The Differences Between a Boiler and a Water Heater

A boiler and a water heater are two important appliances used for heating water in homes and buildings. While they serve similar purposes, there are some key differences between how boilers and water heaters operate. Understanding the differences between these two systems can help homeowners and building managers make informed decisions when choosing the right type of water heating solution.

How Do Boilers Work?

Boilers are designed to heat water for use throughout an entire building. They operate by burning fuel, such as natural gas, propane, or oil, to heat water in a large tank or heat exchanger. The heated water is then circulated through pipes to heat radiators, baseboards, or radiant floor systems located throughout the building.

Some of the key characteristics of boilers include:

  • Large capacity – Boilers have a large water storage tank and are capable of heating much more water than a standard water heater. This makes them well-suited for larger homes or commercial buildings with high hot water demands.
  • Central heating – The water heated by a boiler is pumped through pipes to provide heat and hot water to multiple rooms and zones throughout a building. This allows boilers to provide whole-home or building heating.
  • Multiple fuel options – Boilers can be fueled by natural gas, propane, oil, or even electricity in some cases. This gives homeowners some flexibility on choosing an energy source based on availability and heating costs.
  • High efficiency – The best high-efficiency boilers can convert over 90% of the fuel they use into usable heat for a building. This helps minimize waste and lower heating bills.
  • Requires maintenance – Boilers and their components need regular maintenance, such as annual tune-ups. Failure to properly maintain a boiler can lead to inefficiency and costly breakdowns.

How Do Water Heaters Work?

In contrast to boilers, water heaters are designed to heat and store hot water for more limited domestic uses, not whole-home heating. Most residential water heaters work by heating water stored in an insulated tank. There are some key traits that distinguish water heaters:

  • Smaller capacity – Water heaters have much smaller storage tanks than boilers, usually 30-120 gallons. This provides adequate hot water for domestic needs but not enough for whole-home heating.
  • Point-of-use heating – Water heaters provide hot water to specific taps or appliances, not for heating multiple zones throughout a home. The heated water remains stored until needed.
  • Single fuel source – Most water heaters rely on either natural gas or electricity to heat the water. Some can be dual-fuel but do not have the multiple fuel options of boilers.
  • Varying efficiency – Water heater efficiency can vary widely depending on the technology used. Gas models tend to be more efficient than electric, while tankless water heaters are more efficient than conventional tank models.
  • Less maintenance – Water heaters usually only need basic maintenance like occasional draining to remove sediment from the tank. Overall maintenance is simpler than specialized boiler maintenance.

Types of Water Heaters

There are a few basic types of water heating systems used in most homes:

  • Conventional tank – This is the most common type of water heater. It consists of an insulated storage tank where water is heated by gas burners or electric heating elements. Gas models tend to be more energy efficient.
  • Tankless – Tankless water heaters heat water directly on demand without using a storage tank. This saves space and can reduce energy costs.
  • Heat pump – Heat pump water heaters use electricity to transfer heat from the surrounding air to the water instead of generating heat directly. This makes them 2-3 times more energy efficient than conventional electric water heaters.
  • Solar – Solar water heating systems use thermal collectors and the sun’s heat to warm water. This can lower water heating bills but has a high upfront cost.

Key Differences Between Boilers and Water Heaters

Given their distinct designs and heating capabilities, there are several noticeable ways that boilers differ from standard residential water heaters:


  • Boilers provide central heating and hot water to an entire building.
  • Water heaters supply hot water for faucets, showers, appliances, etc. but do not provide space heating.

Heating Capacity

  • The large tanks or heat exchangers in boilers can heat hundreds of gallons of water at a time.
  • Water heaters have smaller storage tanks in the 30-120 gallon range.


  • The best high-efficiency boilers can operate at over 90% efficiency.
  • Tankless water heaters are over 90% efficient, while tank heaters range from 65-80% efficient.

Fuel Source

  • Boilers can run on natural gas, propane, oil, or electricity. This provides some fuel source flexibility.
  • Water heaters typically rely on either natural gas or electricity. Some tankless models can use both.

Hot Water Distribution

  • Boilers distribute hot water throughout a building through piping and radiators.
  • Water heaters provide hot water to localized taps and fixtures through pipes.


  • Purchasing and installing a new boiler system can cost $5,000 to $15,000 or more.
  • Water heaters have a lower upfront cost, starting around $500 for basic models up to $2,000 for tankless heaters.

Maintenance Requirements

  • Boilers require annual tune-ups and professional maintenance to stay in good working order.
  • Water heaters only need minor periodic draining and anode rod replacement for maintenance.

Choosing Between a Boiler or Water Heater

So how do you decide what type of water heating system is right for your needs? Here are some key factors to consider:

  • For a single family home or smaller building with more limited hot water needs, a standard water heater is usually sufficient. They provide an affordable option for domestic hot water.
  • For larger homes with multiple bathrooms or appliances needing hot water simultaneously, a tankless water heater may be a good choice to provide hot water on demand.
  • In a large residential building or commercial space with high demand for both domestic hot water and heating, a boiler system is generally the best option.
  • If energy efficiency is a priority, tankless or heat pump water heaters offer efficiency improvements over conventional tanks. High-efficiency boilers can also help minimize energy costs.
  • In homes without access to natural gas, an electric boiler or heat pump water heater are cleaner electric alternatives to heating with fuel oil or propane.
  • Climate is a factor. In colder climates, the central heating capability of a boiler may be appealing for keeping a home or building warm in winter.
  • Budget and upfront costs may steer some towards more affordable water heaters if a full boiler system replacement is not feasible.

Key Takeaways

The major points to remember when differentiating boilers from water heaters include:

  • Boilers are designed for whole-home or building heating while water heaters supply hot water for domestic use only.
  • Boilers have a much larger heating and water storage capacity than residential water heaters.
  • Boilers provide central heating and distribute hot water via pipes and radiators while water heaters provide hot water locally at taps and fixtures.
  • The multiple fuel source options and high efficiency potential of boilers give them some advantages in performance over most water heaters.
  • Boilers require specialized maintenance while water heaters have simpler maintenance needs overall.
  • Water heaters present a more affordable upfront investment for basic domestic hot water capabilities while boilers require a higher initial cost.

Understanding these key differences helps ensure you choose the right water heating solution to match your home or building’s specific needs. With their unique strengths, both boilers and water heaters remain essential options for keeping homes warm and providing hot water to taps, showers, and appliances.

Frequently Asked Questions About Boilers vs. Water Heaters

Can you use a water heater instead of a boiler?

No, a standard residential water heater should not be used in place of a boiler. Water heaters are not capable of providing whole-home heating and do not produce nearly enough hot water volume to serve as a substitute boiler.

Is a combi boiler better than a water heater?

Combi boilers combine water heating and central heating into one unit. They provide advantages over a typical water heater like providing heat for an entire home, unlimited hot water through on-demand heating, and space savings without requiring a storage tank. However, regular maintenance is crucial on a combi boiler and they have a higher upfront cost than a typical water heater.

Should I get a new boiler or water heater first?

If both your boiler and hot water heater are nearing the end of their lifespan, it usually makes sense to replace the boiler first. Boilers play a bigger role in home heating while you may be able to continue using an older water heater for a few more years if necessary. Replacing an aging boiler takes priority.

How long do boilers last compared to water heaters?

On average, boilers have a lifespan of 10-15 years, while water heaters typically last 6-12 years. High-efficiency condensing boilers on the upper end of the cost range may last closer to 20 years, while cheaper water heaters are more likely to need replacing every 8-10 years.

Should I get a tank or tankless water heater to pair with a boiler?

Either option can work well paired with a boiler. Tankless heaters provide unlimited hot water on demand which is nice for showers and appliances. But tank water heaters avoid the potential for cold water sandwiches if water flow is interrupted. For recirculating systems, a tank heater ensures constant hot water supply. Cost and energy efficiency are other factors to weigh when deciding.

What provides hot water in a boiler system?

A boiler can heat water stored in its own large tank which can supply domestic hot water. But separate hot water heaters are also commonly installed and connected to boiler systems to provide hot water. This allows smaller tanks or tankless water heaters to supply hot water on demand, supplementing the boiler’s heating abilities.


The decision between installing a boiler versus a water heater comes down to matching the system capabilities with your specific home heating and hot water needs. For whole-home heating and large volumes of hot water, boilers are the clear choice. When only needing limited hot water for sinks, showers, and other uses, residential style water heaters provide an affordable option without significant maintenance demands. Consider fuel source availability, efficiency ratings, upfront costs, climate conditions, and hot water demand to ensure you select the optimal system for providing heat and hot water in your unique situation. With some careful planning, either a high-performance boiler or efficient water heater can serve as a reliable solution.