Multi line header does not play well with geometry headsep parameter

[awa5114]

I would like to use the geometry package alongside fancyhdr. To control the distance between header and main text in my article documentclass, this question recommends use of the headsep parameter:

\documentclass[11pt, a4paper]{article}
\usepackage[includehead, headsep=5pt]{geometry}
\usepackage{fontspec}
\usepackage{fancyhdr}


\pagestyle{fancy}
\fancyhf{}
\fancyhead[C]{A header}%\\a new line in the header}

\renewcommand{\headrulewidth}{1.5pt}

\begin{document}
  \section*{Education}

\end{document}

This seems to work fine, and modifying the headsep indeed modifies the distance between the header and the section title.

However, it so happens that my header is multi-line. as soon as I uncomment line 9 as follows:

\fancyhead[C]{A header\\a new line in the header}

The section title clashes with the header, which I do not want.

How can I get headsep to remain aware that my header is now multi line?

[davidcarlisle]

as noted in comments on this question, the problem is not headsep but headheight it is a user-setable length that you need to set to be at least the height of the actual content. Especially if as here you change the heading adding tall content after the page head has been set up then there is not really anything geometry can do here.