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DayCreek Journal

February 7, 2003


Solar Heating Update

I was recently asked if I was disappointed in the solar heating system considering that the temperatures in the house were rarely in the 70's. I would imagine that anyone monitoring the house temperatures over the last two weeks, you might be drawn to that conclusion.

The truth is I haven't been working on the house over the last two weeks, so the house has been left alone to fend for itself. This has caused the backup heating system to kick in a number of times, but on sunny days the solar heating system has done its job too. Temperatures have typically been in the low to mid 60's inside of the house.

I can say that on most sunny days, the backup heating system will not be called into action overnight. There are exceptions to that, especially in extreme cold weather conditions. For example, last night produced the lowest temperature of the winter at -12 degrees (F). The backup heating system ran for approximately 4 hours. The backup heating system contains (2) 4.5KWH heat elements, so to compute my heating bill from yesterday: 4 hours x 9KWH @ .035/KWH = $1.26. One dollar and 26 cents is pretty darn good considering it was the coldest night of the year. When outdoor temperatures are closer to normal, sunny days produce enough heat to keep the house in the 60's without any supplemental heat. Of course, the sun doesn't always shine, so no matter how efficient the heating system is or how well the house is insulated, there will always be a need for supplemental heat.

Passive Solar
I have been equally impressed with the passive solar (sun through the windows) gains. The house being 16 sided, is not the best shape for passive solar, although most of the windows are facing southeast, south, and southwest. As I write this from our home in Illinois, I am monitoring the temperature inside of the house and at 11a.m., the indoor temperature is in the mid 60's and rising. The heat source during daylight hours is primarily passive solar. Regardless of the outdoor temperature (currently 3 degrees (F)), temperatures will rise into the upper 60's by afternoon. Shortly after the sun sets, the floor temperature over the sand bed will begin to rise.

Thermal Flywheel
I have to say that the sand bed has presented a few surprises. I had been told that sand beds take months to heat up. I was told that I should start charging the sand bed in late August because it would take months to heat up all that sand. I am convinced this is not quite true. Why? After monitoring the temperatures for months now, it has become apparent that it only takes a matter of hours for the heat to radiate through the sand bed and out through the floor. There is no doubt that heating the sand bed for a week or so, will increase the ambient temperature of the sand, but I can't see heating it in August. Maybe late September?

The chart shown below is a good example of what occurs during and after a sunny day. The fluid inbound to the sand bed peaks at midday. Hours after sunset, the floor temperature above the sand bed begins to rise and continues to rise overnight. Out of pure luck, the maximum solar heating occurs overnight when it is needed the most. I couldn't ask for a better scenario. By the time the sand bed temperatures are decreasing, the sun rises and starts the passive/active cycle all over again. The system is quite balanced.

The graph shown above shows that 6 to 8 hours of solar heating, produces a subtle heat that raises the floor temperature by a few degrees that lasts for over 24 hours. During periods of multiple sunny days, the sand bed temperature slowly stair-steps. So far, the highest floor temperature recorded over the sand bed has been in the low 80's (F).

If you were to compare this system to an active solar heating system that uses a water tank to store the heat, I would imagine that the heated water tank would produce warmer floor temperatures, but for shorter periods of time. As I have previously stated, I may switch to a water tank system somewhere down the road, but so far I am quite satisfied with the results.

The sand bed produces a subtle heat that under extreme cold weather conditions will require supplemental heat to bring the indoor temperature into a 70 degree(F) range. Under milder winter conditions (outdoor temperatures in the 20 to 30 degree range), the combination of passive and active solar heating provides the majority, if not all of the heat required to keep the house at comfortable levels (low 70's (F)).

The verdict is still out though. I've only been using the system for a few months and the house is not completely built yet. I also have a concern that over time the sand bed might dry out and lose it's capacity to store Btu's (Based on conversations with Steve Krug.). Like everything else with this house, it's one big experiment that has yet to prove itself. But...so far, so good.