Hot water boilers are manufactured in an array of sizes to serve many different purposes; all share the common factor of heating water. Different boiler sizes and purposes dictate different typical pipe sizes, and because of this it is not possible to suggest a typical pipe diameter size. By the same token, system layouts are determined by each building’s shape and size, so no typical route system can be suggested. There are, however, a number of factors common to all installations that make an understanding of the factors possible. All installations must subscribe to local code.
The Function of Boilers
Electric, oil and gas-fueled boilers all operate on much the same principle: Cold water enters the boiler, the fuel increases the temperature of the water and in some cases changes its state to steam as it passes through the machine, and a system of pipes delivers the heated water to its points of use. A typical system consists of an inlet feed pipe, an outlet pipe and a delivery system. In installations where the boiler provides hot water to faucets as well as to space-heating devices, a storage tank is normal, either integral to the boiler or installed at the highest point of the building.
Highest-Point Storage Tanks
Storage tanks typically are built into smaller boilers, such as those serving the normal home, while highest-point storage tanks are used for commercial and industrial installations. In the latter, the boiler’s pump routes heated water up to the tank; gravity feeds it back downward to the faucets. For such applications, the size of pipe between the boiler and the tank is determined by their outlet and inlet port sizes respectively, and the pipe diameter from the tank to the taps is determined by a calculation that takes head pressure and anticipated maximum delivery into account. In most homes, half-inch pipe is used for runs to taps.
Steam boilers most commonly are used to heat spaces through convection devices such as radiators. Because this application is invariably part of the services in a large or very large building, the boilers tend to be extremely bulky. This, and the amount of space the boiler must heat, dictates the typical pipe size. The pipe that conveys the steam from the boiler to the header is called the boiler riser; the installation also will comprise one massive pipe from the boiler’s header to the system, properly called the system riser. The header is that part of the boiler where residual liquid moisture is removed from the steam before it is routed into the distribution system. Smaller pipes extend from the system riser to loops that feed the radiators.
In large commercial and industrial systems the size of the main system feed pipe, bringing cold water into the system from the utility is determined by code mandates and by the size of the inlet port of the boiler. Such systems typically convert the water to steam, which is then routed to the places where the heat is used. The pipes that transfer the steam from the system riser to the delivery points are much smaller. All pipe sizes, and the size of the boiler, are calculated for each separate installation project according to the maximum anticipated load, using space the be heated and number of radiators as factors.
Water Heating Boilers
Boilers also are used for space heating in smaller buildings, such as homes. These boiler systems function in much the same way as steam boiler systems, but route hot water, not steam, to the radiators. The installations are typically on a much smaller scale than commercial installations, and comprise only an inlet main, a pipe from the boiler to where loops depart to route the heated water to individual radiators and a return pipe. A drain pipe is often installed. The pipes that comprise the main circulation system typically have a larger diameter, or bore, than the loops to the individual radiators.
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