Adding outside air vent to existing HVAC?
Hi all,
I live in Los Angeles and dislike AC. We have AC on the second floor of our home, but in the evening when the outside air is down to a lovely 68-70 degrees, our inside air is still 85. My wife wants to put on the AC. I want to bring in outside air. But simply opening the windows doesn’t really work well. What I really want is to integrate an outside air vent into our HVAC system, thus pulling out the inside air and replacing it with outside air. Trouble is, I don’t know even what this would be called, so I can’t seem to find any information on it by googling.
I bet someone here knows the right term of this and if it’s even possible to integrate with our existing HVAC (which, btw, is ducted through a burning hot attic. Lame).
Thanks,
Franz
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Replies
Franz,
The technique you are talking about is called "night flushing" or "night ventilation cooling" or "night purging."
There are at least three ways to do it:
1. You can open your windows. That method isn't working well for you.
2. You can install and use a whole-house fan. Here is more information on that technique: Fans in the Attic: Do They Help or Do They Hurt?
3. You can install a NightBreeze unit -- a type of fan that distributes outdoor air through your HVAC system's ductwork. For more information on the NightBreeze, see:
NightBreeze
Cost-effective night purging techniques
Supply-only whole house ventilation
One final note: the main problem that you are experiencing appears to be heat gain through a poorly insulated ceiling, exacerbated by the fact that your ducts are located in an unconditioned attic. The best use of your retrofit money would probably be to address these two issues by creating an unvented conditioned attic. For more information on this issue, see Creating a Conditioned Attic.
Martin's answer is terrific as usual. One more note: This isn't a good idea in all climates, because the early evening air is very humid. Bringing it in can exacerbate any moisture problems, and when you turn on the AC again the next time, it will have to work extra hard to remove the humidity. Probably not a problem in LA, but I don't know that climate well. Often the early morning air has lower absolute humidity than the evening air, so doing the purge in the morning is sometimes a better idea.
LA's average summertime dew points are at least a half-dozen degrees lower than typical right-coast cities, and nighttime ventilation works fairly well, with low (or even negative) latent loads 90 days out of 100.
http://weatherspark.com/#!dashboard;a=USA/CA/Los_Angeles
There are those few days when dew points at night are pushing 70F, but on those days the nighttime temperatures aren't as favorable for managing the sensible load either.
Hi all,
Thanks for the replies, Martin, Charlie, and Dana. LA is usually a good place for night purging. This summer we had some high humidity evenings, but that's the exception, rather than the rule. So I'm keen on making this work. As Martin points out, my particular problem is a very poorly ventilated and very poorly insulated crawlspace. It's a bit better with a Title 24 white roof and solar panels; they've cooled the upstairs down by at least five degrees on hot days, but in the evenings that residual heat is still considerable, with inside temperatures being 15 degrees above outside temperatures after sunset.
I appreciate that the ideal solution would be to include the crawlspace as part of the insulated interior of the home, and replace all our ancient, double hung, single pane, wood windows. Given the solar roof, the realistic way to make the ceiling work happen is from underneath and that requires us moving out while the work is done. Plus, windows are quite expensive. This sounds like too much money and trouble to throw at a relatively modest problem. So I'll look further into the alternatives Martin proposes. The Nightbreeze sounds good. But also, as I very rarely use AC during the day, it seems I could get a kind of two-for-one deal by installing an attic fan. While Martin has shown that an attic fan doesn't make sense for many people, perhaps I'm the exception. A powered fan (probably solar) would pull out my hot attic air, replacing it with a fraction of outside air from the tiny vents and a fraction of upstairs air from my ceiling leaks (I could also open the hatchway to the crawlspace, of course). That upstairs air would then be replaced by outside air from windows and other leaks. I wonder if that latter fraction would be large enough to appreciably cool the home.
Clearly, the advantage of the Nightbreeze would be the greater influx of outside air. But the advantage of the attic fan would be the cooling of the lingering heat source in the evening. I suppose I could also do both, but then we're again pushing the boundaries of the cost-benefit equation. Which plan would be better? I guess that depends on multiple factors.