Building pathology: Boilers
- Date: 01/08/2008
Building Magazine: Even the simplest heating system is made up of a multitude of components, all of which need to interact if it is going to operate properly and deliver consistent performance. Some problems can be resolved relatively easily by the facilities manager or homeowner, such as checking that the pilot light is on, the control settings are correct and the fuel or electrical supplies are as they should be.
More complex problems will need the input of a trained person or a specialist and any investigation and remedial work will need to be carried out within the relevant legislative and regulatory frameworks covering issues of health and safety, gas, electricity, pressure systems, water and hazardous substances.
Invaluable references include BSRIA troubleshooting guides, CIBSE and Heating and Ventilating Contractors Association publications as well as specific services plant information. However, here are some of the common things to go wrong:
Cracking of boiler sections
Excessive thermal stresses to the boiler heat exchanger may result in cracking or fracture. High temperature differentials are typically caused by poor water flow, poor water distribution in the boiler shell, burner overfiring or tie rods that have not being tightened correctly in cast-iron boilers. The build up of corrosion between boiler sections can result in stress failures. Thermal stress can also be caused by hot spots owing to waterside scale and deposits.
Short-term overheating with resultant thermal stresses may be caused by operating procedures, for example, water level drop, the incorrect operation of pump over-run, large load swings or changes in the firing pattern. Thermal stresses may also cause differential expansion across boiler sections that results in failure of seals between boiler sections and consequent leakage.
Water side problems
Water quality and the type of water treatment used are critical to prevent scale build up and hence avoid problems of overheating and reduced water flow. Steel and cast iron boilers are particularly susceptible to corrosion (caustic or oxygen) owing to build ups of sludge where the system is not flushed as required. BS 2486 and BS 7593 give guidance on water treatment for commercial and domestic boilers.
Fire side problems
Fire side surfaces may be covered with soot and other combustion residues that are typically high in sulphur content where incomplete combustion occurs, for example, as a result of inadequate air supply. If moisture comes in contact with the soot, highly corrosive sulphurous acids will form that will corrode the heat exchanger. Moisture may be present for a number of reasons but generally as a result of condensation or a humid atmosphere when the boiler is not firing.
Poor fabrication and quality control may result in problems. Assurance may be provided where boilers are to the relevant British and European standards, for example: boilers with forced draught burner to EN 303; gas-fired condensing boilers greater than 70kW (EN 15417) or less than 70kW (EN 677). Consider manufacturers with a third-party accredited quality assurance programme.
Retrofitted devices, for example to improve efficiency, may be incompatible with the boiler. Ensure devices are approved by the boiler manufacturer before installation.
Failure of the lead or most heavily used boiler is normal in advance of failure of less frequently fired boilers. The differential use between lead and lag boilers can be averaged out by a monthly change to the firing sequence. This may not be possible where the lead boiler is a condensing boiler.
Maintenance and operational problems
Maintenance is a critical factor affecting the safety, efficiency and durability of boilers. Thorough cleaning, removal of partly burned fuel and attention to water quality will help prevent corrosion and scale build-up. At best, poor maintenance results in reduced efficiency. At worst, premature failure.
Most manufacturers provide fault charts to assist with problem solving. Well-targeted maintenance and inspection regimes can often catch potential failures and allow the instigation of remedial actions before performance deteriorates too far. Indeed, condition-based maintenance strategies may incorporate monitoring equipment connected to a remote monitoring station allowing automatic checking in real time so that critical changes in performance can trigger an early warning.
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