Condensing Boiler Efficiency Date: July 17, 2012 PRES E NT ED BY DO N L E O NA RDI LE O N A RD I I NC. HV AC T RAI N I N G & C ON SU LT IN G Concepts 1 The current state of evolution in boiler design 2 Boiler control design 3 Hot water heat distribution Terminology Hydronic Heating System Provides heat to a space or area for material and air heating using heated water as the heat transfer medium. The system includes: a boiler which contains systems for combustion and for exchanging the heat of combustion to a fluid; a closed distribution system consisting of piping, pumps, valves that direct the flow of fluid and carry it throughout the building; terminal units to exchange heat from the fluid to building air; thermal sensors that control valves and boiler operation to ensure the delivery of the right amount of heated fluid when needed to heat a space. Boiler A vessel in which fuel is burned to create heat that is then exchanged into a fluid, used for space or assembly heating. Excerpt text from the main text of a story to draw a reader s attention to the page. Burners (also heating elements) The distinct components of a boiler that allow combustion of a fuel to create heat Building envelope - roof, walls, and floors of the building, including doors, windows, insulation, and exterior finishes Reset temperature Energy Saving control algorithm which measures outside air temperature and adjusts mixing valve operation and / or high/low or modulating burners
PAGE 2 Excerpt text from the main text of a story to draw a reader s attention to the page. Hot Water for Space Heating Generally, a fuel is burned in the combustion chamber of a boiler to create heat. Contemporary combustion chambers are sealed, in that there is a dedicated source of combustion air to support the burning of the fuel, and exhaust gases are vented to the exterior of the building all in a sealed system. The Workings of a Condensing Boiler Water or other fluid is introduced to the boiler upon returning from the building. The fluid is first carried through the secondary heat exchanger within the exhaust flue, resulting in a small heat transfer and cooling the flue exhaust gases to the point of condensation. Then, the fluid is conducted through the primary heat exchanger positioned around the fuel burner for the more substantial transfer of heat from the combustion chamber. The fluid then passes out of the boiler and is carried to the various points of use throughout the building to provide distributed heat. After delivering its stored heat to the building, the fluid returns to the boiler, and the cycle is repeated. Internal diagram of a typical condensing boiler Boiler Sizing Boilers come in a variety of sizes, generally rated by the amount of heat in btu s that the boiler can produce over the course of one hour at sea level. Boilers are installed to provide enough heat to a building to keep the interior of the building at a comfortable temperature (generally 70 F) on what would be the coldest day of the year. This amount of heat is determined by calculating how much heat would be lost by the building to its environment through the building envelope. Heat loss is increased by air leakage through cracks and other openings in the building envelope. The Evolution of boiler control strategies Three currently used methods for controlling space heating utilizing a boiler: 1. Control the flow of water (either on or off); 2. Control the temperature of water within a given range (100 F 180 F); 3. Control both water temperature and water flow simultaneously.
PAGE 3 Control Strategy 1 Initially, boilers operated at a fixed combustion rate, regardless of the need for heat within a space. Boilers were turned on and off as needed to supply heat when called for, frequently by allowing or stopping the flow of water. By using this strategy, there was no means to alter the amount of heat output in a boiler it will either delivery 100% output or 0%. As such, a boiler that was sized correctly to deliver adequate heat to a building under the worst case scenario of heat loss would be significantly oversized when the call for heat is lower during swing seasons or even summer. This condition results in very inefficient operation of a boiler system, and significant waste for the fuel. Further, this strategy can result in wide temperature fluctuations in the spaces served. This is a very inefficient operating strategy called short cycling - boilers are most efficient when operated continuously. The cycling of water flow though the system can cause noise in the equipment due to expansion and contraction of piping and terminal units. Control Strategy 2 To prevent overdriving a space and cycling the boiler operation, new strategies were developed that would control the temperature of the water in the supply loop. Initially, water would exit a single stage boiler at full temperature and could then be cooled through the introduction of mixing valves to moderate the temperature. More recently boilers began to be equipped with multiple burners (multiple stage system), which provided a greater degree of control over the amount of heat supplied. One smaller burner would operate continuously to heat the water in the heat exchangers, and additional burners would fire and supply more heat as the need increased in the building. This was a more efficient strategy, as the smaller, multiple heating elements would fire as needed by the call for heat in the building more closely matching the heat production to the building load. This methodology typically will increase boiler efficiency, reduce the amount of wasted fuel and allow the boiler to operate continuously. Heat loss from the building is calculated based on exterior temperature (called the outdoor reset temperature) and the temperature of the water in the return loop.
PAGE 4 Control Strategy 3 Advanced systems controls that monitor building occupancy, interior and exterior environmental conditions, return loop temperature, and other variables add a great deal of sophistication and control to a boiler heating system. Using advanced controls and control system logic, modulating burners, flow control and multiple source mixing valves, the amount of heat supplied to a space through the terminal units can be more closely monitored and adjusted to the specific needs of the spaces being served. Delivery of heat to the space Once the water has been heated in the boiler, it is delivered to individual terminal units distributed throughout the building. Terminal units come in a wide variety of designs depending upon the application. Further, alternative design terminals require a different temperature water to operate efficiently. Style Attributes Design temp F Max F temp High Mass Radiant Low Mass Radiant Fan Coil Fin tube convectors * Radiators, panel radiators * Baseboard * In-floor radiant with concrete / masonry topping Under floor or infloor radiant with staple-up plates or overlay panels VAV Zone boxes with forced ventilation air and heat transfer coils Provide heat primarily through natural convection cycles induced in room air Provide heat through radiation, primarily, but also convection Low profile, provide heat through convection, primarily, but also radiation 120 140 140 160 190 210 180 200 160 180 150 170 * Mixing Valve control systems offer greater control over boiler operation
PAGE 5 Hot water distribution Hot water is delivered to the terminal units via a variety of piping configurations. Control of delivery is through zone control valves, which typically sense the need for heat within a space and adjust the flow of hot water to the space with individual circulator pumps. In North America, there has been a tendency among designers and contractors to specify and install a large number of individual zones, each with their own controllers, pumps and other equipment. Although increasing the number of zones provides a higher degree of control for the interior thermal environment, an over-designed system can cost significantly more at the time of installation. Over the life of a system, the operation of a larger number of circulator pumps can result in a significant power requirement, costing building owners more money, wasting expensive electrical energy, and frequently increasing greenhouse gas production. CO NT ACT I NF O RMATION: DO N L E O NA RDI LE O N A RD I I NC. HV AC T RAI N I N G & C ON SU LT IN G DO N LE O N A RD I @C O MC A ST.NET This information was presented at a Boulder County Energy Smart and Denver Energy Challenge Contractor Training Event and was funded by the American Recovery and Reinvestment Act