Was wondering what the best way for possibly passive cooling on cool nights (in Colorado).   The interior might get to 80F degrees during a sunny day but at night the temperature drops to 55F.    When the windows are open when it's not windy, it takes a long time to cool.   I'm looking for a way to speed up this process.

Attic is unvented, so no whole house fan.   Have ERV systems, but that's recovering the heat. 

Any suggestions on how to do this?   Have 2 "flue sentinel" fireplaces, so was wondering if I could override them to open the flu on a cool night and use the stack effect to pull in the cool air?  Any suggestions for the simplest implementation would be greatly appreciated.

Thanks,

Jack

Some ERVs have a "bypass" mode which doesn't recover heat. Just ventilation.

The stack effect gets more powerful with height. The greater the height difference between the warm exhaust and the cool intake, the better it works. Do you have a two-story home or single level? Does the warm air pool in any particular place?

Yes - sorry - 2 2 two story chimneys I was thinking about. The one chimney on the main floor has a lot of windows so it'll be warmer in the summer.

The bypass mode sounds reasonable though - granted it requires energy to move the air, but at least less then running a condensing unit.

Sorry for the confusion - I'm in construction and haven't chosen the ERV system - the post is based off of my current house but trying to solve the problem before it starts in the new one.

Seems like UltimateAir's RecoupAerator (200DX) would work well for the bypass mode (they call it EconoCool). Thanks for the feedback, made me do some more research (feel free to recommend a different ERV if anyone has an opinion)

Seems like UltimateAir's RecoupAerator (200DX) would work well for the bypass mode

That's the unit I am using on this new build and that's one of the reasons I got it.

IF you are still building, there are a number of things you can do.

1. Put a small window high up in the area in which you expect hot air to gather.
2. Build a light-gathering atrium into your home and put a window, skylight or vent at the top. I have a 30 foot high atrium with a window right up at the top and the stack effect is powerful.
3. Frame a drywalled and insulated "chimney" into your home. You can even block it off at the bottom during the heating season, but open it up when it starts to get hot enough to use it. The top could be a 2X2 motorized skylight or be a cupola with a horizontal window, etc.

Thanks for the info. There is a small window in that room.

Ever hear of the Flue Sentinel http://www.fluesentinel.com/ ? It's a motorized flue for a gas converted wood chimney and keeps the flue closed unless the fireplace is turned on. I'm wondering if there's a way to have it open other than when the fireplace is in actual use (i'll have to investigate that one later). Should do the same as #3.

As far as the sky light - I will consider this. I hate dark entries in homes anyways Wondering how you open that window 30 ft high?

Jack

On a 2-story home, you will have much better success in cooling at night vs. a one-story home. One can build a "clerestory" feature which helps with the chimney effect to create flow through a home. For sure you have to install LARGE operable windows on each end of the home in order to achieve the cooling effect. Preferably large casement windows.

Do NOT install a roof skylight, that thing will really hurt your winter heating bills. Not to mention they eventually leak and are a constant worry during large hail events.

Wondering how you open that window 30 ft high?

Motorized opener. It's a small unit that replaces the regular crank mechanism. Small doesn't mean cheap. They run $350-$700. Skylights have motorized versions that don't have quite so much of a price premium.

Tricking your FlueSentinel to open is worth a try, but my sense is that you won't get too powerful a stack effect because of the nature of the flue and the fireplace opening location.

For sure you have to install LARGE operable windows on each end of the home in order to achieve the cooling effect.

They don't have to be very large. It is all about height and temperature differential. For example, the architect proposed two 4X2 awnings, for a total of 16 square feet, but upon engineering, we reduced it to a single 18" X 48" for a total of less than 6 square feet. Even at that, it is almost twice as big as it has to be. The draw is huge.

Preferably large casement windows.

I don't think it matters much what type of window it is. Mine are awnings. It is the size of the opening that matters.

Not to mention they eventually leak and are a constant worry during large hail events.

Quality skylights, installed properly, are not a source of leaks. I wouldn't buy the cheap plastic "bubbles" for Colorado. Quality skylights have tempered glass and even have security glass options.

Do NOT install a roof skylight, that thing will really hurt your winter heating bills.

There is heat loss to a skylight, but quality units should seal well and there is a summertime cooling benefit to be had as an offset.

Posted By ICFHybrid on 29 Apr 2012 07:25 AM

Preferably large casement windows.

I don't think it matters much what type of window it is. Mine are awnings. It is the size of the opening that matters.

Not to mention they eventually leak and are a constant worry during large hail events.

Quality skylights, installed properly, are not a source of leaks. I wouldn't buy the cheap plastic "bubbles" for Colorado. Quality skylights have tempered glass and even have security glass options.

Do NOT install a roof skylight, that thing will really hurt your winter heating bills.

There is heat loss to a skylight, but quality units should seal well and there is a summertime cooling benefit to be had as an offset.

Wouldn't an awning window basically be a casement window turned on its side?

The reason I suggested casement is because they are THE BEST design window type for air sealing. Sliders, single and double hung, will always allow for more air leaks just because of their design. Plus casement/awning/tilt & turns always offer greater opening areas of a similarly sized slider and single/double hung window.

From what I read, skylights are a huge NO-NO in the home energy world. They are viewed as being a hole in your roof. Whatever the summer breeze benefit it would offer would not offset the winter energy loss. Doesn't make sense to have a R-40 plus roof and then have an area of R-5. All the heat will just pool and chimney right out of your homes roof through the skylight. Heat rises and will be drawn by convection to the cold skylight area. Not to mention the condensation you will experience during winter.

Skylights are probably OK in moderate climates like coastal Hawaii or coastal California but I think they would be a bad idea for Colorado.

Tempered glass will still break, it just won't break into large pieces. Trust me, a baseball sized hailstone or even golf ball sized hailstones traveling at 80MPH will break a tempered glass skylight. It will shatter and stay in one piece which will help prevent water intrusion but it will need to be replaced. The glass in automobiles is tempered and in hail events they break pretty easily.

I'd look at an whole house attic or similar large fan mounted in a wall. With some type of thick foam cover/plug so it doesn't lose btus when not in use.

I have a large window fan upstairs (Air King 9166 20" 3 Speed) in the library in a standard single hung west window, and open the windows downstairs (at level basement) on the east side. This is mounted in the window, but the bracket it comes with allows you to shut the window without removing the fan. This really pulls the cool night air through the house. The fan is not quiet, as it moves a large volume of air and some might find it too noisy.

I like the idea of using the chimneys, but wonder if there would be enough temperature differential for them to draw.
-Rosalinda

I am in a single-story house in Colorado and do not need any fans for cooling at night as open windows work fine, but perhaps I am at a higher elevation than you at 7100 ft., and outside the concrete jungle. Back in the days before air conditioning, double-hung windows were used in let cool air in at the bottom and hot air out at the top. I have double hungs, and they work fine for summer ventilation. I have ceiling fans, but only use them when coming home from biking. If I needed more ventilation, I would just put a box fan in a window blowing outward. As Rosalinda suggested for a two-story house, an exhaust fan in a second-story window would work nicely if that is convenient.

Trying to vent through the fireplace might work, but you would also risk occasionally blowing ash back into the house during some windy conditions (which is 75% of the time in most of Colorado).

Most folks here in the this mountain town open windows at night, and then close them down in the morning when the outside temperature exceeds the inside temperature, or when they leave for work. In a well-insulated house with shades to block the sun, the maximum summer time temperatures that I saw last summer were about 76 F.

The reason I suggested casement is because they are THE BEST design window type for air sealing.

Yep, casements and awnings are better at air sealing. As far as an opening for stack effect cooling, it doesn't matter what kind it is. It's the area and location that count.

Reading about what is "best" isn't all there is to living. If you want a skylight, then put one in.

In this case, it was noted that if providing a vent was the sole purpose, then it might be a good idea to block off the shaft during the heating season. However, I'm pretty sure that most people will choose to have and use the natural light once it is in. In any case, the energy-free venting provided during the cooling season would offset a substantial portion of the heat loss. Colorado does have enough winter sun to provide a certain amount of insolation as well.

As far as being a "bad idea" for Colorado, skylights are used extensively there with good effect.

One thing to remember is that it is a A LOT safer to leave the 2nd story windows open all night on a 2-story home than it is on a single story. A friend of mine had an intruder come in through the open window on his first floor while he and his family slept upstairs. Luckily nobody was hurt.

On my current 2 story home, I only leave the 2nd story windows open at night, all the 1st story windows are closed. Too much of a risk in leaving the 1st story windows open at night while you sleep.