If not installed or maintained properly, forced-air heating and cooling systems can perform poorly, creating air quality problems and unnecessary expense. To optimize a forced-air system, you need to design or refine it to prevent leaks and pressure imbalances, protect it properly during construction (or make sure the ductwork is clean and sound), and use the right filter.
The performance spec of the heat (or cooling) source is also important, but it is critical to address the distribution system first before investing in better furnace and air handler performance. This article is the SECOND of four focused on optimizing forced-air systems on the topics of Distribution, Filtration, Heat Source, and Ventilation.
Some are inclined to believe that forced air systems are more of a cause of air quality problems than a solution. However, one of the advantages they offer is the ability to continuously filter the re-circulating indoor air as well as the outside ventilation air (in a central fan integrated system). While filtration shouldn’t replace efforts to reduce source contaminants, this can be a real bonus to homeowners who have pets (including small children :), live in areas with a lot of particulates in the outside air or have a need for highly-filtered indoor air.
Filters in forced air systems are generally (and preferably) located in the return trunk immediately upstream of the air handler, though sometimes they are located at central return grilles. An important consideration here is accessibility – filters must be cleaned or changed according to manufacturers’ instructions to maintain performance and minimize wear and tear on the equipment. So they should be easily accessible to the homeowner and the access door should be air-sealed in such a way that it will reseal easily and effectively, even after it’s been opened multiple times.
If the air handler is in a garage or mechanical closet, that’s probably a good place for the filter, but if it is in an attic, crawl space or other location that’s hard to reach installing high-efficiency 1-inch filters at the central return grilles might increase the likelihood they’ll get changed regularly. Clogged filters will reduce the air flow through the system, affecting comfort and energy efficiency and potentially increasing wear and tear on the system.
Use the highest efficiency filter appropriate for the system, preferably a filter with a MERV rating of 10 or higher. MERV, or Minimum Efficiency Reporting Value, is a simplified expression of the effectiveness of a filter at removing particles of different sizes from the air stream flowing through it. What you need to know is that MERV is a scale from 1 to 16, that 10 and above are considered to be high-efficiency, and that the non-woven, fiberglass filter that was supplied with your furnace (and may still be in there . . .) probably wouldn’t even get a MERV of 1! (Such filters are installed to protect the air handler from large dirty objects, not your respiratory system from small ones.) If you want more details on what MERV is really all about, visit the National Air Filtration Association website.
If you are upgrading the filter in an existing system, bear in mind that for a given surface area of filter, the higher the MERV rating, the greater the pressure drop across it. One inch filters above MERV 11 may cause sufficient pressure drop to significantly affect the performance of an existing system. If you are upgrading the air handler, the fan/blower spec can be altered to accommodate this, but if you’re adding more efficient filtration to an existing air handler it may be worth increasing the surface area of the filter. This typically means switching to a thicker, pleated filter (perhaps 3” or 4”) which may create an installation challenge in the existing sheet metal. However, if the upgrade also includes full duct sealing and insulation, the reduced losses in the system may offset the reduced flow from a moderate improvement in filter performance. Measuring the pressure drop across the filter and in various parts of the system will provide useful data to support good design decisions. Add a manometer (such as the JMW digital manometer pictured above, and the know-how to use it to your toolbox, or find an HVAC balancing contractor or home energy specialist to assist you.
Make sure the homeowner understands the maintenance requirements of medium- or high-efficiency filters, the benefit of fulfilling those requirements, and the downside of not doing so. The photo to the left shows two filters, one new and one with just a few months of wear that is quite clogged.
Filters with the lowest first cost, such as one-inch (1”) pleated media, require regular replacement (four times per year), amounting to considerable cost over time. Four-inch pleated media filters are now commonly available. Though they cost more individually, they generally require less frequent replacement (so have a lower lifecycle cost) and cause less pressure drop than a 1” filter of similar cross-sectional area. For systems in big homes that move large volumes of air, high efficiency bag-type filters (some of which are washable) may be a worthwhile consideration.
Although electrostatic air filters enjoy a PR buzz, green designers generally don’t recommend them. To maintain their performance, they should be cleaned as often as once or twice per month, and most homeowners are not willing to keep that schedule up. When they’re dirty, their filtration performance drops dramatically, and they may generate and introduce ozone—a significant respiratory irritant—into indoor air stream.
Next week we’ll be looking at heat sources in a little more detail.
Diagnostic HVAC Resources: To find a qualified Home Performance Testing contractor, check out websites for Building Performance Institute (BPI) or RESNET. If you are located in Washington see Home Performance Washington or Northwest Energy Star. To locate a Balancing Contractor, see National Comfort Institute Certified Contractors.
Alistair Jackson, LEED AP, CSBA is Principal in Charge of O’Brien & Company’s Residential Technical Services, and is a LEED for Homes Rater, LEED for Homes Faculty, Energy Star Verifier and Performance Tester, ARSCA Accredited Professional, and Built Green Verifier. Alistair was also a major contributor to the Northwest Green Home Primer where much of the information for this article originally appeared. It has been expanded upon for this publication.
Did you enjoy this article? You might also like these Building Capacity Blog articles:
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 1
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 3
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 4
Mechanical Systems and Fuel Choices for the Warming World
Cracking the Energy Code: What Will It Cost?
Passive House: A Bit of Overkill