Is this open supply duct in my vented crawl space necessary?
Without addressing the moderately unanswerable &quot;do I need this duct&quot; since that depends on a lot of factors specific to your house/insulation/comfort/budget/etc.
Moderately standard freeze protection thermostat setting is 45°F/7°C and &quot;freeze alarms&quot; at 41°F/5°C - running much closer to freezing than that risks freezing in spots that might be colder than where you are measuring the temperature, or in the case of the alarms does not give you time to take action.
The level of engineering you need to &quot;not freeze pipes&quot; while running them as low as 33°F/0.5°C is considerable and rarely engaged in. i.e. standard &quot;heat trace&quot; (the quality electric pipe heating tape that is the thermostat) starts to heat at 50°F/10°C, and (along with an insulation overwrap) is the standard (quality) solution to below-freezing crawlspace pipes down to sub-zero °F temperatures. The cheap heat tape with an external thermostat is much more prone to fail, but is a common approach by those who like to make their own mistakes rather than learning from the mistakes of others.