Page 214 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

The Use of Water in HVAC Systems 211

3. Pressure-reducing or pressure-regulating valves
4. Most crossover bridges
5. Any other mechanical device that regulates water flow
6. Constant-speed pump overpressure

There may be some question about the preceding categories of effi-
cient and inefficient uses of pump energy, particularly temperature-
control valves on heating and cooling coils. Any device that forces
water into a certain path or circuit must be considered inefficient
even if it is useful, as are these temperature-control valves for heat-
ing and cooling coils. A perfect system would be one that did not need
a control valve to regulate the water flow through a coil. Unfortunately,
the inefficiency of small pumps (see Table 7.5) makes the control
valve a more efficient answer than the individual pump on most
installations. Typical of this would be a water system with a variable-
speed pump for each coil and no control valve. The objective here is to
establish a means of determining the overall efficiency of a water sys-
tem. Likewise, it should be our objective to reduce pumping energy by
eliminating, wherever possible, the devices listed above as inefficient
users of energy.

8.1.2 Calculation of system efficiency
There has been little effort to determine how efficient are HVAC
water distribution systems. It is quite simple to develop a coefficient
of performance (COP) for these systems. This, of course, would be
accomplished by determining how much heating or cooling would be
transferred per hour divided by the energy required for this distribu-
tion. Therefore, the following equation would define the system COP.

useful energy transferred to the system per hour
System COP
energy applied to the system per hour
(8.1)

Equation 8.1 is applicable to three basic types of HVAC water sys-
tems. These are: (1) hot water systems consisting of direct radiation
only, (2) hot or chilled water systems utilizing direct radiation and
fan-driven radiation such as fan-coil and air-handling units, and (3)
condenser water systems. This comprises most of the HVAC water
distribution systems.
In all of these systems, if energy is applied in any other form than
electric motors, it must be included in the calculations. In most of
them, the energy is applied only through pump and fan motors. The

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