Alot of imported pipe have a messed up o.d. try tapering the end a little with a grinder,do you have the correct die ie alloy,high speed,stainless,did you install the teeth in the corerect position.
What ever you decide on use their fittings and hose,mixing brands won't work the o.d. of hoses is different manufacturer to manufacturer,series to series.
I've had a 19'5"x8' for three years no problems, did not plan on heating garage but ended up getting a heater so I wish I'd got it with insulated double pane windows.
The combined maintanance facility they just built at FT.Dix has like 40 bays all with radiant heaters on the ceiling,radiant floor heat and forced air heat,unbelievable setup.