I’m not sure the hydrosmart120 can run with outdoor reset.
What controls the modulation ? Do you just pick a output temp ?
Ideally you want constant circulation with the lowest water temp -- better modern pumps are very low power consumption so having them running is not a problem.
With low water temp you get the highest efficiencies out of a condensing boiler.
I'm sure the manufacturer will give you the starting point ..... actual operation is always dependent on the structures insulation and the transfer -- that's a factor of piping amount.
Radiant is very forgiving. It's very common to hear "I did this and it works great" .. that's because you can mess things up and it still works.
A system using higher temp water being piped through a slab with minimal tubing may work about the same as a system using cooler water going to lots of tubing in a slab -- but the later system is going to use less fuel and have the best chance at max comfort w/o overheating.
It's simple math .... what does the building need (this is building size and insulation) based on outside temp. The boiler just makes hot water and the heat transfer is all about how much tubing is in the slab. It's the same as copper fin-tube baseboard needing higher temps than a panel radiator ... more tubing allows more transfer at a lower temp.
It's possible to use more fuel raising and lowering the temp in many systems as the water output has to be too high and the boiler drops efficiency ..
