Hydronic and Steam Equipment and Systems
ASHRAE Technical Committee 6.1

Scope of TC 6.1

TC 6.1 is concerned with all aspects of hydronic and steam systems. This includes the application of boilers, chillers, terminal units, and all accessories and controls making up the total system as well as the design of the integrated system. In addition to comfort applications of both heating and cooling, snow melting systems are included. Cooperation with other TCs is recognized in areas such as control, noise and vibration, refrigeration, pumps and hydronic and service water piping.


The ASHRAE Handbook is published in a series of four volumes, one of which is revised each year, ensuring that no volume is older than four years.

The Handbook can be purchased at the ASHRAE Bookstore by clicking on this link.

TC 6.1 is responsible for the following chapters in the ASHRAE Handbooks:

HVAC Systems & Equipment: Chapter 11 - Steam Systems

Steam systems use the vapor phase of water to supply heat or kinetic energy through a piping system. As a source of heat, steam can heat a conditioned space with suitable terminal heat transfer equipment such as fan-coil units, unit heaters, radiators, and con-vectors (finned tube or cast iron), or through a heat exchanger that supplies hot water or some other heat transfer medium to the terminal units. In addition, steam is commonly used in heat exchangers (shell-and-tube, plate, or coil types) to heat domestic hot water and supply heat for industrial and commercial processes such as in laundries and kitchens. Steam is also used as a heat source for certain cooling processes such as single-stage and two-stage absorption refrigeration machines.

HVAC Systems & Equipment: Chapter 13 - Hydronic Heating and Cooling

Water systems can be either once-through or recirculating systems. This chapter describes forced recirculating systems. Successful water system design depends on awareness of the many complex interrelationships between various elements. In a practical sense, no component can be selected without considering its effect on the other elements. For example, design water temperature and flow rates are interrelated, as are the system layout and pump selection. The type and control of heat exchangers used affect the flow rate and pump selection, and the pump selection and distribution affect the controllability. The designer must thus work back and forth between tentative points and their effects until a satisfactory integrated design has been reached. Because of these relation-ships, rules of thumb usually do not lead to a satisfactory design.

HVAC Systems & Equipment: Chapter 14 - Condenser Water Systems

As part of the vapor-compression cycle for mechanical refrigeration, the heat of compression produced must be rejected to complete the refrigeration cycle. Refrigerant systems may be cooled by air or water. In water-cooled systems, water flows through the condenser and is called condenser water. Condenser water systems are classified as (1) once-through systems (e.g., city water, well-water, or lake/groundwater systems), or (2) recirculating or cooling tower systems.

HVAC Systems & Equipment: Chapter 15 - Medium and High Temperature Water Heating

Medium-temperature water systems have operating temperatures ranging from 250°F to 350°F (120 to 175°C) and are designed to a pressure rating of 125 to 150 psig (860 to 1030 kPa [gage]). High-temperature water systems are classified as those operating with supply water temperatures above 350°F (175°C) and designed to a pressure rating of 300 psig (2000 kPa [gage]). 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 (230°C) increases cost because components rated for higher pressures are required. The design principles for both medium- and high-temperature systems are basically the same. This chapter presents the general principles and practices that apply to MTW/HTW and distinguishes them from low-temperature water systems operating below 250°F (120°C). See Chapter 13 for basic design considerations applicable to all hot-water systems.

HVAC Systems & Equipment: Chapter 28 - Unit Ventilators, Unit Heaters, and Makeup Air Units

A heating unit ventilator is an assembly whose principal functions are to heat, ventilate, and cool a space by introducing outdoor air in quantities up to 100% of its rated capacity. The heating medium may be steam, hot water, gas, or electricity. The essential components of a heating unit ventilator are the fan, motor, heating element, damper, filter, automatic controls, and outlet grille, all of which are encased in a housing. An air-conditioning unit ventilator is similar to a heating unit ventilator; however, in addition to the normal winter function of heating, ventilating, and cooling with outdoor air, it is also equipped to cool and dehumidify during the summer. It is usually arranged and controlled to introduce a fixed quantity of outdoor air for ventilation during cooling in mild weather. The air-conditioning unit ventilator may be provided with a various of combinations of heating and air-conditioning elements.

HVAC Systems & Equipment: Chapter 32 - Boilers

Excluding special and unusual fluids, materials, and methods, a boiler is a cast-iron, carbon or stainless steel, aluminum, or copper pressure vessel heat exchanger designed to (1) burn fossil fuels (or use electric current) and (2) transfer the released heat to water (in water boilers) or to water and steam (in steam boilers). Boiler heating surface is the area of fluid-backed surface exposed to the products of combustion, or the fire-side surface. Various manufacturers define allowable heat transfer rates in terms of heating surface based on their specific boiler design and material limitations. Boiler designs provide for connections to a piping system, which delivers heated fluid to the point of use and returns the cooled fluid to the boiler.

HVAC Systems and Equipment: Chapter 36 - Hydronic Heat-Distributing Units and Radiators

Radiators, convectors, and baseboard and finned-tube units are heat-distributing devices used in low-temperature and steam water-heating systems. They supply heat through a combination of radiation and convection and maintain the desired air temperature and/or mean radiant temperature in a space without fans. In low-temperature systems, radiant panels are also used. Units are inherently self-adjusting in the sense that heat output is based on temperature differentials; cold spaces receive more heat and warmer spaces receive less heat.

HVAC Systems and Equipment: Chapter 44 - Centrifugal Pumps

Centrifugal pumps provide the primary force to distribute and recirculate hot and chilled water in a variety of space-conditioning systems. The pump provides a predetermined flow of water to the space load terminal units or to a thermal storage chamber for release at peak loads. The effect of centrifugal pump performance on the application, control, and operation of various terminal units is discussed in Chapter 13. Other hydronic systems that use pumps include (1) condensing water circuits to cooling towers(Chapters 14 and 40), (2) water-source heat pumps (Chapter 9), (3)boiler feeds, and (4) condensate returns (Chapter 11). Boiler feed and condensate return pumps for steam boilers should be selected based on boiler manufacturer’s requirements. 

HVAC Systems & Equipment: Chapter 46 - Pipes, Tubes, and Fittings

This chapter covers the selection, application, and installation of pipe, tubes, and fittings commonly used for heating, air-conditioning, and refrigeration. Pipe hangers and pipe expansion are also addressed. When selecting and applying these components, applicable local codes, state or provincial codes, and voluntary industry standards (some of which have been adopted by code jurisdictions) must be followed.

HVAC Systems & Equipment: Chapter 47 - Valves

Valves are the manual or automatic fluid-controlling elements in a piping system. They are constructed to withstand a specific range of temperature, pressure, corrosion, and mechanical stress. The designer selects and specifies the proper valve for the application to give the best service for the economic requirements.

HVAC Systems & Equipment: Chapter 48 - Heat Exchangers

This chapter describes some of the fundamentals, types, components, applications, selection criteria, and installation of heat exchangers. Chapter 4 of the 2009 ASHRAE Handbook—Fundamentals covers the subject of heat transfer. Specific applications of heat exchangers are detailed in other chapters of this and other volumes of the Handbook series.

HVAC Systems and Equipment: Chapter X - Chilled Water Plant Design

The ASHRAE HVAC SYSTEMS & EQUIPMENT HANDBOOK may be purchased from the on-line bookstore by clicking on the highlighted text.

Fundamentals: Chapter 22 - Pipe Sizing

This chapter includes tables and charts to size piping for various fluid flow systems. Further details on specific piping systems can be found in appropriate chapters of the ASHRAE Handbook.

The ASHRAE FUNDAMENTALS HANDBOOK may be purchased from the on-line bookstore by clicking on the highlighted text.

Comment on the Handbook: ASHRAE welcomes your comments on the Handbook or a specific Handbook chapter.  To submit a comment about any aspect or part of the Handbook series, you can use the Handbook Comment Form.

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Technical committees develop and sponsor technical sessions at the winter and annual conferences. Information about their future technical program is discussed at each TC meeting and at the TC’s Program Subcommittee meeting

ASHRAE publishes papers and transactions from presentations at its conference events. In addition, ASHRAE records most of the seminar sessions from its conferences on DVD. These DVDs are ideal for use at chapter meetings, in university courses, or company lunch and learns. Products available from the most recent conference may be found here.


Technical Committees are responsible for identifying research topics, proposing research projects, selecting bidders, and monitoring research projects funded by ASHRAE. Information about their specific research program is discussed at each TC meeting and at the TC’s Research Subcommittee meeting.

TC 6.1 has the following active research project:


The objective of this research project is to develop user-friendly windows-based software will greatly accelerate adoption of seasonal efficiency analysis for commercial boiler systems. A significant obstacle to the use of any new standard is the learning curve for users to become familiar with the new terminology and inputs and learn how to do the computations.


ASHRAE writes standards for the purpose of establishing consensus for: 1) methods of test for use in commerce and 2) performance criteria for use as facilitators with which to guide the industry. ASHRAE publishes the following three types of voluntary consensus standards: Method of Measurement or Test (MOT), Standard Design and Standard Practice. ASHRAE does not write rating standards unless a suitable rating standard will not otherwise be available. ASHRAE is accredited by the American National Standards Institute (ANSI) and follows ANSI's requirements for due process and standards development. Standards may be purchased at the ASHRAE Bookstore.

TC 6.1 is cognizant for the following standards:

ANSI/ASHRAE Standard 125: Method of Testing Thermal Energy Meters for Liquid Streams in HVAC Systems

ASHRAE Guideline 22: Instrumentation for Monitoring Central Chilled Water Plant Efficiency

Proposed Standard: 155P: Method of Testing for Rating Commercial Space Heating Boiler Systems

Proposed Standard: 208P: Method of Test for Determining Hydronic System Balance Valve Capacity

Other Activities

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Include other activities, such as MTG involvement, into this section.


ASHRAE Technical FAQs are provided as a service to ASHRAE members, users of ASHRAE publications, and the general public. While every effort has been made to ensure their accuracy and reliability, they are advisory and provided for informational purposes only, and in many cases represent only one person’s view. They are not intended and should not be relied on as an official statement of ASHRAE. Technical questions not addressed may be submitted to the ASHRAE Technical Services department at tse@ashrae.net.