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Where are you getting 20 btu/sqft? That is above what I see in air tight uninsulated doubled brick in zone 5. Anything new build should be around 10btu and easily bellow that with a bit of effort. The issue generaly with most new builds is the floor is not actually hot enough as the loads are so low. One way around it is to only heat high traffic areas so these can now run closer to max temp and only run the rest in the deep freeze.

The flow rate for each zone should be set based on the load to the zones, not what the pipe can do. Generally you would have more flow to zones with more load like two outside walls or ones with large windows.

You want an ECM delta P circ as it works the best with zone manifolds. I use a plug in corded pump and a switched outlet. Turn it on in the winter, off in the summer. Pump runs 24/7 during heating season.

Delta T seems like it should work but the controls on the ones I've tried are not the best. They do vary flow but it is more of on at max for a bit then run at min flow the rest of the time, never at a nice steady state.

Make sure the secondary is not over pumped, this is why you need to figure out actual flow rates you need. For best efficiency, the secondary flow rate has to be bellow the primary off your heat pump.

No zone control can work as long as the house loads track through the day. If you have any room with large west or south facing windows, you will probably want TRVs or zone valves on those to avoid overheating.

P.S. I would put the bath fan outside the shower, I prefer over the throne. A bath fan in the shower generally causes cold drafts plus it is rough on the fan due to condensation. Modern construction surfaces are warm enough that condensation on walls is not an issue.

Don't get hung up on a fixed ∆T. Choose a ∆ for the design 10- 15° typically for residential radiant floors. Then size the pump, piping, etc to supply that. The actual operating ∆ will move around a bit from the design number based on the load on the system at any point. ODR and constrained ∆ are at odds with one another.

Distilled water is expensive and hard to come by in quantity. True distilled water is made by boiling to steam, collecting the condensate as it runs across a stainless plate, so expensive and slow.

RO or DI is fine this comes from either a membrane, RO, or through a media, DI.

For just a one time system, Axiom has a few disposable cartridges for generating DI water on site.

With a two pipe buffer, single temperature system you should only need a boiler pump, and the distribution pump. A 3rd if you run an indirect. With a zoned system an ECM delta P circ is my suggestion.

your circulators will be under 100w each unless they are something massive, one circuit will power all of them or you could get a 240v circulator and power it off the heat pump circuit. Control xfmrs with 240v primaries are common to power the controls off of 240v refrigeration or electric heating circuits are common as well. Could also use 120v circualtors and controls if the equipment requires 120/240v or you run 120/240v to power the accessories.

There are treatment cartridges for filling hydronic systems, i wouldn't go beyond that, not worth dealing with distilled water.

I don't think this will halve the heat. The heat is traveling primarily via conduction, not radiation or convection. That part that is convection is going to tend to stay up in the joists anyway. But as others have said, it depends on your conditions in your basement. Since you say it's a conditioned space below you might find almost all the heat is going to go into the floor that would in the insulated areas.

@hot_rod 's suggestion (that I failed to see at first) is great though, leave the plates out in that area and you'll be gold

Maybe leave the plates out from under the island or any cabinets really. The tube can still go across the area, but without the conduction, the heat output will be minimum.

Assuming you are using water in the radiant loops now?

.3 gpm for a 3/8" loop is the general recommendation, so 3.5' head in a 200' loop

.5 gpm in a 200' loop of 3/8" is around 9' head.

20% is a weak glycol mix, not much inhibitor in that %. There are alcohol based geo fluids available also.

Vertical height from the basement does not add head if the system is pressurized. You only need to account for the length, not the height.