Thermo Pride MHA1-50N Installation And Service Manual page 19

All installations and services must be performed by qualified service personnel.
condensing within the chimney under certain conditions. The following is an
explanation on why and what to do if condensing occurs in a chimney or vent.
Moisture (in the form of water vapor) occurs in flue gases when hydrogen is
combined with oxygen during the chemical reaction produced by combustion.
The flue gas water vapor will do one of two things as it escapes up the chimney:
1. Remain in a vapor state, if the internal chimney wall temperature is above
the dew point, or
2. Condense 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 of the flue gas. However, the water may evaporate once the walls
are warmer than the dew point. If the chimney walls do not reach the dew point
during the cycle of the furnace, the moisture may accumulate in large enough
quantities to cause problems such as corrosion of the chimney or corrosion of
the furnace heat exchanger.
This condensation most likely will not occur at the bottom of the chimney,
because the flue gas is heating the chimney walls as it rises and the bottom will
be heated first. This heating of the walls will cause the flue gas temperature to
drop, which in turn may reduce the wall temperature below the dew point causing
condensation to appear on the upper part of the chimney first. This condensation
may drip back as far as the flue pipe and heat exchanger, where corrosion may
occur if not corrected.
To prevent condensation, it is necessary that the internal chimney wall
temperature always be kept above the dew point. The chimney may have to be
lined with a flue liner if the temperature loss is too great for the furnace. A liner
will act as an insulator and reduce the flue gas temperature loss. Insulation may
be added around the liner for further temperature stability. If the chimney is on
the exterior of the home and condensing occurs, the chimney may be insulated
around its exterior to help the flue hold it's temperature. Also, check to see if the
chimney is too large for the flue gases to heat, if so reduce to proper size by
lining. Be sure to use steel liners such as stainless types 430, 304, 316, or for
the toughest against corrosion type 29-4C.
More detailed information on condensing may be obtained from the 2000
ASHRAE systems and Equipment Handbook Chapter 30. Refer to National Fuel
Gas Code, NFPA 54/ANSI Z223.1-2002, or latest edition, Category I venting
tables, for proper vent sizing.
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