Abstract: A vertically situated head exchanger is mounted externally of a building and insulated relative to the ambient air. Fan units are situated within the building preferably between adjacent joists and in the basement and in alignment with the connections to the heat exchanger. One fan unit draws fresh air from outside the building, through the heat exchanger and into the building. Another fan unit draws exhaust air from the building and passes this through the heat exchanger thus giving up heat to the cool incoming air and is then exhausted externally of the building. A controlled duct may draw warm air from the building and pass it through the heat exchanger to defrost same when necessary.

Abstract: Apparatus for sensing hydrostatic pressure in a fluid, forming a head for a bubble tube for immersion into the fluid. The head is formed of a plate having a flat lower surface for horizontal disposition within a liquid, a first orifice for introducing a gas under pressure from above the plate whereby the gas can form a bubble under the plate within the liquid. The effect of the plate is as a surface on which a stable sessile bubble of a variable size can be grown. This results in a bubble having a substantially increased radius of curvature at its interface, that is, the gas/liquid interface, on the underside of the bubble than previously obtainable. A pressure gauge measuring the gas pressure in the tube determines the pressure at the end of the tube within the fluid.