Things You'll Need
Shut off the power to the boiler and allow to cool. You do not want hot water in the system as you will be opening it up and you could get burned by hot water or steam.
Drain the entire heating system. Most boilers will have a valve on the side that can be opened to drain the system. Be sure you either connect a hose to the valve or have plenty of buckets as there will be quite a bit of water in the system. You want it completely drained so it does not leak and damage anything.
Locate the radiator(s) you are going to remove. Each radiator will have a intake and outlet pipe where hot water comes in and cool water is removed. There should be a threaded nut on each of these pipes connected to the radiator. Unscrew the nuts with a monkey wrench so these pipes are no longer connected to the radiator. The radiator should now be free-standing and can be removed. These are usually made of cast iron and can be extremely heavy. Be sure to have help when removing as one person probably cannot lift it by himself.
Locate the intake and outlet pipes in the basement. Using the hack saw, cut off the part of these pipes that goes through the floor. These pieces will probably be about one-foot long. Remove these pieces and discard. You may need to expose these pipes by removing any sheetrock or tiles that have been used as a ceiling in the basement. You will want the entire area you plan to heat exposed in the basement.
Attach PEX tubing to the underside of the floor. You will want to run the tubing back and forth between the joists in a "wave" pattern so the waves are about 6-8 inches apart. This will provide adequate coverage for the entire room. Us the u-connectors to attach the tubing to the floor. These connectors can usually be found with the PEX tubing at home centers. Be sure not to cut or puncture the tubing as this will result in the system leaking.
Attach one end of the PEX tubing to the intake pipe and the other end of the tubing to the outlet pipe. This can be done using a crimping tool used especially for PEX tubing.
Attach the aluminum diffuser plates with screws to the underside of the floor. You will want to cover the PEX tubing with these plates. These plates help direct the heat from the pipes upward and through the floor to increase the efficiency of the system. Be very careful when attaching the plates that you do not puncture any of the piping.
Refill the boiler system with water. There should be directions on the boiler on how and where to fill the system. Be sure you do not over-fill as this could result in too much pressure in the system and a potential problem. Turn the system on so that hot water runs through the pipes. Examine to make sure there are no leaks.