For the Placitas system, a hydraulic separator—a “flow center” device that eliminates the need for a primary loop—was used instead of a Combi 101-style primary loop. These devices can be purchased from various plumbing equipment manufacturers. They provide a large, open container that is filled with “boiler fluid” and receives the heat and then provides heat to the other equipment. The Caleffi Hydrolink eliminated the need to assemble a primary loop piece by piece. As seen in the piping diagram, a primary loop consists of tees, valves, elbows and connective piping. A prefabricated hydraulic separator comes from the manufacturer with many of these parts built in.
The Hydrolink 2+2 model was configured to provide the same heating functions and advantages of a primary loop system. The result is a piping system that resembles a Combi 101 system with very compact central piping, incorporating a substantial number of collectors (12) and heating zones (8).
This retrofit’s controls were originally designed with common equipment such as conventional room thermostats, mechanical relays for switching pumps and zone valves using several differential thermostats, and set-point temperature controls. This is the conventional way solar/boiler heating systems have been controlled. For the Placitas retrofit, the technically proficient owner was willing to test a control system that included a solar logic integrated control (SLIC)—a computer- and Web-based control of our own design.
The SLIC control system replaces all of the relays and temperature controls with a single box. It is easy to operate using familiar room thermostats and allows both the installers and the owners to monitor and record the heating system’s performance and data, and adjust settings locally or remotely over the Internet. This is great for fine-tuning the balance between comfort and efficiency.
The fuel efficiency and comfort provided by a solar combisystem is only as good as the control system. There are many ways to save energy through the control system. Features that are not needed are simply turned off at the time of installation. The internal software controls have many functions, such as solar-only and backup-only settings, heat dissipation, and room target-temperature control, plus many other settings subtle and not so subtle that affect system performance and monitoring.
Each 4- by 10-foot collector can produce enough heat to offset up to 0.5 gallons of propane per day. But the savings are not entirely from solar heat gain—other factors include a high-efficiency condensing boiler and heat-saving control strategies. Solar priority over the boiler is guaranteed both by the piping configuration and the control logic. Solar heat for the floors has an adjustable priority over heat storage in the water tank, and is controlled by the SLIC using virtual two-stage room thermostats. (The room thermostats transmit the room temperature and the user’s setpoints to the central control, which implements the two-stage functions.)
Heat storage is also optimized in the DHW tank and DHW recirculator by software control. The SLIC controller is programmed to save heating fuel in every way possible, such as stranded heat recovery—routing hot fluid left in the pipes after a heating cycle is completed to a water-heater tank or some other useful load—and intelligent priority control based on temperatures and critical loads.
Past and current performance can be reviewed and analyzed at any time. The graph shows an example from two days in October 2009. On Day 1, the room temperature is kept within a comfortable range, and the solar heat is diverted to the water tank after the room warms up in the morning. On Day 2, the weather is even warmer and sunnier, so the room warms up, the water heat gets very hot, and the intelligent heat dissipation kicks in to cool the collectors all afternoon, typically routing heat to the concrete floor in the garage. This verifies that the control system is set correctly; data like this can be viewed at the house or remotely at any time.
The opportunities for saving electricity in a heating system are sometimes small but worth considering. In this system, circulator pumps are disabled when they are not needed. Multispeed circulators are used and set to the lowest speed that is effective for each job. The number of transformers are limited to eliminate their “phantom load.” “Latching” zone valves are used, which only require power when they change their state. There is no “primary pump”—all circulation through the flow center is provided by the secondary pumps that are smaller and thus require less energy. Solar circulation for collectors using closed glycol loops is achieved with very small pumps that were energized by PV power.