I have the system up and working in my 40X60 shop. I use propane since we do not have natural gas available. Next fall, I hope to add solar panels to heat the water and use the propane as a back up. When I installed the system, we designed for the addition. We will install a reservoir to collect/hold the heated water from the panels for night time. It gets a little involved to drain the panels back into the reservoir at night so the panels do not freeze and to send it back to the panels the next day when the panel temp gets above the reservoir temp. I had this system in Utah and the panels work most every day, even with cloud cover and -15F outside temp. This winter, the thermostat has been set at 55* and I have been working in my shirt sleeves and the feet never get cold!! So far, I have burned 240 gal of propane. Not bad for heating 2400 sq feet and the heat has been on since mid Oct. Of course the mountains of North Carolina are not Canada in the winter time.
By the way, you did elevate the PEC to be in the middle of the slab thickness didn't you?