I'm not a mechanical engineer. But with installing my OWB I chose to go with a primary/secondary system and I had to learn a lot about fluid dynamics to design the system. My OWB is 250 feet from the house.
There is no way you will be happy with the performance you would get with a 700 foot run. Keep in mind, that's not a 700 foot run its a 1,400 foot run because when sizing a pump you're looking at the distance its moving water in the complete round-trip circuit (700 feet to the building, 700 feet back to the boiler). A pump can only move so much water through a given diameter pipe. Distance and the number of elbows have to be factored in because they reduce flow. Now when you run into your heat exchanger, unless you do a primary/secondary design like mine, you're reducing the diameter to probably 3/4 inch or less. Talk about killing the flow rate.
The reason flow rate is so important is because that is how you get your BTUs to the heat exchanger. In other words, if you're only moving 3 gallons per minute you can't pull the same BTUs out of that water that you could if you're moving 10 gallons per minute. You want to have more rate of flow than you need. That way you are able to use all available BTUs for your heating needs. I used Grundfos 3-speed pumps because it allowed me to fine tune my system. I could change the flow rate as easily as moving the switch to a new position.
The reason you won't be happy with your 700 (1,400) foot run is you can't move enough water over the distance and a bigger pump only gets you so far. In your case, not far enough. Don't do it.
