HRV/Central Fan filtration
Hi,
This question was inspired by the Q&A from Cathy who wanted to know about affordable alternatives to HRVs.
We recently bought a house with a bathroom fan unit we can set to run constantly or when humidity reaches a certain level. There’s a separate fresh air intake on the side of the house that flows into the HVAC system, and the fresh air gets minimally filtered through the furnace air filter.
We’re looking for a unit that would provide us with better filtration, such as a carbon-activated filter that would take out VOCs from outside (we live near an Interstate highway). Some of our neighbors also have their lawns treated regularly in the summer, and we don’t want to be drawing those toxic chemicals into our home. We’ve asked a couple of local HVAC contractors, but they didn’t have any suggestions.
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Replies
Reid,
You can't always trust the claims of manufacturers of air filtration devices, so be cautious. I seriously doubt that there is any way to filter the "toxic chemicals" that may be found in the outdoor air after your neighbors have treated their lawn.
Fantech sells a 240-cfm supply fan equipped with a HEPA filter. The manufacturer claims, "Mold spores, pet dander, cooking odors, dust, dust mites and their by-products are all captured in a series of three filters. The prefilter collects the largest particles while the carbon filter absorbs odors. The third filter is a true, certified HEPA filter which has been tested and certified to be 99.97% effective at removing particles down to 0.3 microns."
I'm highly skeptical that you can effectively filter all your incoming air by any means. We live in houses, they get their air from outside through various openings. Your exhaust fan may pull some portion of its makeup air in through the HVAC system's filter, but it pulls other air through all sorts of cracks in your building shell (and probably through your HVAC system by routes that bypass the filter).
It might be worth using a HEPA filter inside the house, one that simply pulls air through a filter, air that's already inside. I don't know if you can remove VOCs that way--I suspect not, and I suspect that there are already VOCs in the house by way of furniture, contents, building materials, and so on.
If you're concerned about air pollution from nearby traffic sources, you should be concerned with both particulate matter (from vehicle exhaust and mechanical abrasion) and VOCs (which are gas phase compounds). HEPA filtration would address only the particles; you would need activated carbon filtration to address gas-phase pollutants.
Many of the particles near roadways are ultrafine particles, meaning less than about 100 nanometers (or 0.1 microns). HEPA filters are still very efficient even in that small size range. Some laboratory testing I've done recently showed that residential MERV 16 filters can achieve about 50-80% ultrafine particle removal efficiency. MERV 13 could get 30-50% efficiency, which is still helpful if HEPA ends up being too expensive. These are all typically extended depth filters (e.g., 5 inch) so you either have to accomodate them within your HVAC system (and do check for airflow issues) or install an add-on product. It appears that the Fantech unit Martin suggested has both HEPA + activated carbon filtration and is installed as an add-on unit. Lennox also makes something similar. I don't know of actual measurements with these but upon first look they seem adequate.
Overall, installing these at your air handling unit (and after your outdoor air inlet) is wise because they will filter not only those pollutants from outdoors but those emitted inside your home. Plus, the amount of airflow through your combined recirculated + outdoor air component is much greater than your outdoor air component alone, meaning you have a lot of air coming in contact with a high efficiency filter -- both are good things.
And if you're worried about pesticides, those are generally considered semi-volatile organic compounds, which means that they exist in both particle- and gas-phase forms. The amount in particle or gas phase depends on some chemical characteristics (like vapor pressure). Either way, HEPA + activated carbon should address both if they are airborne. You also should be concerned about track-in on your shoes if you're concerned about pesticides. A well placed door mat and/or no shoes policy should generally do the trick.