Even the slowest speed on a VS is not all that slow in terms of flow rate (15-20 GPM) and should be more than sufficient to prevent freezing. However, you should have contingency plan in case there is a power outage or the temperature sensor fails. One cost effective method to increase the amount of time it takes to freeze the pipes is to simply cover the equipment with a tarp. This reduces heat loss from the plumbing and can increase the time to freeze by 3x. Adding a light bulb will prevent freezing of the pad equipment all together even without the pump running.

However, if you are only running the pump without some of these extra protection, then make sure water flows through every part of the plumbing. More complex setups often neglect portions of the plumbing when automated freeze protection is used.