Toller wrote:
....
Will a low temperature of 30 overnight freeze anything. 25? 20?

Well, the answer to that is, obviously, yes. Now. is that "anything" a
plumbing line in a protected location and is "freeze" solid enough to
actually do damage--of course not--for one night. Now, what are the
daytime temperatures, where is the pipe, how long a run is it, is it
hot/cold water, is it in sunlight or shade, is ...???

Seems like a common enough problem that there ought to be some information
available on it.

There _is_ information available--water (pure water at atmospheric
pressure, that is) freezes at 32F, the coefficient of expansion owing
to the phase transition is known, the rupture strength of piping can be
estimated (although for old galvanized, for example, the existence of
corrosion weak points is problematical), estimates can be made for what
heat transfer coefficients/rates would be applicable based on geometry,
wind/air speed, etc., from which one could estimate how long it would
take for any given piece of pipe to freeze given a temperature profile.
Obviously, with so many variables, it is essentially impossible to
provide any conjecture that would be of any use to your specific
situation sight unseen.

As I noted previously, I'd not worry about the first occasional frost
at all. But, as critical as the low temperature is what are the daily
high temperatures (and daily temperature profieles) and what
temperature does the house achieve during the day? If it's 25F at
night but warming up to 70F pretty quickly during the day, obviously
that's a _WHOLE_ lot different than 25F at night w/ daily highs in the
low 40s, say, and cloudy and damp and windy. To reiterate yet again,
there are far too many variables and unknowns to provide much
quantitative information. All I can say is it will require getting
below freezing at the location of the pipes for a minimum of several
hours for there to be any significant chance of any damage. What it
will take for your cottage to reach that is anybody's guess, yours
surely should be far better than any of ours.

Seems like if it's such a big deal to winterize and you want to avoid
that until the last possible moment the thing to do would be to either
make some simplifications to the process by modifications to the system
or add some safeguards such as heat tape, insulation, whatever seems
required given whatever it is you have...