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What are my water heating systems options?
Hot Water Tanks
In household and commercial usage, most water heaters traditionally been of the tank type. Also called storage water heaters, these consist of a cylindrical vessel or container in which water is kept continuously hot and ready for use. Typical sizes for household use range from 75 to 400 litres. These may use electricity, natural gas, propane, heating oil, solar, or other energy sources. Natural gas heaters are the most popular since the gas is often conveniently piped throughout cities and towns and currently is the cheapest to use.
Another popular arrangement where higher flow rates are required for limited periods is to heat water in a pressure vessel that can withstand a hydrostatic pressure close to that of the incoming mains supply. These vessels are called hot water tanks, and may incorporate an electrical resistance heater, an air source heat pump, or a gas or oil burner that heats water directly. Compared to tankless heaters, storage water heaters have the advantage of using energy (gas or electricity) at a relatively slow rate, storing the heat for later use. The disadvantage is that over time, the water inside the tank will cool down causing the heating system to activate to heat the water back up. Additionally, once the tank’s supply of hot water has been exhausted, there is a significant delay before hot water is available again. Larger tanks tend to provide hot water with less temperature fluctuation at moderate flow rates.
Tankless water heaters”also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heater ”are gaining in popularity. These high-power 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. Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication. Tankless heaters may be installed throughout a household at more than one point-of-use (POU), far from a central water heater, or larger centralized 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 plentiful continuous flow of hot water (as compared to a limited flow of continuously heated hot water from conventional tank water heaters), and potential energy savings under some conditions.
A common arrangement where hot-water space heating is employed, is for a boiler to also heat potable water, providing a continuous supply of hot water without extra equipment. Appliances that can supply both space-heating and hot water are called combination (or combi) boilers. Though on-demand heaters can provide a continuous supply of hot water, the rate at which they can produce it is limited by the thermodynamics of heating water from the available fuel supplies.