Heat exchanger swaps stale air for freshness

By , Special to The Christian Science Monitor

It seems like a vicious circle to most people. A well-insulated house, tightly sealed against draftiness, saves energy and therefore dollars. Yet environmental studies show that home improvements that block out the cold air are also effective at sealing in stale air, noxious vapors, and moisture produced in daily living. Ventilation is the obvious remedy for polluted indoor air, but opening windows, ripping off weatherstripping, or chipping away at caulking to increase ventilation is certainly not the way to go about it.

A more practical solution may be an air-to-air heat exchanger. Long used in commercial and industrial buildings, a heat exchanger vents stale indoor air, replacing it with fresh outside air, while leaving intact up to 70 or 80 percent of the heating or cooling inside a house.

The principle behind a heat exchanger is easy to demonstrate. Roll a sheet of paper into a tube and blow through it. Notice that the paper warms as your breath flows through the cylinder. Since heat flows to a colder surface or space, the warmth of the air transfers to the colder paper.

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In a heat exchanger, warm, stale indoor air is exhausted through a series of ducts. At the same time, fresh outside air is pulled into the house through a separate ducting system. These two air streams meet in the core unit, a series of channels that keep the two airflows separate yet contiguous.

As the air currents pass, heat from outgoing air is transferred through the walls of the core unit to the incoming air.

Inside and outside fans generate air movement through the system. In most air exchangers, one motor operates both fans and uses from 25 to 100 watts of electricity. The fans are usually equipped with variable speeds to adjust the ventilation rate when needed.

Ducting for heat exchangers today is made of high-tech materials to control heat loss and to prevent the moisture that condenses from working into the structure of the house.

The core unit is the heart of an exchanger system. The two most common designs for this channeling section are the ``sensible type'' and the ``enthalpy type.''

``Sensible'' refers to the fact that the warmth of the incoming air can be felt or sensed. Sensible designs are usually fixed-plate exchangers -- a series of nonporous, contiguous metal pipes, bent into U- or V-shape, which guide the incoming and outgoing air. Heat is transferred between the channel walls.

Enthalpy exchangers, on the other hand, transfer sensible as well as latent heat contained in the water vapor of the outgoing air. In this design, the total energy content, or ``enthalpy,'' of the stale air is converted to the fresh incoming air.

Enthalpy exchangers are manufactured either as fixed-plate or rotary styles. Fixed-plate enthalpy exchangers use water-absorbent separators rather than nonporous channels like their counterparts, the fixed-plate sensible exchangers.

In a rotary enthalpy exchanger, a revolving wheel transfers heat and vapor. Outgoing air exits through the lower half of the wheel while incoming air enters through the top half. As the wheel rotates (about four to five times a minute), heat is transferred between the two air flows.

Like all machinery, air-to-air heat exchangers are not problem-free. Freeze-ups have been reported in some models, most frequently in the enthalpy types. Freeze-ups occur when the moisture from the incoming air collects and freezes in the separator channels, thus blocking the air flow.

To de-ice the channel, the fan on the incoming side must be stopped either automatically or manually. Outgoing air continues to flow through the system, warming the frozen incoming channels and melting the ice. At 20 degrees F., thawing requires only a couple of minutes. The melting process takes longer as the temperature of the outside air drops.

Sensible exchangers have a tendency to reduce the humidity levels of the indoor air as the moisture generated inside the house exhausts with the stale air. Cold outside air contains only limited amounts of replacement water vapor. A humidifier may be needed to maintain humidity levels in a home with a sensible exchanger.

Also, heat exchangers are not efficient unless the house is extremely tight. A house with a natural air-exchange rate of once every hour or two will not benefit from a heat exchanger. In such a case, the unit will only add to the heat loss.

Heat exchangers at this point in their development and manufacture are probably not for everyone. Prices start at between $200 and $300. In homes with serious pollution problems, however, heat exchangers may prove beneficial in providing fresh air and heating, too.

In deciding whether or not to buy an exchanger system, consider these points:

Size. The size of the system needed in a home depends on the total volume of air within the house. An established rule of thumb is to allow 75 cubic feet per minute (cfm) of fresh air flow per 1,000 square feet of floor space in the house.

The American Society of Heating, Refrigeration, and Air Conditioning Engineers suggests allowing 40 cfm for each bathroom, 40 cfm for the kitchen, and 10 cfm for each occupant in the house.

Installation. Exchangers can be mounted either in an attic or in a basement. Window models, which clean the air in only one or two rooms, are as simple to install as window air-conditioning units.

Vents and ducts. In central exchanger systems, ducts must be separate from other ducting systems in the house. Vents, both inside and outside, should be spaced far enough apart (at least 6 feet) to assure that the air flows do not mix.

The usual method is to have fresh air entering certain rooms while stale air is exhausted from other rooms.

While the actual venting of fresh and stale air depends on the arrangement of a house, a typical layout exhausts stale air in the kitchen, bathrooms, or family room. Fresh incoming air pours into the hallways.

Fresh air for all seasons. Air exchangers work equally well in summer. The process works in reverse during the summer with the outgoing air, cooled by the air conditioner, bypassing the incoming fresh hot air. The heat from the incoming air is then transferred to the outgoing cold air.

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