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Community and Q&A

DIY Mechanical Ventilation

frasca | Posted in General Questions on

Hi all –

I’ve lived in in Seattle, WA for 5 years in a house that I’ve retrofitted with minisplits for heating but that has no mechanical ventilation. I’ve dealt with the need to ventilate bedrooms ( by cracking a window at night in my wife’s & my room and the room that our 4 children sleep in. I am fully aware that in the heating months I am adding to the heating load, but a little fresh air from the Puget Sound is nice, we turn the set points down at night because we like it cool when we sleep, and I have used Awair monitors to play with just how little I can open the windows to keep the CO2 from building up. I’m down to less than half an inch on sliding windows that are 2-3′ tall. The resulting load is within the capacity of my minisplits, and doesn’t seem to add a ton to my electrical bill vs previous years where I kept the windows closed through the winter. In the non heating months, sleeping with the windows open is great; the air is cool and dry (our dew points almost never reach 60 deg F) and we aren’t conditioning the house anyway so there’s no impact on energy consumption.

I am moving to Wake Forest, NC in a couple weeks and will be in a rental home for a year. I haven’t seen the home in person but based on photos it has typical ducted AC/furnace units with no mechanical ventilation. I’ve lived in the southeast enough to know that if I try my crack-the-windows technique in the summer I will be bringing in air that is hot and full of moisture which will require increased runtime on the AC units to remove. I’d be open to paying the energy penalty if there were a clear health and comfort benefit, but in my experience air this moist isn’t terribly pleasant to sleeping humans.

I was searching around the internet to find a portable, temporary system that I could use to do some single-room heat-recovery or energy-recovery ventilation. I was thinking maybe somebody would be selling something that looks like a two pipe portable AC unit that does this, but there doesn’t seem to be anything like that.

Any ideas of something I could hack together to make life better for the summer, then remove when I leave? I was thinking maybe each bedroom gets a Lunos fan set in plywood/foamboard and placed in a window like a window-unit AC? I have spent some time thinking about the Panasonic WhisperComfort FV-04VE1 because conceptually I like the idea of having intake and exhaust in the same box, vs a reversing or room-to-room paired system like Lunos. I also could see lots of opportunity to reuse the Panasonic units on a more permanent installation when I ultimately buy a house in the area. But I can’t think of any way to put an FV-04VE1 into a window without having it protrude into the room. Maybe surface-mount it on the ceiling somehow and run flex duct to my “window panel”?

I am willing to spend some time and money (hundreds, not thousands!) on this if anyone has any neat ideas. I know none of it will pay off in the year I’m in the house, but it would be fun, I’d learn something, and it seems like the closest I can get to the windows-open nights that we really enjoy.


[Edit: multiple edits to try to get line breaks and paragraphs to show up right on my browser]

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  1. user-5946022 | | #1

    Suggest you back up a bit. Your goal is bringing in fresh air to keep the CO2 level appropriate, right?
    1. First, measure the CO2 in your new house with all the windows closed. You may find the thing is so leaky that this is not an issue.
    2. Where is the conventional HVAC installed? In the SE part of the country, many are installed in vented unconditioned attics. They system itself is leaky enough to bring in fresh air from that alone.
    Once you have some hard data, then we can work of appropriate solutions. The solution could be as simple as letting a bath exhaust fan run overnight (although I am not at all a fan of making a house negative) or buying an inexpensive ERV and installing it "window air conditioner style" in a window as you describe above. The through wall Lunos would work for that, or something like this:*cwczy8*_ga*NjgwNjgzMTA0LjE2NTMwNjIzNzE.*_ga_MT1J2PK3R7*MTY1MzA2MjM3MS4xLjEuMTY1MzA2MjQxNi4w

    At that point, you may also consider buying one of the new single unit mini-split type window air conditioners. Advantage is they are made to be installed in windows, they bring in fresh air which they dehumidify and cool at the same time, and you can resell it at the end of the year of your rental.

    1. john_m1 | | #8

      I have purchased two of the Midea units and am very pleased. They are very quiet, and effective with a SEER of 15, and the included window mount is best in class. I think some of the units might even have heat pumps.

  2. Expert Member
    BILL WICHERS | | #2

    Note that a "ducted AC/Furnace unit" IS mechanical ventilation. What you're looking for is known as "makeup air", which is basically fresh air being brought into the building on purpose.

    Is there any particular reason you can't consider a normal commercial HRV here, installed into your HVAC system in the usual way?


  3. frasca | | #3

    CL - re: "Your goal is bringing in fresh air to keep the CO2 level appropriate, right?"

    Sort of... it's more generally to have fresh air get to bedrooms at night. But, given that the experience of a cool, year round pacific breeze is impossible to recreate in NC, I'll settle for low CO2, since that seems like a decent proxy for low levels of other bad stuff, and at least some level of outside air :).

    I agree on bath fans, with the caveat that they are better at determining where air leaves the building (from the bathroom), than they are at determining where air enters the building. Still, as you say, might be enough.

    Good call on leaky HVAC in the attic. If the leakiness is on the upstream/suction side of the AHU, then I can see that getting reasonably fresh air AND running it over a cold coil to remove sensible and latent heat. The energy penalty must be massive, but I'd be running it anyway, and at least I'm not buying or jury-rig any equipment.

    Thanks for sharing the US Vents/twinfresh units... not bad for a poor man's Lunos. I still like the idea of a Panasonic FV-04VE1 on a cart, or surface mounted on a wall or ceiling, with flex duct to a window baffle. Given that enthalpy exchange of the core is well <100%, the air coming in would be warmer and wetter than the indoor air, but cooler and dryer than the outside air, and maybe that exchange does enough to take the edge off. I did about a 2min thought exercise on mounting a Santa Fe ventilating dehumidifier like that and using 100% outside air, but that might be too much overkill even for me!

    Also re: "At that point, you may also consider buying one of the new single unit mini-split type window air conditioners. Advantage is they are made to be installed in windows, they bring in fresh air which they dehumidify and cool at the same time..."

    I love what I have read about the U-shaped Midea units (variable speed, quiet, attractive) ... but one thing I did not think they did was bring in any outside air. Is there a make/model you know of that does? That would obviously be perfect.

    @Bill - re: "Is there any particular reason you can't consider a normal commercial HRV here, installed into your HVAC system in the usual way?"

    Mostly just that I don't own the house...

    1. Expert Member
      BILL WICHERS | | #4

      >"Mostly just that I don't own the house..."

      gotcha. There used to be a twin fan that you could close a double hung window onto that might be an option. I've seen it made by various companies, but I'd try Lasko. They aren't very expensive, and you could use one to pull air through the house similar to a whole-houe fan but without the permanent installation. If you don't have double hungs they probably won't work for you though. You could use some 1/4" masonite to build an insert that would set in place of a screen for an awning or casement window, and mount a fan to that. It wouldn't be elegant, but it would work, and it would be easily removeable without any damage or modification to a window.

      You won't get a lot of airflow using bathroom fans. They would work, but I don't know if you'd get that draw of outside air the way you want with the small amount of airflow.


  4. frasca | | #5

    Well, the wall-hanging FV-04V1E is in!

    So far 40cfm is enough to drop the CO2 about 200 ppm at around 6am, the overnight peak. Previously I was seeing around 900-1000 ppm (with a window cracked) and for the past two nights I’m seeing more like 700-800 ppm.

    I was expecting a more dramatic impact from 40cfm of outside air, but I suppose 4 breathing humans create a lot of CO2. Hey, it’s something.

    And the air coming in feels, as expected, slightly cooler and drier than the outside air, if a bit warmer and moister than the inside air.

    It’s reasonably quiet and the flex duct don’t get in the way too much (though it is difficult to keep the duct insulation fast against the window board I made.)

    The main takeaway is that when I own a house I will try to get more than 40cfm of balanced ventilation!

    1. Expert Member
      BILL WICHERS | | #6

      You might try measuring CO2 PPM in several locations around the hosue to see if your higher-than-expected numbers aren't because the 40CFM isn't enough flow, but rather is because the air in the home isn't mixing, so you have areas of high and other areas of low concentration. If you have various concentrations like that, try running your regular HVAC system in "fan only" mode for a little while to mix the air around the house to even things out. Some thermostats will even allow you to program things so that the fan will run a minimum of some number of minutes every hour automatically.


    2. tekjunkie28 | | #11

      Wow. nice install. I am wondering how do you have the outlet facing outside. I would put a 90 on it and turn it down and away from the inlet. If the wind isn't blowing then you might not be getting as much fresh air as you'd think. I am going to have to do a similar setup as this but in my basement. I may do 2 of these units, one for the basement and one for the 1st floor. I am aiming for 800-1000ppm CO2. Right now if I crack a window the CO2 in the house gets to 2000+ppm with TVOC's over 1000. I am using the uHoo air monitor... IDK how accurate those are.

  5. frasca | | #7

    The CO2 monitor is in the same room as the ERV, sadly. So in theory that should already be the lowest concentration I could see...

  6. walta100 | | #9

    I wounder what the monitor would read when placed outdoors?

    Instead of telling us how the air feels please give us all 4 temp and humidity readings.

    Admittedly I am a sceptic please show me numbers that will change my mind from NC.

    Yes, I do believe there can be impressive numbers if you are in the frozen north not so much in NC.


  7. frasca | | #10

    Walta - I've seen Awair read as low as 400ppm CO2, or very close to it, when we're out of the house for a while and/or have windows open. So I have no reason to suspect that the Awair's CO2 reading is miscalibrated or that NC's air differs from most of the world's in CO2 concentration.

    Is your skepticism about the efficiency of the heat and moisture transfer the FV-04V1E is doing, or something about the CO2 levels?

    At this point I'm pretty agnostic about whether the enthalpy recovery is 25% or 75% ... I suspect it's somewhere in that range, but I need to get fresh air to my children, and any recovery is better than none.

    If the heat and moisture %s would be useful to you, however, I coincidentally happen to have an Ambient Weather WS-3000 with three remote sensors that I have yet to unbox. So I could get at least 3 locations, as long as the sensors don't mind having 40cfm air blown over them.... let me know!


  8. walta100 | | #12

    I am skeptical that any measurable energy can be recover when the differential temp is 15 ° or less and at 25° it is less than 15% is recovered.

    My guess is that fewer than 20% of the hours per year have even a 25° differential if you heat to 68 and cool 79. I could well be wrong not having lived in NC or owned this equipment just my gut feelings.


  9. frasca | | #13

    @tekjunkie - I was hoping nobody would ask about the outside of that window insert... photo attached.

    It's ugly but I sort of have the the positioning you described; the one with the hood is the intake side and the flush one with little louvers is the exhaust side. I angled them maybe 15deg away from one another, and have the exhaust one on the west since a majority of the time our wind comes from the east. The surface is a 3/4" EPS sheet. The reason it looks so janky is 1) I thought I could help the rain and UV resistance by putting a coat of rattle-can spraypaint on it, and it turns out whatever the solvents are in spraypaint they dissolved the EPS, and 2) it was the last thing I finished on Sunday under a time crunch and I didn't have time to trim the canned sprayfoam. Suffice to say it's definitely temporary and I will redo it with a better design when I have time. I would consider putting a 90deg elbow in one of the runs and running it down a couple feet. Panasonic recommends having the exhaust and the intake 3' apart, and mine are nowhere close. But I will say that when I reach my hand out the other window and feel the exhaust vs the intake, it is very hard to imagine that that much exhaust air is short-circuiting back into the intake air; the currents are in quite different directions and the mixing with outside air seems to be happen very fast.

    (BTW your 2000ppm of CO2 with windows open seems really high!)

    @Walta you might be right. Panasonic specifies that during cooling seasons, with 95deg outside air, the Total Recovery Efficiency is 36%. (In heating seasons the Apparent Sensible Effectiveness is 66%). They state that "The testing of the energy performance is in accordance with the CSA-C439 standard", which I would read, but the Canadian Standards Association wants $139 for it. For comparison, Panasonic claims that their ducted, 100cfm ERV, the FV-10VE1, has a Total Recovery Efficiency during cooling seasons of 73%.

    Anyway, I'll do some field testing with my Accuweather unit when I have time, and let you know what I find.

    Bottom line is that it's still 40cfm of outside air, at a cost of 23 watts plus whatever I'm adding to the cooling load. And if I am persuaded that ventilation is important to human health (which I mostly am) I can't think of THAT many better ways to get that. Bath fans and box fans both use at least that much power, have zero energy recovery, and are either not targeted to the bedroom or are much louder. Cracking windows has zero energy recovery, relies on the highly-variable stack effect, and lets in too many bugs. A proper whole-house ERV, CERV, or Santa Fe dehumidifier with an outside air intake would be ideal, but also difficult to do in a rental. Someday though when I am a homeowner again!

  10. frasca | | #14

    Just found out that Awair has a nifty export feature, and I was able to do some work in Excel comparing CO2 levels pre- and post ERV. My sample sizes aren't huge but it's pretty clear that the little Panasonic unit has helped reduce the CO2 concentrations. The boys' room is now well below 1000 ppm at all hours of the day. I'll call that a win!

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