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Community and Q&A

Night flush/ Free cooling/ Economizing

ethant | Posted in Mechanicals on

First of all, kudos to the site redesign. My only last quibble is that the search function does not work for multi-word phrases.

OK, so we have a ~2000 sqft home under construction, pretty-good-home, maybe passive house level airtightness (we’ll see). I have currently specified the Panasonic Intellibalance (

I’ve been thinking a lot this summer about how I really live, and how mice fresh night air is. This has put me on a quest for an ERV/HRV with a night flush/economizing function.

Neither GBA or Google searches have helped me, maybe because I am using the wrong terminology. Basically, I would like the ERV/HRV to be able to bypass the core, particularly on cool/dry summer nights.

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  1. this_page_left_blank | | #1

    Assuming you are in North America, you have two options: Zehnder and UltimateAir. I have a Zehnder and am happy with it. Of course, you realise you can simply open the windows at night, right? That will actually give you bigger "fresh air" feeling than what the H/ERV will bring in.

  2. ethant | | #2

    Trevor, I do understand that I can open windows, but in my current (unventilated house) this still only works well with window fans, which are an uncomplicated version of night cooling ventilation. I am not sure the cost difference between the currently specified Panasonic and the Zhender/Ultimate Air. I also don't know the "value" of night cooling... I do know that I can open windows, and I know that my current house/apt layout has pretty bad cross ventilation on the second floor, which may be why the fans have seemed necessary.

  3. Expert Member
    BILL WICHERS | | #3

    Have you considered a whole house fan? These work well on nights that aren’t too cold or hot, and they can usually exchange the entire volume of air inside a house fairly rapidly. They also have the ability to bring in fresh-smelling air which I see really nice.

    Why use a fancy HRV for this function? They usually have much lower airflow. The only benefit I see for the HRV is that you could use it over a larger range of outdoor conditions.


  4. ethant | | #4

    Bill... That's interesting. I wonder where I would put a whole house fan... maybe there is one I could mount on a window? We have a pretty tight envelope and no attic... I see a whole house fan does 15~30 ACH... much higher than recommended .35 ACH, correct?

    1. Expert Member
      BILL WICHERS | | #5

      Normally you’d install a whole house fan in the ceiling of your uppermost floor, usually in a central location like a hallway. The fan needs to be able to pull from the house, so you don’t want to install it in a room with a door, for example. Without an attic the install would be more interesting. Normally whole house fans exhaust into the attic, and the air is blown through the attic and out all the various attic vents that normally ventilate the attic. You’d open windows on the lowest level of the house while the fan was running to allow the fan to bring fresh air through the entire house.

      If you have no attic in which to install a whole house fan, a window mounted fan might be your next best option. Aside from the window mounting location, operation would be essentially the same as for the attic mounted fan.

      Yes, whole house fans provide much higher air flows than an HRV — they serve different purposes. HRVs serve to minimize lost heat energy while providing fresh air into a space. The air they provide is to keep people comfortable by keeping the air quality up for breathability. Whole house fans are intended to bring in cooler outdoor air quickly to cool off a house instead of needing to run an air conditioner. You never run your whole house fan during the heating season, for example, and that’s why insulated doors for these fans are a selling point.


  5. keithhoffman22 | | #6

    I popped onto GBA to ask this question and lol, Ethan has recently beaten me to it.

    First, I wanted to say that Ethan didn't enumerate the multitude of reasons that opening windows might not be an acceptable solution.

    If you are subject to these problems, opening windows is probably a poor option.

    Reasons to want non-window based ventilation:
    1. Loud neighbors. Windows provide a lot of sound protection.
    2. Your own privacy. Same thing, other direction
    3. Irregular night noises beyond neighbors (train whistles are loud even miles away)
    4. Security (though there are solutions to this)
    5. And my top reason, allergies. The best seasons for cool night air is when the days are warm. In a lot of climate zones, that's April/May, September/early October. In other words, peak allergen season. As a bonus, a given household might have one person allergic to trees (spring) while another is allergic to weeds (fall) thus eliminating window cooling for a huge percentage of the available time.
    6. For evening air, ozone is another problem in many areas even in shoulder season but I don't really know how to filter that.
    7. Last, I'm not convinced that running a ceiling fan and a box fan in every bedroom is less expensive than running a high SEER/ECM fan air conditioner. (I'd love to see data, maybe I'll work it up).

    Like Ethan, I don't have an attic for a traditional attic fan solution and it still involves opening windows and introducing unfiltered air into the house.

    So with that diatribe against "why not open the windows" complete, I'm really interested in options for night time air. My own research agrees that built-in bypass air support is limited to Zendenher and UltimateAir.

    The Zendenher seems really expensive (not sure, very non price transparent) while the UltimateAir seems to have terrible fan power consumption (am I misinterpreting this data? Looks like <1 CFM/w?).

    Comparatively, the Broan HE solutions offer multiple CFM/watt performance (depending on flow) but doesn't offer bypass mode.

    I've wondered, assuming you have a ducted ECM blower furnace, whether the cheapest installed and operated solution wouldn't be to drop air into the return air of the forced air furnace (presumably then passing through a high MERV filter on the furnace). With CFM/w in the 2.5 to 8 range (see, the furnace ECM blower is highly competitive with ECM fan HRVs. Obviously, that positively pressurizes the house (just as an attic fan negatively pressurizes the house). I guess I'd assume the positive pressurization will resolve through the kitchen hood and the presumably interlocked and off HRV. If not, then presumably you need to have an outlet off the furnace return to dump hot air (would that have to have an exhaust fan?).

    Of course, I'm assuming a house that requires night time cooling for sleeping and/or is tight enough to hold temperature throughout the day. If the heating loads and design needs are 3 pm to 9 pm, this nighttime cooling might be of limited benefit. However, given many people can tolerate higher temperatures for evening activity than for sleeping, the system still has potential merit for achieving a lower nighttime temperature than the thermostat setting.

    I think most of us here are aware that an old style PSC furnace motor drawing 500+ watts wrecks the whole scheme from a power consumption perspective. I'm only envisioning a system utilizing an ECM blower on a lower speed setting (say ~1 ton, 400-500 CFM)

    So building scientists: here are my questions:
    - What am I missing? Why isn't there a gold standard for free distributed filtered nighttime cooling?
    - How could I calculate comparative COP for such a scheme? How could I calculate COP for running an ac unit when outdoor temperatures are under thermostat temperatures? Maybe someone knows of a blog that describes the methodology.
    - Would such a system require a powered (i.e. fan) exhaust?
    - Does anyone have nighttime temperature data by climate? i.e. could a climate based rating be produced for the effectiveness of such a system? I suspect there is a lot more opportunity to use such a system in the Rockies than in Houston.
    - Is such a system a total waste in humid climates?
    - If you have a high SEER inverter driven AC central ac unit, are you better off just running the ac unit rather than moving air?

    Martin: If you think this question is better addressed separately as "whole house fan vs ecm furnace with ventilation air vs high seer ac" feel free to migrate it.

    1. Expert Member
      BILL WICHERS | | #8

      Regarding your reason #5, allergens, I come from a family with allergies. I’ve looked into this myself. What you’ll find is that indoor air quality is generally much worse than outdoor air quality. You may find that ventilating with outdoor air is actually helpful.

      I don’t think it’s going to be practical to filter out ozone on any kind of large scale. Activated carbon might be able to do it but I haven’t really checked.

      If you provide massive amount of makeup air by venting outside air into your furnace air return, you’ll pressurize your house. You’ll need to provide a way to let the extra air out somewhere.

      I would be VERY surprised if running ANY air conditioner would be more efficient than just using a fan. The air conditioner itself has fans, and even with the compressor only running a little it’s still, from a system perspective (all the fans and the compressor running) is pretty certain to use more energy than just air circulation fans which generally have pretty small motors.

      Regarding your pondering fancy systems, this is something I deal with at work. I actually design large telecom facilities (datacenters) where cooling is one of the dominating design parameters. My most recent project has a design cooling capacity of 1,500 tons. Anyway, the point is it is difficult to deal with relatively small thermal differentials. I hate that we use energy to pump energy out of our facilities at work. Why pay to cool one side and heat the other (office) side of the same wall? I’ve spent some time on this problem and everything comes out poor from an economic perspective. The waste heat isn’t hot enough to heat anything enough, so the costs to run a heat pump exceed the costs to just make heat. It’s frustrating. I’m considering using waste heat to heat the parking lot to avoid the need to plow or use salt in the future. I hate just blowing the energy away.

      The complex systems are too limited in application because they get specialized. It’s generally most efficient to just move air than to pump heat (ie a fan uses less energy than a compressor in a compressor or heat pump). In the mountains around Colorado, evaporative cooling works amazingly well. In Florida, where it’s very humid, it probably will make things worse. You need to think about the overall system and the application where you’ll be using it. Everything gets customized.

      Average night time temperature data is available from NOAA. It’s used in some kinds of thermal system design, although usually for cooling you’re more concerned with the average high.


    2. Jon_R | | #10

      Say a 250W fan can move 1000 CFM and outside air is 10F cooler than inside.

      BTU/hr = CFM x Delta-T x 1.08

      10800 = 1000 * 10 * 1.08

      Divide by 3.4 to get watts = 3176 W

      COP = W/W = 3176/250 = 12.7 (say about double what a mini-split might get in these conditions)

      Open windows do infinitely better, but they take effort and can over-cool. You should also account for moisture (potentially bad) and thermal (good) storage.

  6. GBA Editor
    Martin Holladay | | #7

    See the September 2004 Energy Design Update attached below. Look for the article titled "Nighttime Ventilation Cooling" -- it begins on page 8. Lots of technical issues here, including the need for a massive hole in your home's thermal envelope for the air intake vent, plus large duct sizes. The NightBreeze was a valiant attempt, but there was no market for it -- it is no longer being manufactured.

  7. brad_rh | | #9

    I have a forced air heating system with an ECM blower, and fresh air duct to the outside, and 2 dampers to switch the return air from outside to inside. I just run it manually, but for a couple hundred $ could probably make it automated. This works great where I am where it always cools off at night. I put some insulation and plastic under the louver to seal it up in the winter.
    The other issue with a whole house exhaust fan is pulling in dust. With the supply side economizer most of the dust gets filtered.

  8. LCA_Reaearcher | | #11

    Could you operate an exhaust fan upstairs without the window open downstairs? The HRV would then pull in more air then it exhausts to balance the pressure. This way the fresh air still goes through the furnace filter. May need to run at a lower flow rate to offset the higher pressure drop to keep a resonable efficiency though.

    1. Expert Member
      BILL WICHERS | | #12

      You could, but you may find that that exhaust fan overwhelms the HRV. HRVs are pretty low capacity in terms of CFM when compared to most window fans, and especially whole house fans.


    2. exeric | | #13

      I concur with Bill. One has to optimize all the different angles when attempting to use night flushing to cool a home. To reiterate what other have said, you need a tremendous volume of outside air in CFM to efficiently cool a home. The smaller the delta in temperature between cool nighttime outside air and inside air the harder it going to be to cool your home. I originally was trying to do something similar by using several large (8"" dia) butterfly valves in the walls in lieu of opening windows. It didn't work at all because my WHF couldn't reach the required CFM. Opening windows is really the only solution when running the WHF to cool a home.

      Even then it does not always work. During this latest heat wave on the west coast it's been a problem. The problem is by midnight the outside temperature may still be as high as 75F. After three or four days of triple digit temps the inside temperature can get up to around 85F by nightfall. If the night temperatures aren't low enough for a long enough time then night flushing doesn't work very well. Overall I can tolerate it but it isn't ideal in this kind of weather. The only reason I can tolerate is because of ceiling fans in every room. If you expect only two or three days of high temp then it works well. If it lasts longer than that then all bets are off. Of course, on the plus side I'm saving money and energy like crazy over my neighbors, ha ha.

      And again, night flushing only works when most of the windows are wide open at night.

  9. noequus | | #14

    Fantech has "makeup air" units up to 2000 cfm. These include an air filter (maybe MERV 13) and a damper. They seem appropriate for nighttime cooling (not just the kitchen exhaust make up air they seem to be specifically designed for).

    The number of ACH needed for nightime coolling is unclear to me, though it obviously depends on the outside air temperature and the mass of your house (the source of the heat that needs to be removed). Also depends on other things like the thermal conductance of the walls (interior and exterior; basically, the R value of the plaster/lath/framing or sheetrock/framing).

    Intuitively...30-60 ACH (20 year old DOE suggestion--not sure if its been updated...) seems really high--that's one every 1-2 minutes and the temperature gradient between the wall and the cool night air would be really low so not much heat removed per AC. Quiet Cool, which makes fans, suggests 2-3 CFM per square foot of house, which translates to something like one every 3-4 minutes (ie, 15-20 ACH; eg, see

    Tamarack makes whole house exhaust fans (blowing out, so incoming air is not filtered, like Quiet Cool) and 115 or 20 years ago if I remember correctly they were suggesting something like 800-1000CFM for maybe a 1500 SF (roughly 5 ACH) home. Now they say 2500 CFM (roughly 13 ACH) is "best".

    Having said all that...What is an appropriate number of ACH? Is Quiet Cool right? Tamarack? Still seems rather high to me but my intuition isn't very, we don't want that big a fan in our house (noise, and we have some space limitations).

    1. brad_rh | | #15

      You don't necessarily need a tremendous amount of air. How much airflow really depends on your conditions. Generalizations for this are worse than generalizations for heater sizing. After a hot day, in the 90's I can run my ecm blower at 300 CFM (0.14 CFM/sqft, 0.9 ACH) & never run the AC. I'm in the west, at altitude so the delta T from night to day is relatively large. For a WAG, rule of thumb, I would guess 2 ACH. That's more than I need.

    2. charlie_sullivan | | #16

      It really depends on the climate. Generally though, in low humidity climates, which are the ones where this makes the most sense, the temperature drop at night is big enough that you don't need all that much airflow. I think fans that blow air in, with a filter, make sense, especially if you have allergies, as the season for running these often corresponds to allergy season.

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