Gary,
 I'm not real clear on what you want to accomplish, but I agree with Mike on 
the convection flow. If your storage tank is higher than the heat source, you 
don't really need a pump. You WILL want to size the lines 'up' from what 
they'd be with a pump (go to at least 1" size, maybe 1 1/4".
Water heating boilers aren't very high pressure, almost always rated at under 
15PSI. You naturally want to figure in an expansion tank and pressure relief 
valves.
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