Healthy Indoors Magazine
Issue link: https://hi.healthyindoors.com/i/1066484
Healthy Indoors | 27 I took the photo above in a nasty, damp crawl space in Atlanta. Notice that filter slot near the middle of the photo. Not only is the cover missing but so was the filter. I took the picture on a hot, muggy day in August. Imagine all the nasty stuff getting sucked in there and sent into the house. It doesn't take a fancy filter to get bypass, though. Here's one with a standard one inch fiberglass filter that was jammed into the slot incorrectly. 3. Not enough runtime This one may blow your mind. Filters can clean the air only when there's air going through them. When the system is off, no filtering happens. If you've been reading this blog for a while, you may be thinking, "Aha! Yet another reason not to install an oversized heating and air conditioning system." The problem there is that when you put in a smaller HVAC system, you also cut down the air flow and the filtration vol- ume is the product of those two quantities: Filtration volume = Air flow x Runtime You can double the runtime by cutting the system size in half, but at the same time you're cutting the air flow in half so the filtration volume stays the same. One way that some IAQ advocates tell people to get more runtime is to put the fan in the "On" position instead of in "Auto." Well, that's a nice idea and it will work in some places...at the cost of higher energy bills. But in a humid climate during cooling season, you could be making your in- door air quality worse by raising the humidity. I know. I mea- sured it in my home a few years ago. The best thing to do here is size your system properly, don't put the fan in the "On" position, and get as much fil- tering as you can with your system. If you have an efficient blower motor and are in a dry climate or running your sys- tem with a dry coil, using the fan "On" setting can help. But the best thing to do is focus on minimizing the stuff in your indoor air that needs to be filtered to begin with by doing source control, air sealing, and mechanical ventilation, both local and whole-house. If that still doesn't do it for you, it may be time to add a standalone fan with a filter that will run more and get you a higher filtration volume. 4. Not enough flow Now you know flow and runtime go together. This other way to get low filtration volume is a common problem for a cou- ple of reasons. First, many return ducts and filters are sized too small. That increases the pressure in the system, which reduces air flow. Second — and I know this doesn't apply to you — some people don't change their filters often enough. Again, the result is high pressure and low air flow. 5. Low efficiency (MERV) filter You can install a standard one-inch fiberglass filter. It's not going to do much for your IAQ because it's basically de- signed to keep dog hair, dead spiders, and lost socks from getting into the air handler. The standard rating system for filters is Minimum Efficiency Reporting Value, or MERV. That one-inch filter is about a MERV 2. The higher the num- ber, the more stuff you filter out.