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Consider a Hydronic Solution for Boiler Replacement

Tip Sheet: July 2013

Key Facts

  • Condensing hydronic boilers can operate at efficiencies up to 99 percent
  • The payback on a new hydronic system is typically two to four years
  • Maintaining a hydronic boiler is often easier than maintaining a steam one

Some facilities that convert from a steam boiler to a condensing one reduce their energy bill by as much as 50 percent.


Many facilities are replacing their steam boiler with a hydronic one to increase efficiency and decrease operating costs. Before condensing boiler technology was introduced, boilers operated at a maximum of 80 percent to 85 percent efficiency. Today, condensing hydronic boilers can operate at efficiencies up to 99 percent. Hydronic boilers can be used for either building heat or process hot water applications. 

Some facilities that convert from a steam boiler to a condensing one reduce their energy bill by as much as 50 percent. A condensing boiler is more efficient than a steam boiler because it extracts both latent heat and sensible heat from combustion exhaust.   


Steam systems are prone to heat loss for a few reasons. First, many systems utilize a steam-to-water heat exchanger, while others have direct steam heating equipment. Energy is lost in these systems through steam traps and steam leaks as well as through radiant losses in the piping because steam systems run at a high temperature. Some facilities add more insulation around the piping, which minimizes the heat loss, but it does not eliminate it.   


It is often advantageous to use multiple boilers in condensing applications. Multiple boiler systems are designed to provide proper system turndown to meet the peak load and minimum design load conditions as well as provide sufficient redundancy. By installing multiple condensing boilers, a facility manager can stage the boilers depending upon heating load, which helps save fuel compared to one larger boiler.    


Along with multiple boilers comes the challenge of controlling them to operate at their peak efficiency. In contrast to traditional steam and non-condensing hydronic boilers, condensing boilers have an inverse efficiency curve characteristic. This means that condensing boilers operate most efficiently at lower firing rates. A good control strategy will modulate multiple condensing boilers at lower firing rates to keep the system operating at peak efficiency.   


Are All Condensing Boilers the Same?   

There are many different designs of condensing hydronic boilers, and some designs are better than others.  Published efficiency ratings do not tell the whole story. Heat exchanger design, materials of construction, and effective heating surface area are important factors that should be evaluated when selecting a condensing boiler.  


The most effective heat exchangers are counter-flow, or counter-current, arrangements that deliver the maximum amount of condensing possible. Cold return water is introduced at the end of the heat exchanger with the coldest exhaust gases while the hot supply water is alongside the hottest combustion gases. Since condensing only occurs on the surface of the heat exchanger, some boiler designs will only condense at reduced firing rates due to limited effective heating surface (per BTU input) and compromised heat exchanger design.  


Certain boiler designs will also limit the system piping options, and correspondingly, the potential efficiency gains. Non-condensing designs, such as copper-finned watertube or cast-iron sectional boilers, may be packaged with a secondary heat exchanger, typically made of stainless steel, to obtain condensing performance. These boilers cost less to manufacture and have very little water volume. They must be piped with a dedicated circulating pump and temperature control to ensure adequate water flow and temperature to protect the heat exchanger. Operational efficiency and long-term reliability are compromised with these designs.   


Condensing stainless steel firetube boilers deliver more effective heating surface in a larger water volume design. Besides delivering higher operational efficiencies, these provide the customer and engineer with more flexibility in system piping and variable flow opportunities. Large water volume firetubes can be implemented in either primary pumping or primary-secondary arrangements. The firetube heat exchanger is conducive to counter-flow arrangement and maximizing the effective flue-side heating surface available. The large water volume inherent in firetube designs delivers low-flow tolerance, making them ideal for a variable flow primary pumping system.  


Minimal Maintenance   


Maintaining a hydronic boiler system is often easier than maintaining a steam one. A boiler operator must closely manage the chemical treatment in a steam boiler. As the system evaporates water into steam, the chemicals and minerals stay inside the boiler and can become highly concentrated. As a result, the pH level can spike. Daily boiler blowdown and water sampling are necessary for maintaining safe and reliable steam boiler operation.  


A hydronic boiler has a closed-loop system, so the chemicals that are added are significantly reduced. Unlike in a steam system, there is very little makeup water required. In a closed-loop hydronic system, there is less opportunity for undesirable constituents, such as hardness, oxygen and carbon dioxide, to enter the system. Typically operators only have to evaluate the chemistry in a closed-loop system once a month and make any necessary adjustments. A water meter on the makeup line is recommended to help determine if a leak is present and if adjustments to the chemicals are required.  

The payback on a new hydronic system is typically two to four years; however, it can be less depending on the inefficiency of the existing boiler operation.