There’s a long thread on the topic here:
https://community.glowforge.com/t/lowest-safe-temperature-for-storage-of-glowforge-coolant-does-it-expand-freezing-point/16157?u=markwal&source_topic_id=17673I think we don’t have a definitive answer. I think that what Glowforge says is keep it above 40F for long term storage and warm it to 60F before operating.
The discussion in the topic I linked is primarily conjecture that since shipping temperatures are much colder than that and Glowforge doesn’t stop shipping when the temperatures are very low, the unit must be able to withstand colder temperatures for at least a short time. Problem is, I don’t think Glowforge tests long term storage below 40F so they don’t really know if it will be bad for the machine and if it does turn out to be bad for the machine, it won’t likely be covered under the warranty.
The thread also talked about some folks using heating mats to keep the interior of the Glowforge warm in case the problem is coolant in the tube. However, this technique is also not tested by Glowforge.