Posted on Nov 14, 2012
By Jon Klima
Our off-the-grid house is located in the Colorado Rockies about 35 miles, as the crow flies, west of Colorado Springs at an altitude of about 8900 feet. I designed the house with maximum energy efficiency in mind. But I did not make it so tight as to need an air to air heat exchanger to bring in fresh air. The house faces due south although if I were to be able to do it again, I’d face the house somewhat, maybe 10 degrees, towards the east. This is only because of the local weather patterns that occur here, i.e., there tend to be, on average, more clouds in the afternoon than in the morning.
The house footprint is 1635 ft2 with the lower level tucked into the hill side. There is an unheated, 1348 ft2 attached garage on the north side of the house, half of which is for cars and the other half houses my work shop and the PV system electronics and batteries. Being a bit paranoid about wild fires, I decided to use James Hardy concrete siding and a metal roof. This would at least give the house a better chance of survival should a wild fire move through our area. The house uses both passive and active solar heating. We also use a wood stove for heating and use about 1 ¼ to 1 ¾ cords of aspen and ponderosa pine per year that I cut off of our property. Our final backup heat source is a propane fired boiler which is integrated with the active solar heating system to provide heat via the radiant floor heating system. Over the past two heating seasons the boiler has fired up for only about a total of 35 minutes to provide space heating. I’m estimating it has run for about 10 hours during the same two year period to provide heat for domestic hot water (DHW).
My wife and I are in our 70s so we keep the house warmer than when we were younger. The indoor daytime temperatures are usually in the low to mid 70s and the thermostats are set to keep the night time temperature at about 67 - 68 ºF. Averaged over the past nine years, which is about as long as I’ve been keeping temperature records here, our heating requirements have amounted to 8705 degree-days per year. Our coldest temperature was -23 ºF on Feb. 2, 2011. Since our last propane fill up on Oct. 1, 2008, usage has averaged 45 1/3 gallons of propane per year. We cook with propane, have a propane fired clothes dryer, a Kohler 10RY62, 10 KW propane fired generator (only started and ran for 10 minutes several times each year just to make sure it was still OK), and the boiler (Buderus Model G 124X-18) which, when needed, heats DHW and provides space heating.
Many people still remember the bottle and tire solar homes from the 1980s. Those were the exception, not the rule, but they got the publicity. Here are some pictures of the inside of our solar house.
The outside walls are constructed with standard 2 X 6 lumber and filled with fiberglass insulation. Before the sheetrock was installed I put up 1” thick isocyanurate rigid insulation and ran a bead if silicon calk where each sheet butted up against the next sheet. I also sealed every electrical box and any other penetration in the sheetrock with silicon calk. The ceiling, which has 12” of fiberglass insulation, got a 6 mil polyethylene vapor barrier before the ceiling sheetrock was installed. The interior walls are of standard 2 X 4 construction but are filled with fiberglass insulation to cut down on noise transmission from one room to another.
The mechanical room contains the 400 gallon stainless steel solar thermal storage tank (insulated with 3 1/2” of fiberglass insulation along with 2” of isocyanurate rigid insulation), the boiler, the 40 gallon DHW, and the controls equipment. You may have noted the six 4’ X 10’ foot solar panels (three of each side of the three skylights) in the first photo. These panels are mounted on the south facing (15/12 slope) metal roof. They are sloped slightly towards the sky lights so as to allow for draining the panels. This is a drain back system and only water is used as the heat collection fluid. All pipes must be sloped to allow for complete drainage of the water back into the 400 gallon tank when the circulation pumps turn off.
In order to save electrical energy, all heating thermostats (there are seven zones, three downstairs and four upstairs) are line voltage devices. This way, there is no 120/24 volt step down transformer to be consuming power 24 hours a day. All zone valves are therefore 120 volt valves and their end switches switch 120 volt power to the rest of the heating controls. Therefore, as you can see, all control wiring is contained in metallic flex conduit. The control system is designed to heat the DHW or space, as needed, first with solar, if the 400 gallon tank can provide the necessary heat, and if not, the boiler will fire up to do the job.
For those of you who enjoy these sorts of things, here is the plumbing diagram of our heating system. The controls are set up to provide heat for either space heating or DHW, but not both at the same time. If there is a call for space heating and DHW heating simultaneously, DHW has priority since it can be satisfied relatively quickly. The mixing valve shown is set at 120 ºF so as to keep the water going to the PEX tubing in the floor concrete at a reasonable temperature.
I chose a ground mounted array simply because I wanted easy access to the back of the array in case of any maintenance that might be needed in the future. The array tilt can be changed for optimum summer and winter tilt angles. As shown, the 2680 watt array is at the winter angle. And yes, I do need to clear the snow pile-up at the bottom of the array in the winter. I wanted to mount the array higher to avoid needing to do this but due to very rocky soil conditions I wasn’t able to drill a deep enough hole for the west side pier. Consequently, I felt I needed to keep the piers fairly short and the array close to the ground.
This past April I decided to not change the PV array to the summer tilt angle as I have in the past. I was curious as to how much less energy I would collect as compared to the average of the previous five summers. The energy decrease was only 6.1% and since I did not run short of energy during the summer, I’ll probably just leave the array tilt in the winter angle again next summer.
When we first moved into this house I had installed only 1200 watts (12, 100 watt modules) of Photowatt modules. My original design for a mounting rack and wire sizing took into account future expansion. Wit