ASHRAE Journal - February 2012 - 38

May 2010. The remainder of this article comprises case studies of actual events and experiments with these ﬁve systems.

180 170 160 Temperature (°F) Primary Loop

Examples from the Field Heat from Anywhere to Anywhere

150 Figure 2 (Page 36) is logged data from our system in Virginia from 5 a.m. through 8 p.m. on a single Boiler Off 140 day in the summer. The system has two in-mass radiant ﬂoor zones and one baseboard zone and thus the 130 need for a heat storage tank for the baseboard zone. At this time of year, the only use for the solar heat is 120 DHW Tank making DHW but the storage tank is used to buffer heat so the collectors do not overheat during the day. 110 With primary/secondary plumbing and intelligent, 100 centralized control, Btus are moved throughout the 0 10 20 30 40 50 60 system at will. Minutes The sun rises just after 8 a.m. and by 11 a.m. the 180 ﬂuid from the collectors is hot enough to be useful. Boiler Off At this point, the heat storage tank is 145°F (63°C) 170 and the DHW tank is 140°F (60°C). For this family, the size of the DHW tank is small, so the sys160 tem parameters are set to prioritize solar heat into the DHW tank ﬁrst. The target DHW temperature 150 is 160°F (71°C). With this amount of heat in the DHW tank, the family can be reasonably assured 140 that there is enough hot water (mixed down) to meet their needs until the next day’s sun. 130 After the DHW tank reaches its target at noon, 120 heat is simultaneously put into the storage tank and the DHW tank. (The storage tank heats up quickCapturing the Heat from the Primary 110 Loop and Putting It Into the DHW Tank ly because it is directly ﬁlled with the circulating Raises the Tank Temperature By 5°F. boiler ﬂuid.) The two tanks continue to absorb the 100 unneeded solar heat for most of the rest of the day. 0 10 20 30 40 50 60 Just before 3 p.m., the storage tank is at the highest Minutes temperature we allow for safety reasons and the system stops sending heat to that tank. The DHW tank Figure 3a (top): With normal controls, after the boiler is turned off, the continues to absorb heat until it reaches its maxi- stranded heat is slowly lost to the mechanical room. Figure 3b (bottom): mum at about 3:30 p.m. With central control and integrated logic, the system can keep pumps The sun begins to set and we have successfully running to capture the stranded heat in the primary loop and move it to absorbed all of the unneeded heat into the two the DHW tank. tanks. At 6 p.m. the collectors are used to radiate heat from the storage tank into the night sky2 and within one- trols. Figure 3a shows the temperatures in the primary loop half hour we have cooled the tank enough to have adequate and the DHW tank when controlled in a normal fashion after heat storage capacity for the next day’s buffering. The DHW a brief period of running the boiler for space heating. After tank is left untouched. Both tanks will cool overnight 5°F the boiler turns off, the primary loop dissipates its heat to the to 10°F (2.8°C to 5.5°C) further and DHW will be avail- mechanical room over a period of an hour. For this system, able at all times, with no fuel usage. The system is also set the primary loop plumbing is particularly spread out and we to take heat out of the storage tank and put it into the DHW estimate that there are 2,000 Btus (2.11 MJ) of heat stranded tank overnight if the DHW usage happens to be large that in the primary loop after the boiler turns off that are eventually evening. released to the mechanical room. Meanwhile, the DHW tank continues its normal slow loss of heat. Heat Capture from the Primary Loop Figure 3b shows how the central control can capture that Figures 3a and 3b show how a centrally controlled system stranded heat and put it to good use. The control knows that can perform efﬁciency tasks impossible with ordinary con- the boiler was just turned off and that there are no other calls

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Temperature (°F)

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February 2012

ASHRAE Journal - February 2012

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