Thermo Products OD6FA072D48B Proper Chimney Size, Prevention of Chimney Condensing

A. CHIMNEY:

The furnace must be connected to an adequate chimney or an approved vent in accordance with these instructions. An adequate chimney is one that is sealed and lined with the capability of producing a (-).04" WC flue draft and having the capacity to handle the amount of stack gases that are introduced into it. A chimney with an internal construction of corrosion resistant tile, stainless steel, or some other material that will withstand flue gas temperatures up to 900°F is required.

Qualified service personnel must perform all installations and services.

The following are common chimney requirements necessary for the furnace to operate correctly:

A masonry chimney serving a Thermo Pride oil fired furnace must comply with local codes and NFPA Standard for Chimneys, Fireplaces, Vents, and Solid Fuel Burning Appliances (NFPA211-1996 or latest edition).

1. PROPER CHIMNEY SIZE:

The inside area of the chimney liner should equal, at minimum, the area of the vent pipe exiting the furnace.

EXAMPLE: π x r2 = Area of Pipe (sq. in.)

r= radius of pipe

π= 3.1417

Flue Pipe Diameter = 6" [Radius of pipe = ½ diameter of pipe = ½ (6 in.) = 3in.]

πx 32 = 28 sq. in.

NOTE: This formula calculates the minimum inside area of the chimney. If more than one appliance vent connector pipe is connected to the chimney, the minimum inside area of the chimney should be equal to the area of the largest vent pipe plus one half the area of any additional vent pipes. If the chimney is too large or condensation has been a problem in the past refer to the NFPA Standard for the Installation of Oil Burning Equipment (NFPA31-1997 or latest edition) Appendix E for proper liner sizing.

2. PREVENTION OF CHIMNEY CONDENSING:

Stack gas may do one of two things as it escapes up the chimney:

A.Remain entirely in a gaseous state if the internal chimney wall temperature is above the dew point

B.Condense water vapor on the chimney walls if they are chilled below the dew point.

Condensing will always occur on chimney walls whose temperatures are below the dew point, but the condensate may evaporate when the walls warm above the dew point. If the chimney wall temperature does not exceed the dew point during the heating cycle of the furnace, the moisture may accumulate in large enough quantities to cause problems such as corrosion of a metal chimney (especially plain steel or galvanized steel), erosion and break up of a tile liner in a masonry chimney and, in severe cases, corrosion of the heat exchanger. Condensate also could enter the home through cracks or joints in the chimney in a worse case situation.

Condensation most likely will not occur at the bottom of the chimney because the stack gas heats the chimney walls as it rises and the bottom will be heated first. This heating of the walls will cause the stack gas temperature to drop, which in turn may reduce the stack gas temperature below dew point, causing condensation to appear on the upper part of the chimney first. This condensation may then run down inside the chimney and drip back as far as the flue pipe and heat exchanger, where corrosion may occur, if not treated.

To prevent condensation, it is necessary that the internal chimney wall temperature always be kept above the dew point. If the chimney is a masonry type, it may have to be fitted with a flue liner, when the temperature loss is too great for the furnace. If the chimney is a metal type, then an "all fuel" chimney must be used, such as a Class "A" triple wall or insulated metal chimney. A liner will act as an insulator

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