High-Efficiency Condensing Boilers

Is it time for your older inefficient boiler to be replaced with a high-efficiency condensing boiler? High-efficiency boilers can be over 20% more efficient than existing systems, leading to significant savings. Older boilers may operate at 75% efficiency or below and a replacement system may achieve efficiencies well above 90%.

Deciding to replace a boiler is not a trivial decision and requires a substantial capital investment. SEDAC can help you decide what is right for you by assisting you in answering the following questions:

  • What is the current cost of operation of your existing boiler, including maintenance and fuel usage?
  • Can the existing boiler be made more efficient and how much will it cost?
  • What are the consequences of a failed boiler?
  • What are the anticipated savings of a new high-efficiency boiler?
  • What are your economic criteria?
  • Are there incentive programs to help offset the cost of replacing your boiler?

Advantages of High-Efficiency Condensing Boilers
Replacing large antiquated boilers with modern condensing units with sophisticated control systems will bring energy and cost savings, increased occupant comfort and reliability.

Older hot water boilers lose a significant amount of energy out the flue. To avoid flue gas condensation, boiler flue-gas temperatures are often kept atat 300°F or higher, which also decreases efficiency. Newer condensing boilers achieve higher efficiency through much lower stack exhaust temperatures. Energy efficiency is highest in a condensing boiler when the return water temperature and the firing rate are low.

The figure below, from Fulton Boilers, shows a direct correlation between return water temperature and boiler efficiency. The cooler the return water is, the higher the thermal efficiency. If the system return water temperature is 80°F or lower, you can achieve very high efficiencies; however, in many older systems the return water temperature is considerably higher, leading to a much lower thermal efficiency.


The firing rate of the boiler also impacts thermal efficiency, as shown in the figure above. Lower firing rates for condensing boilers typically have higher efficiencies, particularly when the return water temperatures are low. This is one reason it is preferable to install modular boilers rather than one large boiler. Modular boilers have control systems which adjust as more heat is required. They keep all the boilers at the lowest firing rate possible to meet system loads and maximize efficiency.

High-efficiency condensing boilers have control systems that adjust based on outdoor temperatures. These systems use outside air temperature to determine an ideal water temperature to be supplied to the distribution system. As outdoor temperatures increase, the water temperature supplied to the loop decreases. Non-condensing systems cannot be modulated as low as condensing systems due to condensation concerns. Control systems also increase comfort by maintaining stable indoor temperatures, reduce expansion noises, reduce the possibility of thermal shock, and maintain nearly constant water circulation. Read more about control systems and boiler modulation here.

Not quite ready to replace your boiler?
Above all, schedule annual boiler tune-ups to ensure your existing boiler is as efficient and safe as possible. Read more about tune-ups.  

You may also want to consider retrofitting your older boiler with a stack economizer or boiler flue condenser to capture a portion of the waste heat and transfer it to the boiler feed-water, cold makeup water, hot water return, a hot water storage tank, condensate tank, process water, or potable water.
The difference between an economizer and condenser is that economizers are primarily used to heat a smaller volume of water to a high temperature for boiler feed water, and condenser units heat a larger volume of water to a lower temperature. Economizers may also be used to preheat combustion air.


Boiler Tune-ups

Many people have their automobile serviced on a regular basis to keep it running efficiently and identify issues before they become larger, more expensive, problems.  The same logic applies to the boiler in your building.  Boiler tune-ups keep your building safe and make sure that your system is operating efficiently. Boiler tune-ups require a licensed technician to examine the boiler, perform certain tests and make sure safety systems are functioning properly.  These tests ensure the boiler is working to manufacturer specifications and EPA emission standards.  For boilers that are not serviced annually, tune-ups frequently result in a 1.6% efficiency gain in the operation of the boiler, resulting in payback periods in most cases of less than six months.

Boiler tune-ups make sure that your boiler is running efficiently by optimizing the fuel-to-air ratio.  To ensure complete combustion of the natural gas supplied to the combustion chamber, an excess amount of air is supplied during the combustion process.  Too much excess air wastes energy and increases operating costs. A boiler tune-up measures this excess air, indicated by the amount of oxygen and carbon dioxide in the exhaust gasses.  Technicians can then adjust the fuel-to-air ratio to improve efficiency.  Some boilers have oxygen trim systems which measure oxygen in the flue gases and automatically provide a feedback signal to the combustion air controller. 

How often should your boiler be serviced?  This depends on the age of the boiler.  Many new boilers have sophisticated controls that monitor boiler operation and provide fault codes if there’s a problem.  These boilers may not require annual check-ups for the first few years, but it is still recommended that most boilers be checked annually. 


At what point should you consider purchasing a new boiler instead of maintaining an old one?
Your boiler will last longer if you maintain it. A poorly maintained boiler may fail in two decades while a well maintained boiler can last a very long time.  For example, Rockford schools have 75-year-old boilers that are maintained annually. They continue to function and provide steam, albeit at a low efficiency.

Replacing boilers can be very expensive and complicated.  Deciding when to replace the boiler will depend on whether you have the capital funds for replacement, the length of the payback period, and your desire for energy efficiency. For many people, maintaining an old boiler will be preferable until the boiler fails or it can be shown that it’s financially beneficial to replace it. SEDAC can help you make this decision and will identify energy incentives that can help offset the cost of a new boiler. See “Boiler Modulation” and “High-Efficiency Condensing Boilers” for more information about replacing your boiler.


Modulating Heating Systems

It’s cold outside and your heating system is hard at work.  Should you consider a new energy efficient heating system?
SEDAC frequently recommends replacing an old boiler system with one that modulates. Modulating systems constantly adjust to meet the heat demand.  Modulating furnaces and boilers can save energy, utility costs, and increase comfort by maintaining stable temperatures. 

Why Modulation?
Space conditioning demand, or the amount of heating or cooling required to maintain interior comfort conditions, varies throughout the year.  Engineers design heating and cooling systems to meet the “design conditions” for a specific location.  These conditions include a low or high temperature that the local temperatures are not expected to drop below or exceed, 99% of the time.  These conditions establish the maximum amount of heat that will be required during the heating season, and the maximum amount of cooling that will be required during the cooling season.
The following graph shows what the demand on the boiler will be for a location with a 193-day heating season.  Notice that the boiler needs to operate at peak load (between 80-100%) for only eight days of the heating season. Nevertheless, the heating system needs to be able to supply heat for those rare extremely cold days. For the rest of the heating season, the heating plant needs to provide significantly less heat.

It is during these milder periods that it is desirable to have a heating plant that can modulate the amount of heat provided.  Many older systems, where the heating system cannot modulate, satisfy low heating demand by simply cycling the heating system on and off.  Cycling of heating systems can lead to increased fuel consumption and fluctuating interior temperatures which can compromise occupant comfort.  Modulating systems have the ability to constantly adjust input to meet a specific output demand.  Modulating furnaces and boilers can save energy, utility costs, and increase comfort by maintaining stable temperatures. 
If you have an older heating system that you are considering replacing, contact SEDAC to provide an assessment. SEDAC will help you make the decision that is right for your facility and budget.  Although a well maintained heating system can last a long time (SEDAC recently audited a facility that has 75-year-old boilers), newer heating systems that modulate can be far more energy efficient.  Sometimes, an alternative to replacement is to retrofit older systems with modulating controls and burners.