Bold header (text) row with siunitx S column and column aligment

Question

Ok, I want to have the first row of my table bold faced: Make first row of table all bold, done! But my table has a lot of numbers so siunitx comes in handy with its column specifier S, then the first solution makes the S columns ragged left (why?).

Seems like Bold row in table aligned with dcolumn: A solution with package siunitx could be what I'm looking for, but no. The same thing happens with Heiko's proposal. This is obviously because of the text Header, but why is it happening and how can I get around it?

The following MWE incorporates both solutions and the compilation result is placed below.

\documentclass{standalone}
\usepackage{siunitx,array,booktabs,multirow}
\sisetup{output-decimal-marker={,}}
\newcolumntype{$}{>{\global\let\currentrowstyle\relax}}
\newcolumntype{^}{>{\currentrowstyle}}
\newcolumntype{L}{%
  >{\rowstyle{\relax}}l%
}
\newcolumntype{B}{%
  >{\currentrowstyle}S[detect-weight]%
}
\newcommand{\rowstyle}[1]{\gdef\currentrowstyle{#1}%
  #1\ignorespaces
}

\begin{document}
\begin{tabular}{L^cBB}
    \toprule
    \rowstyle{\bfseries}
    \multirow{2}{*}{Day} & {Running time} & {Water bag weight} & {Frost formation rate} \\
 & [days] & \multicolumn{1}{c}{[\si{\gram}]} & \multicolumn{1}{c}{[\si{\gram\per\hour}]} \\
\midrule
    26.09.2016  &   4   &   1098,2  &   11,4    \\
    27.09.2016  &   1   &   329,8   &   13,5    \\
    28.09.2016  &   1   &   288,4   &   11,8    \\
    29.09.2016  &   1   &   291,7   &   11,9    \\
    \midrule
    \rowstyle{\bfseries}
    Average         &       &       &   12,2    \\
    \bottomrule
\end{tabular}

\begin{tabular}{$l^c^S^S}
    \toprule
    \rowstyle{\bfseries}
    \multirow{2}{*}{Day} & {Running time} & {Water bag weight} & {Frost formation rate} \\
 & [days] & \multicolumn{1}{c}{[\si{\gram}]} & \multicolumn{1}{c}{[\si{\gram\per\hour}]} \\
\midrule
    26.09.2016  &   4   &   1098,2  &   11,4    \\
    27.09.2016  &   1   &   329,8   &   13,5    \\
    28.09.2016  &   1   &   288,4   &   11,8    \\
    29.09.2016  &   1   &   291,7   &   11,9    \\
    \midrule
    \rowstyle{\bfseries}
    Average         &       &       &   12,2    \\
    \bottomrule
\end{tabular}
\end{document}

Answer 1

The issue here is \relax. This is a 'do nothing' command in TeX but is non-expandable. The siunitx package takes this as being 'end of number' in a table cell: there's not reason to expect one in the middle of a number. As such, you need to arrange that \currentrowstyle is something else, for example an empty macro, when it's not 'active':

\documentclass{standalone}
\usepackage{siunitx,array,booktabs,multirow}
\sisetup{output-decimal-marker={,}}
\newcolumntype{$}{>{\global\let\currentrowstyle\empty}}
\newcolumntype{^}{>{\currentrowstyle}}
\newcolumntype{L}{%
  >{\rowstyle{}}l%
}
\newcolumntype{B}{%
  >{\currentrowstyle}S[detect-weight]%
}
\newcommand{\rowstyle}[1]{\gdef\currentrowstyle{#1}%
  #1\ignorespaces
}

\begin{document}
\begin{tabular}{L^cBB}
    \toprule
    \rowstyle{\bfseries}
    \multirow{2}{*}{Day} & {Running time} & {Water bag weight} & {Frost formation rate} \\
 & [days] & \multicolumn{1}{c}{[\si{\gram}]} & \multicolumn{1}{c}{[\si{\gram\per\hour}]} \\
\midrule
    26.09.2016  &   4   &   1098,2  &   11,4    \\
    27.09.2016  &   1   &   329,8   &   13,5    \\
    28.09.2016  &   1   &   288,4   &   11,8    \\
    29.09.2016  &   1   &   291,7   &   11,9    \\
    \midrule
    \rowstyle{\bfseries}
    Average         &       &       &   12,2    \\
    \bottomrule
\end{tabular}
\end{document}