A Heat Recovery Ventilator (HRV) will bring in fresh air from the outdoors into the home, and exhaust an equal amount of stale air from the home.  But unlike opening a window, an HRV can warm or cool that incoming fresh air to bring it closer to the home’s comfortable interior temperature.  The HRV moves heat from one of the air streams to the other, but keeps the incoming and outgoing air streams separated.  Only heat moves between the two air streams.

Let’s assume that you are bringing in fresh air on a cold winter day.  The HRV will bring in fresh air, and exhaust an equal amount of stale interior air.  The HRV will take the warmth of the stale interior air, and move that heat to the incoming fresh air stream.  That warms the incoming air substantially, reducing the need to heat incoming fresh air.  Meanwhile, the outgoing stale air loses heat, before it is dumped outside.

HRV OPERATING ON A COLD, WINTER DAY

The diagram above indicates how the HRV works.  The incoming and outgoing air streams pass by each other (but do not mix).  As the outgoing, stale warm air passes by the incoming fresh air, heat passes from the warm outgoing air into the cool incoming air.  By the time the cold outdoor air is released into rooms in the home, it has been warmed substantially.  So its a lot more efficient and comfortable to use an HRV to bring in fresh air, than to open a window (which allows cold air into a warm house).

On a hot summer day, the same system works in reverse.  Hot incoming fresh air is hot, but is cooled by the outgoing cool, stale air.  The heat from incoming air is passed to the outgoing air stream.

Amazingly, HRVs can be designed to be very efficient at moving heat from one air stream to the other air stream.  Some units claim to work at 95% efficiency at low speeds.  Even inexpensive systems can operate at 70% efficiency.  HRVs range in cost from less than $500 (for a single room) to over $5,000 for the most efficient system for an entire home.  But they save money by reducing heating and air conditioning costs.  HRVs provide more comfortable ventilation than using a vent fans or opening windows when outdoor weather isn’t ideal.

INDOOR HUMIDITY AND HEALTH

Ideally, the home’s indoor air should be moderately humid.  Between 40% and 60% indoor relative humidity is a healthy humidity range.  Below 40%, people can have trouble with dry skin and higher rates of respiratory infections.  Mold, mildew, fungi and dust mites can be a problem with high indoor humidity above 60%.  Note the following chart.

PERCENT RELATIVE HUMIDITY AND AIR QUALITY

Fortunately, in areas where humidity can be a problem, HRVs can be designed to help control indoor humidity as well as indoor temperature of a home’s incoming fresh air. HRVs that help control indoor humidity are mentioned next.

HRV OPTIONS

  1. Heat Recovery Ventilators also can transfer humidity from one air stream to another.  For example, during the summer, the hot, humid incoming air can be both cooled and become less humid by transferring humidity to the outgoing stale air stream.  Heat Recovery Ventilators that transfer both humidity and heat are called Energy Recovery Ventilators (ERVs).
  2. HRVs or ERVs can be equipped with a small electrical heating unit that will boost the incoming air temperature.  Since the HRV or ERV already boosts that air temperature most of the way to the interior temperature, not much extra heating is necessary. Having a heating element in the HRV keeps the incoming air stream most comfortable for home occupants. Note that any kind of interior heating system could still be used to keep the interior home temperature warm without an HRV heating element. Also note that an HRV heating element might eliminate the need for a central heating system, if the home is very well insulated and airtight.  Or a heating element might reduce the size of the home’s heating system, which also could reduce the cost of the heating system.
  3. HRVs and ERVs may have specialized ducts, air registers and other options for installing all parts of the system for an entire home.  Some HRVs operate along an exterior wall at one specific place (e.g., bathroom) and do not need any additional ducting.
  4. HRVs or ERVs can be equipped with air filters, incoming air pollution detectors (with automatic shutoff controls), interior carbon dioxide detectors (to automatically bring in fresh air when needed), automatic operation controls, and other advanced options.
  5. HRVs or ERVs often come with HRV freeze protection options to avoid condensation or icing from extremely cold, incoming air.

SUMMARY

Overall, HRVs and ERVs:

  1. Can save energy while providing fresh air;
  2. Improve indoor air quality;
  3. Improve indoor comfort for occupants;
  4. Improve worker attention and efficiency by providing more fresh air;
  5. Provide an overall health benefit;
  6. Reduce indoor odors;
  7. Reduce the chance of mold and mildew problems;
  8. Can reduce noise compared to opening windows or operating noisier exhaust fans.

Designers and builders of energy efficient homes typically use HRVs and ERVs in high performance homes.  In some colder Canadian climates, HRVs are now required for new buildings.  The following maps shows where ERVs vs. HRVs are recommended.

MORE INFORMATION

  1. Video featuring top quality Zehnder HRV and ERV systems.
  2. Video featuring easier to install Lunos ductless HRV units
  3. Information on home insulation.
  4. Design a Zero Heating Bill Home, with no central heating system needed.

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