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Boiler vs Water heater

How does a boiler work?

A boiler is a closed vessel in which water or other liquid is heated. The heat used can be provided by wood, coal, oil, gas, or electricity. A boiler’s purpose is to create steam, but it can also be used to transfer heat energy from fuel to fluids (such as water), by the process of convection and thermal radiation.

A boiler system consists of an enclosed firebox where the fuel is burned; a “flue” (a chamber) for exhaust gases; and usually a vapor collector/superheater (known as a pre-heater). Basic boilers may only contain these elements. However, they are commonly fitted with extra devices that deliver additional functions, such as water treatment (“scrubbers”), vaporizers, superheaters, burners, pumps, and controls.

Types of boilers:

  • High pressure boiler – Steam is generated at pressures above normal. This may be achieved by having a pressure vessel with a small volume, such as a boiler drum, so that water has to turn to steam more quickly.
    High efficiency types of high-pressure boilers generally give higher efficiency than low pressure boilers when extracting heat from the combustion of fuel. Supercritical steam generators can operate at temperatures and pressures above those typical of high-pressure (i.e., > 15 bar) water boiling and produce steam at much greater efficiency than other types of boiler systems.
    Supercritical turbo generators use very high pressures and temperatures through the Rankine cycle to generate electricity, which is then used in conventional power stations such as coal or nuclear plants where large quantities of hot water are produced.
    Steam is used in power generation and has a number of benefits over high pressure water, such as reduced pipeline leakage and reduced plant corrosion from dissolved oxygen in the water.
  • Domestic heating boiler – A boiler used for space heating in residential applications. The sizes are typically limited to a few hundred thousand British thermal units per hour, with output between 1 to 100 hp (45-75 Kw). Equipment is manufactured according to ASME standards, particularly Section III which sets out construction requirements for low pressure liquid fired equipment .
    It is very common today for boilers to be made that can burn either natural gas or liquid fuel oil or biodiesel blended fuel. Domestic hot water boilers are almost exclusively electric water heaters with a tank added, and they are generally not considered boilers.
  • Vacuum boiler – a boiler that operates without pressure. Pressure is instead controlled by the use of a mechanical or electromagnetic release valve that controls steam output via a throttle valve.
    The fuel used is most often wood, but solid fuels such as coal and coke work equally well. This type of boiler was once known as an atmospheric engine, since it is the same design as the common one-cylinder steam engines found in small model railroad locomotives and boats.
    The first vacuum boilers for commercial central heating were installed by French engineer Georges Imbert in Paris during the 1890s on what were then considered to be “very large installations”.

Boiler components:

  1. Firebox –  The combustion area where the fuel burns. The inner surface is called the “fire bed”.
  2. Water wall –   Typically constructed of masonry for vertical boilers or steel plates for horizontal units, this element structures the fire bed and supports its weight. In most cases they also provide connections to piping systems that carry steam from other parts of the boiler circuit.
  3. Steam drums –  Used to store superheated steam after it has been run through a superheater, which increases its temperature so it can be used to drive turbines in power generation facilities, or converted to mechanical energy using a steam engine.
  4. Downcomers –  Extend from the upper drum to top of the water wall or steam drum, extending into the firebox. They are used to remove steam and water from the boiler that is being fed into it. Side walls – The sides of a water-tube boiler are often equipped with large diameter tubes which run through them.
    More than one such tube may be provided, and these often contain other types of tubes as well: economizer, superheater and so forth. Since this type of boiler uses hot gases inside steam drums or water drums to heat feedwater, care must be taken to insure that the feedwater is not contaminated by anything feeding back down those tubes.
  5. Jacket – A thin outer shell usually consisting of a series of plates, is wrapped around the boiler shell or tubes. The jacket surrounds and insulates the working parts. It is filled with water to form a closed loop system in which steam flows through it and back into the boiler’s main circuit.

How does a water heater work?

A water heater is a heat exchanger used to supply hot water for domestic uses. It is primarily composed of an insulated tank containing either a potable water supply or a heating medium that stores thermal energy from another source.

When hot water is drawn from the tap, cold water passes through the exchange chamber within the tank to be heated before returning to the building’s plumbing system. Water heaters are most commonly made of steel with copper or aluminum-finned elements inside the tank to provide more efficient heat transfer between media. The storage tank also has insulation on its sides to minimize heat loss when hot water needs are low.

Types of water heaters:

  • Residential gas water heaters use natural gas (cubic feet per hour) to generate hot water which flows through copper tubing to radiators, baseboard heaters or convectors.
  • Fast heating Residential electric/oil/propane hybrid tankless water heater circulates two fluids (electricity and antifreeze) through its internal exchanger (boiler). They are very fast at approximately 3-5 minutes to reach maximum temperature because they have no storage vessel; instead it relies on thermal energy captured during continuous circulation of the boiling fluid.
  • Residential electric water heaters are also called “tank-type” by manufacturers. They are usually installed under the kitchen sink, in a basement or on an outside wall.
  • Commercial cold water storage tankless units have larger capacities for use in restaurants, laundromats, etc.. The units are larger than residential units to provide hot water faster with greater efficiency. There is no tank so there is more room for equipment and insulation.
    Cold water enters on one side of the unit, passes through coil(s) filled with heating media (usually some type of metal), then returns to the building’s plumbing system after being heated.
  • Utility Electric Water Heater –  Utility electric water heaters are large scale industrial electrically powered tanks which supply hot water for entire buildings such as apartment complexes, office buildings and hotels.

Components of water heater:

  1. Water-storage tank – A high energy density storage device that contains a volume of water, usually pressurized by an electric/gas thermal storage heater element. It may also have a non-pressurized bottom-feed cold water inlet and top hot water outlet for gravity feed systems.
    Storage tanks are usually made of either steel or fiberglass. Aluminum is used on some models due to its lighter weight; however, it is less resistant to corrosion than the other materials.
    Plastic (or “poly”) material tanks are available but typically considered unsuitable for corrosive fluids such as salt-water systems, although they are often used for fresh water service with proper maintenance and pre sealing techniques. Large diameter copper piping helps reduce heat loss through the thinner sidewalls of the tank.
  2. Refrigerant – The refrigeration cycle exchanges heat between a cold condenser and a hot evaporator, using a compressor to increase the pressure of the working fluid (i.e., refrigerant) and move energy from an area where it is relatively high-pressure/low-temperature to another area at low-pressure/high-temperature.
    In addition to heating or cooling, heat pump water heaters can exchange heat with cool air in summer or cold air in winter.
  3. The compressors used in refrigerator applications are automotive-style reciprocating compressors that have been adapted to the water heating industry. They are designed to handle higher pressures and temperatures than residential or commercial refrigerator compressors.
  4. Element  – The element is the most important part of a storage tank’s operation; it is what removes heat from the water. It can be made out of copper, steel, stainless steel, aluminum, nickel/silver alloy or composite materials such as glass-fiber reinforced plastic. The choice of material depends on many factors including cost, corrosion resistance, and working fluid compatibility.
    Another factor impacting selection is whether the unit will have hot gas bypass capabilities built into it or not (this is usually chosen by the designer based on load requirements). Condensate drains are located below the bottom heater element for gravity drain systems. Heater elements are connected to the hot gas bypass using either copper piping or aluminum tubing.
    The bottom of the tank is where you will find a cold water inlet for gravity feed systems and the top is where you will find a hot water outlet for gravity feed systems. Typically, this is sealed off with metal straps/bands, but on some models, they use gaskets instead.  
    Also present here may be an optional riser pipe used to increase working pressure of lower temperature units, though most higher temperature units do not have them since they tend to reduce efficiency rather than enhance it (the exception being newer “condensing” units which utilize both a riser pipe and built-in heat pump capabilities).

So what’s the difference?

The main use of a boiler is to convert water into steam. The steam produced in a boiler can be used to power equipment or sent directly to the point of usage (like turning an engine). A water heater on the other hand stores hot water which can be heated and released at a time when it is needed.

They do not produce pressurized steam as boilers do; instead, they supply hot water that can be heated with various types of heating systems including gas, oil, electricity, or solar energy. Both boilers and water heaters have reservoirs where liquids are stored until they are heated/cooled.

How can they save money?

A boiler saves energy by enabling the user to make more use of electricity, gas, oil, or other fuels. It will increase their efficiency and save money if they are on a meter or charge per kWh/GJ/therm.

Similar to how a boiler saves energy, if you had an electric water heater that was constantly heating water that isn’t needed then it would constantly be using electricity that could have been saved. Similarly with gas or oil boilers constantly heat up water that you aren’t actively using at the time. They might not actually save you any money but they’ll stop wasting it!

How can they save water?

If your home uses hot water for anything or everything then this could save you a lot of water because it’s not constantly boiling/heating up large quantities which would be wasted.

Water heaters are better at saving water than boilers since the only thing boiled is the necessary amount required to satisfy immediate needs (usually an hour or two) rather than all the time like with a boiler. If there’s no need for hot water then nothing gets heated, but if you use enough hot water during that time period then it will require heating regardless.

Because they’re used more sparingly, both types can save some serious amounts of water over their lifetime.

How can they save energy?

Boilers use a lot of energy because they continually heat water even when it’s not needed. If you have a boiler and make full use of the fact that hot water can be used for more than one thing then it will save some energy.

An electric or gas-heated hot-water heater uses up to 50% less energy than other types, but still should only be used as necessary and not left on for no reason! It might seem like you’re saving money by using it all the time, but energy is lost whenever boiling/heating up large amounts of water which aren’t needed.

Water heating accounts for 25-30% of total household electricity consumption so this could save you serious amounts over its lifetime as long as it’s not overused.

What is better for my household?

A boiler is more expensive and requires grounds (or a gas line) for installation, however, it has higher efficiency than other heating sources.

Electric water heaters are often the cheapest to install but they depend on electricity which can be both cheap or expensive to run depending on your region and time of year. Water heaters should always be set at adequate temperatures so that the water doesn’t freeze in winter and doesn’t burn people in summer.

When this happens the temperature sensors will turn off power to the unit while waiting for conditions to change. If you live in a cold climate where it could get very cold during certain seasons and the house needs hot water to keep it warm then an electric hot water heater would not do well because it’ll constantly need to turn itself on and off.

This is where a boiler would be more efficient and cheaper to run than an electric water heater for extended periods of time because gas or oil heating systems can provide adequate heat without wasting money by turning itself off and on all the time.

What is better for commercial issues?

Large buildings such as hotels or apartment blocks would benefit from a boiler because it can provide adequate heat to keep the building warm and make use of hot water by providing it on demand (rather than having to let it get warm first).

Residential buildings such as homes and townhouses typically only require one or two hot water outlets. This way all your taps and showers can be heated using an electric water heater which doesn’t need to constantly produce hot water as a boiler does.

They’re suited for smaller areas and might not do well in commercial situations because of their low flow rate and inefficient heating system.

In the case of a small water heater running out of hot water, it can be quickly replaced with a new one whereas a boiler cannot run out because there would need to be an adequate supply of water for boiling.


As you can see, both boilers and electric hot water heaters have their own advantages depending on your situation so it’s best to choose the right one for your household or building. Luckily they come in all shapes and sizes so you’ll definitely find one that suits your needs!

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