THERE'S A SOLAR HEATED HOUSE ALIVE AND WELL IN PRESCOTT ARIZONA

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The Winter heating load requirements of the Grieve house are considerable over a six-month period (Degree Days in December, January, and February this year were, respectively, 818, 865, and 686). Heat loss for the, 2,100-square-foot residence in January was 650,000 Btu per day.

This "problem", however, is offset by the fact that more than 80% of the days are sunny (which, of course, is good for solar heating systems) during the coldest weeks of the year out here. Large south-facing windows-that allow the sun to warm a thick, heat-storing concrete floor in the building-have, alone, contributed 250,000 Btu to the residence on a January day.

A total of 1,750 square feet is directly heated by the collector storage system (which is backed up by auxiliary electric radiant ceiling panels) in the Prescott solar house. The sun-powered system has two modes: (11 rock bin heating)

When we have a sunny day (and the south-facing windows mentioned above-are providing all the house heat we need during daylight hours), we're on rock bin' heating. This mode is controlled by thermostat which is wired to a remote sensor in the rooftop collector. The sensor turns on two one-half horsepower motors when the collector's temperature reaches 105° F, and shuts them off when it falls to 100°.

The motors (located on both ends of the storage bins in the basement) turn fans which draw warmed air off the top of the collector, through the return manifold, down a duct, and in and around the rocks which fill the insulated basement boxes. Once the air has been forced through the bins, it's returned through another duct to the supply manifold on the lower end of the, rooftop collector. It then picks up a new load of heat as it flows upward through the collector toward the return manifold from which it repeats the circuit all over again.

To change the solar system to its house heating mode which is done every evening and on overcast days-we manually switch to thermostat t2 and throw four dampers.

what is a degree day?

To estimate average heating requirements, we use the "Degree Day". Degree Days (DD) are figured from a base of 65° F outside ambient temperature. It is assumed with the Degree Day method that if the outside temperature is 65° F or higher, no heating requirements exist to keep the inside temperature of the home at 70° F. Each degree below a 65° F outside temperature is a Degree Day. For example, if in a given month the average mean outside temperature is 55° F, then each day has 10 Degree Days.

The above explanation of Degree Days is taken from Harnessing The Sun, an excellent small handbook which is subtitled, A Practical No-Nonsense Approach to Home Solar Heating. It is that and, at only $1.25 a copy, the book is a rare bargain. The publisher is Conestoga Graphics Publication, 1100 W. 38th Ave., Denver, Colo. 80211, and the book is available for $1.25 plus $ .75 handling and shipping from International Solarthermics, P.O. Box 397, Nederland, Colo. 80466. Thermostat t2 monitors the building's temperature and cycles the two fan motors mentioned above to maintain a preselected comfort level throughout the 1,750 square feet that are heated by the solar system. To do this, house air is heated by drawing it down through the storage bins. The air is then diverted by the dampers into ducts that carry it back to various areas of the building.

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