Generally, products that meet Federal Energy Management Program (FEMP) efficiency requirements and/or those that qualify in terms of ENERGY STAR standards have an input rating of no more than 300,000 Btu/h (which is the measure of British thermal units – the amount of heat energy it takes to raise the temperature of a pound of water by 1 deg. F – in an hour) are acceptable for residential use. Commercial and industrial boilers have input ratings that are higher than this and are excluded from these consumer-orientated guidelines for obvious reasons. Instead, a mechanical, electrical, and plumbing (MEP) engineer will have the knowledge and expertise to make the choice.
Nevertheless, it’s useful to understand the way efficiency ratings of boilers work. The U.S. Environmental Protection Agency (EPA) provides efficiency levels that relate to products and their specifications on its ENERGY STAR website. Federal buyers also use ENERGY STAR data to identify and verify compliant models.
What is important is that the efficiency of boilers is measured by annual fuel utilization efficiency (AFUE). This reveals how efficiently appliances convert the energy from the fuel used to power them over a period of a year. So, for example, if a boiler has an AFUE of 90% (which is considered to be highly efficient), that is the percentage of fuel that is converted into heat while 10% is lost, usually up chimneys that may or may not be ducted.
While heat can be lost in the piping that is incorporated in boiler plants and systems, these heat losses are not included in the AFUE, even though they can be substantial. This is why it is so important for pipework to be designed to maximize efficiency.
The Federal Trade Commission (FTC) requires the AFUE percentage to be displayed on all boilers so that engineers and consumers know what they are buying and can compare the heating efficiencies of different models.
The U.S. Department of Energy (DOE) compares the efficiency of different heating systems:
- Old systems are not energy efficient and their AFUE rating is between 56% and 70%. Generally heavy heat exchangers, old boilers incorporate a continuous pilot light and a natural draft that results in a constant flow of combustion gases.
- Mid-efficiency systems have an AFUE of around 80% to 83% largely because they are smaller in size and lighter in weight, which reduces cycling losses. The flue pipe also has a much smaller diameter and there is no pilot light, instead, ignition is electronic. Exhaust fans in mid-efficiency heating systems control the flow of combustion gases and air more accurately.
- A 90% to 98.5% AFUE indicates that the heating system is highly efficient. This rating usually applies to new designs that feature sealed combustion and a secondary heat exchanger where flue gases are condensed to maximize energy efficiency.
Additionally, the DOE recommends that sealed-combustion units are specified because they avoid the risk of gases back-drafting and escaping into the inside airof buildings, big or small. Boilers that are not sealed-combustion units are also known to waste energy because they tend to draw the heated air into the unit and this air escapes up the chimney.
The DOE recommends ENERGY STAR natural gas boilers that have an AFUE of 90%, or better still, the best available gas boilers that have an AFUE of 97% which have a lifetime cost saving of nearly double the ENERGY STAR models.
However, the DOE also advises in general terms that “efficient products” are only cost-effective when the energy savings of the boiler during its entire lifetime are greater than the up-front costs when compared to the same equation of an option that is less efficient.
While it is safe to assume that ENERGY STAR-qualified products and those that meet FEMP requirements are cost-effective for their lifecycle, negates the need for guesswork and rule-of-thumb assessments. Instead, we will analyze every client’s needs and ascertain in individual situations what the best options are in terms of efficiency and cost.
For example, while it makes perfect sense to invest in high-efficiency boilers if you live in a cold climate area, where there are milder climatic conditions, the additional investment may not be justifiable.
Whether you have one boiler in your home or an elaborate boiler system in your building, if it is old, worn out, inefficient, or undersized, replacing it will save you money in the long term and ensure the system operates more efficiently.
Upgrading old boilers and related equipment can potentially halve not only fuel bills but also harmful emissions that pollute the air in buildings. The simplest solution is to replace old equipment, but it is important that they are correctly sized.
If the chimney or vent pipe is to be retained, it is vital that it is carefully checked when boilers are to be replaced. The liners of older chimneys often deteriorate, if indeed they were lined in the first place. If the building dates back to the late 1980s or early 1990s, non-condensing boilers might have been installed with plastic vent pipes prior to the advent of acceptable PVC pipes. If the plastic isn’t PVC, it must be removed and ideally replaced with a safe stainless steel vent pipe.
Having said that sealed-combustion units are much more efficient, they do tend to produce acidic exhaust gases that old, unlined chimneys cannot cope with. For this reason, the chimney or a new duct will need to be properly lined to accommodate the vented exhaust gases.
It may be preferable to install new equipment that doesn’t require venting via a chimney.
The physical integrity of heat exchangers will also need to be checked. If these leak, there is a real danger that harmful combustion gases will mix with the air inside the building. If the whole system is to be replaced or retrofitted, a technician should be employed to carry out a combustion-efficiency test.
Many different factors need to be considered when selecting a replacement boiler. The challenge is that while it may seem logical to replace it with something similar but better, this is not necessarily the most sensible approach.
For instance, we’ve discussed the different fuel options and said that natural gas is the best option. So if you are currently using a boiler that operates with LP gas or oil, and natural gas is available from your local utility, then it will likely be a better option to switch.
The other factor to consider is whether the existing boiler system runs on hot water or steam. Heating systems that run on steam tend to be older and so you may want to upgrade the system in its entirety.
We discussed vents and chimneys above, and these elements will need to be carefully considered when a replacement boiler is chosen.
Whichever boiler is chosen to replace an old or faulty one, it is essential that it meets boiler standards including those established by the American Society of Mechanical Engineers (ASME).
It is advisable to consult a professional home heating contractor or an MEP engineering firm before you go ahead with any boiler replacement operations.
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