So a helpful note. With these weather extremes your heating system is working very hard. Do anything to give it a break. Want to do some baking? Do it now. Good time. Want to use the oven for anything, good time. Kitchen can generate some heat to upstairs at these times.
Place rolled towels over bottom of back and front doors. Unless you have the best possible doors and seals you'll be seeing some heat loss at doors.
If you have some space heaters it not the worst idea to crank a couple up just to heat the house a bit and aid your furnace. Your furnace doesn't require this, but its something to think about so that the furnace can get a bit of a break.
if you get a furnace break down (and they typically take place right at these lows because its the hardest your furnace ever has to work) then use space heaters around the house, move them around. get some heat in there. You may panic but with oven on, space heaters on (we have 3 just for emergency type use) you'll be able to keep your house warm enough through weekend and until the temps pick up a bit. Furnace call volumes would be a nightmare right now. With some modest work even with a furnace breakdown you can keep your home warm enough for days.
Reducing temp slightly to 18-19C is also a way to give your furnace a break. Get out the sweaters, blankets. Its much harder for furnace to work for that each extra degree C.