Shut off the boiler and allow the system to cool before doing any work on the hot-water furnace. Shut off any water supply pipes to the furnace. Look for a line from the household cold-water system to the furnace boiler or expansion tank, and close the valve.
Drain the hot-water furnace system. First, open the bleeder valves at each radiator throughout the home to allow air into the system. Then open the valve at the lowest point in the boiler assembly at the furnace. Drain the water into a plastic bucket. When the bucket gets nearly full, replace it with another bucket, dump the first bucket, and continue to swap the buckets until all the water has drained. Water will not drain from the radiators and pipes unless air can enter the system. The draining of the hot-water furnace and radiators can take as long as an hour.
Add boiler antifreeze to the furnace. Close the drain valves and locate the valve to the boiler or to the expansion tank. Add the boiler antifreeze to the tank, which will prevent freeze damage from any water left in the boiler after the draining process and also protect the boiler from corrosion. Consult the hot-water furnace owner’s manual for information on the size of the boiler.