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Fault Diagnostic Diagram 1 - Discharge from Pressure Relief Valve


Discharge from Pressure Relief Valve 

Faults are relatively rare in unvented hot water storage systems, the manufacturer’s installation instructions will contain quite detailed procedures for identifying faults for their specific unvented hot water storage systems and these should be used as a key reference when diagnosing faults.

The following flow diagram of key faults, on a system incorporating an expansion vessel as the expansion device, has been included to provide a general idea how to diagnose faults.

Internal expansion type cylinder
With bubble tops or cylinders incorporating an air gap as the expansion device, intermittent discharge from the expansion relief valve will in most cases mean that the air gap or bubble has reduced in volume. A simple solution to this problem is to:
• Turn off the water supply to the cylinder
• Open the lowest hot water tap in the system
• Hold open the temperature relief valve until water ceases to run from the cylinder
• Re-fill the system by turning on the mains supply. The air volume will automatically recharge as the bubble refills
• Turn off the hot tap when a smooth flow is seen

There are a number of manufacturers that incorporate internal bubbles in the cylinder and the principle of recharging the air is basically the same for all types. One brand has an internal floating baffle disc to separate the water and the air, intended to prolong the life of the air. Other brands may reduce the pressure of the incoming water to control the internal water pressure, which in turn will also prolong the life of the air.

The air gap in the internal bubble type has disappeared
The main reason why the air gap in the internal bubble will disappear is that the oxygen in the air is absorbed into the water (H2O). The time it will take for the air to be absorbed will depend upon the amount of oxygen already in the water. The Water Regulations requires that neither type of expansion facilitate wastewater, so periodic maintenance checks are required.