BYPASS BALANCING
With systems where water temperature can be expected to drop appreciably due to long standby periods, or heavy draw down, a bypass pipe of at least 1" size with a balancing cock should be installed between the boiler inlet and outlet (see fig. 10). When the system first starts, the valve should be slowly opened until the condensing ceases. This adjustment remains at a permanent setting to establish required temperature rise across the boiler.
LINEAR-TEMP SPACE HEATING APPLICATIONS
Controlling of these systems is decided mainly by the type of building system controlling that is desired. A single boiler installation might be controlled directly from space temperature thermostat(s). Multiple boiler installations are more effective when the boilers are sequenced in and out of operation by some form of main water temperature controller. With one or two boilers, individual control set at progressive temperature may be used. For more than two boilers, electronic sequencing controlling is recommended.
Individual controls, or the separate stages of a step controller, should fire a boiler and also start the boiler loop circulator whenever that boiler is fired. Some large installations may require the firing of more than one boiler per stage.
The system or primary circulator may or may not be controlled by the boiler sequencer. When this pump is operated through the first switch of any type of step controller, care should be taken to determine if a motor starter is needed due to insufficient switch capacity.
If the primary pump is controlled by a manual switch or any other controllers, the electric current supply to the boiler group should be through the primary pump controller. The fast response of A.O. Smith boilers eliminates any need to maintain boiler temperature when the system is satisfied. Wiring should always prevent firing of boiler(s) when there is no water flow in the mains.
Installation diagrams show safety flow switches in the outlet piping from each boiler as good protection against any boiler being fired when the boiler loop circulator is not in operation.
These safety flow switches will also provide some protection if there is a loss of water.
LINEAR-TEMP multiple boiler installations are especially adapted to the use of outdoor reset for main water temperatures. This feature is not mandatory but offers smooth, efficient operation of a modern system.
Outdoor reset systems should utilize an automatic shutdown control if there is a continuous recirculating main and/or if the entire building is not under control of space temperature thermostats. A single bulb outdoor sensing control will serve this requirement. This precaution will prevent overheating of halls, stairways or other uncontrolled areas. There are occasions when outdoor temperatures are temporarily too warm for even a moderate amount of heating in these areas.
Space temperature controlling can be varied to meet the building requirements. Either the single thermostat, as shown, or multiple zone thermostats should control a common relay. This relay controls electric power to the system primary circulator and to the main water temperature controller. This provides for water movement in the system before the main temperature controller can start the secondary circulating pump or fire the boiler.
Figure 11 shows a typical field wiring diagram for a single stage boiler LINEAR-TEMP installation. The boiler may be controlled by a main temperature controller as shown or may include outdoor reset if desired.
The following fig’s. 11 thru 13 are shown as layouts for various choices of controls often found in commercial heating. These layouts are not intended to be wiring diagrams and only show the relation of one device to another in the system.
Figure 11 is a typical layout of controls for two boilers with two circulators and thermal balancers plus optional outdoor reset control.
Figure 12 is shown as a typical control group for various multiple boiler installations, and is not intended to imply that this is the only arrangement to be considered. Commercial size installations are always best when designed to individual building requirements.
FIGURE 13
LINEAR-TEMP SPACE HEATING INSTALLATIONS
A. New Installation
The A.O. Smith LINEAR-TEMP system has been designed to provide efficient, trouble-free operation of these boilers with any of the following conditions:
•unknown system flow rate.
•varying flow rates as with zoned systems and 3-way valve system.
•multiple boiler installations.
Figure 14 shows piping and accessory arrangement for 1 or more boilers pumped independently of the primary system mains. Pipe sizing and boiler loop pump selection data, are shown in table 4 for several different temperature rises across various boiler sizes.
Total heating requirements for the building can be supplied by a series of boiler loops all connecting to a common pipe joining the system supply and return mains. The supply and return branches of each boiler loop must join the common pipe only a short nipple length apart. The different sets of branches should be installed reasonably close together, but not necessarily to the short nipple length as required for the supply and return of each set. These branches may be made with tees or with welded connections.
The installer is reminded that the total boiler flow rates do not have to match the system flow rate. Flow rates through heat
