Unpressurized Storage Pump not Holding its prime

Are you saying that your open storage tank is below your (pump) and plate exchanger? If so....................
Its a circulator so it cannot pump air. Can only create suction VIA liquid. Whats stopping the piping from gravity feeding and emptying back into the storage if it sits below your pump? Having a check valve in your tank on the suction line side of the pump once primed may work. I must not be understanding your equipment placement VIA elevations.
:coolhmm:

Is it circulating at all?

I suspect the mixing valve is an issue but also would be concerned about a pump with a check valve in it.
All of this puts more frictional pressure drop in the loop.
Of course, regardless of what some others say, the pump should be bronze or stainless, not iron. A cast iron pump will fetch up eventually unless the
water is treated with corrosion inhibitor.

A charging assembly on the outlet side of the pump is necessary. You need to be able to positively push all air out of all
the pump lines. Since you have all this extra stuff with the four way valve, it could be that some air is lurking in one of those loops.

A charging assembly is a boiler drain and a shutoff or two, depending on the location. This ensures that the water is pushed in the right direction. It sometimes takes
a couple times to get all the air out.

You do need to have some excess head capacity beyond whatever the loop is to help prevent cavitation.

Your plumbing is a little more involved, so you will have to push all the air out of both lines that go into the tank.
Both ends in the tank must be submerged.

Do not install any air vents since they will not work the way you want them to. The lines that go to the pump are sometimes under pressure
and other times under vacuum.

When you open the pump, there should be water coming out non-stop.

a circulator is not designed or intended to "lift" water or a fluid. It moves or circulates by lowering the pressure on the suction side and adding that pressure to the discharge side. I believe Grundfos recommends at least 2 psi pressure on the circ to prevent cavitation.

In a closed loop that would be in the fill pressure. In an open loop you need a column of water, not air above the pump to provide the pressure. Known a NPSH net pressure suction head.

I suspect every time the pump shuts down the water flows back down to the tank below, and the circ tries to lift the fluid from the tank, like a sump pump?

The pump will rust and corrode also in that application as it will see a constant supply of o2.

Any way to use a heat exchanger in the tank and build a closed loop pressurized system?

Try not to run it too long without fluid it is a wet rotor pump and needs fluid to lubricate the bearings, and also cool the motor. Trying to pump air will score and eventually destroy the bearing surfaces, and or over heat the windings and let the "smoke" out

hr

A circulator will usually work in this type of installation, if properly primed and if it is low in relation to the height of the water in the tank.
We have done a lot of these types of installations as solar drainback systems with good results. It is critical that the pump is primed--no air in the loop and no
leaks in the plumbing, and no air vents that can leak air into the syphon tubes that go in and out of the tank.

And as HR said and I mentioned earlier, no iron pumps unless you have a good corrosion inhibitor.

Suggestion, reverse your pump to push water in flat plates.

And inlet side requires a minimum pressure.

http://s3.pexsupply.com/manuals/1249544647674/Grundfos_ups15-58fc_Product_Data.pdf