Crawl space-assisted heat pumps

By means of field tests in real houses, it was found that the source air for the heat pump can be preheated (or precooled) by drawing the outdoor air through the crawl space before it is used by the heat pump. The outdoor air is warmed (or cooled) by flowing over the crawl space earth. Most of the data is from winter months, when one would expect the most benefit for heat pump performance. Generally, greater preheating occurs at lower outdoor air temperatures; and, the effect tapers off to virtually no gain at about 10 C. The preheating effect has been shown to be sustained for an entire winter season. Another experiment proved that the preheating does come, in great part, from the earth and not simply from the house above. Analysis had also led to the conclusion that the heat transfer mechanism is by simple conduction within the upper 0.3 m of the crawl space earth.