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Solar water heaters come in a variety of types, including active and passive. Passive systems place the storage tank above the collectors so that the heated water from the collectors rises into the storage tank.
Active solar systems can be direct or indirect systems. The indirect system uses a pump to circulate a heated fluid from the collectors through a heat exchanger to heat the potable hot water in the storage tank. The direct system circulates the hot water from the collectors to the storage tank with a pump.
Solar water heater storage tanks are very well insulated and unlike storage tank type water heaters they do not have a central flue which further decreases the losses.
A temperature controlled mixing valve will then mix water from the solar storage tank with cold water to obtain the desired water temperature at the outlet to the hot water plumbing.
Gas tankless water heaters measure the temperature of the incoming cold water and regulate the burners to produce a steady constant temperature output. The outlet temperature is set by the user. By connecting the inlet of the tankless heater to the outlet of the solar water heating system, the tankless heater will only fire up when the incoming temperature is lower than the outlet temperature setting.
Electric tankless water heaters do the same thing, monitoring the incoming water temperature and adjusting the amount of electric current being run through the heating elements to obtain and maintain the outlet temperature set by the user.
This is an ideal situation, since you will use no additional energy when there is plenty of solar generated hot water, and less than normal energy when the solar is putting out less than maximum temperature water.
If you run completely out of solar hot water your tankless unit will work as normal and still supply you with the hot water you need.
What about using a hot water demand system with a solar water heater or combination solar and tankless installation? No problem.
For a demand type hot water system to work with a residential hot water plumbing system it only needs to have a source of hot water and a source of cold water that also feeds the source of hot water. In other words, the cold water supplied to the fixtures must also supply the water to the water heater. I've certainly never encountered any other way of doing it.
So a hot water demand system will work with any solar water heater and any combination of tankless and solar water heaters, as long as the pump is powerful enough to create sufficient flow to turn on the tankless unit. That usually requires at least a flow of 1/2" gallon per minute.
Some demand systems don't have motors powerful enough to pump that much water such as the Metlund S50-T demand pump. In some cases with relatively short runs the Metlund S70-T pump will suffice. The Chilipepper CP6000 has a big enough motor to work with any tankless water heater in combination with any solar water heater.
Combining a solar water heater and a tankless water heater will provide you with a green plumbing system, saving you a lot of energy, and still providing you with unlimited amounts of hot water when you need it. Make your system even greener by adding a hot water demand system and start saving water as well as energy. It's the green thing to do.