I have an evacuated tube (30-tube) solar water heater that produces more hot water in the summer than we need. I have a Steca differential control that has a setting to circulate the antifreeze to cool the tank if it gets too hot. According to the control instructions, it is supposed to circulate the fluid at night to cool the tank. It seems to turn on and circulate for hours but it loses very little, if any, heat overnight like the instructions claim. Do you have any idea what is wrong?
Bill Noyes • Winnipeg, Manitoba
The vacation mode built into some hot water controls does work—but not with evacuated tubes. The recirculation heat dissipation is effective with flat-plate collectors because they only have a single sheet of glass as insulation on the front of the collector, and the plates can readily lose the excess heat.
The vacuum insulation in evacuated tubes that provides enhanced performance in cold, cloudy weather hinders reradiation losses. In addition, most tubes sold in North America today use a heat pipe inside the tube to collect the solar energy and transfer it to the manifold at the top of the collector. The heat-transfer fluid is then circulated to a heat exchanger and into the storage tank. Heat pipes don’t work in reverse, and this prevents any reradiation in tubes of this type.
Apricus makes a heat dissipator to help prevent summer overheating in evacuated-tube systems. The heat-transfer fluid from the collector(s) is diverted to the dissipator/radiator (liquid-to-air heat exchanger) when the fluid temperature is above 170°F. The dissipator and other overheating solutions are the subject of an article in HP142.
Chuck Marken • Home Power thermal editor