Filter box for regular + charcoal filters
I’d like to buy or build an ERV pre-filter box for a max 150 cfm (typically more like 75 cfm) that can accommodate a regular and charcoal filter, with a filter size that will likely require changing the plain filter no more frequently than 6 months (no pets or kids in the house; not a dusty environment). I’m wondering about using a 14x24x3″ in plain filter and 14x24x1″ charcoal filter. I expect to use the charcoal filter when it’s smokey season and could remove it the rest of year. Attached is a sketch plan for a box I could easily fabricate (or have fabricated). I’d like to minimize the space, put the plain filter upstream of charcoal, and will insulate the box. The drawing is conceptual – I’ll figure out the seals, etc.
My questions are:
– are the proposed filter sizes likely be sufficient for longevity?
– are there issues putting two filters in the same box? (if so, what?)
– what distance between the filters is necessary? Are there standards or recommendations for this?
– are there ‘better’ (e.g. more common/available) filter sizes? I’d like to avoid a ‘specialty’ filter size.
– does anyone make such a box (I haven’t found one)?
– any other problems with such a design?
Thanks
johng
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
It's hard to say how long the filters will last, because that depends on the amount of particulates in the air at your location. During smoke/wildfire season, I would expect significantly less life for the filters compared with "clear" times of year.
There should be no problem putting two filters in the same box, as long as you allow for the higher pressure drop across them. The easiest way to allow for the additional pressure drop is to provide more filter area -- twice the filter area gives half the pressure drop, for example. You'll probably be fine in your case due to the very low airflow though. I would put a differential manometer across the filters though, which will let you measure pressure drop across them. This way you can KNOW when you need to replace the filters based on a measurement, rather than just replacing the filters on an arbitrary schedule. Knowing is alwasy better than not knowing, and lets you maximize the life of the filters by only replacing them when it's really necassary. I would look on Ebay for Magnehelic differential manometers in the low 0.25 to 0.5 inch of water column range. You can usually find used units of this type in good condition for under $50 or so.
You can put the filters close together, but I'd leave enough of a gap that you can slide them in and out without them rubbing on each other. This way replacing one filter doesn't dislodge dirt that may be on the other filter. Dislodged dirt has a tendency to blow down the duct, so that's something you want to avoid as much as possible.
Your filter sizes look oddball to me. I'm used to 24x24 as a "standard", although there are others. Grainger has a good selection of pleated filters, so I'd suggest you look in their catalog and see which filter sizes seem to be the best bang for the buck, then design around those sizes. Note that thicker filters tend to last longer.
Most filter "boxes" I've seen are just built into ductwork or equipment, I'm not aware of any purpose built, stand alone filter boxes. That doesn't mean they don't exist though. I would suggest calling some HVAC supply houses and asking.
I think the diagonal filter placement will make replacing filters tricky. I would try to put the filters in in such a way that the airflow either comes in perpindicular to the face of the filters (the usual arrangement, where the filters would slide out one side of the box), or bring the ductwork in on either end and offset to one another, then have the filters in the middle of the box with the airflow coming in parallel to the face of the filters. You should build things so that the filters have a snug fit, and then use some soft weatherstripping for a seal if you want to make sure there aren't any sneak paths for air to leak around the filter. It's difficult to get a full seal though, so just do the best you can without making things so tight that you can't slide the filters in and out for replacements.
You could potentially use Aprilaire's whole home filter housings that fit their filters. This might be an option for you, you'd just have to fabricate duct transitions to let you install these filter housings into your ERV system's ductwork. These filters are bigger than you really need in this application, but that just means they'll last a long time. They also make combination particulate/carbon filter cartridges for these. I use two of these systems in my own home with their MERV 13 filters in them, and I get well over a year between filter changes without any problems.
Bill
Hi Bill,
Many thanks for your detailed response. The diagonal filter placement is more of a fiddle but is used in in-line filter boxes like the Fantec FB6 (from SupplyHouse.com) and it's much smaller than putting the filter perpendicular to ducting. I have a large Aprilaire filter (I think 24x24x4) and it's great, but it would require a very large filter box and serious overkill for a 150 cfm application, thus the question about smaller filters. I've never used such small filters and have no idea what sizes are 'standard' or 'common' size. The local big box has 12 x 20 and 14 x 20 1" (but no 2, 3, or 4") filters in stock so I thought they might be more readily available. The double filter clearly isn't essential, but it seemed like an good way to get good filtration and (hopefully) much longer effective 'charcoal' filtering without a great increase in the space consumed by filter boxes.
Check with Grainger, they have a lot more filter sizes than the box stores, and their prices are reasonable. I have ordered many, many filter from them for commercial projects :-)
Once you've decided on the size filter to use, you can build everything around that. For such low flow rates, you could get by with 1" filters, but it's a tradeoff of price to lifetime as you get thinner with pleated filters at low flows.
Bill
I've been considering the same thing.
My current thought is to buy this filter box in the 24x24x4 size: https://www.hvacquick.com/products/residential/Indoor-Air-Quality/Inline-Filter-Boxes/HVACQuick-CFB-Series-MERV-13-Inline-Filter-Boxes
And use these 24x24x4 MERV15 carbon filters: https://www.amazon.com/Nordic-Pure-24x24x4M13-C-1-Furnace/dp/B0073V4ETC?th=1
Anybody have any thoughts on that?
Edit: I'd thought about doing the dual filter route as the original poster is describing but I then decided I preferred the simplified route of a single filter. Can use the carbon filters during wildfire season and swap them out for non-carbon filters the rest of the year if desired. Or just keep the carbon filters in all year to also help filter out smells from my neighbor's grill and cooktop hood.
Putting a regular particulate filter ahead of the carbon filter will extend the life of the more expensive carbon filter. That's something to consider if you are thinking about going with a single filter instead of a dual filter design.
Note that if you get a filter housing that fits, for example, a single 2" deep filter, you could also fit two 1" filters in the same slot to double up if you want to try a sort of compromise design. A housing for 4" filters could take a pair of 2" filters too. I would probably tape the two filters together if doing this to make sure they stay in alignment when inserted in the housing.
Bill
For carbon filters what you really want is a high mass of activated carbon, unless you want to change them quite frequently. Those carbon dust coated particulate filters don't last very long at all, maybe a month max with not much odor present.
https://www.instructables.com/Activated-Carbon-Air-Filter/ You'd probably want to construct some kind of similar frame with wire mesh to contain granular activated charcoal. Then you can buy year's worth of refills for like $100 and just dump out the old, and dump in the new. I don't know anything about this vendor but here is an example: https://buyactivatedcharcoal.com/catalog/product/view/id/121/s/granular-activated-charcoal-coconut-8x16-mesh/category/630/
In this case you might want to put the charcoal upstream to the particulate filter, to capture any charcoal dust, although I'm not sure.
The problem with putting the carbon filter upstream is that larger particles will clog it. Carbon filters work by having lots (and lots, and LOTS) of very small pores that can trap tiny things, so you want to filter out big stuff before they get to the carbon filter, so that the carbon filter can focus on doing what it's best at: getting the teensy weensy stuff.
If you're worried about carbon dust, put another particulate filter downstream of the carbon filter, but you really want to keep the primary particulate filter upstream of the carbon filter to protect the activated carbon filter media.
BTW, this goes for water filters too. I have a particulate filter upstream of my carbon cartridge, and another one downstream of the carbon cartridge to protect the water softener that is the last stage of the system.
Bill
One idea would be to use the Zehnder filter housing along with their carbon and particle filters (one box fits both). It doesn't matter what ERV you're using since you can just attach an end plate to each end and use regular ducting. The housing comes in different sizes, so you can choose the size that matches your desired airflow.
I don't know how that compares in cost to the other options you're considering, but it's an off-the-shelf solution, and I think you can be confident that it would perform well.
Andy and Kevin - Thanks! Good suggestions. I particularly like the idea of constructing a super-heavy-duty charcoal filter -- I'll explore this. The Zehnder box looks well designed and could work well for the ERV. I also have a fantasy of venting the stove hood indoors, using a home-built, high-capacity filtering system that avoids the complexity of make-up air. Our range hood could easily be vented to the basement where the air could even be routed through a moderate-sized drum loosely filled with activated charcoal, which ought to last a very long time.
https://www.greenbuildingadvisor.com/question/hrv-erv-inline-pre-filter#comment-192382
Interesting comment: "Because there is a lot of wood smoke in my area, I have a carbon filter on the supply air after the unit. The carbon filter works better after the temperature and humidity are moderated."
I can't help but chuckle at the spare-no-expense approach being something like - coarse washable pre-filter > particulate pre-filter > carbon pre-filter > ERV > carbon post-filter > carbon dust post-filter. Could definitely fill an unvented attic with that!
The usual way this is done in the commerical world is a bag filter ahead of a carbon filter, and nothing downstream of the carbon filter. It's a relatively simple two-stage setup. I wouldn't bother with a prefilter ahead of the particulate filter here, just go with a decent size pleated particulate filter, then the carbon filter, then you're done. I've never had an issue with carbon dust coming off of the carbon AIR filters, although it can be an issue with activated carbon WATER filters.
I would also add that people with respratory problems who live in the fire zones probably would be happy to spend extra money to get some relief, even if that means a fancy filtration system to maintain.
Bill