Mechanical Ventilation for Affordable Existing Housing
Know when you need to add whole-house mechanical ventilation and what to use when you do
Everyone needs fresh air; it’s just hard to figure out how much we need when we are indoors. Enterprise Community Partners’ (ECP) Green Single Family Rehab Specs (2008) requires a ventilation system to meet ASHRAE 62.2 for “substantial rehabilitation” (essentially gut rehabs) and then cites a source for the ASHRAE standard and a source reviewing types of residential ventilation systems. Quite a lot to digest here so let’s break it down.
What is ASHRAE 62.2?
ASHRAE is the professional organization for HVAC engineers and they spent a heck of a lot of time and effort (and pain) hammering out what they thought were reasonable and effective ventilation requirements for homes. This is tricky for homes because volume of space, number of occupants, and contaminant loads (moisture, odors, air-borne chemicals, etc) vary quite widely from home to home. It’s also tricky because you can ventilate too little and compromise occupant health or ventilate too much and waste energy and money. The rate of ventilation in ASHRAE 62.2 turns out to be about that of a typical bath exhaust fan for a 2000 square foot home. For more background, see the GBA Green Basics section on Ventilation Choices.
How tight does a home have to be to require mechanical ventilation?
All homes leak air: we open and close doors and windows, we turn on spot exhaust fans in the kitchen and baths, we run devices that pull air through the home (clothes dryers, fireplaces, water heaters and furnaces with open flues). How do we know when we have made a home, particularly an older existing home, air tight enough to require whole-house mechanical ventilation? ASHRAE 62.2 states that any home tighter than 0.35 air changes per hour needs a dedicated, systematic ventilation approach (that means approximately all the air in a home being replaced every 3 hours). That is pretty much why the ECP green rehab specs for full gut jobs require ASHRAE 62.2 compliance; by the time you get done the specs for air sealing and insulating, you will have tightened up the home enough so that you need to introduce a mechanical ventilation system.
What systems make the most sense for affordable housing renovation?
GBA editor Martin Holladay has a great blog on types of ventilation systems. The two most affordable yet effective ventilation systems are exhaust-only and central fan-integrated supply.
This system can be as simple as an Energy Star bath exhaust fan installed to run at the appropriate rate either all the time or at a higher rate intermittently on a schedule to satisfy ASHRAE 62.2 (see Figure 1, although this is a more complex exhaust-only system). The advantages to exhaust-only systems are: low first and installed cost, and appropriate in all climates and types of homes. The cost chart from the recommended BSC ventilation article lists the installed cost for a single-point exhaust system as $300 (2005). The retrofit installation portion of the total cost is, of course, affected by how easy it is to install the fan and how long or complicated the duct runs are.
The major disadvantage to this system is distribution; with only one point of draw it is easy to over-ventilate one space and under-ventilate others. As a result, exhaust-only systems work best in smaller homes with open floor plans. A key to this system's effectiveness is quality installation; don’t pay for a good fan and then get a lousy installation with loose and lengthy ducts with lots of twists and turns. The straighter and shorter the duct runs, the better.
Some builders add make-up air inlets to their exhaust-only ventilation systems. This means that the air pulled into the home to supply the exhaust comes from these inlets rather than from sundry cracks and random air leaks. Many building scientists argue that how much and which direction these inlets permit air flow is dependent on forces (such as wind) that can make them just as unpredictable in their operation as random cracks. If you do use air inlets, make sure you locate them away from sources of lousy air.
Central-fan integrated supply (CFIS) ventilation
This system runs an intake duct to the return plenum or trunk of your forced-air system and then uses the blower fan to intermittently pull in outside air per ASHRAE 62.2 requirements (see Figure 2). The advantages to this system are: low first and installed cost, appropriate for all climates, and excellent distribution (since the forced air duct system mixes the fresh air with all the other air it is moving about the house). The same BSC ventilation cost chart lists the CFIS installed cost at about $300 as well. Again, the cost for retrofit installation of this system depends on just how easy it is to add a duct running from a good exterior wall location to the return plenum or return trunk.
The disadvantages to this system are: it is really only feasible in homes with central forced-air systems and its operating costs over a year can be much higher than other systems. The higher operating cost is because a large and often energy inefficient central air handler must run to pull in and distribute the outside air. Of course, the more your air handler is running anyway to distribute cooled or heated air, the less the air handler is running JUST to do ventilation. So, the marginal operating costs of the CFIS system can vary by climate.
- EPA Energy Star
- DOE High Performance Scopes of Work
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