Figure 4-50.Electrical switches: A. Snap-action switch;
B. Mercury switch.
heating plant. It transmits the indicating signal to a
primary control for action. This indicating signal is
initiated by closing or opening electrical contacts in the
Thermostats often differ in construction according
to the type of primary control with which they are to be
used. Probably the most used thermostats are the
spiral-bimetallic type and the mercury-bulb type.
An electric clock thermostat has the additional
features of an electric clock and an automatic
mechanism that can be adjusted to change the
thermostat setting at a desired time. For instance, it can
be adjusted to reset the thermostat automatically from
80°F to 60°F at 11:00 p.m. (when 80oF heat is not
needed). Then it will reset the thermostat to 80oF at
6:00 a.m. (when more then 60°F heat is needed).
The location for the thermostat should be
representative of that part of the building in which heat
is needed to maintain a comfortable temperature. The
best location is on an inside wall, just a few feet from an
outside wall and about 4 1/2 feet above the floor. The
thermostat wiring must conform to local electrical
To check the calibration of a thermostat, hang-an
accurate test thermometer within 2 inches of the
device. Allow 15 to 30 minutes for the thermostat and
thermometer to adjust themselves to room
The thermostat contacts should close
when the control knob or dial is set at the temperature
indicated by the test thermometer. You should not try
to recalibrate the thermostat if the closing point varies
1°F or less. When calibration is necessary, follow the
Since there are many types and makes of oil- and
gas-fired warm-air furnaces on the market, detailed
assembly instructions to suit all makes and types
cannot be given in this manual. However, some
general instructions, which apply to both oil-fired and
gas-fired furnaces, except as noted, are given below.
Carefully follow assembling instructions included
with each furnace or blower shipment. Each piece or
casting is manufactured to fit in its proper place. Parts
are seldom interchangeable.
Install furnaces in a level position. If the floor is
uneven, use a steel wedge, a cast-iron wedge, or the
leveling bolts provided on some equipment. Use a
spirit level to make sure the unit is level.
Gas-fired and oil-fired forced-air units, which
have the blower below the heating element or
combustion chamber, should be set on masonry at least
3 inches thick and extending at least 12 inches beyond
the casing wall. Install all other units on a cold masonry
floor. Provide enough clearance to permit easy access
for repairs. Make the clearance at least 18 inches from
wood or other combustible material unless you install
an asbestos board at least 1 inch from the combustible
material. Units may be installed near masonry walls;
however, leave ample room to permit proper servicing.
Furnace cement is furnished with each cast-iron
furnace. Seal all furnace joints with a liberal amount of
furnace cement between sections to ensure the furnace
is gastight. Asbestos rope is furnished with a number
of furnaces; follow the manufacturer's instructions
covering its use. See that projections from the furnace,
such as the smoke pipe or clean-out doors, extend
through the outside of the casing.
In assembling a furnace, be sure to tighten all bolts.
Draw each bolt until it is almost tight. Then, after all