The coefficient of performance (COP) is the heat pump equivalent of efficiency: the ratio of the heat output divided by the electrical input. A COP of 4.0 means that the heat output is four times greater than the electrical energy required to operate it. The higher a heat pump’s COP, the lower its operating cost.
The graph shows the heat pump’s COP dropping rapidly as the hydronic heating system’s water temperature increases. Thus, for the highest possible COP, the water temperature supplied to the hydronic heat emitters should be kept as low as possible.
Wood-fired boilers can produce higher water temperatures, even up to 200°F, but that doesn’t negate the benefits of matching a wood-fired boiler to a low-temperature distribution system. These heat sources are best used with a thermal storage tank. They add heat to the storage tank, and the heating distribution system draws heat out. The lower the tank temperature can go and still supply sufficient heat to the building, the less often the boiler has to be stoked.
For the best efficiency, design hydronic heating systems supplied by either solar collectors or hydronic heat pumps so that the supply water temperature to the load (under maximum load conditions) doesn’t exceed 120°F. This temperature is a reasonable compromise between maintaining good heat source performance, while not overly increasing the cost and space requirements of the heat emitter.
It’s critical to understand what determines the supply water temperature (the temperature being supplied to the load via heat emitters) in a hydronic heating system. Some designers think that it is determined by the heat source—because many boilers come with a dial or digital control that “sets” the temperature produced. Unfortunately, that’s not how it works. That setting is only a high-temperature limit on the heat source output. It does not guarantee that the set water temperature will ever be reached. The water temperature in any operating hydronic heating system climbs only high enough for thermal equilibrium—where the rate of heat release from the heat emitter balances the rate of heat input from the source. Once thermal equilibrium is reached, there is no thermodynamic incentive for the water temperature to climb higher, and it won’t!
It’s the hydronic distribution system’s design, rather than the high-limit setting, that determines the system’s operating water temperature. Almost everyone who designs heating systems wants to maximize thermal efficiency. For hydronic systems, this means moving away from high water temperatures by using heat emitters with larger active surfaces, or other details, such as internal microfans, that increase both convective and radiant heat transfer. This allows thermal equilibrium to occur at relatively low water temperatures during both maximum and partial load conditions.
There are several ways to design modern hydronic distribution systems around RE’s lower water temperatures, starting with the heat emitters—any device that removes heat from water flowing through it, and releases it into the room.
Many homeowners are used to the look of fin-tube baseboard heaters. From the outside, modern baseboard heaters look similar to older ones—but what’s inside is very different. Fins can be about three times larger than traditional baseboard heaters, with multiple copper tubes running through those fins. Tubes can be piped for either parallel or series flow. In the latter case, the hottest water flows in the upper tube, makes a U-turn at the end of the heater, and flows back in the lower tube.
Assuming an average water temperature of 110°F, this baseboard releases about 290 Btu/hr./ft.2 at 1 gpm when the tubes are configured for parallel flow. With 4 gpm, the output increases to about 345 Btu/hr./ft.2 If the two tubes are configured for series flow, the output drops about 10%.
Consider a 12- by 16-foot room in a well-insulated home, with a maximum heating load of 2,880 Btu per hour (e.g., 15 Btu/hr./ft.2). This load could be met using a 10-foot length of Heating Edge baseboard operating at an average water temperature of 110°F at 1 gpm. A 10-foot length of conventional residential baseboard heater would require an average water temperature of about 150°F. This temperature is well above what a typical geothermal heat pump can produce, and would significantly lower the efficiency of solar thermal collectors. An additional 14 feet of conventional baseboard would be required (24 feet total) to deliver the same output at an average water temperature of 110°F.