Enough boiler ?

If it's keeping the thermostat satisfied, it's big enough. If the target water temp is higher than necessary, it's not uncommon to never reach it- very likely your ODR curve needs adjustment. With 6" OC tubing, even on the coldest day of the year your water temp should never need to exceed about 100* so if it's much higher than that, it should come down anyway.

that would be 18 btu/ sq foot. That is a nice number for a large shop. Your boiler is large enough and you have plenty of tubing. ODR on low temperature systems will not add a lot to the efficiency. But if you have it, may as well dial it in.

Getting it perfect takes some tinkering. If you have an actual design it will indicate SWT on design day, start with that number at the high end. If not guess something like 105 as a WAG

On some of the mod con boilers they have a boost function. If the boiler is not keeping up with the load based on temperature rise, it will automatically raise the supply temperature.

So read the control manual to see what functions your control can offer.

you got pics of the install, and what size are the pumps feeding the floor?

Again, that's because the SWT is too high. Any boiler will run 100% until it reaches the desired setpoint so if the setpoint is too high, it'll never modulate which is what you're seeing. Turn the temp down.

70 could be too warm if you are doing any physical work in the shop?

With an 82 degree floor surface, 70 ambient, you get just at 25 btu/ square ft.

With 1/2 tube on 6” spacing you could probably run .5 gpm per loop.
39 loops at .5 gpm 19.5 gpm for the pump spec

a 6” slab that size is around 113 yards of concrete, 452,000 lbs. so it may take time ramp up, and down. The ODR will help prevent overshoot., and having a massive flywheel to tame down

at days end, it’s the load on the building that drives the operating condition of the boiler. Not unlike a tractor plowing a field.

The boiler will seek and find a thermal equilibrium point, based on the load. The delta is fine, and if it continues to stay at that, neither supply or return temperature moving, you have reached thermal equilibrium

If the boiler runs non stop under that condition, the floor is taking 100% of its horsepower.

If the shop heating as you expected? Have you had a cold/ design day?

I dont think you are in bad shape at all. I have a Baxi 133K in a shop thats 60x80. Not spray foamed, just the rolled insulation. It has 3" of foam underneath and we put the tubing on top of the rebar so only 2" down from the top. It works great. The return temp is always low, the slab only needs to get to 65 keep it at 60 in there which is what he wants. Design day is -20 with 20 mile an hour wind up here in Vt. I may have gone a little bigger on the boiler but it was a good used one I had kicking around. I think your setup will run for long stretches if you wanna go up 5 degrees in the building, but if you set it and forget it, you should be ok

To raise the room temperature, the floor temperature needs to be increased. So to add even 1° to that nearly 1/2 million lbs of concrete will take some time. A slab like that behaves more like a freight train than a race car.

Actually, you do want the boiler to run a long cycle like that instead is short bursts.

The ODR control tries to match the boiler supply temperature to the load, by basing it on the outside temperature. By doing this you can help avoid over-shooting the temperature to and in the slab.

It sounds like the system is purring along? Any problems with it's performance?

hi I have the same exact boiler as yours. The discussion board helped me figure out something with the boost function. The boost function turned on and then the call for heat is done and the elapsed time finishes then a boost of hot water goes into the system. The problem is that you are set for 5 - high mass setting. That is perfect for concrete but the boost of hot water will send in 165 plus degree water into the system which is not good for concrete.