There’s broadly two ways a gas fired heating/hot water system work. The traditional method is to use gas to warm up a tank of hot water that users draw on as required and the other way is to heat the water on demand: the gas boiler turning on briefly to heat water as it’s needed.
Stored hot water systems
The hot water tank method means the boiler is usually set on a timer, shared with the home’s radiator heating system, and the tank is often set to be heated at the same times; the logic being that, if you want the home to be warm, there’s a very good chance that’s when you’ll also want hot water. Should the homeowner run out of hot water between the set heating times, there’s often an electric immersion heater that can be switched on to ’emergency heat’ the top third or so of the tank – or the boiler’s timer can be switched into a temporary ‘on’ period and gas can be used. While using gas is significantly cheaper than electricity per KWh of energy, the gas boiler will heat the water in the tank from the bottom so it’ll take longer to bring the contents up to temperature: if you’re in a hurry and only need a small volume of water to be ‘ready’ – for a shower, for example – then the immersion heater is the ideal solution.
Two kinds of storage systems
Stored hot water systems then break down into two kinds: those that refill the hot water tank from a header tank fitted above the stored hot water – the indirect or gravity method – and ones that are refilled directly from the cold main water feed – the direct method. In practical terms, both connect to the boiler the same way and are heated the same way – the main differences are that the direct method means the hot water is under the same pressure as the cold water and so it is easier to drive high-performance showers without needing an additional pump. The indirect method adds an extra tank, usually in a loft space, but does make it marginally easier to undertake some maintenance jobs.
On demand systems
In a system that supplies heated water as and when required, the mains cold water is fed into a small tank (the heat exchanger) in the boiler itself and often maintained at a pre-defined temperature. When a hot tap is turned on, the hot water is fed to the tap, drawing more cold water into the boiler’s tank. Once this water temperature drops below a certain point, the boiler switches on and brings the tank temperature back up. Obviously, if all the hot water taps are turned on simultaneously, there’s a good chance the boiler can’t keep up (over-engineering the system to accommodate for this rare event would be very wasteful). However, direct heating takes up less space in the home: there’s no need for hot water storage tanks, header tanks and so on. One downside is that there’s no storage of hot water beyond the small volume in boiler and, if there’s a power cut, the home is generally unable to offer any heated water (while a boiler runs on gas, it needs a small amount of power to generate the ignition spark, any flue fans, pumps and so on).
On-demand systems don’t use timers on the hot water heating side of the system: hot water is always produced as and when it’s required.
Relationship to Central Heating systems
Direct, indirect and on-demand systems can all work with water/radiator central heating systems with ease. In all cases, the water that flows through through the radiators is a ‘closed loop’ – it’s not the same water that gets fed into the hot taps – and is heated by the boiler, usually controlled by a timer. For stored hot water systems, the heated water from the boiler is passed along a pipe and is then directed into either the hot water tank or radiator network – or both, depending on the timer’s settings. While some systems will have two valves, one for each ‘arm’ of the system and each one is either open or closed (commonly called the ‘S’ system), it’s very common to have a two-way valve that takes a single feed pipe and splits the hot water, feeding the tank, radiators or both (‘Y’ system).
Electrical safety
Because of the way the systems are electrically interconnected, it is important to note that simply switching both systems off on the timer/programmer will almost certainly NOT isolate the boiler. The thermostat – both room temperature and tank temperature ones – run at mains voltage and extreme care is needed to be sure the circuit is isolated before changing or examining any component in the system.