Handbook of air conditioning and refrigeration / Shan K

Nighttime Setback. ASHRAE research project SP43 found that a nighttime thermostat setback
of 10°F (5.6°C) lower for 8 h improved E t slightly (only a 0.4 percent increase). However, there is a
10 to 16 percent annual savings in energy input compared to those furnaces without nighttime
setbacks.
Oversizing the furnace shortens the morning pickup time caused by nighttime setback. Use of an
oversized furnace has a significant effect on the swing of space air temperature when an on/off control
is used for the gas valve. Project SP43’s results showed that when the furnace size corresponded
to 1.4 times the design heating load (DHL), the furnace had a space temperature swing of
4.9°F (2.7°C) and a morning pickup time of about 1 h. If the furnace size was based on 1.7 times
the DHL, the space temperature swing increased to 5.9°F (3.3°C), and the pickup time reduced to
about 0.5 h. A furnace size based on 1.4 times the DHL is more suitable for a nighttime setback
period of 8 h and a setback temperature of 10°F (5.6°C).
Fan Operation. In the past, continuous fan operation in small upflow gas-fired burners was said to
offer the benefits of better air circulation, reduced noise (because the fan did not start and stop), and
an even temperature distribution. Project SP43 showed that continuous fan operation resulted in a
higher furnace efficiency. However, continuous operation consumes more electricity than intermittent
operation in which the fan shuts off and the supply temperature drops below 90°F (32.2°C). In
many locations with a high electricity-to-fossil-fuel cost ratio, an energy cost analysis based on
AFUE may determine whether continuous or intermittent operation is more efficient.
The fan often starts about 1 min after the burner starts. Such a delay allows the heat exchanger
to warm up and prevents a flow of cold air. The fan will shut down 2 to 3 min after the burner is
shut off. The supply of residual heat from the heat exchanger also improves the performance of the
furnace.
Safety. For safety, the vicinity of the furnace should be free of combustible gas, vapor, and material.
Any passage to provide combustion air must be carefully planned. Gas and vent pipes should be
installed according to local and federal codes.
8.3 HOT WATER BOILERS
Selection of Fuel
HEATING SYSTEMS, FURNACES, AND BOILERS 8.9
A hot water boiler for space heating is an enclosed pressure vessel in which water is heated to a
required temperature and pressure without evaporation. Hot water boilers are manufactured according
to the American Society of Mechanical Engineers (ASME) boiler and pressure vessel codes.
Boilers are usually rated according to their gross output heat capacity, i.e., the rate of heat delivered
at the hot water outlet of the boiler, in MBtu/h, or thousands of Btu/h (kW). Hot water boilers are
available in standard sizes up to 50,000 Mbtu/h (14,650 kW).
Natural gas, oil, coal, and electricity are energy sources that can be used in hot water boilers. It is
necessary to provide for an adequate supply during normal and emergency conditions and to take
into account the limitations imposed by any building and boiler codes for certain types of equipment
due to safety and environmental concerns. In addition, storage facilities and cost should be
considered before a fuel is selected.
Whereas natural gas and electricity are supplied by a utility, LPG, oil, and coal all need space
for storage within and outside the boiler plant.
Cost includes energy cost, initial cost, and maintenance cost. A gas-fired boiler plant requires
the lowest initial costs and maintenance costs, oil-fired boiler plants are moderately higher, and
coal-fired boiler plants are significantly higher (although their energy cost is lowest). An electric
boiler is simple to operate and maintain. In addition, it does not require a combustion process,