Ventilation, Old and New
by: Paul Kando
According to energy audit data, 30 to 60% of most Maine homes' heating energy is lost due to air leaks through holes and gaps in the building envelope. New houses must be built to a higher tightness standard and existing ones must be air sealed. The trouble is that, as building efficiency is improved and buildings become more airtight, they can also become poorly ventilated. Insufficient fresh air can lead to stuffiness, odor buildup, and unhealthy air quality. Lack of supply air for combustion appliances can cause incomplete burning, back-drafting and the introduction of dangerous exhaust gases into the living space. Another major, but often overlooked problem in tight houses is excessive moisture buildup which can lead to unhealthy conditions, mold, and mildew. Condensing moisture can also damage the building structure. Therefore, when a professional energy audit report recommends ways to tighten a leaky house, it must also indicate the safe limit beyond which tightening should not proceed without introducing additional ventilation.
Simply opening windows provides ventilation, however it also causes loss of heat and humidity in winter and excessive gains of same in the summer. This is wasteful and costly. It takes energy for the building's heating and cooling systems to compensate. Exhaust fans remove stale air, but they rely on a leaky envelope for replacement air. Thus they cannot be relied on for long term ventilation in a tight house.
Heat recovery ventilation (HRV) and energy recovery ventilation (ERV) introduce fresh air into a building, improving comfort, while they also minimize energy use. HRV and ERV systems recover heat from the exhaust air and transfer it into the incoming fresh air. The difference between the two is that, in addition to heat, ERV systems also recover and transfer humidity. Both systems can be stand-alone devices that operate independently, or they can be built-in, or added to existing HVAC systems.
It would be foolish to build a new house today that does not meet the Passivhaus standard, even if it initially costs more than a conventional building. This is because living in a passive house over the term of a typical mortgage invariably costs thousands of dollars less than the first-cost difference between a Passivhaus and a conventionally built equivalent. Passive houses come with built in central HRV or ERV systems, which exhaust stale air from the home's wet rooms - kitchen, laundry, bath - and introduce fresh air to bedrooms and living areas. These systems consist of low wattage fans, an air to air heat exchanger, air filters, mufflers to dampen sound, and ductwork connecting the central unit to the various rooms and the outdoors. (ERV systems also contain a moisture-transfer wheel.) Designed to run all the time during the heating season these systems often take the place of a conventional heating system which, thanks to the building's high energy efficiency, is no longer needed.
In existing houses the need to install even the small diameter ductwork required for a central HRV or ERV system can be a challenge. In these situations, especially in houses with nearly every room adjacent to an exterior wall, small stand alone HRV devices can provide ventilation for single rooms or combinations of rooms. One ingenious system, originally developed for apartments in Germany, employs pairs of identical ventilation units installed in opposite walls, and controlled by a wireless control device. Instead of a heat exchanger, these through-the-wall units contain a ceramic heat storage element. For 70 seconds one of each pair of wall units will exhaust stale air while the other brings in fresh air. The exhausting unit captures 85 to 90% of the heat contained in the outgoing stale air and stores it in its cylindrical ceramic storage element. After 70 seconds the paired fans reverse direction. Now, upon entering, the filtered fresh air picks up the stored heat accumulated during the prior 70 second cycle. Fans in these units are extremely quiet and draw only about 4 watts of power in operation. The imported devices are reasonably priced, and easy to install in circular through the wall openings. They require only an electrical connection and no ductwork. They are ideally suited for retrofit applications. The number of pairs to be installed will depend on the building's total air volume. Each wireless control device can handle up to two pairs of wall units. One small challenge these units pose in some of our existing houses is that they are designed for thicker walls common in Europe. But, hey, what's Yankee ingenuity for?
One of a pair of through the wall heat recovery ventilation units, showing, from left to right, the shielded interior air inlet/outlet, the reversible, remote-controlled fan,the interior wall plathe, the ceramic heat storage unit, the wall cylinder into which everything fits, and the screened exterior inlet/outlet.