You are on the right track. With the aquastat (with Differential) on the return from the bay loops AND the ability to adjust temperature on the demand heater, you have an infinate amount of variables that control cycle (on/off) timing. I think I remember starting with a 5° differential and 140° heater outlet temperature. The heater short cycled which I didn't like...I then changed the differential to 10° and the cycle doubled in timing 10 minutes on, 10 minutes off. I settled on the 15° differential which powered on the heater pumps 15 minutes on/15 minutes off. It seemed to be directly proportional! I then found that 105° output on the heater was all I needed to keep the bay floors at roughly 40°. The colder it gets from 32°, the longer the On cycle is and the shorter the off cycle is, but is always a combination of 30 minutes. Even down to -16° (below Zero), it still cycles on and off, meaning I still have capacity to go on the heater at these settings and temperature. Even at -16°, it keeps the bays ice free. I have experimented and kept one bay open to this temp with weep flowing across the floor to the drain and it stayed ice free. Once I settled on the settings above, I don't have to adjust anything, it self regulates based on load. Hope this helps...
Every system is different based on pump/zone flow rates and heater flow rates. 105° on my Takagi 199kbtu with (2) Taco 013's feeding it in series at 40psi gives me 6gpm flow output on the heater. When you dial in more heat, say 140°, it restricts flow to about 4gpm to achieve the higher temperature and also modulates to a higher heat setting....So this will change all the dynamics. The single Taco 013 for my (4) bay loops and (1) trough loop is calculated at about 2 gpm flow rates for each loop, which is right in the sweet spot, according to all the documents I read. Flow rates of either the primary or secondary loops will affect the cycle timing and aslo how the loops transfer heat to the slab. Mine just happens to be most efficient at the settings stated above. I have helped others with a system similar to mine, even heating floor loops in the living space of their house along with driveway snow melt, all on the same system utilizing (2) demand heaters. Every project I've helped with seemed to be most efficient once dialed in to the 15 minutes on 15 minutes off cycles.
Here is a link that explains primary/secondary plumbing in case you don't know what it means:
Primary/Secondary plumbing | | DIY Radiant Floor Heating | Radiant Floor Company (radiantcompany.com)
