March 12, 2000
It All Together
The most trouble that I've had with the design has been designing the backup system. There are a number of ways to heat a house. I frequently had to tell myself to keep it simple! Let's start with the primary heating system - the solar system. (We're not talkin' planets here, although some of these system were world's apart.)
Heat Source: Solar
I have to be careful how I phrase this, because if I say that the solar heating system will take care of the majority of our heating needs, it will stay cloudy for the next 50 years. Good ol' Murphy's Law. So let's just say that more than 50% of our heating needs should be taken care of by solar heating. Here's the final design for the primary system:
(Click on the image to see a larger view.)
Although a water storage tank would give me greater control and greater heat storage efficiencies, the sand bed approach is simpler with no maintenance required. A water tank would need to be cleaned every so often and might eventually leak. No maintenance is a wonderful thing. Plus, the system can run itself. A DC pump will only circulate the heating fluid when the sun is available to heat the solar hot water collectors. It's simple and it's a proven design.
During the non-heating season, the majority of the solar collectors will be covered to keep the system from overheating, yet allow enough heat through to heat the domestic hot water. (This is one of the advantages to locating the panels at ground level.)
Heat Source: Wood Stove / Backup Boiler
Keep it simple. I have to keep telling myself this. I really wanted to heat water with some sort of a stove, but the more I thought about it, the more I shied away from the idea. The best way to burn wood is a full burn -- meaning that all the gases are burned. Temperatures need to approach 1,200F in order for a full burn, and newer, non-catalytic stoves can achieve this. This achieves a number of benefits. The wood burns very efficiently producing hardly any creosote at all and it beats the EPA's clean-air standards. It's good for the environment and gets more BTUs out of the wood and into your house.
Most wood boilers do not burn the wood completely and those that claim that they do have not passed the EPA's standards. The key is to getting the most BTUs out of the wood and a wood burning stove does the best job.
In the event that we are not home for an extended period, a backup boiler will kick in to keep the house at a relatively warm level if it's a cloudy day. We may also add a small space heater to heat both bathrooms if needed.
Another piece to the puzzle that I have neglected to mention so far is domestic hot water. An eighty gallon storage tank will be heated via the solar collectors using a heat exchanger. Water heated in the tank will then flow to an Aquastar on-demand (solar version) hot water heater. The heater is quite unique. The heater will vary it's BTU output based on the temperature of the incoming water. It is quite efficient and should work beautifully. If the temperature of the water is hot enough, (on a sunny day) the heater will not turn on at all and if the temperature of the water is luke warm, it will heat it up just enough to the proper level.