How Do Boilers Work?

You can use a wide variety of heating systems to heat your home, with boilers being one of the more traditional options. These systems, which use steam or hot water to generate heat for your home, vary in terms of size, heat source, how they heat the water or steam and several other factors.

In this article, you'll learn about the different types of boilers available, how they function, their common applications and much more.

How Do Boiler Systems Work?

The process by which a boiler generates heat can be summarized in the following steps:

  1. Your thermostat detects a temperature drop: When it senses this temperature drop, it will signal the boiler to turn on.
  2. The boiler turns on: Using gas, oil or electricity, your boiler will power on and begin to heat up the water.
  3. The boiler sends the heated water throughout the house: The boiler will send steam or heated water to areas throughout your home via radiant floor systems or radiators, where it gives off heat to warm the air.
  4. The water returns to the boiler: As the steam condenses or the water cools, it returns to the boiler, where it will be heated again and sent out to continue providing heat to your home.

The above process will continue until your home has reached the temperature you've set on your thermostat, at which point your boiler will shut off.

Parts of a Boiler

As with any heating system, hot water boilers and steam boilers vary in how many accessory components they require. One key difference between the two is that a hot water boiler has a pump, which circulates the heated water throughout your house and then back to the boiler. Steam boilers don't require pumps because steam naturally fills pipes and returns to the boiler due to gravity.

A hot water boiler also needs several other accessory components to function. If one of the components below malfunctions or fails, this can increase your hot water boiler's maintenance costs. These components include:

  • Water pressure regulator
  • Water temperature regulator
  • Manual or automatic bleeders
  • Check valve
  • Circulator
  • Expansion tank

Boilers sometimes have "sealed systems," which means they provide hot water and central heating through a storage cylinder in an airing cupboard. This type of system doesn't use tanks in a loft, unlike normal heat-only boilers.

Boiler Fuel Sources

Most boilers use gas, oil or electricity to create heat, which they then transfer to water. Here's how each fuel source works:

  • Gas: When a natural gas boiler receives a signal from the thermostat to heat, it will use either an electric start or a pilot light to ignite oxygen and gas in a combustion chamber. This chamber is a secured, safe chamber that burns gas to generate heat. This burning gas will heat the heat exchanger, a copper piping web that transfers the heat to the water. Gas-fueled boilers are the most popular boiler type and are generally less costly to install and use.
  • Oil: An oil boiler functions similarly to its gas counterpart — oil will burn in the combustion chamber and heat the heat exchanger, heating the water. The primary difference between oil and gas boilers is that oil boilers require storage tanks. The oil has to be delivered to your house and stored in the tank until it is used. Oil boilers are less environmentally friendly than gas boilers, although some models can be more efficient. Oil boilers tend to be more costly to use than gas ones.
  • Electricity: Generally speaking, electric boilers are only used in cases where natural gas isn't available. These boilers generate heat differently than their gas and oil counterparts — rather than burning fuel, electric burners use electricity to warm elements. Despite "burning cleaner," electric boilers are less efficient and more costly to use than gas or oil boilers.

Hot Water vs. Steam Boilers

As we mentioned above, boilers heat either hot water or steam. Both kinds can use oil, gas or electricity for fuel, and both use heated water as a primary source of heat. How they generate this heat, however, is what makes them different.

Steam boilers heat water to its boiling point to generate steam. Once this has been achieved, the steam will travel through various pipes to radiators located throughout your house. Hot water boilers, on the other hand, heat water up until it's just hot enough, then push it through similar pipes to your radiators. Once the steam has condensed or the water has cooled, it will return to your boiler, where it's reheated.

Energy Usage

Both hot water boilers and steam boilers provide energy efficiency, although during different steps of the process. Steam boilers require more energy for boiling the water but use less energy during the heat transfer step. Conversely, hot water boilers use less energy for generating hot water and more energy when transferring heat. Due to steam boilers' exceptional efficiency when it comes to heat transfer, they're frequently used in extremely cold environments or insufficiently insulated homes. Steam boilers are used in many industrial and commercial buildings.

Many hot water boilers today are thought to have better efficiency than their steam counterparts due to the pumps they have for circulation. However, you can still find steam boilers as efficient as hot water boilers — sometimes even more so. A boiler's efficiency depends on many factors, including the manufacturer, size and heat source.

Draft Method

Boilers can also have different draft methods, which describe how air moves through the flue.

Natural draft boilers don't use any fans or blowers to move air artificially. Instead, air flows naturally based on the different densities of hot and cold air.

Forced and induced draft systems both use fans to move air, but they do it from different ends of the system. Forced draft pushes air into the combustion chamber, while induced draft pulls air out of it.

How Can Boilers Be Used to Heat Your Home?

Residential central heating boilers exist in the following forms:

Steam Radiators

It's not uncommon to find metal steam radiators positioned upright along the walls of older houses. These systems can provide comfortable, consistent heat, although they require some straightforward maintenance to function correctly. The heat they emit can also warp the floor or make grooves in it. If this occurs, the radiator might tilt, resulting in loud banging sounds. Furthermore, if a steam radiator is positioned along an exterior wall, some heat may radiate to the outdoors. It can also limit where you can place furniture in a room.

Hot Water Radiators

Hot water radiators are more commonly found in newer houses, where they stand upright like steam radiators or run along the baseboard of a room. A hot water radiator can lower how much energy your home consumes because it lets you control the temperature of a big space or each floor. Heating professionals can equip your radiators with automatic valves and connect them to different thermostats.

The main issue associated with hot water radiators is the unwanted air in the system. And like steam radiators, an upright hot water radiator limits where you can put furniture.

Hydronic Radiant Floor Heating

These are the most popular type of radiant floor heating systems. In this system, a boiler will heat water and flow it to tubing located under the floor, which serves as a large radiator. A hydronic system can heat up an entire home, although they're primarily used for heating one room or space. Some models can be zoned, allowing thermostats to control separate spaces.

Hydronic radiant heating systems have higher efficiency than baseboard heating, and they're usually more efficient than forced-air heating systems, too. Additionally, they don't spread allergens like forced-air systems sometimes do, and it doesn't consume much electricity. However, installing and repairing a hydronic system can be costly, and if your floor is covered in thick carpet, it may be less efficient.


Note that a boiler can only provide heat. If you want cool air in your home, you'll need to invest in a separate cooling system. However, residential boilers can be supplemented with conditioning units like ductless heat pump split systems.

Applications of Industrial Boilers

Industrial boilers operate in a very similar fashion to standard boilers, with the main difference being that industrial models are designed to provide heat for much larger spaces. To this end, industrial boilers are considerably bigger than their standard counterparts and contain stronger parts. Practically every building or facility in the United States uses a boiler for one use or another. Businesses that use industrial boilers include:

  • Breweries: More breweries are turning to commercial boilers to provide the heat and hot water required for many of their processes, including sanitization, sterilization and pasteurization.
  • Laundries: Laundries represent one of the most obvious ways that steam from an industrial boiler can be used. As you probably know, it's easier to remove spots and dirt when the washing water is warmer. This is why residential washing machines can heat water, but they rely on electrical heating. Using electrical heating for a large laundry business would be horribly inefficient. For this reason, they instead use industrial boilers, which can create the large amounts of heated water they need to achieve their goals.
  • Hospitals: It is perhaps in hospitals that boilers serve the greatest range of needs. The purchase and use of a high-quality boiler are among the greatest expenses for hospitals. Hospitals use boilers for sterilizing operating rooms and equipment, producing clean steam, providing heat, humidifying emergency rooms, preparing food and much more.
  • Food preparation: Food needs to be heated or boiled frequently when being processed, and a sufficient amount of heat is required at practically every stage. Although obvious applications of steam including cooking, warming and drying, steam from industrial boilers are also useful for eliminating microbiological risk in meat, poultry and vegetable processing.

How Long Do Boiler Systems Last?

Boilers don't have many moving parts, which means they can last for many years as long as they are regularly maintained. Yearly inspections are critical for catching signs of rust, corrosion, erosion, overheating and leaks — all potential signs that your boiler isn't working correctly and needs immediate attention.

If you've missed several regular yearly inspections, there are several signs that your boiler needs a replacement:

  • Age: Condensing boilers, although new, are expected to last at least 15 years. Cast iron boilers generally last 20 to 30 years, with some models lasting as long as 50 years. Research the average lifespan of your model.
  • Frequent repairs: If your boiler requires constant repairs, this likely means it has become less efficient and you should consider an upgrade. A noisy boiler is another potential sign that it's time to upgrade.
  • Increase in energy costs: If the costs to run your boiler are increasing despite no increase in use, this is another likely sign that your boiler is becoming less efficient and should be replaced.
  • Discoloration: Black soot on an oil boiler and yellow flames on a gas burner mean the fuel isn't burning properly and is producing carbon monoxide. In this case, you should absolutely upgrade to a newer model.
  • Leaks: If your boiler is leaking or needs extra make-up water, it may be near the end of its life. If you replace your unit, you may be avoiding expensive floods and serious property damage.
  • Comfort: If certain rooms in your home are too cold or too hot or have inconsistent temperatures throughout the day, this may mean the unit's controls aren't effective anymore and you should consider an upgrade.

How Is Boiler Efficiency Measured?

Boiler efficiency is measured by annual fuel utilization efficiency (AFUE), which represents how efficiently an appliance converts the energy in its fuel to heat over a normal year. More specifically, it can be described as the ratio of a boiler's yearly heat output compared to the total yearly fuel that it consumes. If a boiler has an AFUE of 95%, this means 95% of the energy in the fuel source turns into heat for your home and the other 5% escapes out the flue and elsewhere.

The capacity of a boiler is measured in terms of British thermal units (Btu). A BTU represents the energy required to increase the temperature of a single pound of water by one degree Fahrenheit. The required capacity for your boiler will depend on the size of your home and the climate of your region. We'll get into those details later.

What Boiler Ignition Systems Are There?

The most common boiler ignition systems are the following:

  • Electronic/intermittent pilot: The intermittent pilot was the most common type of ignition system and is still used today in some boiler models. It used electronic components to open the valve and create a spark when the furnace turned on. This method was safer and more efficient than standing pilot systems.
  • Hot surface ignition: If your boiler was made in the last two decades, it probably has a hot surface ignition system. They are a vast improvement on pilot light ignition systems in that they significantly reduce wasted fuel by burning fuel only when the boiler is in operation. They also operate more quietly than pilot light ignition systems, which produce a clicking noise when lighting up.
  • Standing pilot light: The standing pilot was an early boiler ignition system. It made its first appearance a century ago, and it was used up until the 1980s. They used gas continuously unless the gas valve and pilot were off, which made them extremely inefficient. This ignition type is no longer found in boiler heating systems.

Which Size Boiler Is Right for You?

A large business obviously requires a larger boiler than the average home. How do you determine the exact size needed for your space?

To determine the boiler size you need, remember this rule of thumb:

  • Do you live in a warm climate? Figure around 20 Btu per square foot.
  • What if your climate is more moderate? Around 35 Btu per square foot is typical.
  • How about those who live in cold region? For that, 50 Btu per square foot is a good approximation.

Determining the right size is extremely important. An excessively large unit will cause just as much trouble as one that's too small. A small boiler won't do its job properly. An oversized boiler will waste a lot of energy, and it will still deliver inferior comfort. When determining the boiler size you need, we recommend consulting a professional boiler technician. They can help you size your unit.

Shop for Boiler Units Online Today

If you need a new boiler, check out the diverse selection of high-quality boilers from Ingram's Water & Air. Some of our boilers boast efficiencies as high as 95% AFUE. If you aren't sure what you need, we can help you choose the best boiler for your situation.

0 comments (view/add)

* All fields required.

Your email address will not be published. Comments will display after being approved by our moderators. Comments may be edited for clarity.