  The first high-resolution X-ray spectroscopy of the planetary nebula
BD+30$^\circ$3639 allowed to study X-ray emitting "hot bubbles" (HBs) of
planetary nebulae in unprecedented detail. We investigate (i) how diagnostic
line ratios are affected by the HB thermal structure and chemical profile, (ii)
if the HB chemical composition of BD+30$^\circ$3639 is consistent with the
H-poor (H for hydrogen) composition of the stellar photosphere, and (iii) if
H-rich nebular matter has been added to this HB by evaporation. We apply an
analytical, 1D model for wind-blown HBs with temperature and density profiles
based on self-similar solutions including thermal conduction. We construct
heat-conduction HBs with chemical stratification. The X-ray emission is
computed using the CHIANTI code. Our HB models are used to re-analyse the
high-resolution X-ray spectrum of BD+30$^\circ$3639. Our models reproduce the
observed line ratios much better than plasmas with single electron
temperatures. All the temperature- and abundance-sensitive line ratios are
consistent with BD+30$^\circ$3639 X-ray observations for (i) an intervening
column density of neutral H, $N_{\rm H} =
0.20_{-0.10}^{+0.05}\times\!10^{22}\rm\ cm^{-2}$, (ii) a characteristic HB
X-ray temperature of ${T_{\rm X} = 1.8\pm 0.1~ }$MK together with (iii) a very
high neon mass fraction of about 0.05, virtually as high as that of oxygen. For
lower values of $N_{\rm H}$, we cannot exclude that the HB of BD+30$^\circ$3639
contains a small amount of evaporated (or mixed) H-rich nebular matter. Given
the possible range of $N_{\rm H}$, the fraction of evaporated H-rich matter
cannot exceed 3% of the HB mass. The diffuse X-ray emission from
BD+30$^\circ$3639 can well be explained by models of wind-blown HBs with
thermal conduction and a chemical composition equal to that of the H-poor and
carbon-, oxygen-, and neon-rich stellar surface.
