\setuppapersize[S8]

\setupbodyfont[24pt]
\setupinterlinespace[height=0.8]

\setuppagenumbering[location=]

\definelayer
  [header]
  [width=\paperwidth, height=\paperheight]

\define[2]\setheader{%
  \resetlayer[header]%
  \setlayer[header]%
    [preset=lefttop, x=10mm, y=5mm, width=15cm, heigth=40pt]%
    {#2}%
}
\define[2]\setsubtitle{%
  \setlayer[header]%
    [preset=lefttop, x=10mm, y=\dimexpr(5mm+40pt), width=15cm, heigth=22pt]%
    {#2}%
}

\setuphead[chapter][command=\setheader, page=yes, before={\setupbackgrounds[page][background={header}, state=repeat]}, after={\strut\blank[-2*line,samepage,disable]}, insidesection=\vfill, aftersection=\vfill]
\setuphead[section][command=\setsubtitle, after={\strut\blank[-2*line,samepage,disable]}, before=]

%\showboxes
\showframe

\starttext

\startchapter[title={Test 1}]

\input knuth
\input knuth

\stopchapter

\startchapter[title={Test 2}]

No page-breaks here.

\stopchapter

\startchapter[title={Test 3}]

\startsection[title={Subtitle}]

\input knuth
\input knuth

\stopsection

\stopchapter

\stoptext