Fresh air instead of AC in the mornings,how??

Discussion in 'HVAC Heating & Cooling' started by garyl53, Jun 20, 2007.

  1. garyl53

    garyl53 Engineer

    Messages:
    54
    Location:
    Colorado
    Hi All,
    I have a general HVAC question. I live in Colorado and even on the hottest days (90+) the evenings are usually cool (<70 degrees) but not until about 2 or 3 am. I have a setback thermostat that is set to raise the temperature to 76 deg. after 12 midnite until 4pm the next day when we return home (to save money). In the mornings it is not uncommon for the house to be 75 degrees inside due to the heat from the previous day but outside is 65 to 70 degrees. Is there a way to allow the HVAC to sense when the outside temp is low enough outside and have it start to draw outside air into the house instead of just recirculating the stale air in house? This would be like a car that has a lever for choosing to recirculate air or bring in fresh air only this would have to be automatic.
    Any ideas or solutions out there?
    Gary
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    Most residential systems don't work that way. Yes, it could be added but you're getting into commercial type controls, I think. Wait for one of the pros.
  3. Verdeboy

    Verdeboy In the Trades

    Messages:
    2,051
    I like the window fans that have a thermostat built in to them. It was over 100 today, but as soon as the sun went down, it cooled off enough to start sucking in the outside air. My electric bill is still only around $25/mo.
  4. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    ANYTHING can be done.

    A lot of commercial units have a dampered vent built in and a servo of some type to open/close it.

    Adding one to your resi unit is feasable but tricky.
  5. PEW

    PEW DIY Senior Member

    Messages:
    487
    Keep in mind that comfort is mostly measured in relative humidity, not temperature.

    Should you introduce moisture, there is the cost of removing it to get back to the comfort level. Moisture is absorbed by the contents of the home which can often end up costing more to remove vs keeping it low in the first place.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    There are specially designed heat exchangers that can help to add fresh air without losing as much heat or moisture transfer as just opening a vent. These are much more prevelant in Canada than here with their more stringent energy rules. They are more optimized to retaining heat than cooling the place off with external air, though. They temper the air differences so the fresh air introduced isn't much different in temp than what was exhausted. Still, might be worth looking into as some can help control humidity as well. I live in a condo, otherwise, I would have researched it more. External modifications are frowned on here.
  7. garyl53

    garyl53 Engineer

    Messages:
    54
    Location:
    Colorado
    My original question is something that would only see summer use. Since I am up at 5280 ft. and on the east side of the Rockies it is generally very dry air so I am not too worried about the humidity. In fact it would be nice to inject a bit of humidity into the home due to the dryness. I would think that a solution to use cooler outside air as part of the AC venting in this type of environment would be a good solution due to the lower night time temperatures. Many times I just shut down the AC and open a window, of course, but there are many hot days that things don't cool off soon enough to do this.
    Gary
  8. Rancher

    Rancher Guest

    Does your system have the ability to use "outside" air, much like the normal setting on a car, as opposed to the Max air setting that recirculates the air?

    If so and your A/C has a fan only setting, it probably uses the outside air, so you would need a differential thermostat, probably many are available.

    Here is a pretty neat presentation on what you are wanting.

    http://www.efficiencyvermont.com/pages/BBBD2004/docs/Luskay%20-%20Optimizing%20Rooftop%20HVAC.pdf

    Here is another:
    http://www.chps.net/manual/documents/2002_updates/HVAC.pdf

    Rancher
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,313
    Location:
    New England
    Commercial units can get quite complex with numerous sensors. In critical situations, they have reheat, where they cool things to reduce the humidity, then warm it back up so it is comfortable. A computer room or other places where the humidity must be maintained within specified range and temperature would be an example. To do this right, you'd want a control that monitored the outside temperature and humidity, and opened up a vent to the outside only when it was both cool enough and the humidity was low enough to provide cooling. You may not want it to drop the house temperature too far, so it might need to modulate, open and close to maintain not only a maximum temp, but also a minimum temp. On a really cold night, you probably wouldn't want the house to get down to 50 or so, which it could. All of this is doable, but the added complexity and cost may not repay your costs to just leaving the a/c on.
  10. Best query ever, Gary! I inquired about this a couple years ago, and I gave up calling manufacturers. They did not have a clear way to handle the simple idea of using nature's own air when it was right temperature and humidity. I think I hammered home a clear message, though. I let them know I'd be back. Time to go on the warpath again.

    David
  11. garyl53

    garyl53 Engineer

    Messages:
    54
    Location:
    Colorado
    Hi,
    Thanks for the links to the articles. I determined that what I am looking for is called a "Residential Economizer" that uses simple temperature sensors (versus humidity sensors) and they do make them! However what I read in several articles is that it probably would not pay for itself in savings since the range of temperature that it would "open" to let in outside air would be just between 65 and 70 degrees and that in a residential setting it would have minimal impact. For a commercial business where there is a lot of heat generating equipment or for schools where there are a lot of heat generating bodies the payback is much larger since you are using "free" outside air to remove the heat instead of the compressor when the outside air temperature is in the appropriate range. It might be worth it in terms of comfort improvement but I think it would be less expensive to just run the AC longer. Oh well, this sounded like a good idea on the surface:eek:
    Gary

    PS: What might be worth looking into is installing one of these instead of a whole-house fan. You wouldn't need to run arround opening windows to gain fresh cool air into the house, you would reduce the electricity use and not have to cut holes in your ceilings. It would be easier to install if you had a fresh air source handy to your HVAC fan!
    Last edited: Jun 26, 2007
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