Optimal Whole-House Ventilation
Choosing the optimal whole-house ventilation system for an existing home can improve indoor air quality and prolong the building’s life.
INCLUDING A MECHANICAL system for removing contaminants is critical for ensuring indoor environmental quality in today’s tighter homes. A report from the Consortium for Advanced Residential Buildings (CARB) offers guidelines for selecting and installing ventilation systems for existing homes. (Note: Although the CARB report covers spot ventilation for bathrooms and kitchens, we are only including whole-house ventilation strategies.)
Operational Costs. In general, ERVs are the most expensive ventilation systems to install but the least expensive to operate.
Choosing a System
Before deciding on a ventilation strategy, the minimum flow rate must be determined. ASHRAE 62.2 has become the standard guideline for both local and whole-house ventilation. The latest (2013) iteration of the standard does away with the built-in “infiltration credit” and uses the following formula for calculating whole-building flow rate:
Qtot = 0.03(Afloor) + 7.5(Nbr + 1) (2)
Qtot = Total required whole-building ventilation rate [CFM] Afloor = Floor area [ft² ] Nbr = Number of bedrooms
Note: If a blower door test is done, the ventilation flow rate can be reduced.
There are three main strategies to achieve whole-house ventilation: exhaust-only systems, central fan-integrated supply and heat or energy recovery ventilation. The choice of system depends on several factors, including existing HVAC system, existing ventilation, ease of accessibility and scope of the larger remodeling project.
In these systems, exhaust fans operate continuously (or on timers) to remove air from the home. Fresh air is introduced through induced infiltration. These systems usually make use of an exhaust fan that provides local ventilation as well, most often in a bathroom.
- Simple and affordable. Nearly all homes have exhaust fans, and upgrading one or more to efficient models designed for continuous operation is very straightforward.
- Units that use very little electricity are available (5–12 watts for 50–80 CFM).
- Low maintenance, other than vacuuming or wiping the fan grille.
- Depressurizing the home draws in air from outside the home. Exhaust-only ventilation should not be used in homes with atmospheric combustion appliances, homes where makeup air comes from damp, moldy crawlspaces or basements or homes with attached garages that are not well air-sealed from the home.
- Ventilation is typically not distributed. A single exhaust fan removes air from one location, and makeup air enters where it will. Different parts of the home are likely to be ventilated to different degrees—especially when interior doors are closed.
Cost. Efficient exhaust fans range from $100 to $250, depending on rated flow rates, special features, etc. If installed as an upgrade (e.g., in a bathroom that already has an old exhaust fan, power and ducting), installation costs could be as low as $100. If installed in a location which did not previously have a fan, costs can be much higher. If installed in a ceiling beneath an accessible attic, installation can be $200-$400. If drywall or finishes must be removed and repaired, costs can be substantially higher.
Energy implications. A 10-watt fan running year-round consumes 88 kWh. At $0.11/kWh, this costs $10/yr. As this system doesn’t include heat recovery, outdoor air brought into the building must be conditioned. These costs vary with climate and HVAC equipment, but they are usually much greater than the cost of electricity to operate the fan.
Location. Usually a single unit utilizing an existing duct run is adequate. In larger homes, it’s better to install two fans, rather than a single fan with a higher CFM rating.
Equipment. Newer, ENERGY STAR-certified fans are more efficient and quieter. The CARB report recommends choosing a fan with 6 to 9 CFM/watt, which far exceeds the ENERGY STAR minimum standard of 1.4 CFM /watt.
Controls. Various strategies are available to achieve the desirable CFM, including continuous operation, adjustable speeds, programmable controls and on-demand controls.
CFIS systems take advantage of an existing HVAC system to distribute fresh air throughout the home.
Central Fan Integrated Supply
CFIS systems make use of an existing forced-air heating or cooling system. A duct is run between the return plenum and outdoors, and CFIS controllers are programmed to turn on the air handler fan and open the motorized damper. Outdoor air is drawn into the return plenum, mixed with return air and distributed throughout the home.
- Simple and affordable.
- CFIS systems distribute outdoor air to all parts of the home.
- Aside from keeping the air intake free from debris, a CFIS system requires little maintenance beyond maintenance of the central heating and cooling system.
- Outdoor air is filtered (through the central air handler filter).
- System is viable only in homes with forced-air heating or cooling systems.
- High electricity consumption from using the air handler fan for modest ventilation needs. This strategy should be considered only when the air handler has an electronically commutated motor (ECM) blower.
- Leaky central duct systems can drive up operating costs.
- Potential comfort problems if cool mixed air blows on occupants during the winter (or warm, humid air blows on them during the summer).
Cost. If the air handler is accessible in a basement or attic—and a duct can be fairly easily run from outdoors—total CFIS cost may range from $500 to $900; these costs include controls, motorized damper and installation. Installing the outdoor air duct and/or removing and refinishing drywall can bump the costs up significantly.
Energy implications. CFIS systems can consume tremendous amounts of electricity. Systems that utilize an efficient air handler fan motor (300 watts) and run an average of 8 hrs/day for ventilation (in addition to operation needed for space conditioning) will consume 876 kWh/yr—an extra $96, assuming a rate of $0.11/kWh. Most central air handlers have motors that are not this efficient; it is not uncommon for draws to be two or three times higher. Basic CFIS systems don’t include heat recovery, so outdoor air brought into the building must be conditioned. If the duct system is leaky, these costs can be even higher.
Intake location. The intake should not be located in an attic, garage, basement or crawlspace. It should be located near the return plenum.
Ducting. Duct runs should be short and straight.
Fan motor. Installing a CFIS system with a PSC fan motor is not recommended, as energy consumption will be high. Fan motor replacements are available for some older furnaces.
Outdoor flow rate. In general, more outdoor air reduces the amount of time that the air handler must operate. But cool air can compromise comfort, and some manufacturers caution that the temperature of the mixed air passing over the heat exchanger should stay above a minimum temperature and/or that the outdoor air should not exceed 15 percent of the total flow rate.
Equipment. Kits often include a controller, a motorized damper and a transformer, though these components can also be bought separately.
Installation. Ducts should be sealed, and in cold climates, insulated.
At least one side of an ERV or HRV system should be ducted separately.
HRVs and ERVs
Heat recovery and energy recovery ventilators are balanced systems; they exhaust air and supply outdoor air simultaneously. These two airstreams cross in a heat exchanger, so during the winter, much of the heat in the exhaust stream is transferred to the supply stream. In the summer, the reverse is true. HRVs transfer sensible heat only; ERVs also transfer moisture.
- Heat recovery reduces conditioning loads. The level of heat recovery (efficiency or effectiveness) varies among manufacturers.
- ERVs/HRVs can potentially distribute air to many areas in a home.
- These systems don’t induce a large pressure (either positive or negative) on the building.
- Outdoor air can be filtered.
- ERVs and HRVs are typically more expensive and require more involved installation procedures.
- Higher maintenance, as filters and heat exchange media typically need to be cleaned or replaced regularly. Some systems have high electricity consumption.
Cost. The cost of ERV and HRV equipment ranges widely. Costs are generally proportional to heat transfer effectiveness and electrical efficiency. Until recently, costs for core hardware ranged from approximately $400 to $2,000. Recently, some higher end European products have become available in the U.S. market. These boast even lower electrical consumption (near 3 CFM/watt) and higher heat recovery effectiveness but with higher price tags of $3,000 to $5,000. Installation costs vary tremendously, depending on several key factors:
- Location of the core unit. If installed in an accessible space (such as a basement), core equipment and duct connections may be fairly simple.
- If ducts can be run in an open space (basement or attic), wall and ceiling finishes may be left mostly undisturbed.
- More and longer duct runs translate into higher installation costs.
- Need for a condensate drain and/or pump. Some contractors have found installation costs of $1,000 to $1,500 when the system is entirely installed in a basement, attic or other accessible space, since very little ceiling or wall removal or finish work is needed. More complex installations will drive up costs substantially.
Energy implications. One of the main benefits of ERVs/HRVs is heat recovery. In colder climates, the savings from heat recovery are more pronounced. The electrical power consumption of these systems can also vary significantly; in milder climates, the electricity costs can actually be greater than the thermal energy savings.
Ducting. Duct runs should be short and straight, and the unit should be accessible. Ducts should be sized for the higher flow rates.
Intake and exhaust. Outdoor air should be clean; it should not come from an attic, garage, basement or crawlspace. Baths should have separate exhaust fans to keep those areas from retaining moisture.
Central duct systems. Integrating ERVs or HRVs with central duct systems is not recommended. Outdoor air will follow the path of least resistance and exit through the return plenum, resulting in higher energy consumption and/or inadequate fresh air.
Equipment. Choose equipment with at least 80 percent ASEF value and a TRE greater than 50 percent. Quieter, more efficient units are typically more expensive. For cold climates, make sure equipment includes frost protection and condensate drains.
Installation. Ducts should be sealed and insulated.
HRV or ERV?
Are typical indoor moisture levels more comfortable than outdoor levels? If yes, than an ERV is more practical. For hot, humid climates, an ERV is the better choice. For other climate zones, other variables come into play:
Colder climates, leaky homes: Indoor air is dry, so ERVs can help retain moisture within the home, improve comfort and possibly eliminate the need for humidifiers.
Cold climates, airtight homes: Indoor air humidity can be uncomfortably high, so HRV will help reduce indoor humidity.
Hot, dry climates, larger or leakier homes with low moisture generation may benefit from ERVs (to retain indoor moisture), while smaller, tighter homes with higher occupancy may benefit from HRVs (to reduce indoor humidity levels).