A.O. Smith BTH 120 - 250 Vent Pipe Preparation, Controls and Switches, Blower Prover Switch

Table 2. VENT LENGTH TABLE

4-inch PVC may be used for a MAXIMUM intake of ONE HUNDRED TWENTY

(120)EQUIVALENT FEET and a MAXIMUM exhaust of ONE HUNDRED TWENTY (120) EQUIVALENT FEET. The maximum number of 90° elbows with the 4-inch venting is six (6) on the intake and six (6) on the exhaust. A 90° elbow is equal to five (5) equivalent feet of pipe. One (1) 90° elbow is equal to two (2) 45° elbows. Any venting configuration using less than 50 equivalent feet should use 3-inch venting. See Table 2.

The 3-inch venting terminals (provided) must be used with the 4-inch venting by adding 4x3 reducing couplings at the venting terminals. A reducing coupling is also needed immediately after the condensate elbow (exhaust) and immediately before the 3-inch blower adapter (intake) if direct venting is installed. See Table 2.

IMPORTANT

When multiple units are direct vented through a wall (3-inch or 4-inch venting), all intake vent terminals should be no lower than the highest exhaust vent terminal.

NOTE: This unit can be vented using only PVC (Class 160, ASTM D-2241 Schedule 40, ASTM D-1785 ; or Cellular Core Schedule 40 DWV, ASTM F-891) , Schedule 40 CPVC (ASTM F-411), or ABS (ASTM D-2661) pipe. The fittings, other than the TERMINATIONS should be equivalent to PVC- DWV fittings meeting ASTM D-2665 (Use CPVC fittings, ASTM F-438 for CPVC pipe and ABS fittings, ASTM D-2661/3311 for ABS pipe. If CPVC or ABS pipe and fittings are used, then the proper cement must be used for all joints, including joining the pipe to the Termination (PVC Material). PVC Materials should use ASTM D-2564 Grade Cement; CPVC Materials should use ASTM F-493 Grade Cement and ; ABS Materials should use ASTM D-2235 Grade Cement.

NOTE: for Water Heaters in locations with high ambient temperatures (above 100°F) and/or insufficient dilution air, it is recommended that CPVC or ABS pipe and fittings (MUST USE SUPPLIED VENT TERMINAL) be used.

4.It is important that condensate not be allowed to buildup in the exhaust vent pipe. To prevent this from happening the pipe should be installed with a slight, 1/8 inch per five feet (5') of pipe maximum downward slope.

5.The vent system should be supported every five feet (5') of vertical run and every three feet (3') of horizontal run of vent pipe length.

NOTE: Stress levels in the pipe and fittings can be significantly increased by improper installation. If rigid pipe clamps are used to hold the pipe in place, or if the pipe cannot move freely through a wall penetration, the pipe may be directly stressed, or high thermal stresses may be formed when the pipe heats up and expands. Install accordingly to minimize such stresses.

VENT PIPE PREPARATION

1.INITIALPREPARATION

A.Make sure the solvent cement you are planning to use is designed for the specific application you are attempting.

B.Know the physical and chemical characteristics and limitations of the PVC, PVC cellular core, ABS or CPVC piping materials that you are about to use.

C.Know the reputation of your pipe and cement manufacturer and their products.

D. Know your own qualifications or those of your contractor. The solvent welding technique of joining PVC, PVC cellular core, ABS or CPVC pipe is a specialized skill just as any other pipe fitting technique.

E.Closely supervise the installation and inspect the finished job before start-up.

F.Contact the manufacturer, supplier, or competent consulting agency if you have any questions about the application or installation of PVC, PVC cellular core, ABS or CPVC pipe.

G.Take the time and effort to do a professional job. Shortcuts will only cause you problems and delays in start-up. The majority of failures in these systems are the result of shortcuts and/or improper joining techniques.

2.SELECTION OF MATERIALS

PRIMER

It is recommended that Tetrahydrofuran (THF) be used to prepare the surfaces of pipe and fittings for solvent welding. Do not use water, rags, gasoline or any other substitutes for cleaning PVC cellular core, ABS or CPVC surfaces. A chemical cleaner such as MEK may be used.

CEMENT

The cement should be a bodied cement of approximately 500 to 1600 centipoise viscosity containing 10-20% (by weight) virgin PVC material solvated with tetrahydrofuran (THF). Small quantities of dimethyl formamide (DMF) may be included to act as a retarding agent to extend curing time. Select the proper cement; Schedule 40 cement should be used for Schedule 40 pipe. Never use all-purpose cements, commercial glues and adhesives or ABS cement to join PVC or CPVC pipe and fittings.

SAFETY PRECAUTION: PRIMERS AND CEMENTS ARE E X T R E M E L Y FLAMMABLE AND MUST NOT BE STORED OR USED NEAR HEAT OR OPEN FLAME. ALSO, USE ONLY IN A WELL-VENTILATED AREA.

CONTROLS AND SWITCHES

The BTH-120 and 250 are provided with four pressure switches. Both the BTH 150 and 199 are provided with three pressure switches. These switches are essential to the safe and proper operation of the unit. All switches are wired in series. The controller is set up to shut the unit down whenever there is a failure of any of the switches. It is important to understand the purpose of each switch.

FIGURE 13

BLOWER PROVER SWITCH

(SEE FIGURE 13)

The Blower Prover Switch is provided on the heater to verify that the fan is operating. It is a positive pressure switch whose electrical contacts are normally open. When the fan increases the pressure in the burner, the pressure switch will allow the electrical contacts to close. The pressure switch is connected to the burner tap by a piece of tygon tubing. This