You can use renewably made hot water for your hydronic system—but designing for low water temperatures is critical to good performance.
Hydronic heating is the technology of moving heat using water. It has been used for decades in millions of North American homes, most of which have a gas-fired or oil-fired boiler as their hydronic heat source. Good hydronic design is also the “glue” that holds together renewable energy thermal systems that provide space heating and domestic hot water. In other words, pick a renewable heat source, do a good job with the underlying hydronics, and you’ll likely be pleased with the results. Treat the hydronics as “whatever,” and you’re likely to be disappointed.
In the past, solar hydronic heating meant using solar collectors as sunny-day substitutes for conventional boilers or water heaters. Designers focused on the collectors, storage, and control aspects of the solar subsystem, but devoted little thought to a compatible means of distributing solar-derived heat within the building.
Most hydronic distribution was designed around high-temperature supply water. Residential systems commonly used fin-tube baseboard heaters with water temperatures sometimes exceeding 200°F.
But those high water temperatures were beyond what solar collectors could produce consistently. Sure, there was an occasional “perfect solar day” in winter when the storage tank got hot enough to heat a home during the following night. However, performance over a typical northern heating season was often disappointing. As a result, after investing thousands of dollars in collectors, storage tanks, and controls, many early systems spent much of their time distributing heat generated by conventional fuels rather than by the sun.
The North American heating industry has a tendency to focus on heat sources rather than overall heating systems. This mindset continues to limit the performance of not only solar thermal, but also heating systems supplied by sources such as geothermal heat pumps and wood-fired boilers.
All renewable heat sources yield better performance when combined with low-temperature distribution systems. To see why, take a look at the thermal performance characteristics of a solar collector and a geothermal water-to-water heat pump. The “Solar Collector” graph below shows how the thermal efficiency of a flat-plate solar collector is affected by the temperature of the fluid entering its absorber plate. On this typical sunny, midwinter day in the northern United States, the thermal efficiency of the collector drops rapidly with increasing inlet fluid temperature.
For example: If the fluid entering the collector is 90°F, the outdoor air temperature is 30°F, and the sun is bright (solar intensity is 250 Btu/hr./ft.2), the graph indicates that the fluid gathers about 56% of the solar energy striking the collector. However, if the entering fluid temperature is 160°F, while the other conditions remain unchanged, the collector’s efficiency falls to 33%—a significant “penalty” when the collector operates at the higher inlet temperature. It’s the result of greater heat loss from a collector to outside air, much like the increased heat loss associated with keeping your house at 75°F rather than 68°F.
The relationship between efficiency and the entering water’s temperature also holds true for hydronic heat pumps. The “Heat Pump” graph shows a similar effect for a modern water-to-water geothermal heat pump operating with a constant earth-loop inlet temperature (at the condenser side of the heat pump) of 45°F.