Residential Boilers: Types, Terms, Technology [NEW Guide]

September 2, 2023
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Most U.S. homes are heated with either furnaces or boilers. As the temperature starts to drop, our natural tendency is to consider if our homes are ready to handle the next heating season.

This guide is specifically geared toward the homeowner who has a residential boiler as their primary heat source or is considering an upgrade to an existing boiler. We discuss top residential boiler recommendations, advantages, boiler system home safety, service, maintenance, and much more. 

Since half of the energy used in your home goes to heating and cooling (according to Energy Star), making smart decisions about your home's heating system can affect your utility bills (and let's not forget your comfort). We want to provide you with the most reliable, energy-efficient brands on the market, so that you can live comfortably.

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How Do Boilers Work?

A boiler system heats the water inside a tank that supplies either hot water or steam, which flows through pipes and radiators to produce warmth.

Most residential boiler systems are hot water heating systems. Some older homes and larger homes may have steam heating systems. Both steam and hot water heating systems are excellent for keeping your home warm during the cold winter months.

A boiler normally sits on top of a burner where fuel is burned to produce heat. The most common fuel for boilers in the U.S. is natural gas, which is usually piped into the house from a pipeline that runs under the streets or road. In rural areas, not served by natural gas lines, the most common fuel used is propane gas, which is kept in a large tank in the yard of the house. Propane is usually more expensive than natural gas.

In some areas of the U.S., there are boilers that are heated by fuel or heating oil. Outside of the northeast U.S., oil-fired boilers are very rare, and many oil-fired boilers have been converted to burn natural gas or propane. Natural gas and propane are a popular fuel source because they are generally more affordable than fuel or heating oil.

It costs about the same to operate a boiler as it does to run a forced-air system, but forced-air systems are usually less expensive to install. Boilers are more common in the colder areas because they can provide more heat. 


Evaluating Types of Boiler Systems


There are two main types of boiler systems: Steam boilers and Hot water boilers. A steam boiler system heats water to create steam, which is pumped throughout the house's radiators, whereas a hot water boiler system heats water and pumps it through radiators or baseboards in the house. In many cases, this will also heat water used for other purposes in the home, such as bathing and cooking, eliminating the need for a separate hot water tank.

When considering a boiler system to purchase, you’ll want to consider the system's operating efficiency. How efficient your new system is will determine the percentage of fuel that is used to create heat. Theoretically, the more efficient the boiler system is, the more you’ll save on your heating costs.

When evaluating a boiler system’s efficiency you’ll find there are two types; Standard Efficiency and High-Efficiency. Standard efficiency systems are slightly less expensive but less energy efficient and are usually around 84% in efficiency. High-efficiency systems refer to a boiler with an energy efficiency of greater than 90%. Such boilers usually qualify for Energy Star. 

Energy Star is a certification from the federal government that a boiler has an energy efficiency rating of over 85%. Contrary to popular belief, not all Energy Star products qualify for federal tax credits.

High-Efficiency vs Standard Efficiency Boilers

If your unit is over ten years old, you'll probably save money by installing a new system. However, it can be confusing to know what type of boiler is best for your home.

What Makes a Boiler High-Efficiency?

The way a boiler works is by heating water, which is conducted through radiators, radiant floor systems or a coil. With a standard boiler, some energy that is used to heat the boiler, whether it is fossil fuel or natural gas, is lost in the process of conducting.

A high-efficiency boiler is designed to trap the escaping heat and direct it back into heating the home.

Why is High-Efficiency Important?

Low-efficiency means that fuel is wasted. Boilers over ten years old only have efficiency ratings between 50-70%. That means they use 50 to 70% of the fuel to heat the house, and the remaining 30-50% of the fuel goes to waste.

Along with saving fuel costs, a high-efficiency boiler will also help the environment. Because it uses less fuel, it creates less pollution. According to the calculations by ENERGY STAR, 29% of your home's fuel use is in heating. In especially cold climates, that percentage can be even higher. The most significant thing you can do to reduce your home's impact on the environment is to have an efficient heating system.

What is the Difference in High-Efficiency vs. Standard Efficiency?

The Federal Trade Commission requires that new boilers display their Annual Fuel Utilization Efficiency (AFUE) so consumers can compare them.

Here are the ratings:

  • Old, Low-Efficiency Boilers have 56% - 70% AFUE
  • New, Minimum Efficiency Standard Boilers have 78% AFUE
  • New, Mid-Efficiency Boilers have 80 - 83% AFUE
  • New, High-Efficiency boilers have a 90 - 98% AFUE.

High-Efficiency Boilers vs. Standard: Energy Costs

Since high-efficiency boilers are so good at converting fuel into useable heat, they reduce the amount of fuel needed to heat your home, which also reduces the cost.

How much you'll save depends on the AFUE of the boiler you're replacing. The U.S. Department of Energy suggests that if you are replacing an old boiler with a new high-efficiency one, you may reduce your fuel bills in half.

High-Efficiency vs. Standard Boilers: Replacement

High-efficiency boilers generally cost more to install than standard boilers. Here are some reasons why:

  • The initial cost of a high-efficiency boiler unit is higher.
  • Retrofitting a house to handle a high-efficiency boiler may cost extra.
  • Repair and installation of high-efficiency boilers may be more expensive because they have more complicated technology.

Offsetting these higher costs is the fact that many homeowners can buy a smaller high-efficiency boiler, which reduces the cost.


Steam Versus Hot Water Boilers: What's the Difference?


Terms You Need to Know 

Shopping for a new boiler system can be confusing because you’ll be confronted with all sorts of terms that you may have never heard of before. Learning what some of these terms mean can help you become more prepared and find the right boiler for your needs.

Here’s a list of some popular boiler terms:

  • Heat Exchanger – This device converts energy into heat.
  • Electric Ignition – Automatically ignites the boiler's pilot light with electricity when the boiler comes on.
  • Standing Pilot Light – The pilot light is always burning in these types of systems.
  • Boiler Pressure Valve – This device releases pressure from the boiler. All boilers are required to have one.
  • Vent Damper – This device automatically closes the boiler off when it isn’t working, which can help save energy.
  • LPG – Liquid Petroleum Gas – The term that boiler manufactures use for propane.
  • Outdoor Temperature Reset – This is an electronic thermostat that adjusts the temperature of the water in the boiler to match the temperature outside.
  • Infinitely Modulating Capacity – This means water is continually moving through the boiler.
  • Water Velocity – This refers to the pressure of water moving through pipes. Higher water velocity usually ensures smoother operation and fewer problems.
  • Heat and Power Boiler – This is a boiler that produces a limited amount of electricity as well as heat and hot water.
  • Hydronic Heating – A fancy term sometimes used to describe hot water heating.

Consumer Reports wrote a Boiler Buying Guide discussing Condensing vs. Non-Condensing and Sealed Combustion vs. Non-Sealed Combustion.

"All boilers are either condensing or non-condensing. A condensing boiler concentrates water vapor produced in the heating process and uses waste heat to preheat cold water entering the boiler. Condensing boilers are extremely energy efficient because they can provide ample heat while operating at lower temperatures.

Non-condensing boilers typically operate at higher temperatures, and some heat ends up being vented outside. Non-condensing boilers are 80 percent to 88 percent efficient, while condensing boilers have efficiencies greater than 88 percent."

"Boilers are either sealed-combustion or non-sealed combustion units. You should buy one that is sealed combustion because it brings outside air into the burner and directs exhaust gases outside. Non-sealed combustion boilers draw heated air in and then send it up the chimney, wasting the energy used to heat the air. Also, sealed-combustion boilers won’t introduce dangerous gases into your home." 


Boiler Buying Guide


What to Consider Before Buying 

There are several things to consider when purchasing new residential boilers. Since there are so many options out there, it is important to know which ones are best for your situation. 

The top 3 things to consider are size, venting style, and efficiency.

Size - Consider Your Climate

When it comes to size, too large is just as troublesome as too small. A boiler that is too small won't do a good job, and one that is too large will end up wasting energy. A general rule of thumb for determining the size you need is actually based upon the climate you live in. Figure about 20 BTU per square foot for a warm climate, 35 for a moderate climate, and 50 BTU per square foot in colder climates. 

Watch the video below for more information about right sizing a new boiler.


Venting - Open Area Importance

Residential boilers that vent directly through the chimney can be located in smaller, closed-in areas. If the boiler is vented by forcing air through a venting system, it will need to be located in an open area that allows it to gather air to use.

Efficiency - Check Your Numbers

An EnergyStar rating on your boiler will indicate that the unit is energy efficient, therefore saving you money. You want to look for an oil boiler that has at least an 80% rating, or a gas boiler that runs at an 89%  to 98% efficiency. Boilers that run on electricity normally run 100% efficient because they produce no waste.


Modern Technology and Advantages of a Boiler System

There are many advantages to using a boiler system vs. a forced-air furnace. Properly installed modern boiler systems are more efficient and provide more heat than forced air furnace heating systems.

If you have an older boiler in your home, you should consider installing a modern boiler system. Modern boilers are smaller and far more efficient than older models.

Some modern boilers are 95% more efficient than older models, allowing you to get more heat and hot water at a lower price.

Also, modern boilers now come with pilot-less ignition. Having no pilot light
eliminates the risk of explosion, and also eliminates any possibility of wind
blowing your pilot light out.

Modern boiler systems now come with combined hot water heating systems
to eliminate the need for a separate hot water tank. This is a significant advantage to homeowners because it saves money by only having one device to heat water. This reduces the amount of oil or gas needed to provide both heat and hot water to your home. Additional advantages include savings with installation and maintenance costs since there is only one device to install and maintain.

The computerized electronic modules in modern boiler systems, which provide infinite modulation, give priority to hot water while keeping your house warm. This modern technology allows you to reduce your energy costs further.

Many modern boiler systems feature an outdoor temperature reset. The
outdoor temperature reset is wired to an outdoor thermostat that monitors
the temperature outside. The boiler automatically raises and lowers the
temperature of the water as the outside temperature rises and falls, which keeps your house comfortable all year round and further reduces energy costs.

Modern systems also provide their own self-diagnosis systems that allow you to monitor the health of your system with the single touch of a button on your control panel. This makes it easier to service your boiler system.

Chapter 6 

Common Problems with Boilers You Could Encounter [and fixes]

Image by Gerd Altman from Pixabay

There can be one or two minor problems when your system is first started for the season. Unfortunately, you usually learn about these common problems when it's already cold outside, making the need to address the issue much more urgent.

Let's face the most common problems you might encounter first before you take a look at a newer energy efficient boiler.

You'll need to know what type of boiler you have. Most produce steam or hot water using natural gas or heating oil, and all are either condensing or non-condensing. While these systems differ in their controls, they both utilize a boiler.

Your first step will always be to determine if you have a hot water boiler or a steam boiler. If you don't see a circulator (pump on a hot water heating system) or if you have a clear glass tube on one side of your boiler, then it's a steam boiler. This information is good to know if you have to contact a technician.

Here's a quick checklist to run through before calling a professional technician.

1. Check Your Pilot Light

The problem could be as simple as a draft blowing the pilot out. Or it could be debris clogging the gas delivery nozzle. Checking the pilot light is something to consider if you have either a hot water or steam boiler.

2. Investigate Loss of Pressure  

This commonly occurs due to a water leak somewhere in the system. However, the cause could also be due to a faulty pressure relief valve. To determine the cause, it's best to contact a professional who will be able to pinpoint and repair the problem quicker than you can on your own.

In the case of the steam boiler, you can check the water level at the sight glass. The sight glass should be filled up 3/4 of the way.

3. Could You Have a Faulty Thermostat?

Thermostat issues are exactly as they sound. If yours is inaccurate or is not operating properly, then you must replace it. This is something that you can typically replace on your own.

4. Is There a Deep Rumbling Coming From the Boiler?

The noise you hear is called "kettling", and it's not the best sign for the health of your boiler. Probably at the top of the list of the most common problems with boilers is limescale buildup within the boiler's heat exchanger. Cleaning the system will take care of that problem. Probably best to call a professional to "err" on the side of caution.


Boiler System Home Safety

If you use a boiler to heat your home, it’s important to check it for safety every fall. It’s recommended to check the boiler at the beginning of the heating seasons and about once a month when it’s in operation.

The main thing you should check is the boiler pressure relief valve. This device lets off excess steam or hot water and keeps pressure from building up and damaging the boiler. These valves usually look like pipes and can be located anywhere on the boiler.

All you have to do is glance at this valve and the area around it. If it doesn't look like water or steam has been coming out of it recently, you should call for professional boiler service. 

You’ll also want to check to make sure the valve isn’t blocked and there is anything that can be damaged by jets of steam or hot water coming
out of the relief valve. Most importantly, make sure there isn’t any way that
steam or water from the boiler pressure relief valve can hit or damage any
sort of electrical wiring or electrical device. 

It’s also important to make sure that people won’t be sitting, walking, or working in areas where they could be hit by steam or hot water from the boiler pressure relief valve.

Modern boilers are extremely safe systems that present no fire hazard when they're operated properly. Unfortunately, carelessness can turn a boiler into a fire hazard. To avoid any fire hazards, make sure nothing is sitting on the boiler or stored right next to it and that materials like paper, plastics, flammable liquids, wood, wooden items, cardboard, cardboard boxes, clothing, and chemicals are stored nowhere near the boiler. Children and pets should always be kept away from the boiler. If you have a separate boiler room, it’s best to keep it locked so they can’t get to it.


Tips to Ensure Your System is Running Properly

Your boiler system requires an annual service check. The reason boilers have to be serviced is to ensure your system is running properly. Boilers can be very dangerous if they malfunction and steam boilers can even explode if they are not properly maintained. This means that a homeowner should have a boiler serviced at least once a year.

Contrary to popular belief, you don’t need to have the boiler service done at the beginning of the heating season. You can have boiler service done during the summer months when the boiler isn’t running. You just need to be sure that the boiler service is completed at some point during the year, every year.

An annual boiler system service check requires a trained and licensed boiler
technician to inspect your system’s performance and check on your boiler; if anything is wrong, it'll be fixed promptly.

It’s recommended that you get the boiler service done at least once a year. You should also get boiler service done if you move into a new house that has a boiler. Never start up a boiler in a new home (especially one with an older boiler) until you’ve had a boiler technician check the boiler.

It is also a good idea to have the boiler service done if a boiler hasn’t been
used for a while. If a house has been sitting empty for more than a few months without the boiler running, you should have the boiler inspected
before starting it. In some cases, boilers in empty houses could be filled with
antifreeze which will have to be drained before they can be started.

In most areas, technicians that work on boilers have to have a specific license from the state. Boiler technicians will also need to acquire special training and tools to work on boilers.

Many heating and air conditioning technicians won’t be able to work on
boilers because they are trained to work on forced air heating systems. This means you will have to find a heating and cooling service that has boiler
technicians available. Remember to ask for boiler technician when you request a service call.

Boiler Maintenance Tips 

Properly maintaining your boiler can save you money and keep your home safe. Boiler maintenance is actually simpler because modern boilers are fairly foolproof devices that don’t require much attention.

1. Inspect Your Boiler Every Year

If you don’t run your boiler during the warm weather months, you should inspect it before the heating season. The best time for this is usually in September or October (or when it gets cold in your area). The inspection doesn’t need to be intense or serious, you just have to give a good look at the boiler.

Make sure there are no cracks or damage and that no water is leaking from the boiler. This shouldn’t be too hard all you’ll have to do is take a close look at the boiler. Keep in mind if it's kept in a dark basement, you may have to use a flashlight. Once this is done you should turn the boiler on and fire it up. Then you should watch the boiler in operation. You have to do this in order to check for leaks and other problems.

Make sure no water is leaking from the boiler if it is a hot water boiler, and no steam is escaping if it is a steam boiler. If you see any sort of crack or leak in the boiler, shut it down and call a maintenance technician immediately. Never run a boiler with a crack or leak in it.

2. Boiler Pressure 

Check the pressure when you start the boiler for the season. There should be a pressure gauge located on your boiler. Your boiler manual should tell you what the pressure should be. If the boiler pressure is different, you may need boiler maintenance. It is a good idea to check your boiler every month it's operating. Look for leaks and cracks, just like you would when you check the boiler in the fall and if you see any damage shut the boiler down and call for maintenance.

3. Keep the Boiler Clear 

Always keep the area around the boiler clear when it is in operation. Never store anything that could melt or catch fire close to the boiler because it's a fire hazard. Make sure furniture, papers, appliances, paint, aerosol cans, gasoline and other such items are kept as far back from the boiler as possible. You should also keep the boiler clear so you can get a good look at it when you’re inspecting it. In particular, you should be able to get to the gauge and the pilot light easily. It is also important to keep the area clear in case of an emergency.

4. Have the Boiler Serviced

It is always a good idea to have the boiler serviced by a licensed technician every year. Many people do this in the fall right before the heating season. Something to remember is that you can have the boiler serviced any time of the year. It can often be hard to get boiler technicians in the fall because it is their busy season, so it might be better to have the technician come later in the season or during the summer.


Top Boiler Choices Based on Consumer Reviews

Furnace Compare has collected thousands of reviews from homeowners and lists its top choices based on consumer reviews.

You can visit their site in the resources below for full coverage of the reviews, but take a look at their top five residential boilers on the market right now.

1. American Standard - Offers three different lines of residential boilers: two are gas-fired, one is oil-fired. 90% of the customers recommend and American Standard is reached #5 out of 82 boilers.

2. Bosch - This received a positive review by 100% of the customers and reached #5 out of 82 boilers. Bosch produces nine models of gas tankless water heaters, including the Greentherm C 1050 Condensing and the Greentherm C 950 ES Condensing, which are both Energy Star Rated.

3. Slant/Fin CHS - #1 of 6 Slant/Fin Boilers. The Slant/Fin CHS is a modulating, condensing gas boiler with an efficiency rating of 95% AFUE. Slant/Fin manufactures a number of high-efficiency residential gas and oil-fired boilers, including several in the Eutectic line that the company says are the quietest in the industry.

Slant Fin Boilers Prices and Reviews 2020 

4. Westinghouse - 89% of consumers recommended this boiler, ranking it #4 out of 82 boilers. A number of air conditioner, furnace and heat pump options are available under the Westinghouse brand.

5. Lennox - Rounding out the top five, it ranked at #19 out of 82. Lennox markets three different series of boilers. 

Residential boilers are growing in popularity and may be the system of choice for your family. According to Lifestyle Comfort Solutions, they are "no longer the monster in the basement with clanking radiators in each room, today’s residential boiler is compact, and comes in a variety of efficiency ratings, at least 80% AFUE mandated by law." 

Energy Star covers the most efficient boilers for 2020. These exceptional gas and oil boilers represent the leading edge in energy-efficient products this year. You can access the full list below in the resources.

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