You will need an Allen wrench and a needle nose pliers for this task. If the heat in the building is on (eg the radiators in the hallway are warm) but the heat in the unit does not seem to be working (even with the control turned all the way up to 5), the pin that controls the flow of water into the radiator pipes may be stuck. This is simple to fix and only takes a few minutes. Locate the control knob which is under the air conditioner.
Step 1: Loosen Control Knob Screw
Using a 5/64 inch Allen wrench, loosen the screw on the top of the control knob
Step 2: Pull Off the Control Knob
Gently twist and pull the control knob until it comes off
Step 3: Pull Pin
Using a needle nose pliers, gently pull on the pin that was covered by the control knob. You don't need to pull it out all the way, just enough to loosen it. You will hear water flow into the pipes as you pull it out. If you accidentally pull it all the way out you can just push it back in to place.
Step 4: Put Control Knob Back in Place
Put the control knob back on the pipe. Push it on as far as it will go and then using the Allen wrench tighten the screw on top until it is firmly in place. You should be able to turn the knob left and right and hear it controlling the flow of water in the pipes if you have it on in the correct position.