On a vented central heating system there is a feed and expansion cistern, usually located in the attic. This cistern supplies water to the system when required – for example when filling. It also serves a more important purpose which is to allow for expansion. When water is heated it expands and in a vented system there needs to be some way of safely accommodating this.
Feed and Expansion Cistern
The water supply to the cistern will have a stop valve allowing for maintenance and isolation. The flow of water into the cistern is controlled by a float valve which opens or closes according to the water level in the cistern. If the level drops below a set point, the valve will open allowing water in from the supply pipe. As the water level rises, the float will rise and gradually shut off the valve.
The water level in the cistern is set when the system is cold and will normally be fairly low. The reason for this is to allow for the water expanding as the system heats up. If you look at the level when the system is cold and again when it is hot you will see just how much expansion takes place. The outlet from the cistern to the boiler should not have any stop valve as this could cause serious problems if it were turned off. The cistern will be protected from overfilling by an overflow pipe. This is fitted above the maximum water level and will allow water to freely escape should the float valve get stuck or fail.
The open pipe running up from the boiler and overhanging the feed and expansion cistern is a safety vent pipe. This rises up next to the cistern and above the water level by about 450 mm before bending back down over the water. The purpose of the safety vent is to allow for increases in pressure caused by the circulating pump. For obvious reasons this safety vent pipe will not have a stop valve fitted.
If you have water continually or regularly flowing from this pipe you will need to investigate and remedy this. This problem is referred to as pump over and there are two common causes
If the pump speed is set too high, this may cause an increase in pressure which pushes the water up and out of the vent pipe into the feed and expansion cistern. Generally the pump speed is set when the system is installed and should not need to be changed. However, it may have been changed at some point to accommodate extending the system or possibly in an attempt to fix a system which has become partially blocked with sludge. The latter is not a good way to deal with this and the only solution is to clean the system properly.
Poor design of the central heating system may be another factor. If the position of the pump is not considered carefully then it can end up creating excessive pressure and force water back up the vent pipe. If this is the case then you may need to relocate the pump, and possibly some pipe work as well, to prevent the problem occurring. Generally speaking the optimum arrangement is where the cold feed from the F&E cistern is to the flow, before but close to the safety vent pipe, and then the pump being mounted in the flow after these.
On modern systems the feed from the cistern and the open vent may be closely connected on the flow with an air separator. Air can get trapped here and needs to be removed from the system. Air getting into a central heating system will encourage corrosion and lead to early failures. The air separator, as its name suggests, enables air to be released at this point and this will also reduce system noises.