The Auxilary Heat Systems
Since the solar panels are designed to provide only half of the heat demands of the house, it is obvious another source is required to make up the difference. This is done by a 20 kw electric furnace. ( A fireplace can also perform this task, more about that shortly.)
Note the original sequencers have been removed.
That function is now performed by the Digital Controller
An electric furnace was chosen over other types due to its ability to easily be sized by the controller to changing heat demands. A heat pump was considered, but at the time was prohibitively expensive and even by today's standards is only a marginal improvement over electric resistive heat in this climate. Besides, with the solar option, the heat pump is less efficient at the extreme low temperatures which is precisely when it would be needed most since that is also when solar efficiency is lowest.
The furnace is sized by selecting the number of 5 kw elements turned on at any given time. The controller measures the temperature in the solar storage assembly then, after factoring in the room temperature
setting, outside ambient temperature, and heat demand, determines how many (if any) heating elements to turn on. The requirement is to turn on enough to ensure the furnace output plenum is at least 105 degrees F. What this means is quite frequently the electric and solar sources are both on at the same time. However, often one or only two electric elements are on. Normally a house of this size (2800 square feet) would require 15 - 20 kw to remain comfortable, but since the return air is being preheated by the solar, the lower furnace output is sufficient. The reduced electric usage is responsible for the energy savings the system accomplishes.
The furnace blower is a dual speed type which allows the controller to adjust the volume of air flow based on temperature and demand.
A conventional central air conditioner is also incorporated into the system. At 20000 btu. it is smaller than would normally be required in a house of this size. (2800 square feet) The heat dump which was discussed on the collector page also contributes to allowing for a smaller air conditioner. Any time the system is in its cooling mode the storage assembly is bypassed by the air handler.
FIREPLACE - While not directly a part of the Solar Heat System, the fireplace is included here because its operation is handled by the same software as is running the rest of the heat system. The fireplace consists of a 2-Speed blower, outside combustion air supply, and the associated ductwork. The firebox includes a heat exchanger to improve its performance. Ductwork connects the fireplace to rooms both upstairs and in the basement. While from appearance it is a masonry fireplace, performance wise it more closely resembles a woodstove with large glass access doors. The page on the Control System explains how the fireplace is integrated into the Solar Heat System and how it affects the heat mode.