HVAC: 'V' Stands

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A properly designed and installed whole-house ventilation system provides fresh air, filters and mixes the air, and then distributes that air around the house, all with minimal maintenance.

October 01, 2002

 

Figure 1: An entry-level ventilation system in the attic draws fresh air into the return-air side of the furnace or central air system.

A properly designed and installed whole-house ventilation system provides fresh air, filters and mixes the air, and then distributes that air around the house, all with minimal maintenance. Normal, noisy bath fans don't qualify, as they're typically run such a tiny fraction of the day that the air exchange they achieve is insignificant. Most homes don't even have a kitchen exhaust fan. If homeowners don't wash out the grease traps in their recirculating range hoods, the crud trapped up there might eventually qualify as a toxic mini-dump.

"I don't think the public really knows much about ventilation and their options," says John Bower, author of Understanding Ventilation and president of the Health House Institute. "They rely on builders, and most builders haven't offered much. Until builders talk about it, ventilation systems will remain the exception. "

"As time goes on and ventilation evolves, people will be willing to pay more for ventilation. In recent surveys, when people are asked about healthier houses, they say they would be willing to pay between $1,000 and $2,000 for a good ventilation system."

Changing current practice

Mark LaLiberte, building science trainer and materials supplier for Shelter Supply in Minneapolis, stresses an angle that seems counterintuitive. "The only way to get proper relative humidity and good indoor air quality starts with tightening up the shell," he says.

For remodelers, the notion of tightening an existing home and then mechanically ventilating it might seem nothing short of nonsense. Why not just equip the house with a few bath fans and a small kitchen fan and call it good? Trouble is, natural leakage is uncontrollable and unpredictable. During some months homeowners will get too much weather-driven air exchange, and on many days they won't get any. Rarely does natural leakage "distribute" the infiltrating outdoor air evenly around the home.

Most bath fans are so noisy that they're used only to mask sound. Typically, they get turned off too soon to exhaust any significant water vapor during or after bathing.

Change is coming. Building codes in three states - Minnesota, Vermont and Washington - already require whole-house mechanical ventilation for new construction. Upgraded codes aren't the only driving force. Eventually, the mold stories all over the news will influence home buyer receptivity to paying for ventilation.

 

Figure 2: The same type of system used in the attic can be used in the basement.

How much ventilation?

The American Society of Heating, Refrigerating and Air-Conditioning Engineers' residential ventilation standard specifies that home ventilation systems should be sized to supply 7.5 cubic feet per minute per bedroom plus 0.01 cfm per square foot of conditioned space. For a four-bedroom, 2,000-square-foot home, that translates into 50 cfm.

With all three of the generic ventilation options described below, you still need to install a dedicated kitchen exhaust fan to exhaust odors and water vapor, plus vent any combustion gases.

Entry-level ventilation: The central-fan-integrated approach (Figures 1 and 2) is a low-cost, whole-house ventilation option installed in homes built to the U.S. Department of Energy's Building America standards. Fresh air is ducted - typically through a 6-inch duct with two in-line dampers - to the return-air side of the furnace or central air system. When the blower operates, it draws in a percentage of air from the outdoors; the HVAC technician presets the amount by adjusting a manual damper. The outside air mixes with the home's return air, is run through the central filter, and gets distributed throughout the home via the central ductwork.

Vary outdoor air intake by climate
Climate
Description
Maximum outside air percentage
Hot-dry and hot-humid
Average monthly temperature above 45 degrees year-round
13%
Mixed-dry and mixed-humid
Less than 4,500 heating degree days (base 65 degrees
10%
Cold
4,500 to 8,000 HDD
7%
Very cold
More than 8,000 HDD
5%

When the system's central fan hasn't operated during mild weather, a special controller - the AirCycler - automatically turns on the fan. According to Armin Rudd, inventor of the controller and consultant to large production builders through Building Science Corp. (Westford, Mass.), the maximum amount of outside air drawn into the system should vary with climate (see table).

Rudd says the desired run time for the ventilation system is about 10 minutes every half-hour whenever the home is occupied and windows aren't open. When the system isn't moving air, the AirCycler closes a motorized damper in the fresh-air duct. This prevents pooling of unmixed cold air within the return-air system, which might otherwise cause warranty problems with heat exchangers in furnaces.

A major comfort benefit of the system is a reduction in room-to-room temperature differences. On sunny winter days, homes with south-facing windows can develop cold zones along the north side if the thermostat hasn't called for heat. But because the AirCycler draws and mixes air from all rooms when it periodically brings in fresh air, uniform temperatures are easier to achieve.

Building America builders have installed roughly 15,000 of these ventilation systems. According to LaLiberte, an HVAC dealer's cost of materials for the system is roughly $125 for the controller, a 6-inch motorized damper, plus the additional ductwork, adjustable damper, vent cap and wiring. Some code jurisdictions allow the AirCycler system to reduce combustion-air requirements by one combustion-air duct, which offsets some of the system's costs.

Exhaust-only ventilation: This approach uses a quiet central exhaust fan and dedicated small ducts to extract air from several rooms simultaneously. It replaces all of a home's typical bath fans, which helps reduce system costs. Mounted in a remote location such as the attic, a product such as an American Aldes or Panasonic fan can barely be heard when it's functioning.

Depending on home size, fan size and the amount of ductwork present, the fan can be set to function continuously or intermittently. Install a switch so the fan can be shut off when the building is unoccupied. During showers and bathing, the Aldes fan's airflow can be boosted for more effective ventilation of water vapor. Costs for these systems vary with features and number of small ducts; a typical range is $600 to $1,000.

One disadvantage of the exhaust-only system: The incoming 'fresh air" comes from uncontrolled locations. If that air enters from a crawl space or an attached garage, it probably brings pollutants with it. The system also contributes a slight amount of negative pressure to the home - not necessarily a good idea if the home has an atmospherically vented combustion appliance (the typical water heater). Exhaust-only systems are most appropriate when homes have detached garages, no crawl spaces, and sealed or power-vented combustion appliances.

Ventilation with energy recovery: These systems cost substantially more - up to $2,000 installed. The amount depends primarily on whether a dedicated complete ductwork system is required, such as in a home with hydronic heating, or whether the home's heating and cooling ductwork can provide a boost.

Heat recovery ventilators (HRVs) use a heat exchanger core to condition fresh air drawn in from the outdoors. On a cold day in Minneapolis, minus-10-degree air is warmed to about 50 degrees by air's being exhausted on alternating sides of the heat exchanger's plates. In hotter climates such as Phoenix's, energy from cool indoor air's being exhausted can be used to cool hotter outdoor air being drawn in.

To reduce an HRV system's ductwork costs, Building Science Corp. recommends taking exhaust air from the master bedroom and supplying the fresh air into the home's central area. When the thermostat calls for heating or cooling, the centrally supplied fresh air is drawn into the home's conventional ductwork and circulated to all rooms. In mild weather, an AirCycler controller (no duct to the outside is needed this time) runs just 10 minutes an hour to help circulate the HRV's fresh air throughout the home.

A relatively new product, the Guardian Plus by Broan-NuTone, has raised the bar for the HRV industry. Priced the same as a standard HRV, the Guardian Plus combines energy recovery ventilation with HEPA (high-efficiency particulate air) filtration. Quotes from distributors in Minneapolis and Denver indicate the Guardian Plus costs a dealer up to $600. The typical cost of installation varies from twice to three times this amount, depending on the amount of dedicated ductwork

Power draw

Will running ventilation systems increase electric bills? Compared with most homes that just run heating and air- conditioning systems, the increase could be $50 a year. But in some cases, they break even.

The HVAC industry often recommends that blowers be run on low speed 24/7. A 1,000-cfm conventional blower motor consumes about 280 watts of power when operated at low speed and 400 watts at high speed.Installing a power-saving ECM motor cuts energy consumption by roughly 75%.

But Building Science Corp. says running a conventional blower continuously at low speed uses more energy than running the system at higher speeds 20 minutes per hour to circulate house air and bring in fresh air.

Rudd has field data indicating that the combination of a continuously operating 90-watt HRV plus 10 minutes of extra operation by the central air system blower costs roughly $75 extra per year.

But that combination system uses less electricity than running the central blower 100% of the time at high or low speed ($310 and $240, respectively, with average heating/cooling operation).

Steve Andrews is a Denver-based energy consultant and freelance writer and a board member of the Energy & Environmental Building Association.

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