What is the most energy efficient way to mechanically ventilate a home to bring in cool night air, after a hot day?


What is the most energy efficient way to mechanically ventilate a home to bring in cool night air, after a hot day?

Asked by Patricia

This is a well-insulated, two-story home with two gas furnaces, one for each floor, but not air conditioning, built 6 years ago. Standard construction techniques were used. Climate is temperate - San Francisco Bay Area. Due to security, noise, and particulate issues, sometimes it is not desirable to leave the windows open at night to cool the home, particularly the second floor. What options do we have, other than adding air conditioning?

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Randy Potter's picture

This is a great question that more people should think about before they install central air conditioning.

I also live and build in the Bay Area and I can tell you that we rarely install air conditioning in the projects that we work on.

  • We have even convinced clients to leave out the a/c unit and we would install it later if indeed they find that they need it and not one has come back to us and wanted one yet.
  • The climate here is absolutely perfect for natural nighttime ventilation.
  • Opening and closing windows strategically is the easiest solution, however I understand your concern for security, noise and particulate protection.

Luckily there are several options for you that will aid in this natural nighttime ventilation without having to add an expensive and costly to operate a/c system.

HRV and ERV Systems

The first technology to consider would be a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV).

These units provide ducted mechanical ventilation by bringing in outside fresh air from an outside vent (generally on the roof) and exhaust stale indoor air in exchange so they are able to maintain nice even air pressure in your home.

  • The nice thing about these units and their true advantage over exhaust fans or open windows is that they are able to recover the heat (or cool for an ERV) from the exhausted air and transfer it to the incoming air, this way you do not lose the energy that you spent conditioning the indoor air.
  • These units run continually at a very low rate so they do not "blow" air like a furnace or air conditioner.
  • The HRV unit will only exchange heat so it works best in heating months, however it continues to exchange air all year long. An ERV provides the exchange of cool air as well but it does this by removing the moisture in the air so they really work best in humid climates, not really the case in the SF Bay Area.

We install HRVs in almost every home we work on in the Bay Area, they work great and people love them.

Whole House Fan

Another technology to consider in order to combat the late afternoon/evening heat build up in your home is a whole house fan.This is simply a large fan that sits in the attic and pulls air very quickly from the living space of your home and exhausts it out through a roof vent.

  • This is a very cost effective way to get rid of the heat build up later in the day simply by turning on your fan unit and opening some low window on the cool side of your house in order to let in the coolest air possible.
  • Again, this technology is "on demand" and works best in the evening of a hot day when it is warmer in your house than it is outside.

Integrated mechanical ventilating system

Finally, perhaps the most complete solution would be an integrated mechanical ventilating system such as the Davis Energy Group's Night Breeze unit.The NightBreeze system integrates heating, ventilation cooling, and air conditioning and provides fresh-air ventilation for maintaining indoor air quality.

  • During the summer, if the house is warm and the outside temperature is cool, the NightBreeze system automatically brings outside air into the house through a special damper and ventilation duct.
  • In the winter NightBreeze heats the house using heat from the water heater or from a conventional gas furnace and also periodically introduces outside air into the system air stream to provide adequate ventilation.
  • The NightBreeze thermostat maintains the homeowner's comfort level by setting minimum and maximum temperatures for the home, maximizing comfort and the use of natural ventilation.
  • This technology needs to be tied into your existing ducted furnace system and requires an adequate attic space so take a look at the requirements on the website and make sure that this would be appropriate for your home.

We also love heat pump technology for HVAC purposes these days since they provide both heating and cooling, however if you already have your heating system which is relatively new it certainly does not make sense for you to look at a whole new system.

Please feel free to visit our websitewww.myearthboundhome.comfor more ideas, or feel free to email me if you have any specific questions.

Good luck staying cool, it has not been that much of a challenge this Summer with the weather that we have been having.

For more information:

Read "How can you add fresh-air ventilation to an old home with a forced-air system and only supply ductwork throughout?" a Q&A answered by Danny Kelly.