Paradise ponders: temperature moderation edition...

Paradise ponders: temperature moderation edition...  My office is located on the second floor of my barn.  It's well-insulated, and last year I installed a heat pump that both heats and cools it.  That heat pump works great, but there are a couple of less-than-perfect things about it.  First, when the weather is very cold (under about 20°F), it stops heating.  The only other source of heat in the office is a little woodstove – which works very nicely indeed, but of course only while I'm there to feed it.  So when I walk into my office after a very cold night, it might well be below freezing in there.  Secondly, in the summertime if I turn off the heat pump when I leave in the afternoon, by the next morning my office is very hot indeed.  So I've been leaving the heat pump running all night, spending the extra money to make it comfortable for me in the mornings.

The engineer in me says “There has to be a better way!”  A couple years ago I thought of one, and actually started to implement it.  This better way depends on the fact that my barn's first floor is heated, and never goes below 64°F.  It's a very nice heating system, using in-floor circulating hot water.  The first floor isn't air conditioned, but it is very well insulated and rarely goes above about 77°F.

So I came up with the idea of circulating air from the first floor through my office in two circumstances.  First, when the temperature in my office goes below about 60°F, and the first floor is warmer than that.   Secondly, when the temperature in my office goes above about 70°F and the first floor is cooler than that.  Last year I installed the fan required to do this, and started with the ducting for the fan's output.  My next big project is to finish that work: the fan ducting, the return and its ducting, and a controller.  The controller will be the fun part, of course. :)

Why not simply use a thermostat?  The challenge there is the thermostat would base its decision solely on the temperature in my office, but I need something different: to take the temperature at the ceiling of the first floor into consideration.  Of course I'm going to tackle that with a Raspberry Pi (diagram of proposed system above right)!  The parts are on order and should be here soon.  I'm going to put a temperature sensor in my office, near my desk, and a second on the ceiling of the first floor.  I'll use a solid state relay to turn the fan on and off.  The Raspberry Pi will implement the fan control algorithm exactly as I described above, and in addition it will act as a monitor for both the first floor temperature and my office temperature.  Once I have that all running, I'll be able to restrict my use of the heat pump to when I'm actually in the office (saving a few bucks a month), and I won't have to worry about the office temperature descending into the deep-freezer realm...