I also have an unheated garage and was worried about that during my first winter. Despite leaky north facing garage doors, and a very severe winter last year, my garage didn't get below 40 degrees. It was not an issue. If there's even the slightest movement of water in the pipes, they should be alright without any amendments.

My FHW pipes in my unheated garage froze and split,, all 26 feet,, what a mess. Had to tear apart the ceiling below where the pipes were inorder to replair them. The builder had not insulated them. After repairing , added 8" of insulation and repaired the ceiling. Have not had a problem since.

My pipes froze and burst twice. My basement flooded until the well went dry trying to replenish the water in the heating system.

There is a product called ThermGuard which you attach to your thermostat and it will circulate hot water periodically. You can program it for whatever rate you want. I have mine set for 3 minutes every 3 hours. I live in Montana and it does the job. I am not worried about freezing pipes with my wood stove.

It does not sense outdoor temperature, but you can easily turn it on and off when you want. It remembers all your settings even when powered off.

Good luck!

John