Heat exchanger swaps stale air for freshness
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.Skip to next paragraph
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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.
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.