The reasoning was that you need to pull the hot air off the ceiling in during the cooling season and cold air off the floor in the heating season. Ideally a system should be designed so that there is no air stratification at any time. That is why many recent designs have non-dampered grills placed specifically to create a balanced air circulation throughout the room. Many ranch style houses with heating ductwork installed in the basement work best by keeping the top grills open year round and opening the bottom for winter operation only. Likewise, most two story houses we recommend leaving the top open upstairs and the bottom open downstairs year round.
Heat pumps operate by removing heat from outside air and transferring it to the conditioned space.
To do this the outdoor coil needs to operate at 20-25 degrees below the outside temperature. Let s assume the air temperature is 45 degrees and the outdoor coil temperature is 25 degrees. This is well below freezing and as the system operates a layer of frost will build on the outdoor coil. This layer of frost can become significant if humidity levels are very high as is experienced during a period of freezing rain or sleet. Every 90 minutes of operation the system will check for frost and if present, will switch to defrost mode for a maximum of 10 minutes.