With the newest state-of-the-art sensors, direct digital controls and computer driven expert systems, buildings now can be completely automated and remotely controlled. This article will review what such systems and their operators can and cannot do, what new risks can be created for the boilers in such buildings, and how to reduce and control those risks.
Benefits of automation
Intelligent buildings today use a building automation system (BAS) and other systems to operate HVAC, boilers, chillers, lights, elevators, building access fire protection and other systems to achieve the twin goals of efficiency and comfort.
A BAS is defined here as a system of sensors, pilot devices, logic elements, controllers, peripheral computers, and monitoring equipment providing data to a central computer for energy-efficient control of a building’s internal environment while also providing maximum tenant comfort and equipment reliability.
BAS can deliver energy savings, comfort and maintenance/reliability benefits. These benefits cannot be achieved, however, unless the systems are properly integrated. The system designer must anticipate and overcome the risks outlined below.
Thermal shock
When boiler controls are improperly integrated with the BAS, thermal shock can occur in several ways.
The most common is low return water temperature. In a recent claim, a cast iron hot water boiler suffered a cracked section because of cold water returning from the system. The building was a single-story school, with portions of the building distant from the boiler room.
During the fall, the BAS was programmed to reduce the temperature setpoint on weekends. Water in the zones serving remote areas cooled, and when the BAS attempted to bring the building back to normal occupancy temperatures, a slug of cold water cracked the hot boiler sections.
The problem was solved by putting timers on the zone valves to prevent the simultaneous opening. Controls also were added to maintain a minimum floor through the longest loops during reduced temperature operations to maintain a minimum temperature.
In another case, a store chain reported damage to boilers and air conditioning equipment due to thermal shock when the BAS alternated between heating and cooling. This situation was made worse because all the store locations around the country were connected to a central BAS.
The central BAS computer did not adequately allow for transition from heating to cooling operation and vice versa, resulting in boiler damage and business interruption.
Another form of thermal shock can occur in systems that use a three-way bypass valve. When controlled by outdoor air temperature, these valves allow a high bypass, with reduced flow through the boiler. This reduces the temperature delivered to the loop, thereby saving energy.
The problem occurs when the control is so aggressive that the boiler water stratifies the same way water stratifies in a water heater. In situations where the boiler is called on to deliver only minimum input, the burner will cycle frequently. Since the overall temperature is low, the metal is subjected to much higher stresses than intended or designed for by the supplier.
Thermal shock also has occurred in boiler rooms having outside air inlet louvers controlled by the BAS. Cold outside air entering the boiler or equipment room will chill the boiler. If inadequate precautions are provided, thermal shock, freezing or condensation will occur.
Single-pipe heating and cooling systems area also subject to thermal shock. Single-pipe systems — one piping system for heating and cooling — sometimes are selected for low initial cost. Should both heating and cooling be called for within a short period of time, there is a good chance that chilled water will be admitted into the boiler. Serious thermal shock will result.
Two-pipe systems — using separate piping circuits for heating and cooling — do not completely eliminate this risk. If the BAS incorrectly sequences air handling units and if both heating and cooling coils are in the same unit, air cooled by the refrigeration coils will cool the heating coils. This also will cause cold water to return to the boiler.
Another more common event occurs when the BAS calls for 100 percent outside air to save cooling or heating energy. This usually occurs in shared spaces of buildings that do not require heating or cooling for comfort. Spaces such as foyers, lobbies, hallways or large conference rooms frequently receive 100 percent outside air to address indoor air quality concerns.
The heating coils in the air handling units can be subjected to low temperature. When the boiler starts or the local zone calls for heat, the cold water will shock the boiler.
In spaces requiring very strict temperature and humidity controls, simultaneous operation of the heating — temperature control — and cooling systems — to reduce humidity — is needed. Careful attention to the programming of such heat/cool operation is necessary to avoid the potential for cold water return to a hot boiler.
Condensation
When boilers cool below the dew point temperature of the combustion products, flue gas condensation will occur during the boiler warm-up, causing corrosion on fireside surfaces, breeching and stacks.
Corrosion is another leading cause of boiler accidents. Much of the corrosion can be avoided if the BAS is programmed to maintain boiler temperature at or slightly above the flue gas dew point.
Boiler start-up
When a boiler is first started from a cold condition, both the metal and water are cold. If the boiler is designed to adjust firing rates in response to load changes, it is very important to heat the boiler slowly.
The metal structure must be allowed to gradually absorb the stresses that develop as the metal expands with temperature. Most boiler manufacturers include low fire hold into their boiler controls to provide boiler warm-up. BAS programming must not compromise this protection by premature transfer to high firing.
Multiple boilers (Modular)
For buildings with multiple or modular boilers, the hazards discussed above are more difficult to handle. In such cases, BAS programming must recognize the practical limits of boiler operation for all units.
Other concerns
In addition to the mechanical aspects already discussed, there can be other concerns with control system integrity. A BAS is comprised of sensors, pilot devices, logic elements, controllers, peripheral computers and monitoring equipment. All of these devices are sensitive to some of the following conditions.
Electrical transients
A number of claims have recently been reported involving failure of electrical and electronic devices caused by electrical transients. Common disruptions stem from lighting, internal power fluctuations, harmonics and electric utility problems. Without proper protection, an otherwise harmless power fluctuation can result in damage to the boilers if the BAS fails.
Major utility power problems are infrequent. Lighting arresters, surge protection and current controls have made power in the United States very reliable. One source of localized disturbances is not uncommon, however.
Short duration voltage collapse — a few cycles — can occur when utilities attempt to regulate power factors by compensating for inductive loads with large capacitor banks. When the capacitor bank is engaged, the current inrush causes the voltage to sag, which can cause localized disturbances in the power system.
Boiler/BAS Solutions
CONSIDER THESE RECOMMENDATIONS to address common boiler problems related to BAS:
Thermal shock
- Prevent cold water return by installing three-way mixing valves as a way of limiting the boiler inlet water temperature.
- Consider a small blending pump to prevent temperature stratification in the boiler.
- Control the low fire hold to warm the boiler according to boiler manufacturer guidelines.
- Avoid high fire cut-off when zones are satisfied to avoid rapid temperature change.
- Don’t overlook the air side as a potential source of thermal shock.
Condensation
- Avoid control logic that will increase the frequency of boiler cycling.
- Avoid contact of humid outside air on cool boiler surfaces, or keep the boiler warm.
- Maintain minimum boiler temperature at or above flue gas dew point.
Electrical transients
- Properly ground not only the BAS but also all other equipment. Follow the manufacturer’s instructions.
- Isolate AC power connection to electronics using isolation transformers or equivalent devices.
- Maintain minimum boiler temperature at or above flue gas dew point.
Recommendations
This article has covered areas of concern when integrating heating and cooling equipment with automatic building controls. BAS control programs must recognize the operating limitations of all controlled equipment, as well as the customary energy optimization, HVAC comfort, and building security objectives.
Building designers and owners and maintenance and engineering managers should always follow the recommended practice of the equipment manufacturers. BAS have the ability to control and monitor equipment and alarms from remote locations. Many states have laws requiring boiler operators to be on site in certain situations.
The ability of the BAS to sound alarms based on preset conditions with actions implemented from a remote location should not replace the actions taken by qualified operators on the scene.
