Water – My plumber installed the hot water baseboard backward. The feed and return are hooked up backward. What problems will this cause

This type of radiator should be warm to the touch, not hot like an cast iron radiator. If you touch the fins with two or three fingers it should feel hot enough to be very uncomfortable. Lastly, go to incoming pipe coming up through the floor again, it should be very hot, too hot to touch for more than a few seconds. Usually the water moving through the pipe is 150F to 180F. Then feel the outgoing pipe again, it should be almost as hot as the incoming side. If the outgoing is not hot you have a blockage or bad circulater. You should not be able to hold your fingers on either the in or out flow side these pipes for more than a few seconds. If you have a automatic bleeder on the radiator get rid of it and install a manual bleeder. Then run heat and bleed again. Then again one hour later.

If the pipe is very hot going in and out of the baseboard then its probably not a problem with the boiler. You can try turning the water temperature up on the boiler. They normally go up to 200 but i would not go higher than 180 for efficiency. Every boiler is a little different on how to do this. Also if you have exposed pipes or radiators it should be lower to avoid burns

It is most likely something in that basement that changed letting in more cold air or its just being extra cold outside.