That's a bit of a monster pump you're looking for.
Are you SURE you need 26GPM??? That's a LOT of flow.
You also have circulators at each zone, so they can be subtracted from that pump head need.
250' is a lot of line, dead into each zone.
Why not create a primary loop between supply and return. Then you use closely spaced Tees for each zone. Each zone takes care of its own needs, your primary is taken care of by the boiler.
I'm looking to "correct" a situation just like yours. The supply and return lines CANNOT FLOW unless a zone is open, then only as much flow as a zone allows. I'm concerned your cold-start flow from the boiler is going to be REALLY cold and ALL of it must pass through a zone, BEFORE it can start providing any heat.
Think of your zone like a sink 200' away. Before you can get hot water, you have to flow all that water down the drain, through a smaller line than the supply.
If you make a primary loop, you can use a "pre-purge" to run the primary pump about a minute before the zone pump (delayed on Tstat call) kicks on, which will greatly reduce the cool down in the zone calling for heat. There are a number of ways to configure this.
Come to think of it, you might want to do something to keep all that primary loop from getting too cold when not flowing, else shock might be an issue (too cool return temp).
You could either run a remote Thermocouple to the inside-the-building (near the zones) and use that to feed into an aquastat, or simply add a timer to run your (new) primary loop and pump every...(15mins?) or so. Whatever the time, run it 5 mins every 20 or something that keeps your return line from getting too cold.
Are you running radiators in your zones? If so, you DEFINITELY do not want to run any more cold water through them than necessary when starting up a zone. Keep in mind that a 3/4" pipe is going to take...maybe 4 TIMES as long to circulate all that 1.25" water through it before it can even start to think about providing heat.
I could be off on some numbers, estimating, but would you consider it an efficient system if it take 5 minutes or longer to start heating a zone after it calls for heat? ...and begins by cooling down the room and radiators first? Any expansion noises could be increased due to a much wider temp swing.
Now...if you're running all the zones and the boiler ALL the time (think very cold day), no problem. All water is hot, full flow everywhere. But when you have intermittent calls for heat, as most systems do, it could go from "not so great" to "downright crappy" in a hurry.
Not trying to sound all doom and gloom. There may be a way to make it work for you. But small changes as I mentioned can solve a number of problems and avoid others- including your monster-pump issue
Please think it through. Ask questions if you have them. There are always multiple ways to do things and lots of reasons to choose one over the other. Just be sure you know what you might be getting into. (including what is likely to be a monster-price to go with that pump...or TWO pumps)
Hope this is helpful!
