Introduction
In the previous two chapters, we discussed single zone and multiple-zone all-air air-conditioning systems. In Chapter 7, Section 7.2, we mentioned that water coils could be used in the main air-handling unit and for the reheat coils in the reheat and VAV systems. In this chapter we are going to consider systems where water-heated and/or water-cooled equipment provide most of the heating and/or cooling.
In some buildings, these systems will use low-pressure steam instead of hot water for heating. The performance is generally similar to hot water systems, with higher outputs due to the higher temperature of the steam. However, con- trol in these steam systems is generally inferior, due to the fixed temperature of steam. For steam systems and boilers see Chapters 10 and 27 respectively of ASHRAE 2000 Systems and Equipment Handbook. The properties of steam, the theory of two-phase flow and steam pipe sizing, are covered in Chapters 6, 4, and 35 of ASHRAE 2001 Fundamentals Handbook.
Throughout the rest of this chapter, we will assume that hot water is being used as the heating medium.
Because of their ability to produce high output on an ‘as-needed basis,’ hydronic systems are most commonly used where high and variable sensible heating and/or cooling loads occur. These are typically
e Perimeter zones, with high solar heat gains or
e Perimeter areas in cooler to cold climates where there are substantial perimeter heat losses.
The entrance lobby of a building in a cold climate is an example of an ideal use for these systems. They are frequently used in office buildings, hospitals, hotels, schools, apartment buildings and research laboratories in conjunction with ventilation and cooling air systems.
Hydronic systems advantages:
Noise reduction—Virtually silent operation
Economy, due to limited operational costs—Large amounts of heat from small local equipment Economy due to limited first costs—Pipes are small compared to ducts for the same heat transfer around a building Energy efficiency—Low energy consumption at low load Hydronic systems disadvantages:
Ventilation—Provision of outside air for ventilation is either absent or poor System failure—Danger from freezing and from leaks Humidity—Control is either absent or generally poor
We will start our discussion with simple heating systems that operate by allowing heat to escape from a hot surface by natural convection and low temperature radiation.