"Toller" wrote in message

But last night there was a frost advisory, so there was a chance the
temperature would hit 32. Given the precision of the weather forecast,
that means there is a chance it would hit 30 for a few hours.

So my question is basically, if they forecast is for a low of 30, are my
exposed pipes likely to freeze?
If not, what overnight low temperature do I have to be concerned about?

It still comes back to that old time/temperature conundrum. The pipes will
freeze at 30, even at 31.5 if they are exposed to that long enough. Chances
are the pipes were at the high point of the temperature of the day or the
temperature of the water coming into them so it still comes back to how long
will the sensible heat last in that much mass. I'd guess overnight with a
couple of hours dip to 30 is not a big deal, but prolonged will be.