(2)  
Water Tank Drain Valve (Fig. 1)

(a)  The passenger water tank drain valve is a two-port valve located aft of the aft cargo compartment and below the passenger water tank. The valve controls draining of the passenger water tank. A drain valve control handle is accessible on the water service panel below the aft entry door.
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C.  Vent Valves
(1)  
On airplanes with turn-to-flow faucets, a vent valve is provided in each lavatory to vent the passenger water system plumbing while water is draining. It is a 2-way valve with an open and a closed position, and it is located below the sink in the sink cabinet (Fig. 3).

(a)  
In the forward lavatory the two vent valve positions are determined by a placard reading ON and DRAIN.

(b)  
In the aft lavatory the vent valve is closed when the handle points directly into the cabinet, and open when the handle is parallel to the airplane centerline.

(2)  
On airplanes with press-to-flow (self venting) faucets, the vent valves are not required for draining.

6.  Water Heaters
A.  A one and one half-quart water heater is installed under each lavatory washbasin in the supply plumbing to the hot water faucet (Fig. 3). Water is heated by one 400-watt heater consisting of three probes inserted in the bottom of the heater tank. A heater control switch and a light which is illuminated during normal operation are mounted on the side of the heater tank. A cycling thermal switch inserted in the bottom of the heater tank regulates the water temperature to approximately 125°F (Fig. 4). A manually reset overheat switch attached to the top of the water heater tank limits the case temperature to approximately 190°F. If overheat occurs, the switch interrupts power to the heater and the light. The light will be out only if the control switch is OFF or the overheat switch is open. The overheat switch may be located by removing the cover at the top of the water heater. The overheat switch is reset by pressing the bubble in the center of the rubber covering on the overheat switch after a sufficient cooling period. A pressure relief valve installed in the side of the water heater is designed to relieve at approximately 140 psig. The primary purpose of the relief valve is to relieve excessive pressure when the heater overheats due to a malfunction of the cycling and overheat switch and the water heater is isolated from the water system by the lavatory drain valve. When the heater is connected to the water system, any excessive pressure will be forced back through the water line into the water tank and relieved overboard. A cap is bolted to the top of the heater tank to protect the electrical connection.
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Sep 20/81  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  38-11-0 Page 7 

F90396 F90461
Sink Cabinet Passenger Water System Components (Typical)  553 
38-11-0  Figure 3  May 01/00 
Page 8 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

515  Water Heater and Water Quantity Indicating System Circuit 
Nov 01/74  Figure 4  38-11-0 
Page 9 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

PASSENGER WATER SYSTEM – TROUBLE SHOOTING
1. Passenger Water System Trouble Shooting Chart
TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Water pressure low  Filter clogged Defective check valve  Check air filter element If pressurized air is provided by a single source from APU or engine bleed air, check condition of applicable check valve downstream of source and replace if defective  Replace filter element (Ref 38-41-11, Servicing) as required Replace applicable check valve 

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Nov 15/78  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  38-11-0 Page 101 

PASSENGER WATER SYSTEM - SERVICING
1. General
A.  It is necessary to completely drain the water system before disinfecting it or when parking the airplane in freezing weather.
WARNING:  YOU MUST DRAIN THE POTABLE WATER SYSTEM AT LEAST ONE TIME EVERY 3 DAYS TO PREVENT THE GROWTH OF BACTERIA IN THE SYSTEM. IF YOU DO NOT DRAIN THE SYSTEM AT LEAST ONE TIME EVERY 3 DAYS, AND YOU DRINK THE WATER, ILLNESS CAN OCCUR.
B.  The water should be drained at least every 3 days and fresh water added to the system to prevent the growth of bacteria.
C.  Use a disinfectant regularly in the potable water system. Use a disinfectant after you complete a maintenance procedure on the water system or if the water system is contaminated.
　
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