What material for filter box

[JackOfDiamonds]

I want to add a 4" furnace filter box and a fan so I can keep my space positively pressurized during smoke and pollen season. I only have a small space, so I was going to build a custom 16x20x4 filter box with 4" duct connections. I think the filter would last effectively forever at 100CFM. I can't decide what material to use though... I'm thinking XPS insulation board or MDF. Anyone done anything similar?

 

[mm08822]

If you have forced hot air or central ac, can't you just run the blower?

 

[JackOfDiamonds]

I don't have forced air, I have a mini split, and this is one of the downsides...now I'm adding a blower and filter anyway. But note not every forced air system will cause positive pressure. You need to have the system pulling in outside air, meaning you need really good filtration or it will be self defeating.

 

[Jackfre]

Not sure how it will work for you but Jet make a nice filter box. 4” duct will likely just squelch any air flow.

 

[mm08822]

Look into outside air makeup units.

Hard to give a useful answer not knowing where it would be located and esthetics required.

Is it permanent or something you may remove seasonally?

 

[JackOfDiamonds]

It will be mounted permanently. Normal HVAC would use a sheet-metal box. Off-the shelf metal filter boxes do exist, but seemingly designed for specific furnaces or air handlers, not as stand-alone filter boxes with duct connections. I will probably whip something up out of MDF.

Yes, I am considering using a proper ERV unit. However, ERV units are designed to ventilate, but not to pressurize. They are designed to be pressure-neutral. An ERV unit won't make my space positive-pressure, which is what I want to do, to keep out pollution during times of wildfire smoke and high pollen, which is common here. That's what we do in commercial cleanrooms; make sure it's always slightly positive pressure. But to do that you have to be capable of pulling in and filtering outside air.

My experience with my house HVAC is that running circulation alone is neutral, or even makes things worse because just circulating your HVAC doesn't pressurize the house ,and may even cause or aggravate infiltration, depending on your air balance.

An ERV system could be part of the solution if you choke off the ERV exhaust to cause a net pressurization. However, ERVs have dinky, expensive filters that are NOT up to the task of specifically sucking in outside air and filtering it. Also ERVs are pretty tame in terms of CFM. That's why I need a filter box to hold a big 4" filter, even if I do use an ERV, which I will probably follow up with a HEPA.

 

[mm08822]

Pricey.....

 

[PoorUB]

It will be mounted permanently. Normal HVAC would use a sheet-metal box. Off-the shelf metal filter boxes do exist, but seemingly designed for specific furnaces or air handlers, not as stand-alone filter boxes with duct connections. I will probably whip something up out of MDF.

Yes, I am considering using a proper ERV unit. However, ERV units are designed to ventilate, but not to pressurize. They are designed to be pressure-neutral. An ERV unit won't make my space positive-pressure, which is what I want to do, to keep out pollution during times of wildfire smoke and high pollen, which is common here. That's what we do in commercial cleanrooms; make sure it's always slightly positive pressure. But to do that you have to be capable of pulling in and filtering outside air.

My experience with my house HVAC is that running circulation alone is neutral, or even makes things worse because just circulating your HVAC doesn't pressurize the house ,and may even cause or aggravate infiltration, depending on your air balance.

An ERV system could be part of the solution if you choke off the ERV exhaust to cause a net pressurization. However, ERVs have dinky, expensive filters that are NOT up to the task of specifically sucking in outside air and filtering it. Also ERVs are pretty tame in terms of CFM. That's why I need a filter box to hold a big 4" filter, even if I do use an ERV, which I will probably follow up with a HEPA.

An ERV or HRV can be set to pressurize, or run the house in a negative. You just need to set it up properly.

As for a filter box like you describe, some 1/4" luan plywood and 1x2 material will work fine. You can build it out of most anything. MDF will work, but probably over kill and heavy. Duct tape and cardboard would be fine if you just want to test the concept. You are just dealing with air flow, no heat and probably no moisture.

My only question is how much smoke a filter will remove? Certainly a higher MERV rating will get some, but not all. Certainly better than nothing. You might need a higher MERV rating and then a HEPA filter after it. As long as you have a good filter before it the HEPA will last a long time, maybe years.

 

[Ohmthis]

I totally get what you are wanting to do. Essentially you want to bring in outside air, filter the incoming air, and put a positive pressure on your enclosure (building). This will potentially keep out the smoke/pollen by constantly pushing out. I will say, I would not use 2 x 4” round ducts. What are you planning on using to move the air? A regular box fan? Some type of blower? To keep the noise down, larger diameter pipe will keep the pressure down and noise while moving the air.

 

[98ssuck]

I’ve used these to pressurize spaces. They work well for pressurization. But with pressurization comes wild thermodynamic changes.

No idea on the cost of one of these. Where I’ve installed them budgets are a suggestion.

Filtration Protection Systems | Clean Air Filter

www.cleanairfilter.com

 

[Denwood]

Use an ERV and build a box like this one outside:

Super Efficient HRV retrofit with smoke filter for $300?

@PoorUB , yes, I've retrofitted an older HRV by removing the single inefficient internal fan (about 350 watts) and replacing with two external ED fans (about 30 watts typical) which can be automated for speed control etc. Typically the outside air is pretty good, but in Spring heavy with...

www.garagejournal.com

It uses a MERV16 16x20x5 filter and a 15lb carbon canister that can be included/excluded from the air flow. We’ve had several severe wildfire events over the summer but PM2.5 inside did not exceed 7ppm. The box outside has been trouble free over two winters with temps down to -35C.

The large filters mean you can remove the ERV fresh air filter inside the ERV and see no reduction in air flow. Ac infinity now sells carbon canisters with refills, but I’m two years now on the same canister. The carbon filter 100% resolves smoke, skunk, backyard fires etc smells when it is included in the air flow.

Commercial buildings regularly use external filtration…

 

[JackOfDiamonds]

I didn't think about putting a filter outside....that gives me something to think about. Not sure if my wife would hate a filter inside more or outside more.

I like the charcoal filter too, but making a mechanism to bypass it seems complicated. Do you know if you could just leave it in the airflow all the time? Also, many charcoal filters can be recharged by heating them up; do you know if those can be recharged?

 

[fitter30]

Make it out of MFD use liquid nails for caulking just for added strength.

 

[Denwood]

I didn't think about putting a filter outside....that gives me something to think about. Not sure if my wife would hate a filter inside more or outside more.

I like the charcoal filter too, but making a mechanism to bypass it seems complicated. Do you know if you could just leave it in the airflow all the time? Also, many charcoal filters can be recharged by heating them up; do you know if those can be recharged?

Outside means no need to insulate (as you must do if doing this inside the home), no condensation or frost issues. You can leave the canister in the air flow if desired. It’s a very simple 8” damper to include/exclude if you look at my box design Carbon works by sequestering contaminants in micro fissures so I figure once that has occurred, it would be easier just to swap out the contents with a refill kit. One refill kit amounts to less $$ than a few furnace filters. The reason I went with the canister and 15lbs of carbon is that anything less didn’t work effectively 110 CFM. You need to slow the air down and give it some time in contact with the carbon…and an 8” unit does the trick due to the surface area and 1” or so of carbon.

The design is also simpler outside as the 16x24x5 filter face is fully exposed and does not need a plenum.

 

[98ssuck]

You are going to find that the inherent problem you are trying to solve. Is that your pressurization medium is contaminated. Also you will need a post charcoal pleated filter to catch the dust that comes off it.

 

[JackOfDiamonds]

point taken the box is a lot simpler if you only need HALF a box...but having the 4" filter outdoors seems slightly sketchy...I'm in a dry climate but I don't know if those filters are designed for our 100+ degree desert heat. I guess the worst that can happen is the filter glue melts or something. I'm definitely giving this some thought because I only have a 10ft x 30in x 18in utility chase and I was trying to fit my ERV, a splitter/plenum to 3 or 4 supply ducts, and possibly a HEPA and/or a RETURN air filter (to keep the ERV clean inside from house dust)...but it all just depends how overkill I want to go.

 

[Denwood]

Consider roof top units HVAC that use a pretty standard filter stack setup. There is nothing special about the filters, just more of them to ensure correct air flow. Temps on a roof in summer are easily 120F and up.

Now my unit is on the south side of the house so will see lows of -35C, and highs in excess of 35C. White box helps.

 

[Denwood]

You are going to find that the inherent problem you are trying to solve. Is that your pressurization medium is contaminated. Also you will need a post charcoal pleated filter to catch the dust that comes off it.

The canister filters from ac infinity etc integrate a dust/filter membrane and in my experience so far have not required any post filtration. PM2.5 levels downstream are typically .5 to 1 ug/m3

You do want to filter before the charcoal though to extend it's usable life. I'm using a MERV16 (16x25x5) in summer, and a MERV 11 (16x25x4) in winter as the pre-filter. Air is typically diverted past the charcoal canister using a bypass damper in the filter box during winter months. These filters are far more effective, cheaper, and have dramatically less impact on air flow vs the typical OEM HRV or ERV filters out there.

 

[JackOfDiamonds]

4" furnace filters are indeed amazing. In my old house I converted to a 4" filter and added a water manometer so I could monitor filter restriction. The 4" filter was still only 1/4" inches of water after over a year, where the 1" filter was nearly 1/4" when new, and quickly increased to 1/2" or more in a couple months. All at the "same" MERV rating, but I also think the 4" was filtering better.

 

[Denwood]

This X6672 filter at MERV 16 flows a lot of air with relatively low resistance. It’s rated at .31” drop measured at 1600 CFM, new. That’s why I use this one in summer combined with the charcoal 15lb canister.