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Heating is not required for the wetwell.

The wetwell shall be adequately and continuously ventilated at a minimum 12 air changes per hour by the use of both a supply fan and an exhaust fan as recommended by National Fire Protection Association (NFPA) standard 820. Volume calculations shall be based on the total air space above the minimum wetwell wastewater level and the bottom of the roof slab. Air changes shall be based on 100 percent outside supply air. The maximum line air velocity through all ventilation piping shall be 1,200 feet per minute.

Fresh air shall be delivered horizontally about one foot above the high wetwell alarm level on the pump side with the exhaust intake near the top of the wetwell on the inlet sewer side.

The system shall be designed so as to maintain a slightly negative pressure in the wetwell in accordance with NFPA 820. Ducting inside the vault shall be of PVC. The ducts shall not interfere with access into and out of the wetwell or with the removal of pump units.

The fans shall be mounted on top of the wetwell slab if space permits or offset from the wetwell in weatherproof enclosures such that they do not interfere with the opening and closing of the hatches, the loading of the pump units onto a truck using the portable hoist, or with the lifting of the pumps out of the wetwell using a truck mounted crane.

The fan enclosures shall be hingable, lightweight and vandal resistant. Fans shall be provided with vibration isolation and flexible connections at the fan inlet and exit.

Bird screens shall be provided on the inlet to the supply fan and at the end of the discharge vent of the exhaust fan. Screens shall be one-quarter-inch square mesh, 16 gauge stainless steel.

As the fans are to run continuously, no controls other than the branch circuit breakers are required in the control cabinet. Near the fans, manual motor switches with thermal overloads, in lockable weatherproof enclosures, shall be provided. Proper functioning of the ventilation system shall be monitored with vane switches in the supply air and exhaust air ducts. Ventilation system failure shall be locally indicated and also transmitted through the SCADA system. (Design Guideline DG-08 § 5.8, dated August 2006)