Hi I have electric forced air Lennox furnace sentinel ec10
Blower runs but no heat coming from elements. What do I do to get heat?

Could be a bad thermostat, bad control board, bad sequencer. What happens when it calls for heat?

When it gets really cold outside (i.e. below -10) the heat does not seem to work as well. And the thermostat turns off the heat sometimes when it's that cold. Very inconsistent.

Well, is it no heat or "no" heat when it is -10, big difference. You need more BTU in than what goes out. A lot can depend on insulation, air leaks, windows etc. I believe capacity is your problem. What is the BTU rating for your heater? How many square feet in your house? Where are you located? Please clarify to entire situation.