AEECE Webinar: Integration Critical – Getting the Most from Condensing Water Heating and Heat Recovery Technologies

Written by: Christian Lobo

The AEE Canada East chapter was pleased to host a webinar titled Integration Critical – Getting the Most from Condensing Heating and Heat Recovery Technologies on January 28, 2022. As part of our mission to provide our regional energy management and sustainability community with a platform to grow, we are grateful to Robert Triebe of Thermal Energy International Inc. for the informative webinar and the engaging technical discussions that followed.

Below are a few of the key takeaways from this presentation that our chapter found particularly interesting:

  • Worldwide, fossil fuels are used at an average efficiency of only 50%, which is exacerbating the current climate crisis. Boiler plants typically only operate 75 to 85% efficiency which means that 15-25% of the energy that is purchased goes up the stack in the form of hot exhaust gas.
    • This presents a compelling financial case for improving combustion efficiency when fossil fuels are used for heating.
  • Historically, condensing technology was intentionally avoided because the boiler exhaust equipment wasn’t designed for the corrosion that is produced when exhaust gases condense.
  • The energy input (latent) required for the phase change of water to steam at 100°C is 6x the energy input (sensible) required to heat water from 25°C to 100°C. By extension, there is more heat available from exhaust gas between 140°F and 100°F than there is between 400°F and 140°F. Unlocking the latent energy by condensing the hot gases is the key to unlocking boiler potential.

The market has several water heating technologies on offer and each is suited for specific applications.

  • Direct Contact Condensing Economizers are the most efficient type of economizer system; however, they are more costly and complex. They should be considered when there is more sink (load/demand) than source (boiler system heating capacity). The advantage is that it has the longest life (~30 years) since acid is immediately neutralized, however the disadvantage is that it requires a fan and a pump
    • This system was described as essentially a cooling tower, but for heating.
  • Indirect Contact Condensing Economizers are considered to provide better savings when there is more source than sink. The advantage is that it is less complex and has a lower cost, however the primary disadvantage is a shorter lifespan as the tubes must be replaced within 5-10 years.
  • Direct Contact Condensing Boilers are the most cost effective of the 4 options presented, as there is no pressure vessel, just a shower and flame. They also have the longest life and can last 30 years. However, they are limited to 200°F and must have return water temperatures below the exhaust gas dew point to run in condensing mode.
  • Indirect Contact Condensing Boilers are not recommended as the other 3 options presented above typically offer a better balance of performance/efficiency, cost, and longevity, depending on the application needs.

Applying a catalogue selected condensing boiler is often considered the energy efficient alternative. In fact, many owners pay a premium for these systems with the promise of reduced operating costs and improved environmental performance. Interestingly, in many applications, these condensing water heating systems are only operating at peak, nameplate efficiency, for short periods of time. It is for this reason that applying critical integration considerations can lead to projects that maximize operational efficiency and minimize capital cost. A few of the special considerations to the selection and pairing of heating system and recovery technologies include:

  • Input fuel. While Natural Gas remains a relatively clean fuel, applications where fuel oil or bunker fuel are used, materials should be carefully selected.
  • The separation of “sinks” between cold and hot users can significantly drive up system efficiency.
  • A firm understanding of the heat “sinks” is required. Heat recovery coils should be sized to specifically pair with the known sink load profiles.

Despite the decarbonization push towards electrification, it is clear that natural gas will be with us for some time. In light of this, it makes sense to optimize the efficiency of these systems. Engaging with a knowledgeable integrator is key to extracting the maximum potential from these systems.


Christian Lobo, P.Eng. is a Mechanical Engineer at Stantec Consulting with 7 years experience in the energy efficiency industry. He is also a long standing chapter board member.

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