The expansion pipe or vent pipe is a pipe associated with a cold water storage cistern or a feed and expansion cistern. Its purpose is to accommodate the expansion of water when it is heated, and to allow any excess water to vent harmlessly into the cistern. It also serves as an escape route for any air bubbles or gases which have formed.
Expansion pipe – cold water storage cistern
On a hot water cylinder, the hot outlet at the top of the cylinder is tee’d off. One pipe from this tee takes delivers hot water to all of the outlets in the home. The other rises up to the loft, up above the cold water storage cistern and bends down into it, without touching the water. On a cold water storage cistern fitted with a Byelaw 30 or Byelaw 60 kit, the lid of the cistern should contain a rubber grommet in order to admit the expansion pipe.
Air bubbles which naturally form inside the hot water cylinder when the water is heated can rise up and out of the cylinder, and up and out of the plumbing system through the expansion pipe. This prevents the formation of air in the system, which may lead to an airlock.
The expansion pipe on a cold water storage cistern is usually a length of 22 mm copper pipe. It should never be fitted with any valves, as this would open the possibility for hot water to be contained without anywhere for it go when it expands. This could be dangerous.
The expansion pipe should also never dip into the water in the cistern. This would create a gravity circuit between the hot water cylinder and the cold water storage cistern. Since hot water is less dense than cold water, hot water in the cylinder would flow up through the expansion pipe and into the cold cistern. The water would circulate in this way until the entire contents of the cold cistern has been replenished with hot water from the hot cylinder.
Expansion pipe – feed and expansion tank
On the feed and expansion cistern for the central heating, the vent pipe is a 22 mm copper pipe which rises above the cistern and bends down into it. The top of the bend should be at least 40 cm (15.75 inches) higher than the surface of the water in the cistern. This helps to ensure that water isn’t inadvertently pumped into the cistern when the boiler is on. This problem is known as pumping over, and will enrich the water in the cistern with oxygen. If left untreated, the oxygen-rich water will damage the system by promoting internal corrosion.
In order to prevent pumping over, the vent pipe should originate on the flow from the boiler behind the pump.
In the event of negative pressure in the vent pipe, air may get sucked into the system. This problem may occur when the feed pipe and vent pipe originate too far apart from each other on the system.