  The energy budget in common-envelope events (CEEs) is not well understood,
with substantial uncertainty even over to what extent the recombination energy
stored in ionised hydrogen and helium might be used to help envelope ejection.
We investigate the reaction of a red-giant envelope to heating which mimics
limiting cases of energy input provided by the orbital decay of a binary during
a CEE, specifically during the post-plunge-in phase during which the spiral-in
has been argued to occur on a time-scale longer than dynamical. We show that
the outcome of such a CEE depends less on the total amount of energy by which
the envelope is heated than on how rapidly the energy was transferred to the
envelope and on where the envelope was heated. The envelope always becomes
dynamically unstable before receiving net heat energy equal to the envelope's
initial binding energy. We find two types of outcome, both of which likely lead
to at least partial envelope ejection: "runaway" solutions in which the
expansion of the radius becomes undeniably dynamical, and superficially
"self-regulated" solutions, in which the expansion of the stellar radius stops
but a significant fraction of the envelope becomes formally dynamically
unstable. Almost the entire reservoir of initial helium recombination energy is
used for envelope expansion. Hydrogen recombination is less energetically
useful, but is nonetheless important for the development of the dynamical
instabilities. However, this result requires the companion to have already
plunged deep into the envelope; therefore this release of recombination energy
does not help to explain wide post-common-envelope orbits.
