How to re-position each chapter at the beginning of the page/

Question

:-) With the following preamble how can I reposition where the chapters start? Meaning each section is defined to start at almost the 2/3 of the page. There is a small space and then goes the "Section 1: blah blah". Can I start each section at the beginning of the page like the other pages?

\documentclass[a4paper,12pt]{book}
\setcounter{secnumdepth}{-2}

\titleformat{\chapter}[display]{\normalfont\huge\bfseries}{}{20pt}{\Huge}

\let\cleardoublepage\clearpage
\usepackage[english,greek]{babel}
\usepackage[utf8x]{inputenc}
\usepackage{blindtext}
\usepackage[pdftex]{graphicx}
\usepackage[skip=2pt, font=small, labelformat = empty]{caption}

\newcommand{\gr}{\selectlanguage{greek}}
\newcommand{\en}{\selectlanguage{english}}
\usepackage{wrapfig}
\usepackage[paperwidth=17cm, paperheight=24cm]{geometry}
\usepackage[onehalfspacing]{setspace}
\usepackage{textcomp}
\useshorthands{;}
\defineshorthand{;}{?}
\usepackage{fancyhdr}
\fancyhf{}
\fancyhead[LE]{\nouppercase{\leftmark}}
\fancyhead[RO]{\nouppercase{\rightmark}}
\fancyfoot[LE,RO]{\thepage}
\pagestyle{fancy}

\renewcommand{\chaptermark}[1]{ \markboth{#1}{} }
\renewcommand{\sectionmark}[1]{ \markright{#1}{} }

Answer 1

You can patch \@makechapterhead and \@makeschapterhead to suppress the 50pt space inserted:

\documentclass{book}
\usepackage{showframe}% just for the example
\usepackage{etoolbox}

\setcounter{secnumdepth}{-2}

\makeatletter
\patchcmd{\@makechapterhead}{\vspace*{50\p@}}{}{}{}
\patchcmd{\@makeschapterhead}{\vspace*{50\p@}}{}{}{}
\makeatother

\begin{document}

\chapter{Test chapter}
\section{Test section}
\subsection{Test subsection}
\subsubsection{Test subsubsection}

\end{document}

The showframe package is not essential for the solutions; I used it only to draw a frame around the text area as visual guideline.