Modulating Condensing Boilers


Modulating condensing boilers (mod-con boilers) can increase combustion efficiency to about 96%, up from the 80% efficiency of non-condensing boilers. They provide a range of firing rates to match the variable heating load of the building.

However, mod-con boiler efficiency depends on the temperature of the water returning to the boiler. The lower the return water temperature, the higher the efficiency. Low boiler return water temperature depends on the overall boiler system design, not just the boiler.

Here is a numerical example of the benefits of a condensing boiler system. A facility using 10,000 therms per year for space heating is estimated to save around 1,489 therms/year and $819/year at $0.55/therm. The savings estimate assumes all operating hours shift from 80% to 94% combustion efficiency. In practice, the boiler would have varying return water temperatures at different times of the year.

With the boiler market shifting toward mod-con boilers, designers, installers and operators need to understand the new design and operating rules. The rules are different from those for older non-condensing boiler technology.

This article provides general background information on hydronic mod-con boilers and additional detailed references for hydronic system professionals and operators.

Progression of commercial hydronic boilers

The following describes the progression from non-condensing to condensing boilers for commercial space heating.

1. Non-condensing

Non-condensing hot water boilers are the traditional commercial facility default option. These boilers do not allow combustion gases to condense in the heat exchanger, which is typically cast iron. They require a return water temperature of around 140ºF to ensure no condensation forms and that there is no thermal shock to the boiler. A primary secondary piping loop is often used in commercial facilities to ensure the boiler always has at least 140ºF water. Non-condensing boilers provide a combustion efficiency of around 80%.

ASHRAE Handbook, 2016, HVAC Systems & Equipment


Low-temperature radiator for buildings where the radiators will be placed along a wall.

2. Modulating and Condensing Boilers, low mass

Condensing boilers capture otherwise wasted energy by condensing flue gases to extract the heat and increase their thermal performance. Burners and heat exchangers have been improved. Rather than exhaust warm flue gases out a chimney, a condensing boiler uses the heat from the flues gases to provide heat. However, heat exchanger condensation and increased combustion efficiency only occur if the boiler return water temperature is less than 140ºF and preferably under 100ºF. The boiler, piping system and radiators all need to be designed to attain high performance. Most of these boilers have become smaller and the low-temperature radiators larger.

Original modulating and condensing boilers did not call themselves low mass. This term has been added in recent years to distinguish common sub-optimal performance issues and highlight the need for up-to-date boiler system design and operation. Short-cycling often accompanies a poorly designed low mass condensing boiler system. See more on short-cycling below.

One unfortunate outcome of adding a condensing boiler without an engineering review of the entire system is potential boiler short-cycling and high return water temperatures. In this case, the premium-priced boiler wears out prematurely (from short-cycling) without providing any energy-saving benefits because the piping system, including the radiators, was not updated. This does not help condensing boiler sales and may affect the perception of boilers in general.

3. Modulating and Condensing Boilers, high mass

Newer mod-con boiler designs include higher mass internal components. The higher mass of the boiler minimizes short-cycling and allows primary only piping. Primary only piping can be lower cost as there are fewer pumps and valves and less piping. In addition to higher mass, the burner and heat exchanger passageways are often more open, which minimizes clogging and corrosion.

Unfortunately, there is no precise definition of high- and low-mass condensing boilers at this time.

Internal operating systems of a modulating condensing boiler.

Illustrations show the internal operating systems of a mod-con boiler. Low-temperature return water enters the boiler, is heated, then sent back out through the piping system to heat the building.


Concurrent progression of piping designs

The previous material reviewed trends in boilers. Boiler system piping design is equally important. Condensing boilers do not necessarily need the same piping design as non-condensing boilers. To ensure condensing boiler high performance, the piping system needs to be evaluated, designed or modified as well.

1. Non-Condensing Boilers and Primary Secondary Piping

Traditional commercial non-condensing boilers require a primary/secondary piping loop. The primary loop provides boiler water recirculation and ensures the water returning to the boiler is not less than 140ºF. This prevents thermal shock to the boiler and corrosive heat exchanger conditions. The secondary loop is interconnected to the primary loop and functions to distribute water to the various terminal coils and radiators. The interconnection between the primary loop and the secondary loop needs to be well designed to support proper water flow and heat transfer.

2. Condensing Boilers and Primary Secondary Piping

Condensing boilers are designed to accept low return water temperatures. They do not necessarily need primary secondary piping. As noted, primary secondary piping was used to support 140ºF return water to the boiler. Adding a condensing boiler to an existing primary secondary loop can be done. However, it is now common practice to add a designated hydraulic separator to help minimize short-cycling. Adding a low mass mod-con boiler to this piping arrangement without an engineering review often results in boiler trips and low load mismatches. A hydraulic separator provides a better match between boiler heat generation and the heat delivered, especially in low load conditions.

However, it remains unclear how well condensing boilers with this arrangement perform as they are rarely tested.

3. Condensing Boilers and Primary-Only Piping

In theory, a high-mass mod-con boiler with primary-only piping and low temperature radiators will provide the optimal hot water heating system. The boiler will be able to modulate down to low load conditions; it will not trip off on high temperature; it will only need one set of pumps, albeit with variable frequency drives (VFDs) or EC motors; it will only need a primary loop and no hydraulic separator; it will return low-temperature water for high-combustion efficiency; and it will be a lower cost option. Ideally, it will also trend supply and return water temperature, so it is possible to know if it is actually performing as a condensing boiler.

For an existing building, this type of design means re-engineering the piping, pumps and radiators. To achieve the low temperature return water temperature and high efficiency, the radiators will also likely need to be replaced.


A short-cycling boiler rapidly starts and stops regardless if it has satisfied the load. The boiler may be oversized or not have modulation capability or it may trigger its internal high-temperature safety sensor. Short-cycling significantly reduces boiler efficiency and reliability and is most common in modern light-mass mod-con boilers. As the market shifts to mod-con boilers, boiler system designers must address this design issue.

To prevent short-cycling, the designer must understand the turndown capability of the boiler in relation to the minimum load. A boiler should have its heat carried away as fast as it is generated. It is common to oversize the boiler to meet the absolute worst-case heating condition, but the boiler also needs to match the lower capacity of the smallest load. To prevent short-cycling, designers should size the boiler system (boilers, pumps, piping and controls) for the lowest load as well as the largest. Modulating firing rates help solve this problem.

New Construction

Most of this article addressed issues with adding a condensing boiler to an existing non -condensing installation. Designing a condensing boiler for a new building is often easier than for an existing one. The piping configuration is designed to match requirements of the mod-con boiler and the high and low heating loads of the facility. Primary secondary piping is not required, and there is no existing piping to rework. Triple-duty valves can be omitted with VFDs controlling the flow and backflow prevention valves directing the flow. Correctly sized pumps, piping and low-temperature radiators are designed into the project from the start. The combination of these steps ensure the condensing boiler has a long life and receives the low water temperature it needs to support an efficient, high-performance, comfort enhancing, low maintenance building.

What to look for when purchasing a condensing boiler

To help achieve a successful boiler project, consider discussing these items with your provider and installer or using the list as a line item proposal for the contractor to provide pricing on.

  1. Demolition and removal of existing equipment, if applicable.
  2. Condensing boiler(s).
    1. Stainless steel heat exchanger, large passageways for the burner and large passageways for the heat exchanger. Consider a down-flow burner design to allow debris to fall through the burner.
  3. Associated equipment and proper placement: Expansion tank, makeup water system, air separator and hydraulic separator, if needed.
  4. System piping, sized for low-temperature baseboard.
  5. Low-temperature baseboard.
  6. Controls and smart circulator (pump) with electronically commutated motors (ECM).
  7. Combustion air and controls.
  8. Flush and water treatment.
  9. Supply and return water temperature trending.
    1. Consider adding, if necessary, temperature sensors to the supply and return water lines. They may be factory installed within the boiler. Trending and logging the return water temperatures will indicate the efficiency of the boiler. The chart above can be used to define the boiler efficiency for a given return water temperature.
  10. System commissioning.
    1. This is a quality control step to ensure the system is working properly.
    2. Use professionals who are familiar with the supporting material noted below.


Modern Hydronic Heating for Residential and Light Commercial Buildings, John Siegenthaler, P.E.
Idronics, Journal of Design for Hydronic Professionals, Caleffi Hydronic Solutions
Designs Tips to Avoid Boiler Short-Cycling, Kent W. Peterson, ASHRAE Journal, July 2018