Cost-effective night purging techniques
We’re experiencing an unusual weather espisode in Melbourne. In March it’s now autumn (southern hemisphere) but we’re on track to have the longest period on record with maximum temperatures above 30C. This started a few days ago and is forecast to last for around 10 days. As an interesting aside, one should always be careful not to confuse weather with climate but this kind of anomaly is bang on the money for what the climate modellers have been forecasting for SE Australia for some time under anthropogenic global warming scenarios.
Our double-brick house stays reasonably cool for about 3 days when temperatures get above 30C. (Forecast for Wednesday is 37C and today it hit 36C in the CBD; we’re about 12km from the CBD; our summer temps are often a bit hotter and our winter temps a bit cooler.) After 3 days or so of elevated temps the thermal pulse has made it’s way through the walls and starts to seriously heat up the interior. As I write it’s 29C inside.
So to night purging.
There were some great posts a while ago on GBA about installing an attic fan to bring cooler air from the outside through the house into the attic. Appealing idea but this depends on the house either being leaky (which is something we’re addressing) or opening windows. We’ve been experiencing night-time temperatures in the mid to high 20C’s until the wee hours of the morning during this period. To get best benefit from the attic fan I guess we would need it to be thermostatically controlled driven by the difference between inside and outside temperatures. We would face the issue of having windows open to allow cooler air to be drawn in.
The manual night purging process at the moment is largely based around me getting up in a not particularly cheerful mood at about 6am and opening all the windows and doors. With a bit of a breeze – also something of a rarity at the moment with this persistent atmospheric high sitting to the south – this can cool the house 3 degrees Celsius or so fairly quickly. Of course, given the thermal mass of the house the interior temperature can rise once the windows are shut.
Another possibility would be some thermostatically controlled automatic windows which opened and closed by a controller driven by actual interior temperature, desired interior temperature and outside temperature. To make a significant difference as things stand we would need a number of such windows but I’m hoping that as we get some aspects of out house thermal performance a bit more under control even one or two such windows would be useful. This would help us either avoid installing an AC unit or at least getting a smaller unit than otherwise.
So does anyone have some recommendations or experiences to share about such windows? I understand they are available commercially
Thanks
David
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Replies
David,
An automatic ventilation system that performs night purging exists; it is called the NightBreeze system. Here is more infomation on the NightBreeze:
NightBreeze brochure
NightBreeze: Integrated Heating, Ventilation and Cooling System
NightBreeze Product and Test Infomration
NightBreeze Automates Ventilation Cooling
However, since the NightBreeze system costs almost $3,000, and is probably not available in Australia, it may not be conidered "cost-effective" or useful in your case. For cost effectiveness, it's hard to beat a whole-house fan and the window-opening routine.
I wrote an article on the NightBreeze that was published in the September 2004 issue of Energy Design Update. In that article, I wrote:
"The NightBreeze includes a variable-speed air handler with an ECM blower, a hydronic heating coil, a large motorized damper to control the intake of exterior air, and a wall-mounted control unit. The thermostat-like control unit also regulates the operation of the air conditioner, if any. In addition to providing nighttime ventilation cooling, the NightBreeze system provides year-‘round whole-house ventilation.
"In order to pull in enough outside air to cool the house at night, the NightBreeze requires a large exterior intake grille—usually about 3’x3’ (assuming the grille has 50% free area). If the air handler is located in the attic, the air intake grille can be mounted in a gable, in a doghouse dormer, or at the top of a false chimney.
"The air-intake duct is connected to a damper box containing a large hinged motorized damper blade. Damper leakage is minimal—at 25 Pascals, it’s about 1.4% of total fan flow. According to David Springer, the president of Davis Energy Group, one of the main consulting groups that developed the NightBreeze, “It is possible to achieve 6 percent duct leakage in a system that includes the damper.”
"...The NightBreeze sells for $2,850. The price includes an air handler equipped with a hydronic heating coil, a motorized damper unit, and the wall-mounted control. The ECM-equipped air handler delivers a maximum of 2,200 cfm, adequate to handle the needs of a 3,600-square-foot house."
Thanks, Martin. Looks like a useful piece of kit. They might not be available here at the moment but they could always be brought in by an entrepreneurial type. Same with the whole-house fans.
I note the comment above about year-'round whole house ventilation. If you had one of these NightBreeze systems installed, would this remove the need for an HRV system?
What would be really handy would be an article comparing costs, benefits and other considerations of various solutions in this space. Perhaps looking at HRV's, a whole house fan, the NightBreeze system, thermostatically controlled windows etc
Thanks again
David