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T H E M E C H A N I C S O F H O M E B U I L D I N G

howitworks

Heat

RecoveryVentilator

BY ROB YAGID

An insulated shell increasesefficiency of the HRV by kehot or cold air in the unit.

1A fan draws freshoutdoor air intothe HRV.

Fresh air entheat exchanger

is made of a condmaterial like aluminplastic. The energ

the exhaust air conthe intake air. Air streaseparated to keep exh

contaminants from enincom

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5 Indoor air isexhausted outsidethe home.

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FINE HOMEBUILDING16 Drawings: Do

COPYRIGHT 2009 by The Taunton Press, Inc. Copying and distribution of this article is not permitted.

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oday, homes are tighter, and thats a goodthing. A tight houseone free of air leaks

saves money and reduces the need to scrapeearth of its resources to fuel our lives. Howevas explained in 16 Ways to Improve the Air YBreathe on p. 62, a tight house can affect indair quality, and not always for the better.An airtight home has a lower air-exchange r

than a drafty one, which means indoor-air pol

ants can more easily accumulate to unhealthflevels. Homes with a low air-exchange rate, ually anything below 0.35 air changes per hour(determined by a blower-door test), need prowhole-house mechanical ventilation. If notdesigned properly, these systems can be grosinefficient, pumping conditioned indoor air oside, and sending heating and cooling dollarsstraight out the vent pipe. A well-designed vetilation system uses a heat-recovery ventilato(HRV) to retain a portion of a homes energy wstill providing the air exchanges needed to cra healthful home. Heres how it works.

HRVS ARE SMART, BUT SIMPLE

An HRV can keep up to 80% of the energy inexhaust air inside a house. Some manufacturesay HRVs are as efficient at reclaiming the enein cool air as they are at capturing the energyin hot air. Others say HRVs are most efficient capturing heat energy and should be used oncold climates. In either case, HRVs are most ointegrated into balanced whole-house ventilasystems, or are directly ducted (drawing left).The technology behind an HRV is astonishin

simple. An insulated box houses a couple of f

some particulate filters, and a heat-exchangincore in which intake and exhaust air flow. As cditioned exhaust air flows through the exchanit heats or cools incoming air.Manufacturers such as Fantech, Carrier, and

Honeywell recommend HRVs in most parts ofthe country, but not all. Fantech, for examplerecommends HRVs in all but the hottest, moshumid regions. In the Southeast, where its bohot and humid for much of the year, they recomend the use of an energy-recovery ventilato(ERV). An ERV operates the same way as an Hbut has a vapor-permeable heat-exchanging c

The permeable heat exchanger allows a moisexchange between intake and exhaust air to control indoor-humidity levels. ERVs, which coroughly $800 and up, are only slightly moreexpensive than HRVs.

Rob Yagid is an associate editor.

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Particulate filters wrap the heatexchanger. HRV filters should becleaned every six months.

Semiconditionedoutdoor air ispulled to thecentral blower ordirectly into supplyductwork whereit is circulatedthroughout thehouse.

Conditionedindoor air ispulled into theHRV, where itheats or coolsincoming air.

A control box adjuststhe rate at whichthe HRV draws andexhausts air. HRVstypically have a 115vpower demand.

www.finehomebuilding.com AUGUST/SEPTEMBER 2009

COPYRIGHT 2009 by The Taunton Press, Inc. Copying and distribution of this article is not permitted.