Replies

1. 
   exeric | Oct 15, 2014 06:20pm | #1

   Manny, I live about 100 miles northwest of you and it gets pretty hot here too. I think it could work very well, but only under certain conditions. I'm assuming, you didn't actually spell it out, that you are using a forced air ducted air conditioning system. I've been thinking and planning to do the same thing as you, but not with a forced air ducting system, but with a whole house fan. It would work on the same principle though and it sounds like I could get that thermostat to turn the WHF fan on and off.

   The basic problem with this setup is that it requires a fairly thermally efficient house to begin with. Very tight and reasonably well insulated. Along with that it would require a house without extreme southern or western window exposures, or shaded if they already exist. The reason this would be required is that for the cool night air to be most effective in cooling your house you would want to have the thermos bottle effect: cool air is brought in at night and minimal infiltration of heat during the day. In this way you could (theoretically) leave the air conditioning off completely during the day, perhaps augmented by ceiling fans. If you think of it the way I've described it the temperature chart in the house would look much like a rectified capacitor filtered power supply. However, if you have an inefficient house then it would become unbearably hot during the day and you would be forced to use the air conditioning. The result would be much less effectiveness of your plan.

2. 
   KeithH | Oct 15, 2014 10:11pm | #2

   Do you have an air conditioning that you use during the day? What's the humidity like in your area? I know Sacramento is mostly pretty dry. If it isn't dry at night in your area and you use a/c during the day you might introduce enough humidity into the house to cause your daytime a/c unit to run more.

   Also, how does this unit seal up in the winter? I know Sacramento isn't especially cold but presumably you don't want something leaking cold air directly into the heating system in the winter.

3. 
   exeric | Oct 15, 2014 11:07pm | #3

   California isn't humid at night, at least during the summer and shoulder months, except perhaps in marine locations. Definitely not in Sacramento.

4. 
   Busflier | Oct 16, 2014 02:34am | #4

   Hi thanks for the replies. I have two HVAC units that cool my house, 2.5 and 3.5 tons with flex in the hot attic. Sacramento is very dry...normally inside the house is 34% humidity. Today it rained a bit and it went up to 38% :). The nights are cool and dry. In July it can get to 110F during the day and then cool to 65/70F at night if we get the "Delta breeze". So inside my house is 81F and outside is great. So when I saw this "free cooling" idea I immediately though that it might work...but then again...I've been obsessed with HVAC for the last couple of weeks, reading all I can find, and if there is one thing I learned is that HVAC is completely counter-intuitive...at least with my intuition :). It seems to be like "opposite day".

   So it seems to make perfect sense on paper, but there must be a catch somewhere no? There is no info online about this "free cooling". Only HRV/ERV shows up in searches...and it is not the same thing.

   I have a whole house fan, and it seems to me that it is the same concept, only better. No need to open windows, the outside air is filtered, it is automatic, no noise (my whole house fan from the 90's is very noisy), no pressurizing the attic and getting hot attic air back in the house etc etc.

   When I found this blog I was sure they would mention this concept somehow, but I think it is not mentioned at all (other than HRV/ERV).

   I'm really interested in all your thoughts/opinions/advise. Thanks!

5. GBA Editor
   Martin Holladay | Oct 16, 2014 05:19am | #5

   Manny,
   This question comes up regularly at GBA. Here is a link to a recent thread on the issue: Adding outside air vent to existing HVAC?

   The equipment that I am familiar with is the NightBreeze unit (see the links in the other Q&A thread), which was developed by engineers at Davis Energy. (Good people.)

   There are ways to get this wrong -- especially if the dampers aren't high quality, or the blowers waste energy -- and I don't really know about the Venstar equipment.

6. 
   Busflier | Oct 16, 2014 03:57pm | #6

   Thanks Martin, somehow after all the searching I never run into the "nightbreeze" system, which is exactly what I was looking for. The Venstar is only a thermostat, but it included the fresh air cooling as part of the instructions, I assume in case one already has the system miraculously installed :). I was able to find a paper about the system and amazingly one of the two test homes was in Sacramento...couldn't be better than that! Back in 2003 and with tiered electric rates, it saved about $400 in a summer in Sacramento...for just one system (I have two).

   I have to look at the paper in detail to see if it would be worth it at my rates, which are lower than in the paper.

   Thanks so much for leading me in the right direction!