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Boilers provide heat by distributing heated water or steam through radiators or radiant floor systems. High efficiency boilers are relatively new to the marketplace, and for many homeowners they are a welcome change. If you’ve ever wondered why your old boiler ran all day and it seemed the fuel truck was parked in your driveway every week, it’s because many older boilers had efficiency ratings of 50-70%. This means that only 50-70% of the fuel you purchased was used to heat the house while the rest, 30-50%, was wasted up the chimney and elsewhere.
High efficiency boilers, which have an annual fuel utilization efficiency (AFUE) of 90% and above, use condensing technology. These boilers work by recovering heat from the flue gases that simply exit a house from the flue of a conventional non-condensing boiler. They condense the water vapor from those gases and recover heat that would otherwise be lost. One indication of the extra heat extracted is that the gases coming out of the flue in a condensing boiler have a temperature of 50-60° F as compared to the 120° or more in a non-condensing boiler.
Which Boilers are High-Efficiency?
Here is a short list of high-efficiency boilers. All burn natural gas, with the exception of those that explicitly state “oil-fired”.
This is not a comprehensive list — there are many high-efficiency boilers that we have not included.
- Bosch Greenstar
- Burnham Alpine
- HTP Elite FT
- Laars Mascott II
- Lennox GWM-IE
- Navien NCB
- Peerless WB90 (oil-fired)
- Triangle Tube Prestige Trimax Solo
- Viessmann Vitodens 200W
- Weil McLain ECO
How Efficiency is Measured
The measure of a boiler’s efficiency is its AFUE. AFUE is the ratio of annual heat output of a boiler (or furnace) compared to the total annual fuel energy consumed by a furnace or boiler. For example, a boiler with an AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes up the chimney and elsewhere.
The current standard for residential boilers, included in the Energy Independence and Security Act of 2007, requires a minimum efficiency of 82% for gas-fired boilers and 84% for oil-fired boilers manufactured after September 1, 2012. In January 2016, the Department of Energy (DOE) adopted new standards for boilers, raising minimum efficiency levels to 84% for gas-fired boilers and 86% for oil-fired boilers. These new standards take effect in 2021 and will result in consumer savings of $.35-1.2 billion over 30 years, according to DOE estimates.
The Energy Star Program, run by the U.S. Environmental Protection Agency and Department of Energy, awards an Energy Star label to boilers that have an AFUE of 87% or greater for oil boilers and 90% or greater for gas boilers. There are now several boilers on the market that claim an AFUE of 97%.
Boiler efficiency has a direct impact on your energy bill. The more efficient your boiler, the less fuel you use, which results in lower bills. A boiler’s maximum efficiency is reached when it operates at full load.
Benefits and Drawbacks of a High Efficiency Boiler
The main benefit of a high efficiency boiler is it can give immediate savings on fuel costs, possibly reducing your bill by as much as half. These units are also better for the environment, with far lower emissions. Another benefit of a high efficiency boiler is that it does not require a traditional chimney; instead, it uses a short PVC pipe inserted through the side of the house.
One drawback of condensing boilers is their higher upfront cost. The purchase price is higher, sometimes double the cost of a standard, mid-efficiency unit. One way to determine whether the higher cost is justified is to calculate the payback period. Elsewhere we provide detailed instructions and examples on how to calculate the payback period, but in short, the payback period is the time taken to recover an investment. You can think of it as the ratio of investment to annual savings.
For example, if a new high-efficiency boiler costs $10,000 to install and saves $2,500 per year in fuel costs, the payback period will be four years. But when estimating payback period, one must also consider maintenance and repair costs. Because a high efficiency boiler has more complicated computers and circuit boards, they are more likely to have problems and require servicing. Not only are these boilers more likely to need repairs, but repair costs are much higher with these units, often double the cost to repair a mid-efficiency boiler. So when you combine the higher purchase price with higher maintenance and service costs, the payback period of a high efficiency boiler is usually over 10 years, and often between 10-15 years.
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What Is A Combi Boiler?
High-efficiency condensing “combi”, or combination boilers, are very popular in homes today because they can save space and money. A combi boiler is a compact and efficient unit that provides all the heat and hot water a small home needs while saving on operating and installation costs. Combis work like both central heating boilers and hot water cylinders and are so compact they can be hung from your kitchen wall.
How Combi Boilers Work
Unlike conventional systems, combi boilers do not store domestic hot water. They heat water from the cold mains directly even as you use them. With combi boilers, you do not have a hot water cylinder, loft tank, or pipework. Combis are like standard unvented boilers but they have a second heating circuit, which is what gives you the instant hot water whenever you need it. If fed directly from the main water supply to the heat exchanger, the combi boiler can have an unlimited hot water supply as long as the boiler is firing. Combis manage to extract a larger amount of heat from the flue gases that escape from the flue system, which increases the combi’s efficiency.
When Combi Boilers Make Sense
Combination boilers are sometimes preferable because they eliminate some of the requirements of conventional vented heating systems. Combi boilers don’t require a hot water cylinder in the airing cupboard or the cold-water tank. They also don’t require the small header cistern. All the parts of a combi are in a single unit, making it very easy to install.
Combi boilers make sense in properties where space is limited, as they don’t require separate tanks for hot and cold water. If there is more than one bathroom, it is better to have storage systems. The standard combi boilers heat water in about 40 seconds. Some combis control the flow of water until it is hot enough. Storage combis can store some hot water permanently.
According to a 2005 Green Affordable Housing Coalition report (which is no longer available online), combination water and space heating systems can reduce utility costs by 15 percent or more compared to standard installation of separate furnace and water heater systems.
Condensing combination boilers are condensing versions of the standard combination boiler and are more efficient, giving you the advantage of a combination boiler as well as a condensing boiler. They can replace an existing combi boiler.
Major combi boiler manufacturers include:
- Axeman-Anderson Co
- Hamilton Engineering
- Heat Transfer Products
- Newmac Mfg
- Thermal Solutions
- Trianco Heatmaker
- Viessmann Manufacturing
Should You Buy a High Efficiency Boiler?
The answer to this question varies from homeowner to homeowner. Here are some issues to consider:
- Are you willing to pay a premium to reduce your home’s impact on the environment?
- How confident are you in your installing contractor’s skills? A poor installation is the cause of many service calls.
- Assuming that your contractor does a good job on the installation, how long is your payback period?
- How long do you plan to own your home? Is that longer than the payback period of the boiler?
Many buyers place a high value on upgrading their home’s appliances to be as environmentally friendly as possible. If you are not price-sensitive, then a high-efficiency boiler is a great choice. Just be sure that you’re confident in your contractor’s skills.
If you are price-sensitive, than a high efficiency boiler can still be a great choice: replacing a low-efficiency boiler with a high-efficiency condensing boiler could help you save up to 30% on your heating bills. However, in this situation, it’s important that you calculate the payback period to make sure that you’ll see the benefit of your investmetn. And, it’s critcal that you vet your contractor thoroughly: because high-efficiency units are more complicated than mid-efficiency units, there’s more that can go wrong. And, if your contractor makes a mistake, you’ll more than likely bear the cost of that mistake for years to come.