Quality, Affordability & Reliability
Storage water heaters (tank-type)
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.
Continuous Flow Tankless heaters
Tankless water heaters—also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters—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.
Solar water heaters
Increasingly, solar powered water heaters are being used. Their solar collectors are installed outside dwellings, typically on the roof or walls or
nearby, and the potable hot water storage tank is typically a pre-existing or new conventional water heater, or a water heater specifically designed for
The most basic solar thermal models are the direct-gain type, in which the potable water is directly sent into the collector. Many such systems are said
to use integrated collector storage (ICS), as direct-gain systems typically have storage integrated within the collector. Heating water directly is
inherently more efficient than heating it indirectly via heat exchangers, but such systems offer very limited freeze protection (if any), can easily heat
water to temperatures unsafe for domestic use, and ICS systems suffer from severe heat loss on cold nights and cold, cloudy days.
By contrast, indirect or closed-loop systems do not allow potable water through the panels, but rather pump a heat transfer fluid (either water or a
water/antifreeze mix) through the panels. After collecting heat in the panels, the heat transfer fluid flows through a heat exchanger, transferring its
heat to the potable hot water. When the panels are cooler than the storage tank or when the storage tank has already reached its maximum temperature, the
controller in closed-loop systems will stop the circulation pumps. In a drainback system, the water drains into a storage tank contained in conditioned
or semi-conditioned space, protected from freezing temperatures. With antifreeze systems, however, the pump must be run if the panel temperature gets too
hot (to prevent degradation of the antifreeze) or too cold (to prevent the water/antifreeze mixture from freezing.)
Another type of solar collector is the evacuated tube collector, which are intended for cold climates that do not experience severe hail and/or
applications where high temperatures are needed. Placed in a rack, evacuated tube collectors form a row of glass tubes, each
containing absorption fins attached to a central heat-conducting rod (copper or condensation-driven). The evacuated description refers to the vacuum
created in the glass tubes during the manufacturing process, which results in very low heat loss and lets evacuated tube systems achieve extreme
temperatures, far in excess of water's boiling point.
Air-sourced heat pumps absorb heat from air and transfer it to heat water. They run on electricity but are roughly three times more efficient than conventional electric water heaters, so when used in the right environment they save energy, save money and reduce greenhouse gas emissions. Heat pumps work on the same principle as your refrigerator, but instead of pumping heat out of the fridge to keep it cool, they pump heat into water. Electricity is used to pump a refrigerant around the system, which picks up heat from the air and transfers it to the water.