health House Workbook, American Lung Association, 1995

Healthy by Design, David Rousseau & James Wasley, Hartley & Marks, 1997