A Prototype Alternative Ventilation System
Bristol Bay Housing
Authority (BBHA) currently has under development a model home project
consisting of four prototype houses to be built in Dillingham, Alaska
later this year. Included in this project is a prototype residential ventilation
system developed by the author, intended to provide a simpler, lower cost
alternative to traditional Heat Recovery Ventilation (HRV) systems while
addressing the inherent limitations of passive, or exhaust-only systems.
This paper briefly describes ventilation systems currently used in rural
Alaska housing, discussing advantages and disadvantages of each, and describes
an alternative system intended to solve problems inherent to each. An
interactive on-line "Ventilation System Economic Calculator"
is provided at http://www.biorealis.com/wwwroot/VentLoadCalcs5.html,
allowing users to evaluate ventilation system options for their own site-specific
conditions. A version of this paper formatted for printing is available
for download at http://biorealis.com/ventsystem/coldcomfort.pdf
Traditional HRV systems have been installed in hundreds, perhaps thousands, of rural Alaskan houses in the last twenty years or so -- long enough to get an idea of their long term performance under real-world conditions. Results are mixed. There continues to be a wide variety of opinion among designers, installers, vendors, policy makers, contractors, end-users, maintenance personnel, etc. regarding their effectiveness, operating costs, and the cause of reported problems.
Those who argue against HRV systems typically cite high installation costs, difficulty of installation, increased electric usage, and problems with frosting and condensation. Proponents, on the other hand, typically cite poor design, improper installation and/or lack of homeowner understanding as the cause of reported problems. In fact, both sides are probably right. The fact remains that (1) to work properly without problems, all of the pieces must be in place (including proper installation and homeowner education), and (2) in practice, history has shown that it can be very difficult to put all of the required pieces perfectly in place.
In moderate climates, and/or where homeowners clearly understand and appreciate their purpose and usage, HRV systems may be the recommended choice. However, in remote locations with extreme climates, poor cash economies and lack of technical support, the difficulty of installation, added cost and complexity, and lack of understanding may justify consideration of simpler systems even though they may not perform as well.
and rehab projects present an even greater set of challenges. It can often
be very difficult, if not impossible, to find adequate space to locate
equipment, route the required ductwork, and/or properly locate inlets
and outlets. Yet inadequate ventilation is one of the major contributors
to moisture damage, mold, mildew, and associated health problems in existing,
older housing stock. It is a prime candidate for inclusion in many renovation
Passive Ventilation Systems:
In recent years, a variety of alternative ventilation systems and components, alternately described as "passive", "exhaust-only", or "hybrid" systems, have appeared on the market, in an attempt to fill the need for a simpler, lower cost alternative to conventional HRV systems.
These systems typically combine one of a variety of exhaust methods (wind, thermal or mechanical), with an independent passive make-up air system. There is no heat recovery per se, but some efficiencies can be achieved by providing tightly controlled "spot" ventilation - i.e. providing ventilation air only when and where needed (typically as determined by humidity or occupancy sensors in each room). Typical components include humidity-controlled air inlets and outlets which may be utilized on exhaust and/or supply air sides, and passive air inlets located on exterior walls or windows, which automatically regulate airflow in spite of variations in wind pressure.
Advantages over HRV systems typically include (1) greater simplicity, (2) lower first cost, and (3) reduced electric usage. Disadvantages include (1) dependence on external conditions which may be highly variable, (i.e. wind speed, outside air temperature), (2) higher heating energy costs, and (3) negative building pressures.
A passive air intake
system is not recommended for arctic climate projects, for two reasons:
(1) outdoor air temperature is too cold to introduce directly into the
space, and (2) makeup air flow depends on negative indoor air pressures,
which could create hazardous backdraft conditions in the woodstove and
boiler. At arctic and subarctic interior Alaska design temperatures, condensation
or frost would be likely to form on air inlets, and/or homeowners would
experience cold drafts, and be likely close them off, defeating the purpose.
Proposed Alternative System:
Following is a description
of a ventilation system which incorporates the basic features of the passive
systems described above (i.e. spot ventilation, humidity controlled intermittent
operation) but overcomes some of their inherent limitations. With this
system, cold supply air is tempered before reaching the space, and neutral
air pressure is maintained in the building. The relative simplicity of
a passive system is combined with the positive control provided by a balanced
mechanical system. Installed cost should be similar to the passive system