If the outside air temperature is above about 30 degrees F, then yes; if the temperature is below that, then you should open the faucet to a slow trickle of water. If the water is moving, it probably won't freeze in the pipes and break them.
water expands when frozen.
if you put screens to keep birds out of the pipes the moisture from the exhaust will freeze, and the exhaust motor will not be able to vent there by tripping the furnace on a pressure switch fault. the pipes ie intake and exhaust should have a minimum distance of 15 inches apart, and the same above the ground to prevent snow drifts blocking them.
The gas pipes of a belt continuous furnace are typically made of stainless steel or copper.
There is no single best way to keep pipes from freezing. Insulating (wrapping) pipes is a common method. Leaving the cold water faucet open just enough for a slow drip is another method to prevent freezing.
At least one degree above freezing?
Could be a drain for the inducer motor if it has one and it depends do you have a a/c coil on your furnace if so it could be a drain for it.
Pressure has nothing to do with pipes freezing. It is just exposure to cold.
Anti freezing agents are used
Yes, they can
One can prevent pipes from freezing during cold temperatures by ensuring water runs through them. A good idea to do this is to let the tap drip. Draining the water system when going on vacation during cold spells can help pipes from freezing as well.
Result is broken pipes, so symptoms are: 1) no Water at faucet 2) leaking water elsewhere.
A furnace is a device to make air hot and spread it through a home via heating pipes.