A tale of condensation: The story of one high-rise boiler failure
By David Brooks, P.E., Sean Allard, CPD, LEED GA McGuire Engineers, Chicago
Sometimes plumbing designs can look great on paper, but once installed out in the field, an en-tirely different scenario may ensue. Whether the unintended consequence of value engineering, specifying inappropriate equipment for the space, an equipment malfunction or not installing the piping per the manufacturer’s recommendations, a failure in the field can be challenging for owners, end users and designers alike. In some cases correcting one issue may lead to another. The key is to ensure that systems are strategically set up and installed as intended, in order to ensure appropriate equipment performance, longevity and public safety.
Case In Point: Chicago condominium high-rise
As a case in point, one 60-plus floor Chicago high-rise condominium building was designed to code, yet two domestic hot water heaters broke down after just a few years of operation. What went wrong? The building’s domestic hot water system is comprised of two non-condensing domestic water boilers and two hot water storage tanks in the penthouse of the high-rise. The hot water heaters are controlled by the feed hot water into the boilers from the tanks. A pump is attached to each boiler that circulates water between the boilers and the tanks. On a typical day, the lead hot water boiler maintains the storage tank temperature and the lead and lag boilers operate during peak usage.
The first domestic water boiler failed within five years, so the building had the equipment ser-viced and some components replaced. McGuire was asked to analyze the situation and discov-ered a number of air pressurization problems as well. In order to address these, the team modi-fied the combustion air inlet so that it would draw air from outside, and not from the space it-self. This was done by adding a duct from the outdoors and connecting it directly to the boiler combustion air inlet.
Thinking the latter solution solved the problem, the designers were surprised to discover a sec-ond domestic water boiler failure just a few more years down the road. The heat exchanger and burners were removed from the failed domestic water boiler for investigation. It was at this time that the condition of the equipment was revealed and the McGuire team saw first hand that the burners had practically disintegrated, leading to the conclusion that the combustion air inlet wasn’t the only issue, but there was something greater at play as well.
Upon further analysis, McGuire’s team discovered that the domestic hot water was being stored in the two 600-gallon storage tanks at 120°F. When the hot water boilers were firing, hot water less than 120°F would return to the domestic water boilers and cause the combustion gases to condense on the burners, which was slowly destroying them. Additionally, the piping configuration between the boilers and the storage tanks was not installed to allow the water flow to be balanced, which meant that more hot water was passing through one boiler before it reached the second tank. This meant that the temperature of the water in the second tank was even lower than 120°F, further exacerbating the problem.
Of course, the decision to keep the water at 120°F wasn’t arbitrary. It was likely done for the sake of the residents’ safety, as scalding hot 140°F water can cause a second-degree burn with-in five seconds. However, a negative byproduct of this situation could be the unwanted growth of bacteria, which can occur in water stored below 130°F. While the condo association was protecting the residents by keeping the temperature down, unfortunately, at the same time, they were unknowingly destroying their equipment and exposing their water supply to elevated bac-teria counts with no means to control it.
Solving the problem
In order to address this myriad of issues, the building had two main options: changing out the existing non-condensing domestic water boilers with new condensing domestic water boilers or alter the existing systems with new blending valves, piping configuration and controls.
Condensing boilers would have been an ideal fit for such a project during original design, as condensing domestic water boilers are made out of materials that aren’t affected by acidic con-densation. However, it’s likely that this project was focused on reducing the initial infrastruc-ture cost and the condensing boilers were value engineered out during the design phase. In-stead, the non-condensing water heaters installed in the building couldn’t handle the acidic condensation that eventually destroyed the burners.
For the repairs, the owners chose to work with the existing system in lieu of a complete re-placement of the boilers. Therefore, McGuire specified a new sequence of operations that in-cluded raising the temperature in the storage tank to 145°F, which offered the added benefit of providing a means to help control bacteria growth within the system. McGuire worked with the boiler representative to integrate a new control panel with the existing boilers and the building automation system. This provided the building’s engineering staff with the ability to monitor the operational conditions of the boilers regularly.
Next, a three-way non-electronic thermostatic valve was installed on the inlet and outlet mani-fold of each boiler to ensure that the water entering into the boiler is always more than 130°F. Acting as a control valve, the mechanism senses when the inlet temperature is too low and al-lows a portion of the hotter outlet water to mix with the cooler inlet water. This eliminates the condition that previously allowed condensation to form on the burners.
The last item that needed to be corrected was the existing the piping between the boilers and storage tanks. This piping was improperly sized and arranged. The plumbing contractor installed the new piping in a reverse return configuration in order to balance the flows between the equipment without the need for any balancing valves or carefully measuring piping lengths. This corrected the temperature differences between the two storage tanks and the workloads of the boilers when both are in operation.
McGuire had addressed the factors that lead to the failure of the boilers, but in doing so, had also changed the existing operational dynamics of the building. Now that the building was stor-ing hot water at 145°F, the remaining question was how to safely distribute it. We did not want to send 145°F hot water to building occupants who had previously been sent 120°F hot water. To prevent the potential of the hot water scalding the building occupants, a mixing valve was added at the discharge of the storage tanks. This brought the hot water temperature down from 145°F to 120°F to be delivered to the building occupants. McGuire also specified an electroni-cally controlled thermostatic mixing valve as well to provide an added level of protection against temperature creep during long periods of no or little hot water usage.
Plumbing design best practices
Though challenging, projects like the one described above deliver a number of important les-sons learned. Here are some of the best practices important to employ in every plumbing pro-ject.
Existing facility: If a heat exchanger and domestic water boiler is failing in an existing facility, take a close look at the combustion air configuration, the inlet and outlet water temperatures and equipment con-struction. Pay attention and think about how one device affects another.
TIP: Discuss options or equipment features that could make jobs easier with the building’s facilities personnel, such as automatic monitoring or alarming.
New facility: The plumbing design needs to account for the cold water return temperature, either by specify-ing equipment which can handle it or providing a design that can properly operate and minimize condensing conditions.
TIP: If equipment is changed as a result of value engineering and there is an opportunity to re-view the new equipment, take the time to properly evaluate it to try and determine all of the changes that could affect other parts of the system as a whole.
Hot water: Make sure the domestic water is hot enough — which requires storing it above 140°F. The hot water system design needs to account for mixing down the hot water to a usable temperature by the building occupants while also providing a way to control the potential growth of bacteria within the system.
TIP: Distributing hot water at 140°F and up is not practical for most applications and can cre-ate its own unsafe conditions.
Equipment selection: When selecting water-heating equipment, be sure to design to the limitations of the specific equipment that will be in the space. For example, is the system going to be in condensing mode? If so, it is important to specify condensing boilers. If not, make sure the water is set at the right temperature.
Electronic controls: Electronic valves offer a good way to prevent temperature creep. With some mixing valves, very hot water may be forced through the thermostatic valve during long periods of no demand, exposing building occupants to dangerously high hot water temperatures. Electronic valves do a much better job of controlling flow and ultimately delivering water at a more consistent temperature. Granted, electronic valves are more expensive than manual thermostatic valves, so designers have to be able to justify the added expense. Today, most buildings are developer driven and while developers are concerned about safety, they may not be as focused on system longevity.
Buy-in: Make sure the building engineers are on board. They must be educated to understand how the equipment installed is meant to be operated, what maintenance is required and what the limita-tions of the equipment are.
TIP: Write into the actual specs that time should be set aside for the equipment manufacturers to train the building operators and engineers so they understand exactly what regular and long-term maintenance is required.
Ultimately, it’s a plumber’s job to protect the public and that means that the engineers specify the right equipment for the job and ensure that is has been correctly installed. We also must make sure that the plumbing equipment is being used in the way in which it was intended. Value engineering may be good for the bottom line, but it has to make sense for optimal building operations as well.
David Brooks, P.E., is a principal and mechanical engineer, at McGuire Engineers. Sean Allard, CPD, is a plumbing designer at McGuire Engineers. They can be reached at email@example.com and firstname.lastname@example.org.