Our Tahoe Donner house is a vacation house, occupied only part of the time, like many houses in Tahoe Donner. After buying it, one of the first things we wanted to do was to ensure that owning a vacation home would not lead to doubling our carbon footprint (or more, given the Tahoe Donner climate). So, we looked for oppurtunities to save energy, and one of the most obvious ones was to control the heating settings when away.
Most normal thermostats can be set to a low temperature before you leave the house for some time, but there are a couple of problems with this. The first, and more easily solvable, is that many thermostats (such as the old and beloved round Honeywell ones) cannot be set below some temperature, such as 50F.
But the second problem is that you cannot simply set the thermostat on a mountain house to 40F and leave. The thermostat is going to maintain 40F in the house, but what about the crawl space? Will the pipes there freeze? Also, what about the humidity in the house? Will you find a cold and moldy house the next time you return?
The freezing pipes problem can be solved, to some extent, via the use of heating tape and insulation on the pipes. But it is hard to ensure that the heating tape heats all sections of the pipes that could freeze. Moreover, the heating tapes in my house consume 350W when on! — not something you may want to leave on all the time.
So, we started to look for a better solution. We decided to implement a control system that ensures not only that the house stays above freezing, but that also ensures that the crawl space does not freeze, that the humidity in the house is within acceptable levels, and that the pipe heaters are used only when truly necessary.
The good: we estimate that this system has cut by more than half our use of heating. It is not uncommon for the heater to switch on only for some 40 minutes in total, in a typical Winter day at Tahoe Donner.
The bad: you need to have several skills to pull this off. You need to be able to solder wire, to run wire through walls, to connect thermostats to a house heater, to connect the controller to a home ethernet network, and to configure the controller (we will tell you how). If you want to control the heater remotely, you also need to open up your ADSL or cable router to incoming internet connections, and you need to set up a dynamic DNS service so that you can access the controller state conveniently from a remote location.
If you do all that, however, you will save hundreds of dollars in heating each year, and more than that, you will know that your use of a vacation house has led to acceptable energy use!
We wrote these detailed instructions; we hope they can be useful to you!