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Community and Q&A

Exhaust-only vs. Lunos/TrueFresh in a ductless small home

doughpat | Posted in Green Products and Materials on

I am considering my options for ventilation of a relatively well air-sealed 750 square foot 2 bed/1bath home (heated via radiant floor heat with a mini-split backup/cooling). 

Budget is a major concern — if it weren’t, I’d go for the Lunos E2 system and mount one fan in each bedroom (probably the closet of each room), which are on opposite sides of the home.  This would probably be the best possible solution, as it would guarantee fresh air in the bedrooms, high efficiency, and low noise. But, wow, $1100 — thats a lot of change compared to an exhaust-only system. 

I hate the idea of spending all of this time/money to air seal every little crack I can find, only to stick a bath fan in there and suck air through whatever cracks remain. I understand that 20 or 40 CFM is a relatively small amount of air, but still!  

I’m assuming the ROI of the Lunos system is probably painfully long….I can’t imagine the additional $1000 for this system is justifiable from a cost perspective. 

One other benefit that I can see of the Lunos system is that it seems like it would help to keep the house closer to neutral pressure when the range hood (400 CFM) is operating.  Yes, the air would potentially either “overspin” the fan, or “fight” the fan, but I would imagine they are built to tolerate this.   

Anyone have any advice here?  I even considered “pre-wiring” for the Lunos system (i.e. run a low voltage wire from future-Lunos-spotA to future-Lunos-spotB, and make sure line voltage is near each), but just putting in a Panasonic Whispergreen fan in the master bedroom walk-in closet  and “giving it a try” for a year to see if it performs satisfactorily.  Some drywall repair would be needed if I went to the Lunos, but in a walk-in closet, I’m not too worried about it. 


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  1. joenorm | | #1

    I just installed the Panasonic Whispercomfort Spot ERV. In a house as small as yours it might be a good option. I too, could not justify the cost of the Lunos and figured the Panasonic would be better than nothing. It was around $500 after I bought other supplies necessary for install.

    I would rethink having a 400 CFM hood in such a small, tight house, its too strong IMHO

    1. doughpat | | #9

      Joe -- are you in a cold climate? I was under the impression an ERV would ice up in my climate (Zone 5B, Central Oregon--cold/dry winters, hot dry summers)

  2. user-2310254 | | #2

    If budget is a concern, you could skip the radiant and just go with mini-split.

    1. doughpat | | #8

      Yeah....the radiant was probably on the list of "could have gone without", but at this point, I'm committed (i.e. tubing is installed). Live and learn (though I'll live with warm feet, at least!)

  3. Expert Member
    Akos | | #3

    A small house would need somewhere between 40 to 60 CFM of fresh air.

    When it is 10F outside, it would take 60*1.08*(70-10)=3900BTU. My electricity is $0.16 so a COP 3 heat pump would consume ~400W (~$1.5/day) to heat that much air. In colder climates, there is a reasonable ROI on heat recovery.

    A decent exhaust fan still sets you back about $150. Since you don't know where the leaks are and where the fresh air is making it into the house, it still doesn't guarantee fresh air in the bedrooms, which is where you need it the most.

    The Whispercomfort ERV is a good budget option in climates that are not too cold. It switches to exhaust only mode bellow 20F.

    There is nothing wrong with a 400CFM in an small, tight, electric only house. You'll just get a pretty good whoosh when you open the front door while the hood is running.

    1. doughpat | | #6

      Thank you for doing the math on the cost of exhaust-only ventilation. $1.50/day seems way too high though....that'd be $45/month. If that's true, then I will definitely go for a heat-recovery system!

      My concern with the Whispercomfort ERV is that we have cold winters (zone 5B, Central Oregon, single digits happen regularly, though teens are more common) and I was under the impression an ERV can ice-up in those conditions. I didn't realize that it switches to exhaust-only -- I suppose that is to prevent that exact problem. Are you talking about this model, by chance? whispercomfort erv fv-04ve1

      By the way, I'm not sure if this changes anything, but this is not an electric-only house. It has a gas range, gas water heater. Ductless heat pump is mainly there for cooling, and as a backup source of heat.

      1. Expert Member
        Akos | | #10

        The ventillation costs really depend on your energy costs. On gas, it will probably less. In the warmer months it won't be that high, so the monthly average will be less. But definitely not negligible.

        That is the Panasonic unit I was talking about. A better option for cold climate is their Intellibalance unit, much higher efficiency and you can eliminate the bath fan.

        All cold climate ERV/HRV have controls to prevent core ice up. ERVs transfer some of the moisture from the exhaust to the intake, thus there is less moisture on the cold side, so are actually less susceptible to icing up than HRVs. Any unit that is rated for your outdoor temperature should work just fine.

        Depending on how tight your house is and whether the appliances are sealed combustion, the 400CFM hood can create trouble. It doesn't take much to depressurize a small house to 20PA which is usually the limit for sealed combustion appliances. Might be good to have provisions for a passive make up air intake.

        As others have said, a much bigger cost save is getting rid of the hydronic heated floors. Unless your are DIYing it, that is big budget item for not much benefit. You can get most of the heated floor benefits with strategically placed resistance mats for much less money and very little additional heating cost.

  4. Expert Member


    Exhaust only ventilation needs passive air intakes. On larger house they don't work very well as the exhaust fans still pull a lot of air from wherever they can, but in small houses they work fine.

  5. Robert Opaluch | | #5

    Besides delivering fresh air, an HRV (or ERV) will provide some saving some on heating and cooling, can filter the incoming air, and would make the room(s) a bit more comfortable thermally (incoming air temperature) compared to an exhaust fan. The exhaust fan doesn't help these factors. Some of this involves health and comfort, not just $ savings on heating.

    Is anyone sleeping in the other bedroom? Especially given your SQFT, the other bedroom will likely run lower on oxygen level (with only an exhaust fan in the main bedroom) if someone is sleeping there with the door closed. The Lunos system will keep fresh air in both bedrooms, and especially during the day, that fresh air will migrate into the larger living space.

    An HRV/ERV upgrade may not be free, but its benefits, given the cost of the project...

    I also agree you might consider going with only a minisplit and no radiant floor heating if money is a concern.

    Have you considered cheaper HRV/ERV alternatives?

    1. doughpat | | #7

      Thanks for everyone's help here.
      The radiant floor heating is already installed -- and yes, it is "out of place" in terms of budget. It was/is a luxury that was "ok" until we ran into major overages during utilities connections. Anyway--what's done is done and I am just trying to make up for things where I can.

      And yes, we do want the ability for someone to sleep in both bedrooms. Unfortunately for me, I think that the Lunos system is the appropriate (though expensive) selection. It will guarantee air flow through the bedrooms and help save on energy costs.

  6. Expert Member
    Dana Dorsett | | #11

    >"...I’d go for the Lunos E2 system and mount one fan in each bedroom (probably the closet of each room....),"

    That would pretty much ventilate only the closets. The Lunos e2 runs in one direction for about 70 seconds, then reverses. In that 70 seconds even at high speed it doesn't exchange even 1/4 the volume of a closet, and unless there's a pretty good door cut there would be even less exchange between the room and the closet. To ventilate the room the Lunos is counting on good air mixing when pushing outdoor air in, so that it's not just exhausting barely-diluted same air that it just brought in when it changes direction.

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