Pump?

How much flow

A typical hydronic circulator needs to have water above it and a few psi pressure

It cannot lift out of a pit or tank

Sump pumps or well pumps can lift but are not usually sized for solar flows

They are not rated for temperatures above 140f

Ideally a hydronic circ, they are rated to 225F, located below the water level

Interesting Tank

What you have illustrated here is an open system. The type of circulator pumps we use on boilers, like the UP15 series you referenced, are usually designed for closed systems. Those pumps are not designed to pull water from a open system tank and lift it above the tank to be let to flow by gravity back the the tank.

Now your open system is nothing like the closed system that is illustrated by the Ferris Wheel. That is because your open system's return water is NOT allowing the weight of the water coming back down help the pump "circulate". You will need a pump that is self priming and delivers enough head to push that water up to the top. And that is when you will get the noisy operation described in the article above.

How easy would it be to design a closed system that is filled to the top? This way you can use a regulator circulator pump designed for a closed system. You will need to add an expansion tank to allow for the increase in volume when the water heats up. the system can also be drained (manually or automatically) if the water temperature gets too close to freezing. You are just not using the pump to get the water that high. You will be using what is called "static pressure" to get the water that high, and the pump head can be very low. just enough to overcome the friction loss of the system.

Here is how you can use a hydronic circ or two to pump from an open, unpressurized tank.

Come out the top, drop 1" down to the floor, mount the pump(s) as low as possible. The first time you need to prime the pump, after that they have the "head" columnPfor inlet pressure.

If the pump is 5' below the water lever .433 X 5 = 2 psi positive pressure on the pump.

Two small pumps in series doubles the head. Run them both for a minute to fill the collector, shut one off and let just one flow. Most I solar controllers have the dual pump option. Two small circs tend to be less $$ than a high head version.

Probably around 27' of height is about as high as you can maintain a siphon. We struggled at high altitudes to keep a siphon much over 25'

A Grundfos 15-100 is a great drainback pump,, high head, low flow, it will do 30’ of lift. It is a 1.1 amp, but we have run them for years with that Resol control in the variable speed mode

5-6 gpm should move all that collector can produce

If you use an ECM type of pump, you cannot use the variable speed function, just on/ off

if your tank is in the ground, how will you get the pump mounted below it?

The 15-58 with 3’ of water above it will work at 12’ of lift

If you can’t get the pump below the water level, you could try a small submersible sump pump. Maybe a 1/4 hp from the box store. Something inexpensive to see how it flows

It depends on how how that tank will get. 1000 gallons us a lot to heat in a 6 hour solar day

And also how much heat you pull out of the tank every day

The control allow you to program a max tank temperature. Maybe start at 150f

What type of liner do you have in the tank? EPDM or pvc?

This 1/4 hp

will do 25’ of lift, 50 bucks on Amazon

hydronic glycols are rated to 325f, some solar specific versions a little higher

If it can come in contact with potable water, it needs to be propylene glycol

Don’t use RV antifreeze, it doesn’t have any inhibitors to protect the metals.

Solar harvest drops quite a bit in winter, shorter days, lower sun angle, and the cold ambient air steal much more heat energy from the panels

The tank temperature you will attain is related to tank capacity, and insulation. And the array square footage. For dhw use we use 1-2 gallons of tank per square foot of actual absorber size. Even in cold winter climates that will get the tank to 120f.

There was a modular foam underground tank made in Utah called the Cocoon. The one installation I recall had a vault next to the tank to put the pump down at the bottom level of the tank

Looks like you may need to use a submersible pump if some sort. A pump with a foot valve will not work, the collector will not drainback! No check valves of any type in a drainback, on either supply or return

Contractor trash pumps are self priming, but they tend to be 1-1/2” and larger. Way to big for what you have.

It is a nice looking HX tank. I think a sump inside the tank would have too much moisture for a typical hydronic pump.

Why so many coils? Is that corrugated stainless flex? Beware it has a considerable pressure drop, if you intend on pumping through it.

You really do not need a solenoid valve or a vacuum breaker on a drainback solar piping. It adds more complexity and things to fail. So you have that second relay in the controller for any other function.

A pump and collector is all that is requires, a sight glass is handy to assure you are draining down.

Also not needed is an expansion tank or safety relief if this is an open system, which yours is.

See the simplicity in these two drawings? Purple line comes off the botton of the open system tank, loops over the side, down to the pump,then up to the collector. The pump is always "flooded" and will start without priming everytime :)

On the smaller tank schematic, this is a pressurized drainback, yes water drains back when under pressure, so it has a relief valve. PRV on the top of the tank. DHW is genertyated in the coils. A reverse indirect tank is what this is called. Similar concept to what you are doing.

Sounds like you did what I was intending to suggest: a check valve to hold water in the pump up to the Tee above, and a normally open zone valve (powered closed when pump is energized) in the Tee's side pipe to allow everything above to drain when the pump turns off. The hope is that should the zone valve motor or wiring fail it will fail in the open position to prevent this causing potential for freezing.

I have installed several large drainback tanks piped the way Hot Rod showed where the solar pump is mounted off the side, below tank water level. I have two suggested additions to that setup:

1. In several cases the pump mysteriously lost its prime. I finally figured out that the high collectors were draining back so fast the water broke the seal like a toilet and sucked air into the siphon portion between tank and pump. I installed a swing check with a 3/8" hole drilled through the flapper, pointing up, above the water level (just for easy service access) between pump and collectors. This allows full flow while the pump is running but slowed the draining flow enough to stop the air intrusion.
2. At the collector return into the tank we use a Tee with a small side port (to restrict water dumping out but allow air in) and extended the below-water horizontal portion (shown as a gray elbow in Hot Rod's "Figure 5-16" diagrams) as a pipe with many holes drilled in the sides and a cap on the end. This diffused the return flow so it did not break up the stratification in the tank.

Tom