A.O. Smith BTH 300A, BTH 400A Vent Pipe Preparation, Controls and Switches, Blower Prover Switch

VENT PIPE PREPARATION

1.INITIAL PREPARATION

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 EXTREMELY 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-300 & 400 are equipped with four 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.

BLOWER PROVER SWITCH

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 tubing must be connected in order for the switch to change the electrical contacts. The controller requires that the electrical contacts on this air flow switch be open before it will allow the blower to come on. See Figure 13.

BLOCKED OUTLET PROVER SWITCH

The Blocked Outlet Prover Switch is set up to shut the unit off when a build-up of positive pressure in the exhaust vent pipe occurs. This switch is a positive pressure switch that requires an increase in pressure to change the electrical contacts from normally closed to open. When this switch prevents the unit from igniting, most likely the exhaust is blocked by some means. Check to see if the condensate is allowed to flow freely from the exhaust elbow and for obstructions in the exhaust venting and exhaust vent terminal. See Figure 13.

BLOCKED INLET PROVER SWITCH

The Blocked Inlet Prover Switch is set up to shut the unit off when a build-up of negative pressure in the intake vent pipe occurs. This switch is a negative pressure switch that requires an increase in negative pressure to change the electrical contacts from normally closed to open. The switch is connected to the pressure tap on the flange connected to the inlet of the blower. When this switch prevents the unit from igniting, most likely the intake is blocked by some means. Check to see if there is no more than eighty (80) equivalent feet (24.8m) of four inch PVC vent pipe on the intake. Also verify that the intake and intake vent terminal is free of obstructions that may prevent air from entering the unit. See Figure 13.

LOW GAS PRESSURE SWITCH

The Low Gas Switch (LGS) is a single-pole, normally open pressure switch that will close its contacts when a rising pressure of 4.6 in. W.C. (1.25 Kpa) is encountered. The contacts will open when the pressure falls below the fixed set point of 4.6 in. W.C. (1.25 Kpa) The LGS monitors the gas supply pressure to the heater. If the gas supply falls below 4.6 in. W.C. (1.25 Kpa), the main burner is extinguished (if heater is running) or the heater will not start up.

NOTE: LOW GAS SWITCH IS LOCATED ON INLET SIDE OF GAS VALVE. See Figure 13.

FIGURE 13.

ON/OFF SWITCH

The ON/OFF Switch is a single-pole, single-throw rocker switch. This switch provides 120V from the line source to the heater.

CAUTION

THE WATER HEATER IS POLARITY SENSITIVE. BEFORE APPLYING ELECTRICITY TO THIS HEATER BE CERTAIN THAT SUPPLY NEUTRAL WIRE TO GROUND CHECK INDICATES ZERO VOLTAGE.

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