  We consider the homogeneous heat equation in a domain $\Omega$ in
$\mathbb{R}^n$ with vanishing initial data and the Dirichlet boundary
condition. We are looking for solutions in $W^{r,s}_{p,q}(\Omega\times(0,T))$,
where $r < 2$, $s < 1$, $1 \leq p < \infty$, $1 \leq q \leq \infty$. Since we
work in the $L_{p,q}$ framework any extension of the boundary data and
integration by parts are not possible. Therefore, the solution is represented
in integral form and is referred as \emph{very weak} solution. The key
estimates are performed in the half-space and are restricted to
$L_q(0,T;W^{\alpha}_p(\Omega))$, $0 \leq \alpha < \frac{1}{p}$ and
$L_q(0,T;W^{\alpha}_p(\Omega))$, $\alpha \leq 1$. Existence and estimates in
the bounded domain $\Omega$ follow from a perturbation and a fixed point
arguments.
