Hot water systems are primarily used for heating in both residential and commercial buildings. There are two main types; a gravity fed system and a hydronic heating system. Hydronic systems use a pump to pressurise water and send it through pipes and into radiators. This system has all but phased out the much older style gravity heating system. Bleeding each of these systems means locating the bleeder valves on the heating pipes, and purging them of trapped air.
Bleeding hot water systems
Locate a bleeder valve on the hot water system piping. Ideally, locate the valve that is along the pipe run which is on the opposite end of where the heating pipe enters the building. The valve will be found adjacent to baseboard radiators, and will look like a jutting pipe nipple with a nut attached to it.
Turn your thermostat up 15 degrees so that the system calls for heat. This action gets the water flowing through the pipes, and is essential for proper bleeding.
Place a small container or cup beneath the valve that is to be bled.
Turn the bleeder nut slowly to open the valve.
Listen for hissing air to escape and be bled out of the pipes. This is a good sign..
Catch any water with your cup or container that begins to flow out of the bleeder valve. The hissing will stop at this point, and the valve can be closed.
Repeat this procedure on all the bleeder valves, working your way along the pipe run towards the point where the heating pipe enters the building.
Once the bleeder valve is closed, wait a few minutes and then feel the surrounding pipes for heat. If there is no heat, proceed backward along the loop, bleeding out each section of pipe until all the air is purged from the system. Once the air is purged, water will flow and the pipes will get hot.
Be careful. You will be dealing with very hot water. Take precautions to limit any spraying water by always making sure to open the bleed valves slowly and carefully.