That's a loaded question, Chris. What has to happen is for the water to freeze solid. According to what I've studied in physics, water starts to expand around 34 degrees, and the expansion rate increases once the fluid is totally frozen, thus causing the rupture. Since water freezes at 32 degrees, theoretically a freeze rupture can take place at that temp if it stays there long enough. Obviously as the temp drops, it takes shorter time frames to freeze, and alot of the answer in this case would be dependent on surface area of the HX, what is it mounted on, what is it insulated with, etc. Takes a LONG time at 32* to do damage, but who's to say at 10* (which is what it's been on my front porch the last three mornings). If you're concerned, you may want to stick a heat lamp on it temporarily - that's what we do. Better safe than sorry.