Pulling ambient air through forced air system - http://www.hvacmechanic.com/ Forums
Original message
| rkk
 |  "Pulling ambient air through forced air system"
, posted Thu 17 Apr 21:28  
Wondering if anyone knows of a product available for pulling cooler nighttime ambient air into my forced air HVAC system?
Here's what I'm trying to accomplish: our family likes to keep it cool at night to sleep but we don't like the dust, noise, and tiny gnats that seem to have a way of gravitating through the screens of open windows in the evenings. Consequently we run our AC a lot and in the spring and fall I'm sure there are many times we are running the AC because the indoor temp (from residual daytime heat) is higher than the ambient temp outside.
I know they make some sort of heat exchangers for people worried about their houses being too tight (to give several fresh air changes per hour) but I'm really just looking for a way to pull in fresh cool evening air, filter it, and circulate it without having to run the compressor on the A/C.
I'm guessing that one of the downfalls of doing something like this is the humidity that would be introduced at night, then the A/C would be running during the day to remove it and this might be bad for the wood trim, etc., in our house.
Somewhere I thought I heard that someone makes something like this, but I don't know what it's called, cost, etc. The simple solution would be to turn on our whole-house fan and open the windows, but as I mentioned above, the noise and dust are the reasons I get resistance from the rest of the family. I know you can get smaller screens for the windows to keep the tiny bugs out, but I don't like the darker look of those screens (from the curb or from the inside looking out).
Thanks for any advice.
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Replies:
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| vandvmechanical
| "Re(1):Pulling ambient air through forced air " , posted Fri 18 Apr 22:20
Hmmm. I would install an outside air duct (mentioned previously as make up air), with a motorized damper and temp. controller, and then install a home dehumidification system. Expensive, but problem solved.
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| dwcaveney
| "Re(1):Pulling ambient air through forced air" , posted Thu 17 Apr 23:54:
I think you have a terminology issue with the word "heat exchanger" (but that is another subject).
You could put in a "make-up air duct" from your return air plenum to the outside air.
Make sure to install a good damper in the duct so that you can close it tight when not needed.
Then just run your system with fan on. (no HEAT no A/C).
You may need a new thermostat if existing does not have a fan only option.
Another issue: I don't understand your concern regarding "the humidity that would be introduced at night" Cool air holds less moisture than warm air. Maybe you should get a book on thermodynamics.
Oh one more thing. What's more important, keeping the bugs out or the look of the screen? If you don't like the "look" of a properly sized bug screen, don't look at it!
DW
[this message was edited by dwcaveney on Fri 18 Apr 00:55] |
| | theduke03
| "Re(2):Pulling ambient air through forced air" , posted Fri 18 Apr 19:53
DW, It is true that warm air holds more moisture than cool air but when the air cools, humidity% will increase. In fact so much so that when humidity reaches 100%, dew will form. Here in CT where summers are hot and humid, I will not open my windows at night unless it's below 60 deg. The air blowing from an a/c vent is nearly at 100% hum. , this air warms when mixed with your house air and that is when the hum% drops.
In addition I don't think it's a good idea. In a residential setting the outdoor air would have to be below 60 deg. to be comfortable and at that point you just open windows. Commercial spaces use economizers that will automatically use outside air for cooling when it's below a preset temp. This is useful cuz commercial spaces get hot in mild weather so it's like free a/c. For a residence, open windows are free a/c.
Some of the pictures displayed here are brought to you by Houston204. Be Safe.
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| | rkk
| "Re(2):Pulling ambient air through forced air" , posted Fri 18 Apr 02:01
Good point that the cooler air is less humid, but let's say I want the house to stay at 70 and the A/C has been on all day (effectively dehumidifying the house down below 50% RH). If the outdoor temp happens to drop to 60 that night (where I could benefit from pulling in some of that cooler air), I think it could quite likely have a RH greater than 50% when it is pulled in? So one of my concerns is whether a straightforward solution like this would actually be bad for the contents of the house because of constant fluctuations in the RH.
I do have a stat with a fan mode and basically run that constantly in an effort to keep the conditioned (or heated) air circulated throughout, so that would work with the make-up duct.
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| | dwcaveney
| "link to air modeling" , posted Sun 20 Apr 01:18
rkk,
This link might help you determine the method to use for cooling your home at night.
http://www.bfrl.nist.gov/IAQanalysis/docs/IA02-4.pdf
DW
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| | dwcaveney
| "Humidity" , posted Fri 18 Apr 11:43:
I live in California and have never heard of the humidity "problem" you are referring to.
I suppose if I were a curator at a museum I might know something about the subject.
However, the simple answer to your question is: I don't know.
DW
[this message was edited by dwcaveney on Fri 18 Apr 11:45] |
| | Freon
| "Re(1):Humidity" , posted Fri 18 Apr 15:08
The relative humidity of outdoor air will increase at night. Your house air has a given relative humidity which, I suspect, is lower than the relative humidity of the night air (at the same temperature). If you bring in night air and thereby force out the house air that had a lower relative humidity (at the same temperature), then the moisture content of the house after a night of outside air will be higher. Then your AC will spend more energy (dollars) transferring latent heat (moisture) than it otherwise would. I think you might be better off running the AC at night.
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| | theduke03
| "Re(2):Humidity" , posted Fri 18 Apr 19:55
What he said^.
Some of the pictures displayed here are brought to you by Houston204. Be Safe.
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| | dwcaveney
| "Humidity" , posted Sat 19 Apr 00:27:
Duke,
By what Elmo says, it sounds like the OP might be able to go any number of ways, depending on the micro climate at his particular location?
IMHO, your old sig line was superior:
"You can't put a price on comfort! Be Safe."
BTW, I laughed like a hyena at that right hand, left hand post you made a while back.
Best Regards,
DW
[this message was edited by dwcaveney on Thu 24 Apr 11:17] |
| | dcdraftworks
| "Re(3):Humidity" , posted Fri 18 Apr 23:55:
For a given dewpoint and its corresponding absolute humidity, the relative humidity will change inversely, albeit nonlinearly, with the temperature. This is because the partial pressure of water increases with temperature – the operative principle behind everything from hair dryers to dehumidifiers.
Due to the increasing potential for a higher water vapor partial pressure at higher air temperatures, the water content of air at sea level can get as high as 3% by mass at 30 °C (86 °F) compared to no more than about 0.5% by mass at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, indicated by dry skin, itchy eyes, and persistence of static electric charges. Even with saturation (100% relative humidity) outdoors, heating of infiltrated outside air that comes indoors raises its moisture capacity, which lowers relative humidity and increases evaporation rates from moist surfaces indoors (including human bodies.)
Similarly, during summer in humid climates a great deal of liquid water condenses from air cooled in air conditioners. Warmer air is cooled below its dewpoint and the excess water vapor condenses. This phenomenon is the same as that which causes water droplets to form on the outside of a cup containing an ice-cold drink.
A useful rule of thumb is that the maximum absolute humidity doubles for every 20 °F (11.1 °C) increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 °F (11.1 °C) increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 70 °F (21.1 °C) and 50% relative humidity will become saturated if cooled to 50°F (10 °C), its dewpoint and 40 °F (4.4 °C) air at 80% relative humidity warmed to 70 °F (21.1 °C) will have a relative humidity of only 29% and feel dry.
By comparison, a relative humidity between 40% and 60% is considered healthy and comfortable in comfort controlled environments (ASHRAE Standard 55[5]).
Exerpt from: [http://en.wikipedia.org/wiki/Relative_humidity]
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DWCaveney, Inc.
Elmo
dcdraftworks@sbcglobal.net
CAD Drafting of Mechanical Systems
Marin County California
[this message was edited by dcdraftworks on Mon 21 Apr 17:38] |
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