The Differences Between a Boiler and a Water Heater

Modern Heating Boiler

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Boilers and water heaters are important to the heating and water supply operations of the home. While they provide different services to the house—one for space heating, the other for domestic water usage—there is some slight overlap in the form of combined boiler/heater devices.

What Are Boilers and Water Heaters?

A boiler provides non-potable hot water throughout the house to radiators for space heating. A water heater provides domestic hot water for cleaning, personal use, and appliances.

What a Boiler Is

A boiler is part of a type of residential space heating device that performs the same functions as other home heating devices like HVAC systems, modulating furnaces, in-wall heaters, baseboard heaters, or any other system or device that heats space.

Oil, gas, or electricity is the energy source that heats water to near-boiling (145 to 190°F) in a large sealed tank. A circulating pump pushes the hot water through a network of pipes in a loop throughout the house. Some types of boilers push steam through the pipes.

Radiators are termination points or waypoints along the hot water loop. With either, the water cycles back to be heated in the boiler again, where the process starts all over.

Types of Boilers

  • Water boiler
  • Steam boiler
  • Combination boiler water heater
  • Oil boiler

Uses for Boilers

On a residential level, boilers are used for just one thing: producing steam or hot water for radiant heaters. 

What a Water Heater Is

A water heater is a device that heats cold water to a specified temperature for domestic water use.

Cold water enters the water heater from the top and is deposited toward the bottom of the sealed tank with a dip tube. The water is heated by coils with electric models or by a gas flame with gas models. 

Hot water leaves the water heater from the top. The hot water is delivered to various parts of the house through 1/2-inch pipes, usually PEX or copper.

What Is Domestic Water?

Domestic water refers to water used for household purposes like drinking, cleaning dishes, washing clothes, watering plants, or washing the car.

Types of Water Heaters

Uses for Water Heaters

A water heater heats water for:

  • Bathing at bathtubs and showers
  • Personal cleaning at bathroom sinks
  • Washing dishes at kitchen sinks
  • Dishwasher's hot water source
  • Clothes washing machine's hot water source
  • Consumption, if desired

How Boilers and Water Heaters are Similar

Though they do different things, boilers and water heaters share a few similarities:

  • Water is heated in sealed tanks
  • Water is delivered throughout the house
  • Both tanks are pressurized

How Boilers and Water Heaters Are Different

Boilers and water heaters are more different than they are similar:

Boiler
  • Water not safe for consumption

  • Water or steam heats rooms

  • Water heated close to boiling

  • Can use oil as energy source

  • Circulating (loop)

Water Heater
  • Water safe to be consumed

  • Water not used for space heating

  • Water heated far below boiling (120°F)

  • Natural gas or electricity as energy, never oil (unless for a commercial space)

  • Non-circulating

Boiler and Water Heater Services Combined

Boilers heat water for space heating. Water heaters heat domestic water. These two separate systems do the same thing: heat water. 

This redundancy means wasted energy. Plus, having two separate large devices uses more room in the basement, garage, or utility room. Can the two be combined?

While not common, sometimes the boiler and water heater are combined either with combination boiler water heaters or with indirect water heaters used with boilers.

Combination Boiler Water Heater

High-efficiency combination (or combi) boilers serve both functions: space heater and water heater.

Domestic water is always prioritized with combi heaters. So, when a tap is opened in the house, water flows through the heat exchanger. When hot water is needed for space heating, the water is diverted to a separate continuous heating loop for the radiators.

As single, compact units, combi heaters save a considerable amount of space over a separate boiler and water heater setup.

Indirect Water Heater

An indirect water heater has no direct power source like a flame or electric coil. Instead, the water is heated indirectly, using the boiler's heat.

A pipe from the boiler bearing hot water enters the bottom of the indirect water heater. The pipe coils in loops inside the water heater tank, then leaves the water heater to be redeposited in the boiler. 

Both the boiler and the water heater remain separate devices. Water from the boiler's pipe never touches water in the water heater tank.

Can You Use Boilers and Water Heaters Interchangeably?

No. As separate devices, boilers and water heaters cannot substitute for each other since they perform different functions.

Do You Need a Boiler or a Water Heater?

Homes do not need boilers but they do need water heaters.

Most homes are required to have a dedicated heating source, even in areas that rarely experience cold weather. That said, the heating source doesn't necessarily need to be a boiler since many other heating methods are available.

Most homes are required to have hot water supplied to plumbing fixtures for bathing, washing, and kitchen-related purposes. The most practical way to supply hot water within residences is with a water heater or through solar water heating.

Article Sources
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  1. Minimum Housing Standards. Building Enforcement, Miami-Dade County.

  2. Chapter 20 Boilers and Water Heaters. 2018 North Carolina State Building Code.