Backwashing

Backwashing is the process of reversing water flow in the sand filters, flushing trapped particles and material (debris) to waste (sanitary sewer).

When to Backwash

Per PHTA guidance, backwashing the sand filters is necessary when the gauge pressure increases 8 to 10 PSI above the starting pressure (typically about 9 to 10 PSI).

In other words, when the tank gauge shows 17 to 20 PSI, backwashing is necessary. That said, depending on other parameters (such as flow rate), backwashing before this threshold may be necessary. To maintain proper filtration, the system must have sufficient flow to fully turn over every 6 hours (4 times daily).

Excessive Backwashing

Excessive backwashing will result in an inability to remove smaller particles. Sand filters are most effective at trapping small particles when they are at the latter (higher pressure) part of the cycle. Therefore, premature backwashing may result in lower water clarity.

Backwashing also dumps a tremendous amount of water into the sanitary sewer, often leading to a necessary fill post-backwash. Reducing the frequency of backwashing (to only when it's deemed necessary versus on a too-frequent interval) conserves water and can improve overall water clarity.

Procedure

Preparation

1. Disable the call for heat by lowering the temperature setpoint on the temperature controller to a value several degrees below the sensed temperature (in the future, a switch may be added to do this).
2. If the boiler was operating in Step #1, wait for the boiler circulation pump to stop. This may take several minutes. Observe that during this period, the manifold temperature will drop, approaching the boiler inlet (sensed) temperature. It is important to allow the boiler to complete a proper cool-down prior to cutting water circulation. Failure to do so could result in an over-temperature trip, or damage to the boiler itself.
3. Stop the main pump motor. Switching off the disconnect (the lever on the enclosure above the motor starter) may be necessary in some cases (see below).

NOTE: These steps are identical to the first three followed when cleaning the strainer basket.

Set Valve Positions

1. Open Valve #1
2. Close Valve #2
3. Open Valve #3
4. Close Valve #4