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The vault shall be adequately and continuously ventilated at a minimum six air changes per hour by the use of an exhaust fan and a supply fan as required by NFPA 820. Air changes shall be based on 100 percent outside supply air. The exhaust intake shall be near the underside of the top slab opposite the location of the fan.

The fans shall be mounted on top of the valve vault slab in weatherproof enclosures such that it does not interfere with the opening and closing of the hatches or with the removal or installation of valve vault equipment.

The fan enclosures shall be hingable, lightweight and vandal resistant. The fans shall be provided with vibration isolation and flexible connections at the fan inlet and exit. Fan housings shall be of cast iron, cast aluminum, or fiberglass reinforced plastic (FRP).

The ventilation exhaust vent shall be taken through the side wall of the vault. The ventilation exhaust vent shall terminate with a 180-degree bend at least 18 inches above grade. The inlet vent shall also terminate with a 180-degree bend at least 18 inches above grade. All ventilation piping located above grade shall be Schedule 40 steel pipe. The ventilation ducting inside the vault shall be Schedule 80 PVC pipe. The maximum line air velocity through all ventilation piping shall be 1,200 feet per minute.

The 180-degree bends shall be fitted with 24 mesh stainless steel insect screens. The ventilation pipe ends inside the vault shall be fitted with four-mesh stainless steel screens.

As the fans are to run continuously, no control other than the branch circuit breaker is required in the control cabinet. At the vault, manual motor starters with thermal overload shall be provided near the fan motors. This is based on the assumption that the fan motors will be fractional horsepower motors.

An adequately sized industrial quality electric heater with integral fan shall be provided to protect the equipment inside the valve vault from freezing. The electric heater shall be constructed to withstand vault washdowns. A design temperature difference of 40°F and the ventilation rate of six air changes per hour shall be used to size the unit heater. The unit heater shall be controlled by a thermostat with a preferred range of 35°F to 65°F. The heater shall be wall mounted using wall brackets.

(a) Acceptable ventilation fan manufacturers:

(1) Chicago Blower, 38-CPB design.

(2) Ceilcote, CLM Series.

(3) Cincinnati Fan and Ventilator Company, LM or PB series.

(4) Greenheck Fan Corporation, series 21 or 41.

(5) Howden Buffalo, Baby Vent set.

(6) New York Blower, Pressure Blower series.

(b) Acceptable electric unit heater manufacturers:

(1) Chromalox, model HD3D.

(2) Indeeco, Triad model.

(3) Marley, W series.

(4) Qmark, type JUW.

(5) Ruffneck, type CR1 Triton.

(6) Trane, model UHRA. (Design Guideline DG-08 § 6.4, dated August 2006)