I am in VB, VA and I keep my FG pool open all year as well. You don't need to keep the heater running. Just run the pump maybe 1/2 hr per day or enough time to clean out leaves, etc. I usually will fire it up manually if it gets really really cold but in VA the weather is rarely cold enough to affect underground pipes.

Thanks for this. One thing I'm still confused about is what you do about the pipes if you're going to leave the heater off. If the pump's going, isn't water running through the heater? And if so, isn't there a risk of the water freezing in there if you don't have it on? I didn't notice any obvious lever to shut off water to the heater alone. (I'm a new pool owner and finding it all somewhat bewildering!)

Yes there is always a possible concern of having pipes freeze up if they are outdoors and exposed. You would want to run the pump when the temp is below freezing so you would want to run the pump when the temp is going to be below freezing for a considerable time. Hayward is just telling you not to fire up the heater when the temp is below 40. Fortunately we have not really had any extreme cold in VB for a few years. You also could winterize / drain the pipes but still leave your pool open so that you can see it.