In many generic combustion models, one finds that a combustion wave will de
velop with a specific wave speed. However, there are possible initial tempe
rature profiles which do not evolve into such waves, but rather die out to
the ambient temperature. There can exist, in some models, a clear distincti
on between those initial conditions that do evolve into combustion waves an
d those that do not; this is sometimes referred to as the watershed initial
condition. When fuel consumption is considered to be negligible, analytica
l methods can be used to obtain the exact watershed. In this paper, we cons
ider the problem of determining pseudo-watersheds and ascertaining the rela
tionship between these pseudo-watersheds and the exact watersheds. In the p
rocess a novel weight-function approach for infinite spatial domains is dev
eloped.