Question

What is the most effective air exchange system I should employ in a new three-level townhouse in Texas?

Asked by Chuck, Texas

What is the most effective air exchange system I should employ in a new three-level townhouse in Texas?

Answer

Mick Dalrymple

Answered by Mick Dalrymple

Scottsdale, AZ

Eco-Friendly Building Center

June 5, 2008

First work with a mechanical engineer or mechanical contractor to determine the proper size of the system.

  • If the house is under-ventilated, you may still end up with indoor air quality issues.
  • If it's over-ventilated, your energy bills will be higher than they need to be.
  • A whole-home ventilation system should be sized per ASHRAE 62.2-2007. Some jurisdictions have adopted ASHRAE 62.2 into code.

In most places, indoor air quality is only required to be handled correctly in commercial buildings (covered under ASHRAE 62.1). It is kind of ironic, given that we spend more time in the home.

HRV vs. ERV

Second, you’ll want to work with your mechanical engineer to determine if a Heat Recovery Ventilation (HRV) system is appropriate or if an Energy Recovery Ventilation (ERV) system makes more sense.

I assume your home in Texas is either in a hot/dry or a hot/humid climate, which suggests an ERV system will work the best.

Choosing an ERV or HRV system

Finally, check with the manufacturer of your HVAC system to see if they offer an ERV system that is efficient and will tie in seamlessly with the air handler.

If they do not, you'll have to go to a third-party system that will sit next to the air handler and do the same job. The Home Ventilating Institute has tested most HRVs and ERVs on the market, so I suggest reviewing their test results (PDF) for the specific model number prior to purchase.

The key in a hot climate is to review the Total Recovery Efficiency, which is the measure of the unit's effectiveness in a cooling situation (95-degree outside air).

  • Most EVRs are rated 15-55% for Total Recovery Efficiency.
  • For winter effectiveness, you want to look at the Sensible Recovery Efficiency. Some units rate as high as 95% SRE.
  • You have to evaluate your climate, and find a unit that has a nice balance between the summer and winter effectiveness ratings.

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