As a general rule of thumb, boiler economizers allow boiler facilities to recover flue gas enthalpy, but what are they, and how is efficiency increased?
A boiler economizer is essentially a heat exchanger typically installed on the stack that makes a boiler system more efficient by taking feedwater and exchanging heat enthalpy from exhaust flue gas. Boiler systems can be installed with economizers or retrofitted later to recapture this heat. When it comes to boiler system operation, it is typical that energy is the largest cost, as water has a high latent heat of vaporization. Figure 1 depicts the general structure of a steam boiler economizer that allows for the recycling of excess exhaust heat.
While the savings may seem evident, the return on investment will depend on several factors. For an economizer to be effective, there must be a temperature difference between the stack flue gas and feedwater. Normal temperatures of fuel gases in high-pressure boiler systems range from 400°F to 650°F. Conversely, a proper feedwater tank will provide 190°F water and 225°F from deaerators. In an ideal scenario, the flue gas temperature would be relatively low as a result of an efficient boiler, and boiler technology continues to increase efficiency. However, it is still the case that the highest heat loss is related to dry flue gas, estimated at around 12% of the total heat loss. The US Department of Energy has stated that by recovering waste heat, an economizer can often reduce fuel requirements by 5% to 10% and pay for itself in less than 2 years. Generally speaking, boiler efficiency can be increased by 1% for every 40°F reduction in flue gas temperature, so economizers generally consist of baffles to control the heat exchange.
There are some notable drawbacks to economizers. Although economizers rarely suffer from corrosion or erosion, the loss of tube thickness in economizers can be caused by gas-side corrosion, sulfidation, acid corrosion, or water-side erosion-corrosion. Vertical tube economizers have limited capability compared with horizontal types due to the requirement that each coil can be drained. Venting may also be an issue where upper return bends in economizers can get vapor-locked, resulting in reduced or even no flow scenarios.
Stack economizers are offered in a range of sizes for both firetube and watertube boilers, so cost and efficiency will vary. Depending on the location, there are also state programs that help incentivize these energy-reduction projects as well, which can offset the initial capital costs. While there are some minor ongoing maintenance requirements, the investment in an economizer is still usually worth the heat!
List of Resources:
 Lane, R. W. (1993). Control of Scale and Corrosion in Building Water Systems (1st Ed). McGraw-Hill.
 Kotz, F. (2020, May 22). Does Water Softening Promote Corrosion of a Hot Water Heater? Kotz Heating, Cooling & Plumbing. https://kotzheating.com/blog/does-water-softening-promote-corrosion-of-a-hot-water-heater/
 Saturation and Water Balance. (n.d.). CHEMTROL Australia. https://chemtrol.com.au/knowledge-centre/saturation-water-balance/