ART TEC - Solar Domestic Water heating

Things to notice:

If the weather report above shows overcast or precipitation during daylight hours that explains why there has been no heat gain in the storage tank. Solar heating requires sunlight! You can mouse-over the charts above to see specific time stamped readings, or click+drag to expand a section of the chart. On a partially sunny day you might see the collector temperature jump around a lot as clouds pass over. Whereas on a completely sunny day there should be a very clean curve representing the collector temperature.

The 2 circulation pumps are activated by a differential temperature controller. They are only activated if the collector temperature is more than 12°F above the storage tank temperature to account for the temperature drop in the 25 ft. plumbing run from the collector to the tank. It turns off when the difference drops below 10°F difference. This prevents the system from losing heat when the collectors become cooler than the storage tank late in the day.

The heat builds up in the tank as a stratified layer with the hottest water rising to the top that then fills downward. If you look at the tank top and bottom temperatures during the time that the tank is gaining heat you will see that they are very similar because the pump is mixing the water in the tank. A few hours after sundown the tank temperatures begin to stratify so that the top will be 10-30°F hotter than the bottom.

In the evening and overnight the tank temperature begins a slow decline of about 1°F/3hours. This is the heat loss through the walls of the tank. This loss is why I replaced the tank style water heater with a tankless on-demand unit. See this blog I did about that. Most of the homes in Europe and Asia use demand water heaters -- America is way behind the curve on this energy efficiency strategy.

A sudden dip in the storage tank temperature represents hot water usage such as a shower, dishwasher or clothes washer. The dip will be more pronounced at the bottom of the tank as this is where the cold 50F well water enters. These dips become more noticeable at the top of the tank as the overall heat in the tank reduces.

If there was no solar gain, the tank temperature falls to the incoming well temperature of about 50°F. But the tank temperature will rise slowly because the pressure tank and storage tanks are in a heated part of the basement at around 70°F, this was an unexpected bonus that I had not considered until I installed the monitoring system.


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