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Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating. In industry, both hot water and water heated to steam have many uses.
Domestically, water is traditionally heated in vessels known as water heaters, hot water heaters, hot water tanks, boilers, heat exchangers, calorifiers, or geysers depending on whether they are heating potable or non-potable water, in domestic or industrial use, their energy source, and in which part of the world they are found. In domestic installations, potable water heated for uses other than space heating is sometimes known as domestic hot water (DHW).
In many countries the most common energy sources for heating water are fossil fuels: natural gas, liquefied petroleum gas, oil, or sometimes solid fuels. These fuels may be consumed directly or by the use of electricity (which may derive from any of the above fuels or from nuclear or renewable sources). Alternative energy such as solar energy, heat pumps, hot water heat recycling, and sometimes geothermal heating, may also be used as available, usually in combination with backup systems supplied by gas, oil or electricity.
In general, there are two main types of Water Heating Systems:
On-demand water heaters
Stand-alone appliances for quickly heating water for DHW (Domestic Hot Water) are known as tankless heaters. A common arrangement where hot-water space heating is employed is for the boiler to also heat potable water giving a continuous supply of DHW without any extra equipment required. Appliances capable of supplying both space-heating and DHW are known as combination (or “combi”) boilers. Although on-demand heaters can give a continuous supply of DHW, the rate at which they can produce it is limited by the thermodynamics of heating water from the available fuel supplies.
Another popular arrangement where higher flow rates are required (although for limited periods) is to heat water in a pressure vessel capable of withstanding a hydrostatic pressure close to that of the incoming mains supply. (A pressure reducing valve is usually employed to limit the pressure to a safe level for the vessel.) In North America these vessels are known as hot water tanks and may incorporate an electrical resistance heater, an air source heat pump or a gas or oil burner heating the water directly.
Where hot-water space heating boilers are used DHW cylinders are usually heated indirectly by primary water from the boiler, or by an electric immersion heater (often as backup to the boiler). In the US, when connected to a boiler they are known as indirect-fired water heaters.
Tankless water heaters, also called instantaneous, continuous flow, inline, flash, on-demand or instant-on water heaters, are also available and gaining in popularity. These water heaters instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil.
Tankless heaters are often installed throughout a household at more than one point-of-use (POU), far from the central water heater, or larger models may still be used to provide all the hot water requirements for an entire house. The main advantages of tankless water heaters are a continuous flow of hot water and energy savings (as compared to a limited flow of continuously heating hot water from conventional tank water heaters).
How tankless water heaters work
When there is a demand for hot water (e.g. a hot water tap is opened for a sink, shower, tub, or washing machine) the tankless water heater’s water flow turbine senses the flow and starts the heating process. The water flow turbine sends a signal to the control board which looks at multiple factors: incoming water temperature, desired water temperature as set on the temperature controller, and the calculated difference between the two temperatures. Depending on the calculated incoming and desired water temperatures, the gas or electric flow into the burner assembly is modulated and the electronic ignition sequence begins. Water is heated to the desired temperature as it circulates through the copper heat exchanger providing continuous hot water. When the hot water tap is turned off, the tankless water heater shuts down and is placed in a standby mode pending the next call for hot water.
Combination or combi boilers, combine the central heating (CH) with (tankless) domestic hot water (DHW) in one box. They are not merely infinitely continuous water heaters having the ability to heat a hydronic heating system in a large house. When DHW is run off, the combi stops pumping water to the hydronic circuit and diverts all the boiler’s power to instantly heating DHW. Some combis have small internal water storage vessels combining the energy of the stored water and the gas or oil burner to give faster DHW at the taps or increase the DHW flowrate.
Combi boilers are rated by the DHW flowrate. The kW ratings for domestic units are 24 kW to 54 kW, giving approximate flowrates of 9 to 23 litres (2.4 to 6.1 US gal) per minute. There are larger commercial units available. High flowrate models will simultaneously supply two showers.
A further advantage is that more than one combi unit may be used to supply separate heating zones, giving greater time and temperature control, and multiple bathrooms. An example is one combi supplying the downstairs heating system and another the upstairs. One unit may supply one bathroom and one another. Having two units gives backup in case one combi is down, provided the 2 systems are connected with valves that are normally closed.
Installation cost is significantly lower and less space is required as water tanks and associated pipes and controls are not required.
Combination boilers have disadvantages. The water flow rate is likely to be less than from a storage cylinder, particularly in winter. The power rating needs to be matched to heating requirements; heating water ‘on demand’ improves energy efficiency but limits the volume of water available at any moment. The water supply pressure must not be too low. It has been proposed that a flow regulator valve can control the amount of water used. Additionally, a combination boiler has more moving parts that can break down, so can be less reliable than a tank system.