infloor radiant hydronic heating

I have a 30’ x 40 ’ building, slab on grade. 2" rigid insulation under slab and around the outsidefrom grade to 4’ below grade. The building is divided into two spaces. One residential side (30 x 20 - 2 story) and the other 30 x 20 (open to top 18’ high). We have 1" closed cell insulation covered by R20 fiber glass and an additional 2x6 wall insulated with additional R20 on the residential side. In the shop area we have the same 1" spray foam with R20 fiber glass over that and that is it. In the 4" concrete slab we have 1/2" PEX tubing. Each side is ran the same, 3 loops per side of about 200 to 210 feet of tubing per loop, ran 6" apart. So we have two zones each with 3 loops. For our heat source we use an LP High Eff HWT. This is an open loop. When the floor is calling for heat from the Line Voltage T/S we have the problem of only moderately hot water for shower, acceptable. The upper story is still under construction, but fully insulated although there is no heat run up there yet. As well I still have a 4 x 8 opening into the shop area to allow for easy material delivery. This will be closed up shortly. OK now the issue. We can not get the temp over 20C, which is comfortable enough for me but I am concerned about colder weather coming. The HWT seems to be firing for a very long time and when I go to the T/S and see where it is it turns off just by adjusting it down slightly. It is almost there but not quite. Do I require a better HWT or maybe an additional one, or ???

You have not provided the information needed (your insulation type etc. really doesn’t matter).

What heating zone are you in?
I don’t care if your water heater is high efficient; what’s its output. What’s its input?
What’s the temperature of the loop going out and coming back in?
What’s the temperature differential OA versus IA?
What are the engineered design parameters?

If you’re stealing hot water from the domestic hot water side to heat your building, and the temperature falls, you are obviously not properly sized.

If your thermostat doesn’t reach setpoint then you’ve obviously reached your balance point for the OA conditions at the time you notice this.

Is it going to get worse? Of course it will.

Did we mention that this type of heating is “radiant” not “convection”?

Your thermostat is measuring convection air temperatures and operating a radiant heating device. They don’t speak the same language.