Your questions answered: Retrofitting systems in existing buildings with boilers

Hot water condensing boilers offer an excellent replacement solution in many applications and their theory of operation, advantages and disadvantages are well documented. Read this Q&A to learn more.

By Consulting-Specifying Engineer September 22, 2023
Courtesy: Consulting-Specifying Engineer

Boiler insights

  • The webcast and this Q&A review what to look for when retrofitting a boiler system.
  • A relatively long service life, as well as the significant amount of energy a boiler uses, makes all decisions related to boiler replacement critical from financial, operational and sustainability standpoints.

While the mechanical engineering team has much more control over how HVAC systems are designed and how they fit into the facility for new construction, retrofitting HVAC systems to accommodate existing buildings is considerably more challenging.

During the “Retrofitting systems in existing buildings with boilers” webcast on Aug. 17, 2023, several items were left unanswered. Read about the additional information here.

Experts responding to questions:

  • John Mayeda, PE, Mechanical Regional Director, Jordan & Skala Engineers
  • John J. Skinner, PE, LEED AP BD+C, Vice President, WSP USA

Question: If the boiler and hot water supply/hot water return is original to the building, during boiler replacement, should the old piping distribution also need to be replaced or a piping flush/clean will be sufficient?

John Skinner: Depending on the age of the system, cleaning is a great idea. However, unless there are known blockages/clogs/scaling in the system, replacing of the existing pipe should not be required. Coil strainers should be cleaned and checked at all existing coils during the project to ensure any debris is removed from the system. A grit and air separator sized for full flow of the system can also be installed at the plant to clean debris over time and they do work very well.

John Mayeda: A flush/clean should be enough. If there are concerns about the piping and it has experienced leaks, a small section of straight pipe and a few elbows can be removed and tested for piping thickness and corrosion to evaluate the condition of the piping.

Question: Decarbonization and boilers?

John Skinner: Decarbonization or electrification is a big push in more than a dozen states and several large cities at present. You must understand if the existing utility and infrastructure can provide the power required to use electric boilers or equipment with integral electric steam generators. Many municipalities allow a plan to electrify existing facilities over a period.

When considering boilers, there are several options. Courtesy: Consulting-Specifying Engineer

When considering boilers, there are several options. Courtesy: Consulting-Specifying Engineer

Question: Boiler delta-T is generally irrelevant for the system since condensing boilers are always decoupled from the loop flowrate.

John Skinner: We generally agree that heating water systems are less susceptible to low delta-T syndrome, however, the entering water temperature of the system is critical to keep the system in a condensing state.

Question: Are NOx emission requirements for boilers set state by state or are there other codes that describe the limits like occupational health and safety (OH&S)?

John Skinner: NOx emissions do vary by location. In southern California for a period, 12 parts per million (PPM) was acceptable unless the boiler was within a certain distance of a school. Now 9 PPM and 6 PPM are now common requirements. It’s useful to remember that NOx emissions will also vary by fuel.

John Mayeda: Low NOx emissions also vary by the size and type of boiler used. Different states and counties within states, have different air quality compliance standards. Air emissions are also regulated more at the federal level by the Environmental Protection Agency (National Ambient Air Quality Standards) and Department of Energy in lieu of OH&S.

Question: When is de-aeretor necessary? Many boilers are currently operating without one.

John Skinner: De-aeration is necessary on most steam boiler plants to control the amount of dissolved oxygen and is sometimes used as a location to treat and control other boiler water components. For fully hydronic systems, de-aeration is not needed.

Question: What is your typical life of the condensing equipment boiler installed?

John Skinner: About 20 years is a typical service life for boiler equipment.

Question: The residual value is the scrap of the new boiler not the scrap of the equipment replaced?

John Skinner: The residual value used in the example is the weight of the boiler equipment at $0.07/dry pound.

Question: Do you usually recommend electric condensing instead of a gas condensing boiler?

John Skinner: No, I do not usually recommend electric boilers. The electrodes seem to me to be even more sensitive to water quality. The electric boilers I’ve used in health care applications have been focused to a particular set of equipment, like the central sterile department and frequently a cleaner water source is desired, like reverse osmosis (RO) water. When steam is made from RO, the equipment and steam distribution pipe needs to be stainless steel due to the aggressive nature of the steam and water.

Question: If you replace the hot water loop from HX’s with a set of modcons and end up with a limited steam load for only a few things left that cannot keep a huge steam boiler stable (like steam sterilizing for surgical, maybe a few coils only in an AHU) what do you recommend?

John Skinner: The choices are either replacing the large steam boilers with smaller equipment, like maybe two machines at 60% of full capacity or replacing the steam-fired sterilizers with new equipment that has integral electric steam generators. AHUs would need replacement coils and possibly air blenders to minimize freezing concerns.

Question: Do you have a model example of a tandem boiler life cycle cost analysis (LCCA)?

John Skinner: No, I do not, but an LCCA can be completed on any system. It’s just a matter of tracking the same kinds of costs across the same period of time and converting to present value dollars in order for an equal comparison. For tandem boiler systems, you would have two different performance curves, one for each stage, to account for as well as the maintenance.

Question: What are some strategies to minimize scaling due to hard water?

John Skinner: The best strategy is a contract with a good water treatment company. The chemicals in the system and how well they are kept up will make or break a boiler system. Ensure the contract includes regular testing and performance incentives.

Question: Is there commercially-available design software you recommend for preliminary boiler sizing?

John Skinner: Not that I am aware of. We normally take our projected heating demands to use in preliminary sizing. At the concept level, we choose what will be on a hydronic heater and what will need steam. From this information, we can get a boiler size to aid in determining the size of central utility plant.