Do people use whole house fans (like Tamtech HV1600) in high performance homes?  We live by ours in the summer here in central VT where it seems to dip into the 60s most summer nights.   I love a cold bedroom and lots of fresh Vermont air.   What I've found is that if I leave the downstairs windows and doors closed, and open windows/door upstairs, I can get the upstairs cold for sleeping while keeping the downstairs relatively warm which is comfortable when you wake up. 

But I don't know if it's needed in  a high performance home (R43 walls, R80 roof, etc.) with mini split.  I suppose you could turn on the A/C for a few minutes ?

The Tamtech  whole house fans best automatic doors are "only" R50. 

Alternatively could we possibly just run higher power Panasonic bathroom fans?  Those go up to 130CFM. Three of those would be 390CFM.  Two of them would be upstairs on a ~900 sq ft. level.

Opening windows and utilizing stack effect for passive cooling or running mechanical ventilation (e.g., ERV/HRVs) works very well for cooling high performance homes in low humidity climates that have a significant diurnal temperature variation. When closed up, high performance homes don’t heat up very much during the hot daytime hours, but can be quickly cooled down during the cold evening/nighttime hours. If passive or mechanical ventilation will be your cooling strategy, the temperature gradient that you will need to address should be considered when determining the insulation specification for your home.

If you are located in a non-diurnal climate where the temperature is continuously above comfort level, but the humidity is continuously below comfort level, you may be able to use an evaporative cooling system. If you are located in a non-diurnal climate where the temperature and the humidly is continuously above comfort level, you will most certainly need an air conditioning system which might make a mini-split the preferred HVAC solution.

The stack effect is the greatest. If I wanted to use it upstairs without affecting the main floor, i would try to do some kind of a vent system that could be closed off in the Winter. Of course, you'd probably need to have a sealed upper floor, too. The door that sealed it might even need weatherstripping. The stack effect is powerful.

A potential advantage of fans is that they are easier to automate. You probably don't want to open the windows at 9pm because it's too hot, then get up a 3am to close them because it's getting too cold.

How fast one can cool down a house (in degrees/hr) by opening windows is a function of internal thermal mass (and temp/flow) and isn't assisted by good insulation.

If you want the evaporative cooling provided by moving air, ceiling fans are easier.

Relative to using passive solar cooling stack effect versus using active ventilation cooling, we will always use the building's architecture design to take advantage of any passive solar stack effect to the maximum extent possible. However, some high performance clients rightly avoid multi-story buildings, high ceiling styles, and large vertical corridors which are great for maximizing the passive solar stack effect, but unfortunately also tend to increase the overall heating/cooling requirements.

We prefer and we always recommend active ventilation cooling accomplished by using an ERV/HRV system even when we can take full advantage of passive solar cooling stack effect. While we have successfully designed automated window opening/closing systems and automated drape opening/closing systems at the specific request of clients, it is really hard to beat the operational simplicity of an autonomous ERV/HRV system and a properly designed passive solar roof overhang. The energy usage and associated cost of operating an ERV/HRV system the small amount of time that is necessary to actually cool a building in a diurnal temperature, low humidity climate is negligible. Furthermore, high performance buildings will require active ventilation anyhow and these buildings will preferably use an energy efficient ERV/HRV system to accomplish this.

More exterior insulation reduces the rising temp rate of the building interior during the hot daytime hours...this is goodness. More interior thermal mass buffers and reduces the rising temp rate of the building interior during the hot daytime hours...this is more goodness. So unless you like large temp swings, you would be a fool to do otherwise...

…and to complete the aforementioned insulation and thermal mass discussion/education…

A 2000 SF house with 9’ ceilings has a volume of 18,000 CF. 300 CFM ventilation results in 1 one air change per hour (ACH) in this example house. So 3000 CFM of ventilation will result in one complete house air exchange in 6 minutes.

If you quickly exchange the entire volume of air in the house that was at the original warmer indoor temp with air that is at the colder outdoor temp, the indoor air temp will quickly reach the outdoor air temp. So it isn’t very challenging to drop the indoor temp of a house to very nearly the outdoor temp in less than 10 minutes and this will occur whether you have a lot of interior thermal mass or zero interior thermal mass. Controlled ventilation and uncontrolled infiltration can quickly exchange BTUs if there is a significant delta T between the indoor and outdoor air. This is why high performance houses are well-sealed…you don’t want uncontrolled infiltration…you only want controlled ventilation!

It is true that if you then stopped the ventilation after a very short period of time and you have a lot of interior thermal mass that is still at a higher temp than the outdoor air temp, the thermal mass will begin to slowly heat and warm the indoor air. However, since the thermal mass is still at the initial indoor temp, the worst case scenario would be that the thermal mass heats the indoor air back to the initial indoor temp...which should have been a comfortable temp if there was adequate insulation.  Furthermore, you would not allow that to happen anyhow. You would continue allowing the ventilation to occur at a lower rate that maintains the indoor air temp at the lower temp that you desire and which also allows the thermal mass to drop in temp so it is ready to buffer the indoor temp again when the sun comes back up.

Posted By sailawayrb on 23 Sep 2014 01:36 PM

If you quickly exchange the entire volume of air in the house that was at the original warmer indoor temp with air that is at the colder outdoor temp, the indoor air temp will quickly reach the outdoor air temp. So it isn’t very challenging to drop the indoor temp of a house to very nearly the outdoor temp in less than 10 minutes and this will occur whether you have a lot of interior mass or zero thermal mass. Controlled ventilation and uncontrolled infiltration can quickly exchange BTUs if there is a significant delta T between the indoor and outdoor air. This is why high performance houses are well-sealed…you don’t want uncontrolled infiltration…you only wanted controlled ventilation!

It is true that if you then stopped the ventilation after a very short period of time and you have a lot of interior thermal mass that is still at a higher temp than the outdoor air temp, the thermal mass will begin to slowly heat and warm the indoor air. However, you would not allow that to happen. You would continue allowing the ventilation to occur at a lower rate that maintains the indoor air temp as you desire and also allows the thermal mass to drop in temp so it is ready to buffer the indoor temp again when the sun comes back up.

The high-elevation desert SW is a perfect area for this to occur. We see 30-40 degree swings from sunset to sunrise during summer and even winter. It's not uncommon to see summer temps in the mid 90's at 7PM and then by 5AM it's in the mid 50's or lower 60's.

Do you recommend having more operable (tilt & turn/casement) than fixed windows in order to take advantage of stack effect? In other words, how big do the windows have to be in order to flush out the interior air?

Yes Lbear, we also often see 40-50F swings during our dry southern Oregon Summers. It isn't unusual to see 100F+ during the day and 45F at night.  A coat is often required after 9 PM when on the patio stargazing.

If you have a two-story house with significant stack effect, only a couple square feet of open window area will generate quite a lot of ventilation. Stack effect is surprising powerful at first when there is a significant delta T. As the the indoor air cools off, the delta T becomes smaller and the stack effect becomes less powerful...which is not a bad thing as this reduces the effective ventilation rate as you will likely desire.

You obviously need to have an open window ideally in the highest portion of the second-story, and more than one is better. You also need to have some other open windows on the first-story to get all the air moving and fully flush the house. There is some advantage to having casement windows so as to better capture any wind opportunities.

The only con with this approach is that you will need to manually open and close windows...unless you take the next step and automate this...in which case, casement windows are preferred for this.  I believe ICF uses this approach very effectively and can provide some guidance too.

more than one is better.

Yes, you need an opener at the end of the path for each "section" that you want to ventilate. But, as long as you can trace a pathway through all the areas you want ventilated, you only really have to have one. Like Sailor says, it doesn't have to be very large at all. In fact, it might even be a good idea to split a large window up into a smaller transom top that opens. It'll be cheaper than a big opener and it will get the aperture up higher. The higher it is, the more power you develop.

If I was doing a desert home, I would absolutely put a "tower" on it at least one story higher than the top floor with an opener or vent at the top to utilize the stack effect. Incorporate it with a "crow's nest", reading nook, or stick a clock on it, but it sure beats running a fan.

Yes, we have heard the passive solar tower approach works very well, but we have not experienced it first-hand.

i haven't had a lot of luck with this but maybe it is because we're opening too many windows along the way rather than just lower and upper windows?. It's not a very tall building, however the upper window we have open is about 12 feet above 2nd floor floor level...maybe the temperature difference isn't enough.

The house we're building is just a regular old 2 story shoebox so I'm not sure how much potential it will have.

You can use this to estimate flow rates vs size. More than some think, especially when you don't want to wait for higher temperature deltas(later into the night) and you need to overcome thermal mass effects (yes, heat transfer from this source is significant, even at 3000 cfm).

i haven't had a lot of luck with this

It has to be the highest point without any other areas for the warm air to pool. And, yes, you do not want to open windows along the way. Same problem crops up if you do not have a sealed envelope.

Sounds right. The initial temperature differential creates the initial pressure differential that starts the air to initially move and flow. Having the air flow via one dominant path would tend to maximize the rate and duration…a body in motion tends to stay in motion…

I hadn’t given much thought about the effect of excessive building envelope leakage on stack ventilation, but I can see how that could degrade the performance. In fact, it is largely a similar effect that is responsible for uncontrolled infiltration. This is why it is more challenging to obtain well-sealed, low-infiltration, multi-story buildings. The taller the building, the greater will be the pressure differential (in this case, caused by a chimney effect resulting from wind blowing across the building). For a given building envelope leakage area (i.e., an area that was NOT completely sealed), greater pressure differential results in greater air infiltration.

The lofted ceilings of the 1970's and 1980's were gargantuan energy hogs because they were unsealed and poorly insulated. Nothing like having an extra-high temperature differential up there and losing that air to infiltration. Same with having some big skylights high up.

You can't believe how hot it gets up in my atrium. You cannot work up there, even on cool days without venting it. In summer, a power-driven awning window vents the whole place. The winter solution is the Big Ass fan which drives the heated air up there back down for recirculation.

I'll have to try stack ventilation when we get the right weather. In my case, I would open one of the basement windows and the stairs to the main floor, and open a window upstairs. I have a large double-hung that I could open the top half. We've been opening windows lately with the arrival of cooler and drier weather. It's interesting that the house heats back up when closed up. The other day, the house got down to 68 degrees. I closed the windows when I went to bed, and in the morning, the house was up to 72, despite it being 54 outside. I'm not sure where the heat comes from, other than two people and the only thing generating heat is a refrigerator. Thermal mass?

In the absence of insolation, thermal mass is about the only thing that would explain it. Is it ICF?

Right, interior thermal mass will tend to drive the indoor temp to whatever temp the thermal mass temp happens to be. Thermal mass will either cool or heat the indoor air. Thermal mass will either decrease or increase the amount of time that it takes the cooling/heating system to reach the desired indoor temp. This is why you need to carefully maintain the temp of your thermal mass to be close to your desired indoor temp...just to the other side of where you want to move the future indoor temp. This becomes more important as the amount of your interior thermal mass increases. Even a room full of furniture can add significant interior thermal mass.

Yes, ICF, which people say doesn't factor in as thermal mass because of the interior insulation.There's not much in the house right now; I guess the drywall would be the main contributor.

Our old house in FL was CMU, and it was slow to change temperature despite poor insulation and a very leaky envelope. The thermal mass was the obvious situation in that house; it was also on a slab. I would guess our new house is a combination of thermal mass and and a tight and well-insulated envelope. I'm looking forward to seeing how it performs in the winter.

Posted By jdebree on 27 Sep 2014 06:59 AM
Yes, ICF, which people say doesn't factor in as thermal mass because of the interior insulation....

But it really does factor in.

In the case you posted above, say the core of the wall heated up to 74F or higher during the warmest part of the day.  In the evening when you opened the windows when the outside air was cooler, you lowered the inside air temperature to 68F in a relatively short amount of time.  The mass of the concrete in the wall would not have dropped to 68F during this time - it probably barely changed at all due to the insulation on both sides of the concrete and low delta T.  Then during the night when you had the windows closed, the warmer concrete wall began to give up its heat to both the inside of the house and the outside and contributed to the rise in air temperature inside the house even though it was cooler outside. 

This is when the mass of the concrete in the ICF wall provides its greatest benefit - when the outside air temperature is swinging above and below your desired interior temperature.  The mass of the concrete tends to dampen the effects of the peak temperatures day and night so that the wall core temperature tends to stay within a few degrees of the average day and night temperature.