a.In the absence of local requirements, the confined space shall be provided with two permanent openings, one in or near the top of the room and one near the bottom. The openings shall communicate by means of ducts, with the outdoors or to such spaces (crawl or attic) that communicate with the outdoors.
i.Where communicating by means of vertical ducts, each opening shall have a free area of not less than 1 sq. in. per 4,000 Btuh (35 sq. in. per gph.) (5.5 cm2 per kw) of total input rating of all appliances in the enclosure.
ii.If horizontal ducts are used, each opening shall have a free area of not less than 1 sq. in. per 2,000 Btuh (70 sq. in. per gph.) (11 cm2 per kw) of total input of all appliances in the enclosure.
5.CHIMNEY OR VENT (Be sure to read below WARNINGS.)
The Model MPC™ Series boiler is designed for forced draft firing and may be used with a conventional natural draft stack (15’ minimum height) or a stub vent, sometimes called a diesel stack (see Figure 3a). See Table I for the proper vent outlet size. For low silhouette vent terminations, see Figure 3b. Draft controls are not normally required, although they may be used on installations where a natural draft stack is used or on multiple boiler installations with a common stack. For proper operation, boiler must maintain positive pressure of 0.1” W.C. at the breech in the cast iron flue collector during burner high fire operation (see breeching pressure sensing port in Figure 1b). To obtain the above, a factory supplied MPC Flue Outlet Damper Assembly (see Page 56 for details) must be attached directly to the Rear Flue Collector Outlet.
If the venting system is designed for positive or forced draft venting, the boiler, vent connector and stack will operate under positive pressure. Gas tight vent systems designed for pressure systems must
Figure 3a: MPC with Rear Outlet Vent
be used to prevent flue
If the venting system is designed for negative pressure (natural draft), the boiler still operates with positive pressure in the chamber and up to the fixed damper on the flue collar. However, if the venting system is larger than what is required, the stack will provide a surplus draft (or negative pressure) that may require the use of a barometric damper to maintain the positive 0.1” W.C. pressure at the flue outlet. Multiple forced draft boiler stacks should always be designed as negative to ensure the products of combustion do not exit a boiler that is not firing.
NOTICE
When an MPC gas fired boiler is connected to a venting system that is designed so that it will operate under a negative pressure, the use of Type B, or other manufactured vent systems designed for negative pressure is acceptable.
When an MPC oil fired or combination gas/oil fired boiler is connected to a venting system that is designed so that it will operate under a negative pressure, the use of Type L or other manufactured vent systems designed for negative pressure is acceptable.
Unlined masonry chimneys are not acceptable. Lined masonry chimneys are acceptable with the appropriate vent connectors using materials described above.
MPC oil or combination gas/oil boilers should be vented using Type L vent, regardless if the vent pressure is positive or negative.
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