SOLAR WATER HETERS

Solar water heaters

Direct-gain solar heater panels with integrated storage tank

Flat-plate solar thermal collector, viewed from roof-level

Main article: Solar water heating

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 solar thermal.

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.)

Flat panel collectors are typically used in closed-loop systems. Flat panels, which often resemble skylights, are the most durable type of collector, and they also have the best performance for systems designed for temperatures within 56 °C (100 °F) of ambient temperature. Flat panels are regularly used in both pure water and antifreeze systems.

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 (i.e., over 94 °C [201 °F]). 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.

Geothermal heating

In countries like Iceland and New Zealand, and other volcanic regions, water heating may be done using geothermal heating, rather than combustion.

Gravity-fed system

Where a space-heating water boiler is employed, the traditional arrangement in the UK is to use boiler-heated (primary) water to heat potable (secondary) water contained in a cylindrical vessel (usually made of copper)—which is supplied from a cold water storage vessel or container, usually in the roof space of the building. This produces a fairly steady supply of DHW (Domestic Hot Water) at low static pressure head but usually with a good flow. In most other parts of the world, water heating appliances do not use a cold water storage vessel or container, but heat water at pressures close to that of the incoming mains water supply.

Point-of-use (POU) vs. Centralized hot water

A locational design decision may be made between point-of-use and centralized water heaters. Centralized water heaters are more traditional, and are still a good choice for small buildings. For larger buildings with intermittent or occasional hot water use, multiple POU water heaters may be a better choice, since they can reduce long waits for hot water to arrive from a remote heater. The decision where to locate the water heater(s) is only partially independent of the decision of a tanked vs. tankless water heater, or the choice of energy source for the heat.

Other improvements

Other improvements include check valve devices at their inlet and outlet, cycle timers, electronic ignition in the case of fuel-using models, sealed air intake systems in the case of fuel-using models, and pipe insulation. The sealed air-intake system types are sometimes called "band-joist" intake units. "High-efficiency" condensing units can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cooled and are mechanically ventilated either through the roof or through an exterior wall. At high combustion efficiencies a drain must be supplied to handle the water condensed out of the combustion products, which are primarily carbon dioxide and water vapor.

In traditional plumbing in the UK, the space-heating boiler is set up to heat a separate hot water cylinder or water heater for potable hot water. Such water heaters are often fitted with an auxiliary electrical immersion heater for use if the boiler is out of action for a time. Heat from the space-heating boiler is transferred to the water heater vessel/container by means of a heat exchanger, and the boiler operates at a higher temperature than the potable hot water supply. Most potable water heaters in North America are completely separate from the space heating units, due to the popularity of HVAC/forced air systems in North America.

Residential combustion water heaters manufactured since 2003 in the United States have been redesigned to resist ignition of flammable vapors and incorporate a thermal cutoff switch, per ANSI Z21.10.1. The first feature attempts to prevent vapors from flammable liquids and gasses in the vicinity of the heater from being ignited and thus causing a house fire or explosion. The second feature prevents tank overheating due to unusual combustion conditions. These safety requirements were made based on homeowners storing, or spilling, gasoline or other flammable liquids near their water heaters and causing fires. Since most of the new designs incorporate some type of flame arrestor screen, they require monitoring to make sure they don't become clogged with lint or dust, reducing the availability of air for combustion. If the flame arrestor becomes clogged, the thermal cutoff may act to shut down the heater.

A wetback stove (NZ), wetback heater (NZ), or back boiler (UK), is a simple household secondary water heater using incidental heat. It typically consists of a hot water pipe running behind a fireplace or stove (rather than hot water storage), and has no facility to limit the heating. Modern wetbacks may run the pipe in a more sophisticated design to assist heat-exchange. These designs are being forced out by government efficiency regulations that do not count the energy used to heat water as 'efficiently' used.[3]