So, my plan was to be able to use outside air to cool the house at night during the summer without leaving doors and windows open on one hand, while being able to boost or shutoff air to the master bedroom depending on the time and boost HVAC air to the family room which gets the warmest but is also the hardest to cool since it's at the end of the line.
Boosting air is simply done by adding booster fans to the right ducts, but controlling the fans at the right time is where it gets interesting: I wanted to be able to decide not to turn on the booster fan to the bedroom during the day (waste of air) and not bother with trying to heat/cool the family room at night either since the master bedroom needs the air more than the family room.
The astute reader will note that this is usually done with dampers, but the furnace we have is older and would require a pressure return for extra air if only one room were targetted, and dampers just add more money and complexity when I already wanted booster fans. The trick is that booster fans can act as semi dampers when they're not running, which is good enough.
Now, the main goal was however to be able to pull air from the outside and bring it in the house. This was accomplished with the same 2 booster fans to create suction this time and 2 dampers do redirect the air from the furnace output to ducts that go outside the house.
that was a few parts :)
So, to cool down the house (or bring warm day air in during the day in the winter), I need to:
close the air coming from the furnace with a damper
open the air coming from outside with another damper (both happen at the same time)
turn on one or two of the booster fans to suck air from outside
make sure that at no time my HVAC system actually decides to turn on too, and if it does reset the dampers to allow air from the furnace (which is also the AC).
automatically do the above when there is a suitable temperature difference between inside and outside.
For normal operation, I have 2 HAI RC-80 and RC-90 thermostats, one for the house and one for the master bedroom. Depending on which one triggers, the booster fan to the appropriate zone should activate too.
Do you have a zone board that can do all this?
damper opened (notice the 2 small wires which got to my 1-wire 8 channel IO board)
If so, how many hundred dollars is it and do you need to call an HVAC person with proprietary software to reprogram it?
Do the inside and outside temperature probes cost $3 each, or more? :)
If you want to skip ahead, this is the misterhouse hvac code I was able to write to control the hvac system.
Implementation with Misterhouse
This is where misterhouse comes in again, I can program absolutely any logic I want and mostly with sensors and actuators that cost a lot less than typical overpriced HVAC parts.
The bits that came in play were:
HAI RC-80 thermostat which I can query for status (cooling/heating/HVAC fan on or off?) from misterhouse thanks to the code I wrote earlier (see my misterhouse HAI code). Actually I also use that code to know the temperature in the bedroom (which I could also get with a 1-wire sensor).
1-wire temp sensors inside and outside the house. This is obviously useful to know whether air outside is the right temperature for bringing in, or not (see the Temperature, moisture, humidity, and UV monitoring and graphing with 1wire devices, owfs, and cacti for details on the owfs setup.
1-wire 8 Channel I/O board which I use to control 24VAC to the dampers. One is normally closed and the other one normally opened. They each only need about 250mA to be held in the other position, so a single 500mA 24VAC power supply going through a relay on the 8 Channel I/O board is good enough. See my owfs 8 channel I/O page
2 X10 (or Insteon) remotely controlled plugs to turn the booster fans on or off.
Here's a picture of the 1-wire 8 Channel IO board
I then added some 1-wire temp sensors inside the ducts to confirm the temperature of the air going through them, and that's it. All that was left was some code, which you can find below.
When both duct fans are running, the power graph shows they use 300W:
Here is a temperature graph of cooling with outside air without using the AC showing a 5 degree decrease for dining room/family room:
and for comparison cooling with just the windows open, which I don't quite want to do anyway (compare the family and diving room temps which only go down by 2 and 1 degree).
For yet another comparison, the following link is a test with only the family room duct fan running (and not the bedroom one): that was a 3 and 1 degree decrease for the dining room/family room. Not as impressive.
Yes, booster fans do require some electricity to run (150W a piece or so), but for comparison, AC is 3000W just for the AC, to which you have to add 1000W for for furnace whole house fan and then the 150W for the booster fan:
At least, as a bonus, we do now get more cold air to the family room with the booster fan when we do run the AC, so we reach our target temperature more quickly.
Top/Hvac Damper Open to Close Video (click me)
Here is my misterhouse hvac script that controls the booster fans and outside air intake depending on inside, outside temperature and HAI thermostat status.