Building Design and Construction Handbook - Merritt - Ventech!

HEATING, VENTILATION, AND AIR CONDITIONING 13.49
Gas-fired heaters and boilers are usually provided with a draft hood approved
by building officials. This should be installed in accordance with the manufacturer’s
recommendations. Oil-fired heaters and boilers should be provided with an approved
draft stabilizer in the vent pipe. The hoods and stabilizers are used to prevent
snuffing out of the flame in extreme cases and pulling of excessive air through the
combustion chamber when the chimney draft is above normal, as in extremely cold
weather.
Flues for the products of combustion are usually connected to a masonry type
of chimney. A chimney may have more than one vertical flue. Where flue-gas
temperatures do not exceed 600�F, the chimney should extend vertically 3 ft above
the high point of the roof or roof ridge when within 10 ft of the chimney. When
chimneys will be used for higher-temperature flue gases, many codes require that
the chimney terminate not less than 10 ft higher than any portion of the building
within 25 ft.
Many codes call for masonry construction of chimneys for both low- and hightemperature
flue gases for low- and high-heat appliances. These codes also often
call for fire-clay flue linings that will resist corrosion, softening, or cracking from
flue gases at temperatures up to 1800�F.
Flue-pipe construction must be of heat-resistant materials. The cross-sectional
area should be not less than that of the outlet on the heating unit. The flue or vent
pipe should be as short as possible and have a slope upward of not less than 1 ⁄4 in/
ft. If the flue pipe extends a long distance to the chimney, it should be insulated
to prevent heat loss and the formation of corrosive acids by condensation of the
combustion products.
All combustion-type heating units require air for combustion, and it must be
provided in adequate amounts. Combustion air is usually furnished directly from
the outside. This air may be forced through ductwork by a fan or by gravity through
an outdoor-air louver or special fresh-air intakes. If outside air is not provided for
the heating unit, unsatisfactory results can be expected. The opening should have
at least twice the cross-sectional area of the vent pipe leaving the boiler.
(H. E. Bovay, Jr., ‘‘Handbook of Mechanical and Electrical Systems for Buildings,’’
and D. L. Grumman, ‘‘Air-Handling Systems Ready Reference Manual,’’
McGraw-Hill Publishing Company, New York.)
13.12 HOT-WATER HEATING SYSTEMS
A hot-water heating system consists of a heater or furnace, radiators, piping systems,
and circulator.
A gravity system without circulating pumps is rarely installed. It depends on a
difference in density of the hot supply water and the colder return water for working
head. Piping resistance must be kept to a minimum, and the circulating piping
system must be of large size. A forced circulation system can maintain higher water
velocities, thus requires much smaller pipes and provides much more sensitive
control.
Three types of piping systems are in general use for forced hot-water circulation
systems:
One-pipe system (Fig. 13.5). This type has many disadvantages and is not
usually recommended. It may be seen in Fig. 13.5 that radiator No. 1 takes hot
water from the supply main and dumps the colder water back in the supply main.
This causes the supply-water temperature for radiator No. 2 to be lower, requiring