That's progress!

Now I want you to get a pencil, paper, calculator, a clock w/second hand, a comfy seat and a six pack of beer.

Sit down for a while in front of your system and log the on/off times of those idiot lights on the tankless while the circ is running (crank tstat up). I need to know the on time ratio (on time/total time). I will also need deltaT across your radiant loops and across the tankless separately. Try to do this after the slab has warmed up (probably right at finish of last heat cycle of normal tstat setting).

Post results. If you drank all the beer don't bother, your results will be skewed. Try again
later. You were supposed to send the beer to me.

OK, no beer this time, but it was fun. Any way, here are some results;

When I ran the system the heater never turned off, even though it got to 125-126 output. The temps across the heater were 125 and 85, up to 126 and 91. The loops were 86-87 out and 69-73 return up to 87-89 out and 71-76 return.

That's all for now, I've got to go nurse my hangover....

You are getting about a 15F deltat across the loops. This is nice.

As long as you can maintain thermostat setpoint I wouldn't change anything right now.

The wildcard in all this is the tankless heater. It must have some kind of temp control ( I assumed the idiot lights would tell us about on/off status). It is time to whip out a clampon ampmeter if you feel comfortable with it and electricity. It is either turning on/off or sequencing elements or modulating voltage to the elements to control out put and we aren't "seeing" it. Or don't do anything.

If you have a 34kbtu (10kw x 3.414) heater with a 20kbtu load your system will run 58.8% of the time on a design day. I suspect in actuality your 10kw tankless is really 9kw with 2-4500 watt water heater elements. Without schematics I can't know how it controls output.

There is only one led on the control board and it never went out. It does if the water flow stops. I do have an ammeter on it. When I first hooked it up, it would go to 49.8 amps. I'm not sure if that's what it was drawing because it's a 50 amp coil and it may be topping out. Later, I noticed the current was down around 30 to 36 amps. So either it's variable or I've burnt out my coil. I have a hand meter that I will varrify the reading with. The heater has two 5000 watt @ 208 vac elements. It's powered by a 240vac cirrcuit.