System Design with Storage

[Mushroom Man]

I've settled upon a a primary secondary design with the boiler and storage both isolated like zones and 3 heating zones.

I don't understand the storage loop. I have a 1500 gallon unpressurized tank built into the basement. The EKO 60 is 6 feet higher in the garage.

1) Will 1 heat exchanger and pump be sufficient for the storage loop or do I need 2 heat exchangers and 2 pumps?

2) What kind of heat exchanger(s) should I use for such a large storage tank?

Here is some background for your consideration:
There is a forced air oil furnace with 117K BTU rating (it heats house well)
DHW is oil fired 40 gallon tank (never ran out of hot water yet)
The Barn is 215 feet from house with 500 sq. ft. already developed (to be heated). It can be heated with 2000 watts at present
Additional development space in the barn could reach 5000 sq.ft. over several years

I appreciate your input to my steep learning curve.

 

[Nofossil]

The size of the heat exchanger is based on the rate of heat transfer rather than the size of the tank. You'll need enough heat exchanger surface area to transfer the output of the EKO into storage with an acceptable temperature difference.

In a primary / secondary setup you'll need two circulators for storage and / or some complex valving. In order to acheive and maintain maximum stratification, you want to charge the tank with top to bottom flow and discharge with bottom to top flow.

There are a few threads here that discuss stratification and plumbing schemes.

 

[WoodNotOil]

For unpressurized you have the choice of copper coil hx or flat plate. If you use one flat plate heat exchanger, you may need to be able to reverse the flow through both sides of it. One direction for charging, another for drawing. That may require 2 pumps on each side of the hx depending on your specific design. If you use copper coils you still need to reverse the flow through them. If you look at my webpage in my signiture, you will see I found a way to pump through the pumps avoiding the need for several lines or zone valves. (Ignore the rest of my piping as it is a series hookup and doesn't apply to your P/S design)

The plate hx will need to be pretty large. I would say 70 plates or more. Pybyr and deerefanatic picked up a 70 plate hx somewhere for a reasonable price. Ask them about it. Copper coils would require quite a bit of tubing to charge up 1500 gallons. I have a plate hx so I can't comment on specifics there.