There are two main types of vented hot water system. The older type is the direct hot water system and the more modern is the indirect hot water system.
Hot Water Cylinder
With both, there is a hot water cylinder which supplies hot water to the taps in the house. The hot water cylinder is fed directly by its own supply from the cold water storage cistern (usually in the attic) and this feed incorporates a stop valve allowing for maintenance and repairs. This is the valve used to switch off the hot water supply when you need to carry out work on the system – for example when changing a tap. Some people find it a little confusing that the way to turn off the hot water supply is to close the valve on the cold supply feeding the cylinder. The reason is that the pressure which forces the hot water to flow at a hot tap is caused by the head of water from the cold water storage cistern. The weight of water acting down via the cold feed forces the water out of the top of the hot water cylinder.
The hot water distribution pipe is connected at the top of the cylinder and will run horizontally for at least 450mm before it connects with the vent pipe. This vent pipe runs back up and over the cold water storage cistern. It keeps the system at normal air pressure and also allows for overheated water to escape. The horizontal section of pipe is there to ensure correct circulation. The distribution pipe then continues on to each of the hot water outlets in the house.
As the water heats up it expands and pushes back up the cold feed so the water level in the cold water storage cistern will rise and fall to accommodate this.
Combined Hot Water Storage Cylinder
On some systems, the cold water storage cistern is mounted immediately above the hot water storage cylinder in a combined unit. From an installation point of view this has some advantages such as the reduction in pipe work required. However, there are some significant disadvantages as well which should be taken into account. Because the cold water storage is not in the attic it will likely be significantly lower and this reduces the head of water. The head of water, or the weight of water acting down, determines the pressure at the hot water taps. For this reason, the combined unit needs to be positioned so that it is as high as it can be above the uppermost hot tap in the house. If it were positioned at the same level, there would be no water pressure at the taps.
The feed from the cold water storage part is similar to that for a regular system and a float valve also controls water coming into it. The open vent is still required to maintain atmospheric pressure in the system and this runs from the cylinder up into the cold water storage part so that it terminates above the water level.
Direct Hot water Supply
Seen in older properties where the hot water supply system has not been modernised, this works on the gravity system. Hot water is less dense than cold and this creates a flow within a circuit. Water may be heated by a boiler in which case the primary flow goes from the boiler to the hot water cylinder, heating the water within, and returns from the bottom of the cylinder to the boiler again. The hot water cylinder may not be connected to a boiler at all, and simply have an immersion heater element inside to heat the water.
Indirect Water Supply
With an indirect hot water system, a primary circuit from the boiler feeds a heat exchange element in the hot water cylinder. The hot water in the cylinder itself is totally separate from the water in the primary circuit. The two do not mix at any point. The water in the cylinder is heated by the coiled hot water pipe – the heat exchanger - from the primary circuit.
Indirect Hot Water Feed and Expansion Cistern
This type of indirect hot water circuit is the most common with vented systems. It should be noted that the primary circuit from the boiler has a completely separate feed and vent system from the one used for the hot water itself. As was explained earlier, the hot water cylinder is fed by the cold water storage cistern. But with the indirect system, a separate cistern is also required for the boiler’s primary circuit. This is known as the feed and expansion cistern. The purpose is similar but it ensures that water from the two separate circuits cannot mix. If the primary circuit was vented over the cold water storage cistern, it would be possible for the two to mix resulting in contamination of the water supply in the house.
The feed and expansion cistern has two main functions. It is used as a supply of water to fill the primary system, but more importantly, it allows for expansion of the water as it heats up. Water expands by some 4% when it is heated so some way of accommodating this is needed. With the feed and expansion cistern, the additional volume is forced back up the supply pipe and into the cistern. If you look at the water level in this cistern when the system is cold and when it is hot, you will see how different the water levels are. This is the amount by which your water has expanded. It follows that when the system is cold the water level should be set fairly low down in the cistern. A float valve on the incoming water supply to this cistern will automatically top up the water should it fall below the set level. In addition, there will be an overflow from this cistern to the outside so that, should the float valve jam or fail, excess water can be discharged in an obvious place so that you know there is a problem.
The open vent from the primary circuit runs back up past this feed and expansion cistern by at least 450mm before looping back over it. The open vent has two purposes – maintaining normal air pressure in the system, and allowing for pressure surges within the system.
If you have an immersion element in the cylinder it is well worth checking that this has a secondary thermostat to guard against overheating. Old style immersion heaters only have one thermostat and there have been cases recently where this has lead to disastrous consequences. If the immersion heater fails to switch off when the water is at the required temperature, the water will continue to heat and overflow into the cold water storage cistern. If this continues, the cistern can fail and the boiling water pour down through the ceiling.
Many hot water cylinders have two elements – one at the bottom and one near the top. The idea behind this is to use the lower one during off peak hours to heat the whole cylinder and the top one at other times when only a small amount of extra hot water is required.