Providing Fresh Air in Our Home
A quick overview of ventilation systems
One of the features in our new house that I’m most excited about barely raises an eyebrow with some of our visitors: the ventilation system. I believe we have the highest-efficiency heat-recovery ventilator (HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. ) on the market — or at least it’s right up there near the top.
I’ll describe this Zehnder HRV and its impressive specifications and features — but not until next week. This week I’ll provide a little background on ventilation.
For centuries homes weren’t ventilated, and they did all right, didn’t they? Why do we need to go to all this effort (and often considerable expense) to ventilate houses today?
There are several reasons that ventilation is more important today than it was long ago. Most importantly, houses 100 years ago were really leaky. Usually they didn’t have insulation in the walls, so fresh air could pretty easily enter through all the gaps, cracks, and holes in the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials..
Also, the building materials used 100 years ago were mostly natural products that didn’t result in significant offgassing of volatile organic compounds (VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production.), formaldehydeChemical found in many building products; most binders used for manufactured wood products are formaldehyde compounds. Reclassified by the United Nations International Agency for Research on Cancer (IARC) in 2004 as a “known human carcinogen.", flame retardants, and other chemicals that are so prevalent in today’s building materials, furnishings, and belongings.
Ventilation can take many different forms. Very generally, systems can be categorized into about a half-dozen generic types:
No ventilation. This is almost certainly the most common option in American homes. There is no mechanical system to remove stale indoor air (and moisture) or bring in fresh outside air. In the distant past, when buildings weren’t insulated, this strategy worked reasonably well — relying on the natural leakiness of the house.
It’s worth noting, though, that even a leaky house doesn’t ensure good ventilation. For this strategy to work there has to be either a breeze outside or a significant difference in temperature between outdoor and indoors. Either of these conditions creates a pressure difference between indoors and out, driving that ventilation. On calm days in the spring and summer, there might be very little air exchange even in a really leaky house.
Natural ventilation. In this rather uncommon strategy, specific design features are incorporated to bring in fresh air and get rid of stale air. One approach is to create a solar chimney in which air is heated by the sun, becomes more buoyant, and rises up and out through vents near the top of the building; this lowers the pressure in the house, which draws fresh air in through specially placed inlet ports. The rest of this blog will focus on mechanical ventilation.
Exhaust-only mechanical ventilation. This is a relatively common strategy in which small exhaust fans, usually in bathrooms, operate either continuously or intermittently to exhaust stale air and moisture generated in those rooms. This strategy creates a modest negative pressure in the house, and that pulls in fresh air either through cracks and other air-leakage sites or through strategically placed intentional make-up air inlets. An advantage of this strategy is simplicity and low cost. A disadvantage is that the negative pressure can pull in radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. and other soil gases that we don’t want in houses.
Supply-only ventilation. As the name implies, a fan brings in fresh air, and stale air escapes through cracks and air-leakage sites in the house. The air supply may be delivered to one location, dispersed through ducts, or supplied to the ducted distribution system of a forced-air heating system for dispersal. A supply-only ventilation system pressurizes a house, which can be a good thing in keeping radon and other contaminants from entering the house, but it risks forcing moisture-laden air into wall and ceiling cavities where condensation and moisture problems can occur.
Balanced ventilationMechanical ventilation system in which separate, balanced fans exhaust stale indoor air and bring in fresh outdoor air in equal amounts; often includes heat recovery or heat and moisture recovery (see heat-recovery ventilator and energy-recovery ventilator). . Much better ventilation is provided through a balanced system in which separate fans drive both inlet and exhaust airflow. This allows us to control where the fresh air comes from, where that fresh air is delivered, and from where exhaust air is drawn. Balanced ventilation systems can be either point-source or ducted. With ducted systems, it makes sense to deliver fresh air to spaces that are most lived in (living room, bedrooms, etc.) and exhaust indoor air from places where moisture or pollutants are generated (bathrooms, kitchen, hobby room).
Balanced ventilation with heat recovery. If there are separate fans to introduce fresh air and exhaust indoor air, it makes a lot of sense to locate these fans together and include an air-to-air heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. so that the outgoing house air will precondition the incoming outdoor air. This air-to-air heat exchanger — more commonly referred to today as a heat-recovery ventilator or HRV — is the way to go in colder climates. A slightly different version, known as an energy-recovery ventilator (ERV), is similar but transfers moisture as well as heat from one airstream to the other, keeping more of the desirable humidity in the house in the winter and reducing the amount of humidity introduced from outdoors in the summer.
Tight homes need mechanical ventilation
I’ll focus more on HRVs in next week's blog, especially our new high-efficiency Zehnder system. Following that I’ll address why commissioningProcess of testing a home after a construction or renovation project to ensure that all of the home's systems are operating correctly and at maximum efficiency. an HRV is so important and how that’s done — or at least how it was done with our system.
I’m a firm believer that all homes should have mechanical ventilation. With better-insulated, tighter homes, that ventilation is all the more important. But even in a very leaky house, one can’t count on bringing in much fresh air or calm days in the spring and fall when there isn’t a pressure differential across the building envelope.
If budgets allow, going with balanced ventilation is strongly recommended, and if you’re doing that in a relatively cold climate, like ours, then providing heat recovery is a no-brainer. Mechanical ventilation always takes energy; with heat recovery the energy penalty of fresh air is minimized.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.
- Alex Wilson
- Green Building Advisor
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