Why has no owb company not put in an element or two inside the water jacket to be a backup in case the fire dies? Seems they could kick on at 120 or something and it would keep the water from freezing. Put them at bottom of water jacket to avoid having evaporation bare them.
I am guessing because it isn't necessary. The moving water through my system would be kept well above freezing when my oil furnace kicks on. I inadvertently tested this theory a few weeks ago when my fan quit and I didn't have my back up for a few days. I didn't feel like letting my gasser up-draft so I just let the fire go out and waited for UPS. Temp dropped to a number above zero, oil furnace kicked on and that was that.
Should a pump quit I might have different results. My house began heating the boiler water jacket at some point. It would be ill advised to not have a backup pump I suppose but an element in the waterjacket with no pump is not going to prevent my lines from freezing.
On another note I am not sure I would want the added risks involved in an element. Any current leakage through element to waterjacket housing will promote cathodic corrosion and you could end up springing a leak.