The Expansion Pipe Position in Central Heat Systems

Boiler System Operation

• A central home boiler heating system uses a hot water tank to circulate heated water through a network of pipes that run to baseboard or radiator register units. The registers emit heated air into different areas within the home. The boiler tank operates under specific temperature and pressure conditions designed to maintain needed supplies of water in the tank. As the water in the tank warms up, the tank pressure increases as water molecules expand. This increased pressure may cause water amounts to exceed a boiler tank’s capacity.

Function

• An expansion pipe runs from the boiler tank to the expansion tank. When water pressure levels exceed the boiler tank's capacity, excess pressure and excess water exit through the expansion pipe and into the expansion tank. For most central heating boiler systems, normal operating pressures inside the boiler tank are below 30 pounds per inch, or psi. Without an expansion system in place, pressures that exceed 30 psi would cause excess water to drain through the system’s relief valve. On a repeated basis, this will result in ongoing water loss from the system and eventual erosion of system components.

Open Expansion Systems

• Open expansion systems are used with older boiler heaters or cast iron boiler systems. On these models, the expansion tank sits on top of the boiler tank unit, typically in a loft or attic area. Since the tank sits on top of the central heating boiler, the expansion pipe position -- also known as the safety pipe or the vent pipe -- is on the top part of the boiler tank. The pipe has a hook shape that extends from the top of the boiler tank and hooks upward to the expansion tank's location. The pipe position makes it easy for excess pressure and water to rise upward through the pipe and into the expansion system.

Sealed Expansion Systems

• Sealed expansion systems are generally part of newer boiler units. A sealed system consists of an expansion vessel, a safety blow-off valve and a filling loop. The safety blow-off valve replaces the expansion pipe found in open system designs. Sealed systems differ from open systems in that open systems use an open air reservoir tank as the expansion tank. Sealed systems operate from a pressure gauge that monitors boiler tank pressures on an ongoing basis. Sealed expansion systems typically sit directly on top of the boiler tank and are part of the overall boiler system unit. Instead of connecting to the expansion tank, the expansion pipe runs upward and into the relief valve line before entering the expansion tank. In effect, the expansion pipe and the boiler relief valve are both tied into the boiler expansion system.