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Filtered Fresh-Air Intake

ialohcin | Posted in GBA Pro Help on


I’ve been listening to the FHB podcast and thought I might send up a question here, since they so highly recommend your site.

We are located in lovely smoke-filled Portland, Or.  We need to replace the AC on the upper floor (~3500 sq feet), and I am planning to have a Trane electrostatic filter system added (would have been nice during the recent fires…).

I would also like to have a fresh air intake ahead of the filter so that the cool night air could be cleaned up and sent into the house at night, which would be quite pleasant at night and also reduce AC use during the day due to latent heat reduction.

I was all set to the pull the trigger on this, but came across some posts that made me concerned that pulling in this cool night air during the summer could corrode my nice metal air filter and furnace, or cause mold in the ducts etc.

This would probably be nights with 55F 60% RH when I would use this feature.

I was looking at something like the Aprilaire Model 8145 Fresh Air Intake Ventilator, which would according to its specs can supply enough air for my square footage, and has an easy remote control option (system is in attic).

Am I crazy?  Should I get a dehumidifier too or just scrap the whole idea.


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  1. Expert Member
    Akos | | #1

    Without AC, that 55F 60%RH (40F dewpoint) air is equivalent to 30%RH at 70F. So as long as your furnace is cycling house air, the dewpoint inside the ducts would never get cold enough to condense.

    The one issue with the fresh air kit is that it will also need some additional filtering. If you want to remove smoke smells, it will also need a filter box that can take an activated carbon filter.

    1. ialohcin | | #2

      I was hoping to cool the overall house temp with that cold night air. But it seems like I need to cut the relative humidity in half to avoid condensation risks.

      1. exeric | | #3

        Hi, I think you may be making an error in using that calculator at dpcalc. That's a really nice calculator, btw. The warmer the air the more moisture it can hold. For instance if the outdoor temperature is 55F and the RH is 60% it means that the outside air is not even close to its full capacity of holding water molecules in vapor form. As Akos said, that outside temp would have to go down to 40F or lower to make that vapor condense out. That's not going to happen in Portland at night in summer.

        Another way of putting this is that the outside air in summer at night in Portland Oregon does not hold very much water. You are in no danger, unlike in a place like Portland Maine, or anywhere in most of the Eastern USA for that matter. Many people on this forum need to worry about that but you don't.

      2. exeric | | #4

        I googled the Aprilair ventilator you mentioned and the nearest one i found was the 8144 at 220 CFM. That's going to add ventilation but it isn't going to do much to cut down AC use. You probably need 10 times that CFM to do any real cooling from night time outside air. That would require a whole house fan. However, the benefits of doing that are reliant on how tight and well insulated your home is. I think you will need to do some more personal research both here, and at FHB, to get a full understanding of the complexities. I use a WHF and my summer utility bill, according to my utility, is 1/3 of my neighbors bills with similar houses. It works but requires some flexibility such as using ceiling fans during repetitive hot days. Still, my house only gets up to 83F even when its gotten up to 111F outside air temp on two consecutive days.

        Getting a whole house fan and using it (in the western USA) is one of the biggest ways to cut down your carbon footprint. But it requires lifestyle modifications and a Pretty Good House to start with.

        1. ialohcin | | #5

          The higher Oomf of a WHF certainly makes sense. But I am not thrilled about just opening the windows and pulling in a bunch of unfiltered air. Since I got a proper Dylos laser particle counter I have been shocked at how bad the air can be even when the nominal AQI is under 20. So a bunch of MERV13's with adaptor frames in the windows?

          1. exeric | | #6

            Yes, it's a problem. One thing I noticed here is that as wildfires turned into infernos they created their own weather. After those two 111F days it quickly turned cooler just from the blotting out of the sun from the fires. So even though the fires continued to burn it cooled down significantly during the day. So the WHF didn't really need to work and was turned off, and had to be turned off because of the ash and smoke. That's where the ERV came in. I don't know if that's the way it's going to work during all firestorms but it was how the last one worked. The combination of the ERV and WHF kept me somewhat OK

            We're all basically improvising and adjusting to the new "normal".

  2. ialohcin | | #7

    It looks like there was "Nightbreeze system" some years ago that would do exactly what I want, with the exception of relying on media filters. Seems to have gone out of business and been taken over by folks selling upjumped window fans.

    Came across this airspace company selling whole house fans. Their air makeup system has a mode where it can pull in external air, filter it, and send into a house with 1800 cfm. Probably don't need the actual "make up" function in my leaky 90's house (another project hehe). But would be nice, though a bit spendy compared to a WHF.

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