The main component in the schematic (above) is a well-insulated drainback/storage tank equipped with an electrical element or integrated gas burner for backup, with an internal heat exchanger. The element/burner keeps the water at the top of the tank warm enough to provide domestic hot water (typically 120°F to 130°F).
A drainback-protected solar collector feeds the hydronic heat distribution system. The collector circulator runs when the collectors are a few degrees warmer than the water near the bottom of the storage tank. No antifreeze is required in this system, and no heat exchanger is needed between the collectors and the storage tank. These features reduce cost and increase collector efficiency. The same water that flows through the collectors also flows through the heating distribution system. The system is completely “closed” from the atmosphere.
The captive air at the top of the tank is under slight positive pressure. This airspace provides a drainback reservoir, and acts as an expansion tank. The water in the tank provides thermal storage for the solar collectors, and it provides thermal mass to buffer the zoned space-heating distribution system. The latter function protects the burner against short operating cycles, which would otherwise decrease efficiency and increase maintenance. Short cycle protection is very important in a hydronic system with extensive zoning.
A flow switch detects whenever domestic hot water is being drawn at a flow rate of 0.5 gpm or higher, activating a small circulator that moves hot water from the top of the thermal storage tank through a plate heat exchanger. Cold domestic water is heated as it passes through the other side of this heat exchanger, and sent to the taps.
An antiscald thermostatic mixing valve protects against high domestic water temperature when the tank is very hot, like at the end of a sunny, warm day. For the fastest possible response, the piping between the thermal storage tank and heat exchanger should be short and insulated. Combination isolation/flushing valves should be installed on the domestic water inlet and outlet of this heat exchanger. They allow the heat exchanger to be isolated and flushed if necessary to remove scale.
A single variable-speed pressure-regulated circulator feeds the home run distribution system for space heating. One circulator can supply the entire distribution system in a typical 2,500-square-foot house using no more than 40 watts under maximum heating load.
Each panel radiator has an adjustable thermostatic valve that monitors room temperature, and adjusts the flow rate to maintain that temperature. No thermostats, batteries, transformers, or programming—just simple, effective, and reliable room-by-room temperature control.
The mixing valve upstream of the manifold station protects the distribution system from what could be a very hot storage tank following a sunny spring or fall day. It also adjusts the water temperature supplied to the panels based on outdoor temperature, known as “outdoor reset,” and stabilizes room temperature for optimum comfort.
John Siegenthaler is a mechanical engineering graduate of Rensselaer Polytechnic Institute, a licensed professional engineer, and professor emeritus of Engineering Technology at Mohawk Valley Community College. “Siggy” has more than 32 years of experience designing hydronic heating systems. The third edition of his textbook, Modern Hydronic Heating, will be released in January 2013.
Hydronic Heating System Products:
Caleffi • caleffi.com • Solar collectors, storage tanks, controls, thermostatic valves
Grundfos • grundfos.com • Hydronic circulators
Heatlines • heatlines.com • Panel radiators
HTP • htproducts.com • Hydronic heat sources, including solar integration
Jaga • jaga-usa.com • Low-temperature hydronic heat emitters
Smith’s Environmental Products • smithsenvironmental.com • Low-temperature baseboard
Wilo • wilo.com • Hydronic circulators
Xylem/Bell & Gossett • completewatersystems.com • Hydronic circulators