High Temperature Water Heating
High-temperature water systems are classified as those operating
with supply water temperatures above 350°F and designed to
a pressure rating of 300 psig. The usual practical temperature
limit is about 450°F because of pressure limitations on pipe
fittings, equipment, and accessories. The rapid
pressure rise that occurs as the temperature rises above 450°F
increases cost because components rated for higher pressures are
required . The design principles for both medium temperature and
high-temperature systems are basically the same. In this chapter,
HTW refers to both systems.
The following characteristics distinguish HTW systems from steam
distribution or low-temperature water systems:
• The system is a completely closed circuit with supply
and return mains maintained under pressure. There are no losses
from flashing, and heat that is not used in the terminal heat
transfer equipment is returned to the HTW generator. Tight systems
have minimal corrosion.
• Mechanical equipment that does not control the performance
of individual terminal units is concentrated at the central station.
• Piping can slope up or down or run at a variety of elevations
to suit the terrain and the architectural and structural requirements
without provision for trapping at each low point. This may reduce
the amount of excavation required and eliminate drip points and
return pumps required with steam.
• Greater temperature drops are used and less water is circulated
than in low-temperature water systems.
• The pressure in any part of the system must always be
well above the pressure corresponding to the temperature at saturation
in the system to prevent flashing of the water into steam.
• Terminal units requiring different water temperatures
can be served at their required temperatures by regulating the
flow of water, modulating the water supply temperature, placing
some units in series, and using heat exchangers or other methods.
• The high heat content of the water in the HTW circuit
acts as a thermal flywheel, evening out fluctuations in the load.
The heat storage capacity can be further increased by adding heat
storage tanks or by increasing the temperature in the return mains
during periods of light load.
• The high heat content of the heat carrier makes high-temperature
water unsuitable for two-pipe dual-temperature (hot and chilled
water) applications and for intermittent operation if rapid start-up
and shutdown are desired, unless the system is designed for minimum
water volume and is operated with rapid response controls.
• Higher engineering skills are required to design a HTW
system that is simple, yet safer and more convenient to operate
than are required to design a comparable steam or low-temperature
• HTW system design requires careful attention to basic
laws of chemistry and physics as these systems are less forgiving
than standard hydronic systems.
Elements of High-Temperature Water System
High-temperature water systems are similar to conventional forced
hot water heating systems. They require a heat source (which
can be a direct-fired HTW generator, a steam boiler, or an open
or closed heat exchanger) to heat the water. The expansion of
the heated water is usually taken up in an expansion vessel, which
simultaneously pressurizes the system. Heat transport depends
on circulating pumps. The distribution system is closed, comprising
supply and return pipes under the same basic pressure. Heat emission
at the terminal unit is indirect by heat transfer through heat
transfer surfaces. The basic system is shown in Figure 2.
The principal differences of HTW systems from low-temperature
water systems are the higher pressure, heavier equipment, generally
smaller pipe sizes, and manner in which water pressure is maintained.
Most systems are either (1) a saturated steam cushion system,
in which the high-temperature water develops its own pressure,
or (2) a gas- or pump-pressurized system, in which the pressure
is imposed externally.
HTW generators and all auxiliaries (such as water makeup and
feed equipment, pressure tanks, and circulating pumps) are usually
located in a central station. Cascade HTW generators sometimes
use an existing steam distribution system and are installed remote
from the central plant.
Selection of the system pressure, supply temperature, temperature
drop, type of HTW generator, and pressurization method are the
most important initial design considerations. The following are
some of the determining factors:
• Type of load (space heating and/or process). Load fluctuations
during a 24-h period and a 1-year period. Process loads might
require water at a given minimum supply temperature continuously,
while space heating can permit temperature modulation as
a function of outdoor temperature or other climatic influences
. • Terminal unit temperature requirements.
• Distance between heating plant and space or process requiring
• Quantity and pressure of steam used for power equipment
• Elevation variations within the system and the effect
Usually, distribution piping is the major investment in an HTW
system. A distribution system with the widest temperature spread
(Dt) between supply and return will have the lowest initial and
operating costs. Economical designs have a Dt of 150°F or
The requirements of terminal equipment or user systems determine
the system selected. For example, if the users are 10 psig
steam generators, the return temperatures would be 250°F.
300 psig rated system operated at 400°F would be selected
the load. In another example, where the primary system serves
predominantly 140 to 180°F hot water heating systems, an HTW
system that operates at 325°F could be selected. The supply
temperature is reduced by blending with 140°F return water
desired 180°F hot water supply temperature in a direct-connected
hot water secondary system. This highly economical design has
140°F return temperature in the primary water system and a
Because the danger of water hammer is always present when the
pressure drops to the point at which pressurized hot water flashes
steam, the primary HTW system should be designed with steel
valves and fittings of 150 psi. The secondary water, which operates
below 212°F and is not subject to flashing and water hammer,
be designed for 125 psi and standard HVAC equipment.
Theoretically, water temperatures up to about 350°F can be
provided using equipment suitable for 125 psi. But in practice,
push-pull pumping is used, maximum water temperatures are limited
by the system design, pump pressures, and elevation characteristics
to values between 300 and 325°F.