Solar Hot Water Part III: Our System Architecture

As mentioned in the previous post, the system architecture we decided on was to have the cold water from the city main feed into the solar tank, preheat with solar, and then feed into the gas-fired hot water tank. In addition, we decided to get a solar tank with a second heat exchanger coil. This would allow an external boiler, for example a geothermal heat pump or efficient closed combustion gas boiler, to be substituted for the gas-fired tank in the future if we wanted.

Here is a brief description of the system we had installed:

Collector: Schueco Slim Line II-80
Tank: Superstor SB 80 gallon w. backup heat exchanger
Balance of system: Pumping station PS 1.3 from Schueco, expansion tank, DeltaSol BS thermostatic control, etc. was part of the Schueco package

Normally, the Schueco system comes with a double walled steel tank from Rheem, but we dropped that and ordered the Superstor instead.

We decided on Schueco instead of another collector such as Heliodyne because the Schueco collectors are designed to force the heat transfer fluid out of the collector and into the expansion tank when the collector stagnates and begins to overheat. The water in the heat transfer fluid flashes to steam increasing the pressure and forcing the fluid into the expansion tank. This keeps the propylene glycol in the fluid from cooking, increasing the lifetime of the fluid. I've heard anecdotally that Schueco has tested their collectors for up to ten years without needing to change the fluid.

The diagram below shows the system in schematic form (note: the diagram is edited from the Schueco installation manual and is copyright Schueco, used here under the Fair Use exemption):

 The diagram shows the hot water coming into the solar tank, being heated by the solar heat exchanger and flowing from there into the gas hot water tank. The gas hot water tank heats it further if necessary then the water goes into the house. There is a temperature control valve on the house side of the gas hot water tank to keep domestic hot water temperature below 120F, since hotter temperatures are dangerous and can scald. The hot and cold heat transfer fluid flow to and from the collector is shown above the pump station.

The numbered items on the diagram are the following:

1. The Schueco Slim Line II-80 collectors (2).
   
2. The expansion tank and overflow hose. If the pressure gets too great and while filling the system, glycol can run out into the drain through the overflow hose.
3. The Schueco PS 1.3 pump station.
4. The Superstor tank. The bottom heat exchanger is connected to the pump station, the top exchanger is not connected to anything at this point.
5. The DataSol BS thermostatic control. It compares the collector temperature at T1 and the tank temperature at T4 and only turns the pump on if the collector temperature is hotter than the tank, and turns the pump off if the tank temperature exceeds 180F.
6. The existing hot water backup tank.

There are also two pressure release pipes coming out of the solar tank and the gas hot water tank that are not shown on the diagram, these are standard for hot water systems to avoid having the tank overpressurize  and rupture.