How to pressurize circulator pumps for an open radiant heat system

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We are installing a solar water heating system with a below-ground cistern to store heated water (an open system). We purchased Grundfos UPS15-58FC, 3-Speed Circulator pumps to send the water from the cistern to the panels, house, and barn and then realized that the pumps are supposed to be placed at least 3 feet below the lowest possible water point in order to work. Digging an outdoor location for the pumps seems like a bad option because of having to waterproof the space where they go and still keep it accessible for maintenance.

Now we're brainstorming options for what to do next and would like some advice. Should we:
- buy the expensive submersible pumps and put them in the cistern
- buy cheap submersible pumps for the cistern and use them in series with the indoor pumps (and if so, how can we calculate the amount of cheap pump we need to get the Grundfos ones working?)
- use two pressure tanks (about 20 gallons capacity) we scrapped from someone else's project to create a closed, pressurized system (and if so - how do you do this without the pressure tanks just running dry)
- some other approach?



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Open systems aren't a very good idea due to stagnation issues during the shoulder seasons.

Iron pumps like the UPS15-58FC aren't suitable for open systems, since they are susceptible to corrosion from oxygenated water, and aren't rated for potable use. Only stainless or bronze pumps are going to cut it. There are stainless & bronze pumps with similar power and pump curves to the UPS15-58FC, but I suspect the math hasn't been done on how much pump is really needed.

Is the solar the only heat source here, or will there be some sort of boiler or water heater as backup?
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