At the heart of every boiler heating system is the boiler itself. It's responsible for producing and sending hot water (or steam) to the radiators that then distribute heat throughout the entire home.

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Choosing the right home boiler will ensure your comfort and save money on energy bills in the long term. Many factors go into the decision, so use the guide below to learn more. If you need help, you can always call us for assistance.

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Boiler Heating System Basics

A boiler heating system consists of the following components:

• The fuel source that powers the boiler
• The boiler, which burns the fuel source to generate heat
• The heat distribution system consisting of the pipes and radiators that deliver heat throughout the home

In this guide, we’re focusing on the boiler itself. If you need help selecting a radiator, please read our radiator buyer's guide.

Boiler Uses

Traditionally, boilers were used to heat an entire home via cast iron radiators installed in various rooms. Many of these same systems still exist in older homes, particularly in the northeastern United States. As a result, if you’re installing a boiler, chances are you’re simply replacing one in an existing system.

For those designing a boiler heating system from scratch, they're typically installing a hydronic radiant floor heating system, modern panel radiators, or even towel warmers. These often use high-efficiency condensing boilers that sometimes double as water heaters. If they provide hot water for both space and water heating, they are known as combi boilers.

Hot Water vs. Steam Boiler

Boilers can provide heat using either hot water or steam. While a hot water boiler needs a pump to move the water throughout the system, the steam from a steam boiler can flow naturally without mechanical help.

If you're replacing an existing boiler, then you'll need to identify whether it's a hot water or steam type. A steam boiler will have an air vent similar to the one shown on the right. This vent allows air to escape and steam to enter the radiator when the system first starts up. If you don't see this vent, you likely have a water boiler. 

Also, check the number of pipes connected to your radiator. Steam boiler systems usually just have a single connection, while those with two pipes could be either steam or hot water. If your home has radiant floor heat, it almost certainly uses a hot water boiler.

If you're still not sure after checking the radiators, you will need to check the boiler. Steam-fired boilers will have a small glass tube, called a sight glass, mounted on the side of the boiler. This sight glass shows the level of water inside the boiler.

Hot water boilers will not have a sight glass but are likely to have other accessories installed nearby, such as a pump or expansion tank. In some older systems, a hot water system may not have a circulator pump, and the expansion tank might be in the attic.

If, upon inspecting both your radiators and boiler, you are still unsure about what type of system you have, please contact us or call a professional to check the system.

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Boiler Fuel

There are a variety of fuel options available to power your boiler. These include natural gas, liquid propane (LP), fuel oil, and electricity.

Natural Gas Boilers

Natural gas is the most common fuel, especially in urban areas. You will most likely need a natural gas boiler if you have gas plumbed to your home from a utility main.

Propane Boiler

In areas without a natural gas supply, propane is the next most common fuel. Propane is usually stored in a tank on the property where it will be used. This tank can be located either above or underground.

Typically, you will need to have propane delivered a few times per year. Many natural gas boilers are convertible to propane, and vice versa, with an appropriate conversion kit. 

Fuel Oil Boiler

Oil is also a conventional fuel in areas where natural gas is unavailable, particularly in northeastern states and eastern Canada. As with propane, there will be a tank located on the property, and periodic fuel deliveries will be required.

In rural areas that have available wood, coal, or other biomass, solid fuel boilers may be an option. These boilers need to be manually loaded whenever a heating operation is required. They can be a huge source of savings in areas where the necessary fuel is available. Often an oil, natural gas, or propane boiler will be installed alongside a solid fuel boiler to supply heat when no one is available to load fuel.

Electric Boiler

Electric boilers are an option where other fuel types are unavailable or prohibitively expensive. Electric boilers are also useful for handling small applications, such as adding heat to a garage.

Boiler Venting

An essential part of any boiler system is to eliminate combustion gasses resulting from burning your fuel. The three main vent types are chimney venting, power venting (direct exhaust), and direct venting (sealed combustion). Choosing the correct option for your home depends on your existing boiler, your home’s construction, and the efficiency you want from your new boiler.

Chimney Venting: As their name implies, chimney-vented boilers eject spent combustion gasses through a vertical stack. Each boiler will have specific requirements about the size, height, and construction of the chimney.

Only by having a qualified professional inspect your chimney can you determine whether it is suitable for a particular boiler.

Chimney venting relies on the principle that hot air is less dense than cold air, making it naturally buoyant. This warm, lighter air travels up the chimney while fresh air for combustion is pulled into the boiler to replace it.

Because this is a passive process (i.e., there is no motor forcing the hot air out), any obstruction in the chimney or an inadequate supply of fresh air will likely result in improper operation. This could potentially lead to a fire or carbon monoxide entering the home.

Power Venting: For homes without an existing chimney or homes where the existing chimney is in disrepair or inappropriate for the necessary boiler, power venting could provide a reasonable alternative. Power-vented boilers, also called direct exhaust, use a built-in blower to push spent gases outside.

The built-in blower gives you the flexibility to vent through a sidewall or the ceiling without needing to use a chimney. Also, because the air is being actively forced through the boiler, the vent piping used is substantially smaller than what is required for chimney venting. Because power-vented boilers still draw combustion air from inside the home, it is vital that there be an adequate supply of fresh air to ensure proper operation.

Direct Venting: Direct-vented boiler heating systems, also called sealed combustion, are ideal for modern, tightly-sealed homes. Direct vent boilers draw air for combustion directly from outside the house and exhaust it back outside. This means the entire combustion process is “sealed” from the indoors, and you are not exhausting air you had already heated.

Because they do not rely on indoor air for operation, direct vent boilers can often be installed in a closet or other confined space. They can be exhausted through either a side wall or ceiling, and use venting of a similar size to power vented boilers.

Power and direct-vented boilers must be vented directly to the outdoors. They cannot be used in conventional vent systems with chimney-vented appliances, and cannot exhaust into a chimney.

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Efficiency

Home boilers are typically available in efficiency ratings ranging from 80% - 96% Annual Fuel Utilization Efficiency (AFUE). A boiler's AFUE rating estimates the percentage of fuel that is converted into usable heat for the home. By comparing AFUE ratings, you can get an idea of how much a boiler may cost to operate.

Standard Efficiency Boilers: Boiler systems with an efficiency rating of between 80% - 84% are commonly termed standard efficiency. Most boilers in this range use a cast-iron heat exchanger and chimney venting, but power-vented and direct-vent models are available.

Boilers in this range are designed to meet federal minimums for efficiency and are generally not eligible for any rebates or incentives. However, they are relatively inexpensive and use commonly available vent materials.

Due to their proven design and construction, there is little to separate boilers in this range from each other. Choosing a boiler that is correctly sized and one with which your installer is familiar is the best way to ensure efficiency and reliability from standard efficiency boilers. 

Mid-Range Efficiency Boilers: These are boilers that have had their firing characteristics slightly modified to achieve AFUE ratings of over 85%, often sacrificing output and increasing cost in the process. The purpose of these boilers is to qualify for incentives and rebates offered by certain utilities and localities.

In addition to the increase in upfront cost, these 85% AFUE boilers often require the use of costly stainless steel venting materials. We suggest avoiding these special-purpose boilers unless there is a rebate available in your area to offset the increased cost.

High-Efficiency Boilers: True high-efficiency boilers are those with an AFUE rating above 90%. Boilers in this range typically use stainless steel or aluminum heat exchangers for the best balance of efficiency and longevity.

Nearly all high-efficiency boilers are direct-vented, although some are convertible to power-vented operation. Venting materials are less common than for standard boilers but are still less expensive than the stainless steel often required for 85% AFUE models.

High-efficiency boilers have a higher upfront cost and more stringent installation requirements than standard options but can offer operating cost savings of up to 30% and a reduced impact on the environment.

Condensing Boilers

High-efficiency boiler heating systems take advantage of a process called condensing to maximize the amount of heat they can deliver to your home. This takes advantage of the chemistry of the combustion process and the energy released by condensing water vapor into liquid water.

While condensing boilers are often 10% more efficient than non-condensing models, they are better suited for some applications than others.

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Condensing boilers have only recently become a common alternative to non-condensing models in the boiler marketplace. As such, the most common boiler heating systems in use today, such as ones using cast-iron or baseboard heaters, are not designed to maximize their benefits. This is because these systems were designed to operate with high temperature (~180° F) water, and the efficiency of condensing boilers is maximized with water temperatures less than 140° F.

Condensing boilers are still efficient with these high-temperature systems but lose some benefits compared to conventional boilers in those situations. Newer system types, such as radiant floor or panel radiator heating systems, are better suited for use with condensing boilers. These systems, especially radiant floor, use lower-temperature water, which maximizes the efficiency of condensing boilers.

Overall, condensing boilers are a more efficient option than non-condensing models, regardless of the application in which the boiler is being used, but their benefits are maximized when they are used in a system that was designed to operate with lower water temperatures. Learn more about condensing boilers.

Modulating & Staging

Staging boilers have two or more distinct levels of heat output called stages. The boiler will operate at the lowest stage that meets your home’s heating needs. This will ensure the boiler runs longer, delivers heat more evenly, and uses less fuel than a non-staging, on/off boiler.

"Modulating" refers to a control method where a boiler can produce any output between a defined minimum and maximum level. This gives you even finer control than is possible with a staging boiler and results in constant, even heat.

A modulating boiler can maintain a nearly constant temperature and reduce fuel usage significantly by continuously adjusting to your home's heating needs. While staging is rare in residential applications, modulating controls are often built into high-efficiency condensing boilers.

By combining these two energy-saving technologies (along with other sophisticated control methods), modulating, condensing, high-efficiency boilers offer unparalleled comfort, performance, and efficiency.

Sizing Your Boiler

Even with modulating and condensing technology, a boiler can’t offer consistent, efficient comfort if it is not sized correctly. If you are replacing a boiler that you were comfortable with in the past, you can likely replace it with a boiler of the same size.

However, proper sizing of heating equipment is a complicated science, so it is best to have a professional compute the sizing for you. To get a better idea of what goes into sizing a boiler and why it is so vital, check out our HVAC sizing guide.

How Much Does a Boiler Cost?

An average boiler replacement can cost anywhere from $5,000 to $8,000 but can fall below or above that range, depending on a host of factors. Typically, boiler systems are older, which means you are often replacing more than just the boiler itself. For example, your installer may need to add new piping, valves, an expansion tank, etc. All of this doesn’t even include the cost of new radiators if you need them. The cost of the boiler itself can vary substantially based on its construction, efficiency, and output. 

For a more personalized boiler cost estimate based on your specific needs, please reach out to one of our boiler experts today.

The Ultimate Residential Boiler Guide

If you just bought a house in the northeast, chances are it gets heat from a residential boiler. While furnaces are more popular in the Midwest and western United States, residential boilers tend to dominate both new and existing homes out east.

You may benefit from a home boiler if:

• You live in a consistently cold climate.
• Your home lacks ductwork for a furnace system (although we recommend a mini split in that case).
• You own an old home with an existing residential boiler system.
  
  

If the above sounds like you, then read the following guide to get started on your boiler buying journey.

How Does a Boiler Work?

A residential boiler works by using hot water or steam to heat a home. The hot water or steam flows through piping to radiators throughout the building. The radiators transfer the heat to the room, providing warmth.

Here are more specifics on the different types, or mediums, of boilers.

Residential Steam Boilers

In steam boilers, a boiler furnace heats water until it becomes steam. The steam then naturally flows via supply pipes to radiators in different rooms of the house, providing heat. Eventually, the hot steam cools and reverts into water before flowing through return pipes back to the boiler where the process starts again.

Steam boilers are often identifiable by a sight glass, which is a small glass tube partially filled with water. You can also check your radiators to see if they have an air vent, which is another sign that you have a steam boiler.

Because they are among the oldest boiler medium, steam boilers are often necessary when renovating historic homes if you want to keep that vintage look and feel. They also provide higher outputs of heat than hydronic boilers but cost more to run.

Hydronic (Hot Water) Boilers

Hydronic boilers use a fuel source to heat water to a designated temperature and then manually pump it to radiator panels around the home. They require less energy than steam boilers and have become more common in modern homes.

You can often identify a hydronic boiler by a circulator pump. Unlike steam, which naturally expands throughout the piping system, water needs a “push.” A circulator pump provides that push and distributes the hot water around your home.

Hydronic boilers also require an expansion tank. As water becomes hot, it expands and adds pressure. The expansion tank relieves the added pressure from expanding water so that the system doesn’t burst.

You can also use a hydronic boiler in combination with an indirect water heater for greater efficiency. The hot water from the boiler circulates to a coil in the indirect water heater and heats the domestic water for your shower, dishes, etc.

Combination Water Heaters

An emerging type of water boiler is a combination boiler and hot water heater. These units provide both water for central heating and domestic hot water. The domestic hot water side never mixes with the central heating side to avoid contamination.

Combination boilers are highly efficient and save you money on installation and space. However, they are not designed for large homes or heavy demand.

Boiler Fuel Type

Residential boiler systems require a fuel source to heat the water. There are four main boiler fuel sources:

• Natural gas is the most common fuel source, especially for homes with a natural gas supply.
  
  
• Liquid propane is a common fuel source in rural areas that don’t have piped natural gas.
  
  
• Oil is a common fuel source in the northeast. Oil boilers require an oil burner, which is often sold separately.
  
  
• Electricity is popular in areas where electrical power is inexpensive. Otherwise, the high voltages and energy costs make electric boilers impractical for many homes.
  
  
  

AFUE and Boiler Efficiency

A home boiler system's efficiency is determined by its annual fuel utilization efficiency percentage or AFUE. The percentage indicates how much of the boiler’s energy is converted into useable heat.

For example, a boiler with an AFUE of 90% converts 90% of its energy into heat and 10% is wasted as exhaust. Steam boilers are among the least efficient, typically with AFUE ratings of between 80% and 90%. Condensing hydronic boilers are the most efficient with AFUE ratings well into the 90th percentile.

A condensing boiler is so efficient because it recycles additional heat from the exhaust gases to preheat incoming water. As a result, the exhaust gases are cooler and allow for less-expensive venting to be used, such as PVC.

The higher your boiler’s AFUE, the more you will save in energy use over time, although upfront costs will most likely be higher. High efficiency or condensing boilers will also qualify you for incentives and rebates from certain utilities and localities.

Modulating Boilers

Another aspect of residential boiler system efficiency and performance is whether it modulates heat output. A single-stage boiler provides only one output of heat, meaning it’s either off or running at full power. This can lead to uncomfortable swings in temperature and short-cycling, which is when the boiler turns on and off repeatedly, causing wasted energy and unnecessary wear and tear.

A modulating boiler can operate across a range of heat outputs. It will use the lowest heat setting possible to keep you comfortable, saving energy and providing consistent temperatures throughout the day.

Venting

There are three options for venting exhaust gases from a residential boiler: chimney, power, and direct venting.

1. Chimney Venting: This is the oldest form of venting. The hot exhaust gases rise out of a home’s chimney, which must be the proper size and height. Fresh air flows into the boiler from inside to replace the exhausted air and feed combustion. This form of venting is typically used with lower-efficiency boilers.
   
   
2. Power Venting: Power venting adds a blower to the boiler to pull combustion air in from the indoor space and force exhaust out a sidewall or ceiling. This allows for more efficient air movement, flexible installation, and smaller vent pipe than in a chimney.
   
   
3. Direct Venting: The most efficient type of venting, direct venting pulls fresh air directly from the outdoors to feed combustion and exhausts gases through a separate pipe. This is ideal for tight homes and confined spaces and is also known as sealed combustion.
   
   
   

Boiler Sizing

Sizing a new residential boiler depends on the heat output (measured in BTUs) you need to effectively warm your home. This requires a  Manual J Load calculation done by a professional, especially if you are switching to a higher-efficiency model.

If you are simply replacing an existing boiler with one of the same efficiency and size, locate the output ratings on the old boiler's heating plate or sticker. Many boilers were oversized in the past, so it's possible you can go a size smaller, but you should check with a professional to confirm.

Boiler Cost and Maintenance

A boiler's costs take into account both upfront and long-term expenses. Boiler medium, AFUE, output, venting, and local labor rates all impact upfront costs. On the other hand, efficiency and fuel type can impact lifetime costs.

For example, a 95% AFUE condensing hot water boiler may cost more upfront than an 81% AFUE steam boiler, but it will likely save you on monthly heating bills. Alternatively, if you choose a fuel type that is expensive in your area, you will likely negate any savings you would have otherwise enjoyed.

When it comes to maintenance, it is important to have a professional inspect your boiler once a year, preferably right before the heating season. This will ensure there are no gas leaks and that you're getting the proper heat output.

Find a Home Boiler

AC Wholesalers has a wide selection of hot water and steam boilers available with various heat outputs, fuel types, and AFUE ratings. Now that you understand how boilers work and the different options you have, feel free to browse our inventory.

Of course, if you have any questions, please don't hesitate to contact us. We'll be happy to walk you through the selection process and go into further detail on what to consider.

For more Commercial Boiler Systemsinformation, please contact us. We will provide professional answers.