The dampers include bidirectional 24VAC hysteresis motors with switches the close when the dampers are fully open or fully closed. The blowers use 120VAC
motors. The heat pump is a 3-ton unit. The rock boxes contain 35 tons of 3/4 inch rock, with space above and below the rock so the air flows equally through all the rock. The water heating system feeds a domestic hot water
preheat tank to reduce electric consumption in the electric water heater.Functions
The following discussion represents the results of the system analysis performed by Endecon Engineering, using design notes provided by
Mike Kliment as a starting point. The original controller simplified these functional descriptions somewhat, which slowed system response and prevented certain low priority functions from working optimally.
The control computer
must coordinate the operation of the system components to satisfy the needs of several different functions. The following functions are presented in order of decreasing priority. Lower priority functions may
not be able to activate (they are "blocked") if higher priority functions need to use the same components.
Zone Controller
In a small central space heating and cooling system of "conventional" design,
a furnace would supply heat and an air conditioner would supply cooling to the central ducts. If we treated the house as a single temperature zone, a thermostat would be connected directly to the furnace and air
conditioner. Expanding the system to include four different zones requires that a zone controller arbitrate between the (possibly) different requests from each of four thermostats. This arbitration process decides whether hot
or cold air is delivered to the central ducts, and activates dampers to disable delivery of that air to zones that did not request it.
In this system, any request for heat overrides all requests for cooling (since it is not
possible to both heat and cool at the same time, as there is only one central delivery duct).
Venting
This function uses blower 1 to vent the whole house (all zones) using outside air as the source. This function is
activated by turning any of the thermostat "fan" switches on. This function has a very high priority, but is still subject to the constraints of the damper activation speed.
House Heating
House heating is
performed by choosing the warmest available air and warming it if necessary using the heat pump. If the hot rocks are hotter than 95°F then air is drawn from the house return air inlet through those rocks, through blower 2,
and into the central delivery duct. If the rocks are warmer than the outside air, then the heat pump draws heat from the rocks and puts it into the house return air using the condenser coils in blower 2. If the rocks
are cooler than the outside air but warmer than the return air, the heat pump will draw heat from the outside air to heat the evaporator and will draw the house return air through the rocks before heating it with the condenser
coils. If the rocks are not significantly warmer than the outside air, then the heat pump will simply draw heat from outside air.
One final mode is used if the heat pump is running, the boxes are split (summer mode), box 1
is warmer than the outside air, and box 2 is warmer than the house air. In this (rare) case, the heat pump will draw heat from box 1 through the evaporator coils and draw return air through box 2 through the condenser coils.
House Cooling
House cooling is performed by choosing the coolest available air and cooling it if necessary using the heat pump. If the cool rocks are cooler than 65°F, then air is simply drawn from the house return air
inlet through those rocks, through blower 1, and into the central delivery duct. If the cool rocks are warmer than 65°F but still cooler than the return air, then the same air flow is used but the heat pump
is turned on and blower 2 is used circulate outside air past the hot side of the heat pump. If the rocks are not cooler than the return air, then the house return air bypasses the rocks, entering blower 1 through damper 1.
Note
that rock box 1 is only used for cooling if the "Summer" operating mode is turned on.
The 65°F rock temperature setpoint is a compromise between the typical target house temperature and likely available nighttime
outdoor temperatures from which the cold box must be "recharged". Setting it too low will reduce the effectiveness of the thermal mass in stabilizing the daily temperature swings, and setting it too high will raise
the minimum temperature to which the system can cool the house.
Heat Collection
This function is always active, since the system cannot circulate hot air through the domestic hot water pre-heater without also passing
that air through rock box 2. Since it only uses damper 1 and the circulating blower B3, and rock box 2 is always used for heating, there is no conflict between this function and any other functions.
Heat collection is activated
whenever the collector temperature is significantly greater than the bottom-draw duct temperature from rock box 2.
Water Preheating
This function is always active, since it does not interfere with any other functions,
and hot water is always needed. When the water heat exchanger coil is hotter than the preheat water tank, then the circulating pump is turned on.
Greenhouse Venting
This function is always active. When
the greenhouse temperature is greater than 85°F, the greenhouse vent is opened.
Cool Collection
When the summer mode is activated (the rock boxes are separated) and the outside temperature is significantly cooler than
the top of box 1, this function will attempt to deliver cool air into rock box 1.
This function will be blocked if house cooling or any use of the heat pump is needed.
