One of the first steps in ventilation system design is to determine the capacity needed. The best guidelines currently suggest that a general ventilation system should be able to provide a continuous air-exchange rate of either 15 cubic feet per minute (cfm) per person or 1/3 of an air change per hour (ACH), whichever is greater. If the average occupancy of a house is 4 people, then 60 cfm (4 x 15) of continuous ventilation should be sufficient.
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To determine ACH, you first need to calculate the volume of the house. A 1,200 sq. ft. house with 8' high ceilings will contain 9,600 cu. ft. (1,200 x 8). One-third of an air change per hour would be 3,200 cu. ft. per hour (9,600 ÷ 3). This translates into 53.3 cfm (3,200 ÷ 60) of continuous capacity.
Sometimes it’s advantageous to oversize a ventilation system. For example, a system may be sized for the needs of three occupants, but what happens when there are a dozen family members over for dinner? The solution can be as simple as installing a higher-capacity system and running it on slow speed when there are only a few people at home, then high speed when company arrives.
Many ventilation systems are designed for intermittent operation. For example, suppose a neighbor’s wood smoke is a problem at night during the winter months. You may choose to only operate your ventilation system for 8 hours a day when the outdoor air is clear. In such a case, the capacity of the system should be sized to overventilate when it’s running. If this is done, the average 24-hour rate should meet the above recommendations. Of course, if a house is unoccupied during the day—when people are at work or school—the average ventilation rate can be reduced accordingly. These factors should all be taken into account when determining the equipment capacity.
The 15 cfm and 1/3 ACH figures were arrived at by placing a person inside a closed room (made of fairly healthy materials) and exchanging the air in the room at different rates. Average people off the street were asked to stick their head in the room and report how fresh it smelled. For 80% of the people, an exchange rate of 15 cfm or 1/3 ACH was enough for the room to smell fresh. At lower ventilation rates, many people said the room seemed stuffy. Their reactions were not to pollutants typically found in houses because the room was made of fairly healthy materials but, instead, to the metabolic by-products released by the person sitting in the room. So, these guidelines are in actuality a 'body-odor standard', not a health standard.
If a house is built, furnished, and maintained with healthy materials, then 15 cfm or 1/3 ACH will be sufficient for most people. If unhealthy materials are used, it can be very difficult to guess at what rate is sufficient, simply because so many different pollutants are possible, and they occur at such varying concentrations. So, because the air inside each house is different, it’s virtually impossible to state a universal rate for all unhealthy houses, but in some cases it could easily be 60 cfm per person or 1 1/2 ACH—or more. Of course, ventilating at a high rate can have drawbacks. For example, it will mean higher heating/cooling bills, and it can make a house excessively dry in the winter. A high-powered ventilation system will also be expensive to install and operate—and it will be noisy.
One study looked at how much air exchange average houses, without mechanical ventilation systems, actually got. It found that 90% had an average “natural” ventilation rate less than 1/3 ACH for at least a full month. Seventy percent of the houses averaged below 1/3 ACH for the entire heating season. Based on this study, it’s obvious that most houses need mechanical ventilation.
There are many different controls that can be used with ventilation systems. No single control is ideal for all situations—all have advantages and disadvantages. Fairly simple controls include no control (the ventilation system runs 24 hours per day, every day), an on-off switch, and a variable-speed controller. Low-cost, spring-wound crank timers are popular, as are the fancier electronic timers that can turn a fan on or off at specific times of day. There are also controls that can turn a ventilation fan on when the relative humidity gets too high, or when the carbon-dioxide concentration rises above a certain level. Most ventilation-equipment suppliers offer a variety of different controls.
Exhaust air is air that’s leaving a house. It’s often called stale air because it’s been contaminated by people, activities, or materials inside the house. The outdoor air that enters a house is either called make-up air (because it makes up for what was exhausted), or intake air. It’s also often called fresh air even though it may be contaminated with outdoor air pollutants.
It’s always a good idea to pull intake air from a clean outdoor location. For example, don’t have the fresh-air intake near a garage door because it will pull exhaust gases indoors. And don’t put it too near the stale-air exhaust, or a clothes-dryer exhaust, or a chimney, or stale, contaminated air will be pulled back indoors.
Inside the house, fresh air is often introduced into bedrooms, living rooms, and family rooms because that’s where people spend the most time. Stale air is often exhausted from service rooms like kitchens, utility rooms, and bathrooms because they are typically more contaminated. This results in an exchange of air through all of the rooms. Actually, there are as many variations to ventilation-system layout as there are different floor plans.
The cost of ventilation varies widely. For example, a cheap bathroom exhaust fan might cost only $25 to purchase, while a high-quality, energy-efficient, quiet version will be over $100. A top-of-the-line heat-recovery ventilator could be in the $1,000 plus range, plus installation.
Operating costs also vary, but they typically aren’t exorbitant. For example, a 50-cfm bath fan running two hours a day costs less than $15 per year to operate in most parts of the U.S. This cost is determined by the local electricity rate and the cost of heating or cooling the incoming make-up air.
Because heat-recovery ventilators conserve energy, they can be more cost-effective than other ventilation strategies—but only in some climates. For example, the operating cost of 80 cfm of continuous ventilation in Minneapolis might be $188 a year without heat-recovery or $86 with heat-recovery. The annual savings of $102 (188 - 86) could easily make the extra cost of a heat-recovery ventilator worthwhile in this cold climate. On the other hand, in Los Angeles the annual operating cost might be $82 without heat-recovery or $54 with heat-recovery. Saving only $28 (82 - 54) a year probably wouldn’t make a heat-recovery ventilator cost-effective in Southern California.
The cost of ventilation isn’t exorbitant, and it’s definitely something that most houses need. In a 1998 report, it was determined that the total cost (annualized equipment cost plus operating cost) of a whole-house ventilation system in most parts of the U.S. could be in the $200 range per year. These costs were based on a fairly simple system. There are also more elaborate (and more expensive) systems that offer various advantages, but the fact is that ventilation does not need to be expensive. And if you build a tight, energy-efficient house, your overall energy savings will generally be more than the cost of ventilation—so you will be dollars ahead.
Ventilation equipment takes up a certain amount of space, and while an attic or crawl space might seem like a good, out-of-the-way location, they should only be considered if they are easily accessible. This is because ventilation equipment requires regular maintenance in the form of cleaning and lubrication. Over a washer/dryer in a utility room is often a good location, as is a heated basement.
Generally, metal ducts are preferred, but plastic-lined flexible ducts are often acceptable. Some ducts will need to be insulated for energy-efficiency, or to prevent condensation. If ventilation is considered early in the planning process, the actual location of the ducts can be coordinated with the framing to minimize interference.
(Note: This article is part of the original HHI Archives, and was believed to be accurate at the time of writing. The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.)
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