Post date: Aug 9, 2015 5:46:36 PM
The stucco on my house is essentially concrete. Concrete is formed when Portland cement reacts with water. This reaction (called hydration) is fast at first, and slows over time. Most of the hydration is complete in three days (for ordinary cement) and essentially complete in 28 days.
But when the stucco is in the blazing hot sun in an arid climate, the water evaporates before it can combine with the cement. Thus it is best to keep the new concrete wet for at least three days.
The floor slab was easy to keep wet. We just covered it with plastic sheet and the water could not evaporate. And we poured the slab in the rain in a cooler time of year.
But the walls cannot be easily covered, and the sun is hot now, and the wind drying.
So three times a day I climb onto the roof and spray the parapets and upper parts of the walls with water. I go back and forth, ensuring that the water has time to soak in. Then I go onto the ground and spray the walls again — especially the side with the sun shining on it.
Can I actually cool my house at night with the furnace fan?
My floor slab is comparable to 1 foot thick over a bit less than 1,000 square feet, or about 1,000 cubic feet. At 100 lb/ cubic foot, this much concrete slab weighs about 100,000 pounds. Assuming the specific heat of concrete to be 0.21 BTU/lb °F, it will require 200,000 BTU to raise or lower the temperature of the slab by 10°F.
Can I transfer that much heat to night-time air? Assume the slab is 80°F in the evening and the air is 60°F at night and I want the slab to be 70°F by morning and the thermal transfer is perfect (the air comes out at 70°F). The specific heat of air at about room temperature and sea level is about 0.24 BTU/lb °F with a density of 0.080 lb/cu ft, or about 0.0192 BTU/cu ft °F. Can I move enough air thru the slab at night to cool it by 10°F?
200,000 BTU/0.02 BTU/ cu ft °F means that 10,000,000 BTU are required to achieve this temperature change in the slab. If the fan and ducts can move 1,000 cu ft/minute, it will require 1,000 minutes or 16 hours to change the temperature 10°F. It seems feasible, because the fan can actually move 1,500 cu ft/min and I don’t think (from my stay in Taos) that the slab will rise to 80°F during the hottest days.
I received the LED strip. Saturday I spliced wires onto the ends of the cut pieces and tested them with an old plug-in 12V DC power supply. Those strips are bright!