\documentclass[border=3mm]{standalone}

% Configure siunitx before LuaCoolProp.  LuaCoolProp deliberately preserves
% these choices and routes all generated LaTeX quantities through them.
\usepackage[
  output-decimal-marker={,},
  group-separator={\,},
  group-minimum-digits=4,
  per-mode=symbol
]{siunitx}
\usepackage{luacoolprop}
\pgfplotsset{compat=1.18}

\begin{document}
\LCPPHDiagram[
  fluid=R134a,
  enthalpy symbol={\mathcal H},pressure symbol={\pi},
  quality symbol=x,temperature symbol={\theta},
  entropy symbol={\sigma},specific volume symbol={\nu},
  quality values={0,0.5,1},
  isotherm=true,temperature values={-20,0,20},
  isentrope=true,entropy values={1.6,1.8},
  isochore=true,specific volume values={0.01,0.05},
  labels=true,label every=1,
  axis options={
    width=10cm,height=7cm,font=\small,
    title={User-configured \texttt{siunitx}: \num{12345.6}}
  }
]
\end{document}

.