Linda,
I have a AquaHot which is different so I am not sure of all your options. Cold does affect the spray pattern of the nozzle due to fuel viscosity change, the pump pressure may be lower due to lower voltage available and more mechanical drag, and ignition by spark may be affected if the mist of fuel is not quite right along with the air flow being provided.
If there is an option to use an engine coolant loop to heat the HydoHot or an auxiliary heater with an electric element that can circulate and heat the HydroHot you will have a much better chance at getting ignition. If you do not have that option then I would have the generator running or some source of charging voltage on the batteries for a reasonable period of time before I tried to fire up the HydroHot. Warm batteries will provide more output and charging will raise the temperature some. Also the voltage level will be higher which will provide a better chance of ignition of the HydroHot. If you had a ceramic plug in heater with a fan, like a Polonius Disk furnace, and let it run overnight in the HydroHot compartment it could possibly help. It would take quite some time to bring the temperature up of the furnace given the cold but every bit of advantage might help.
Later Ed