DEVELOPMENT OF CONTROL PROGRAM

VARIATIONS IN BUILDING LOAD:
The project house is well insulated, well shaded, and has very low infiltration. Most load changes are gradual and cyclical with the diurnal cycle. Even though the roof/ceiling assembly is well insulated, the ceiling surface presents the largest liability both in summer and winter. Its temperature can rise to 83.4°F (28°C) in the summer afternoon and drop to 64.4°F (18°C) at sunrise on a winter morning. As for the internal loads, the house is occupied by an individual (and a large dog) whose daily routine results in consistently minimal consumption. So, the internal loads have been predictable. Early load calculations showed a need for a cooling capacity of 60-70 W/m2

THE CONTROL SYSTEM:
In general, the best practice is to apply radiant heating from a floor panel. A warm floor provides two forms of heat transfer, radiation and convection. Heat rises by convection, thus providing a relatively even temperature distribution in the vertical plane. For a cool ceiling panel, similar effects are true. It absorbs heat by radiation and receives provides a portion of its heat transfer by convection. There is a saying in the comfort business: "Cold head, warm feet." The head, being the warmest exposed surface of the body, benefits from a cool ceiling when trying to lose heat.
Both the floor and ceiling panels can be provided with either warm or cold water. There are three zones that will allow for flow through either the floor panel only, the ceiling panel only, or both the floor and the ceiling panels together. If the operative temperature in a zone is above the set-point, the appropriate valve(s) will open and this will trigger the appropriate zone pump, which will circulate water from a 60 gallon buffer tank, through the appropriate panels, and return. The tank temperature is maintained by the heat pump; heating in winter, cooling in summer. The tank set-point temperatures were varied during a season based on the average outdoor temperature for the past day (seasonal temperature). A sensor that is buried in the middle of the garage's shaded adobe wall determines the seasonal temperature. This slow changing temperature rarely varies more than a degree or two over a one-day period and provides a good approximation of the average daily temperature.
Satisfactory operation of radiant systems, particularly for cooling, is not possible without recent developments in microprocessor driven control. For best results, the control should be divided into central control and individual zone control (Olesen 2001). The central control can adjust the supply water temperature according to the outside climate, while the individual zone control will control the flow of water to that zone according to the desired set point of the occupant(s).

EVOLUTION OF THE CONTROL PROGRAM:
The control program has evolved over the past two years (2 heating seasons and three cooling seasons). The control strategy is intended to use the mass of the envelope (walls and floor) for thermal storage so as to keep the spaces within the comfort envelope while using the minimum amount of on-peak electrical energy. Following is a brief recap of the evolvement of control strategies over the past two years. Further details are found later under "Performance."
Summer 2000. As the season progressed and the ground heated up, there was poor cooling capacity because of undersized ground loop. Unable to keep interior within comfort zone without use of ceiling fans.
Winter 2000/01. Used floor panel only. Adequate capacity. All electrical energy for heating was used off peak.
Summer 2001. Installed evaporative fluid cooler on ground loop that provided adequate capacity to keep interior within comfort zone. All electrical energy for environmental control was used off-peak. Kept supply water temperature above interior dew point to prevent condensation. As water temperature increased, used floor panel to increase cooling capacity.
Winter 2001/02. Adjusted supply water temperature linearly inverse with average daily outdoor temperature. Warmed ceiling panel in early morning to boost optemp just before "wake-up" time.
Summer 2002. In addition to the ceiling panel, the floor panel was used to take advantage of its thermal storage ability. It was cooled during the off-peak whenever its surface temperature rose above 73.5°F (23.1°C). Some on-peak energy was used to keep the op-temp below 78.8°F (26°C). The ceiling was operated late in the day. Its low mass cooled quickly and kept the operative temperature within the comfort zone with a minimum of energy consumption.

EFFECT OF MASS ON CONTROL:
The response of a radiant panel's surface temperature to adding or removing energy (by hydronic flow) varies depending on the mass of the panel. Both high mass and low mass components have their advantages. High mass provides for thermal storage (during off-peak times) that can reduce the on-peak requirement to run the compressor. But, its surface temperature responds slowly to changes in load. On the other hand, the surface temperature of a lightweight panel can be quickly changed with the application of hydronic flow and bring conditions back into the comfort zone.
Cooling with floor panels or ceiling panels?
Even though the roof/ceiling assembly is well insulated, the ceiling temperature can rise to the mid eighties by the start of the off-peak period in the cooling season. When this happens, the ceiling acts as a radiant heater and the effects can be difficult to counteract with a colder floor -- the lowest advisable floor temperature is 68°F (20°C) due to comfort limitations (ANSI/ASHRAE, 1992) -- when the ceiling's temperature rises to 83.4°F (28°C) late in the afternoon. The slow responding cool floor panel can take 8 hours to show a change after the start of the chilled water flow. In the summer of 2002, mildly chilled water to the ceiling panel was used to reduce the ceiling's temperature and restore comfort conditions. The tank set point is raised from the off-peak value to maintain 72°F (22.2°C) water so that minimum compressor operation is required. This quickly lowers the ceiling temperature from the 80's to the 70's. Initial data shows that this peak load cooling can be accomplished for about 1 hr of compressor run time.
Low Mass Ceiling Provides Quick Warm-up Response
The lightweight well-insulated ceiling panel is quick to respond (15-30 min) when either chilled or warm water is supplied. The ceiling has primarily been used for cooling, but on several occasions during the winter of 2000/2001 the operative temperature fell below the 68° F (20°C) set point, and as an experiment, the ceiling was activated with warm water. Within 30 minutes, the operative temperature was back within the comfort range. On the other hand, because of its mass, the floor's response to hot water flow is very slow. It could take as much as 8 hours after beginning the flow for the surface to respond by 1 degree. During the 2001/2002-winter season, the ceiling was activated from 5 AM to 7 AM with 85°F (29.4C) water (the "wake-up mode") when the ceiling temperature was below 70°F (21.1°C). The ceiling, even though well insulated, can drop as low as 65°F (18.3°C) in the early morning hours. Not a problem if you are under the covers, but provides a chill when you are getting out of your pajamas, taking a shower and getting dressed. Warming the ceiling surface to the mid-seventies tempers the master bedroom/bathroom zone for "wake-up" activities.
Floor Mass Provides the Most Storage Capability
It was intended to just use the ceiling for the summer load, however floor operation proved very beneficial during the high humidity periods when low water temperatures were not allowed because condensation would occur. When the indoor dew point was elevated to the 62°F (16.7°C) level (note: this was the highest observed dew point during the 2001 cooling season), the supply water temperature had to be raised to 62°+F (16.7°+C) to prevent condensation. At these higher chilled water temperatures, the ceiling capacity alone was insufficient to extract enough heat during the nighttime to allow the house to "glide" through the on-peak period without compressor operation. As the humidity rose late in the summer, the ceiling temperatures could not be brought down to as low a temperature. So starting on July 23, 2001, the floor valves were also opened at night. Floor temperatures that, over the first part of the summer, had slowly risen to the 80°F level were lowered to below 75°F (23.9°C) during the night. The additional capacity provided by the floor's thermal storage capacity was sufficient to keep the spaces within the comfort envelope (expanded zone by use of ceiling fans) during the higher humidity on-peak periods. As a result of this strategy, dehumidification was not required during the 2001 summer season. This supplemental floor cooling strategy was continued in the summer of 2002, and controlled using an upper surface temperature of 73.5°F (23.06°C). In other words, if the floor surface temperature was above 73.5° (23.06°C) during the off-peak period, the floor valve would open. Later in the summer, this value was lowered by a half degree to try to reduce daytime operation. The benefits of cooling the floor's thermal mass is shown in Figure 7. This compares 2001 performance with no-floor cooling with 2002 performance that utilizes floor cooling. The lower floor temperatures 2002 keeps the operative temperature within the comfort zone during the entire on-peak period.

figure 7

CONTROL OF TANK SETPOINT BASED ON SEASONAL TEMPERATURE:
A sensor located in the middle of the shaded south adobe wall of the unconditioned garage measures the "seasonal temperature". Over the 2000/2001 annual cycle, the measured temperature at this location has varied from a high of 95°F (35°C) to a low of 56°F (13°C). This variable is used to indicate the season. It triggers the heat pump's reversing valve to switch from cooling to heating, and is used to adjust the set point of the supply water buffer tank. Thus, the seasonal temperature adjusts the temperature of the warm and chilled water flowing to the house's radiant panels in the floor and ceiling. As the seasonal temperature gets warmer, the tank set point is lowered, and vice, versa. During the 2001/02 winter season, the warm water to the house could be as high as 105°F(40°C) or as low as 93°F(34°C) During the 2002 summer season, the off peak chilled water temperature could be as low as 52°F(11°C) or as high as 64°F(18°C).

DEW POINT CONTROL:
The control program calculates the dew point from the RH and the air temperature. It operates the dehumidifier so as to keep the dew point below 62°F (17°C). No condensation problems are foreseen since even at lower tank set-point temperatures, the ceiling temperatures should remain above 62°F (17°C) even at the colder tank temperatures. The tank set point is automatically adjusted upward so that the water supplied to the house is above the dew point in the space. During the year 2001 and 2002 (through Aug. 21) cooling season, the dehumidifier was not required.

CURRENT LOGIC DIAGRAM::
Below is a description of the control protocol that is currently in use for both the heating and cooling periods.
Heating control for the 2001/2002 season: For zone control of floor panel heating during off-peak -- If the optemp is below the designated setpoint, the floor valve opens and flow continues until the surface temperature drops to 2°F below setpoint, or until the on-peak period starts. For ceiling panel heating, if the ceiling surface temperature is below 70°F at 5 AM, the ceiling panel is operated. (supply water temperature < 85°F). During the on-peak period there is no floor operation, unless the floor drops to the unlikely temperature of 64°F (18°F). There is no ceiling operation during this period. For control of the central system during the off-peak period, the supply buffer tank set point varies linearly with "seasonal" temperature and triggers the heat pump operation. At 56°F seasonal temperature the tank setpoint =105°F. At 66°F seasonal temperature, the tank setpoint = 93°F. When the ceiling is operating, the tank setpoint is lowered to 85°F to prevent plaster expansion and possible cracking.
Cooling control for the 2002 season (3rd cooling season). The strategy is to use nighttime (off-peak) for discharging of the thermal mass (active and passive mass) so that conditions stay within the comfort zone with a minimum of on-peak compressor time. For zone control of cooling during the off-peak period, the ceiling optemp setpoint is lowered (from on-peak value) to 76°F (with a 2°F range). For the floor panel, if floor surface temperature is above 73°F, the floor valve opens. Floor is cooled until surface drops to 71°F, or until the end of the off-peak period. For on-peak ceiling operation, the optemp setpoint is raised to 78.8°F (with a 2°F range). No floor operation is allowed during the on-peak time. For control of the central system during the off-peak period, the supply buffer tank set point varies linearly with "seasonal" temperature and triggers the heat pump operation. At 95°F seasonal temperature, the chilled water temperature setpoint = 52°F. At 80°F seasonal temperature, the tank set point = 64°F. Limitation: setpoint must be greater than 3.5°F above the center zone dew point. During the on-peak period, the supply water buffer tank set point temperature = 72°F and only the ceiling is operated to keep the optemp below 78.8°F (26°C).

< click here to download the current control logic diagram >

OCCUPANT CONTROL:
The occupant interface with the control system is by a liquid crystal display terminal located in the laundry room, as shown below. All control parameters are available and adjustable by the occupant on the various menu screens

ENERGY RATE:
To take advantage of the high thermal mass of the house, the time of day rate was selected as the most favorable. It has an energy charge of $0.141/kWh on peak and $0.04kWh off-peak. On-Peak Period: 9 a.m. - 9 p.m., Monday through Friday, summer and winter. Holidays are like any other day. Another electrical service provider in the Phoenix metropolitan area has more favorable on-peak windows; summer is 1 PM till 8 PM while winter on peak times are 5 AM till 9 AM and 5 PM until 9 PM. Times of day rate structures are critical to the design of thermal energy storage to shift the load from on peak to off-peak.

VENTILATION CONTROL:
The system can control the Energy Recovery Ventilators (ERV) in two of the three zones but has been disabled because the homeowner felt that the ERVs were too noisy. Enthalpy control is a possible option. The homeowner has manually opened doors and windows when the outside conditions were favorable. During the 2002 cooling season, the homeowner left the skylights open, thus providing fresh air to the house by gravity when the outside temperature dropped below the inside temperature. So far, no automatic ventilation control has been maintained.