Hot Water Expansion Tanks - Sizing (2024)

Required hot water expansion volume in open, closed and diaphragm tanks.

Expansion tanks are required in heating, cooling or air condition system to avoid unacceptable increases of system pressures when water expands during heat-up.

Explosive Power of Super-heated Water

  • 1 lb (0.45 kg) of nitroglycerin > 2 000 000 ft lbf (2 700 000 J)
  • 1 lb (0.45 kg) of water flashed into steam > 750 000 ft lbf (100 000 J)

Hot Water Expansion Tanks - Sizing (1)

Expansion tanks are in general designed as

  • open tanks
  • closed compression tanks
  • diaphragm tanks

Net expansion volume of water when heated can be expressed as

V net = (v1 / v0 ) - 1 (1)

V net = expansion volume of water ( ft3, m3 )

v0 = specific volume of water at initial (cold) temperature (ft3 /lb, m3 /kg)

v1 = specific volume of water at operating (hot) temperature (ft3 /lb, m3 /kg)

Open Expansion Tanks

Hot Water Expansion Tanks - Sizing (2)

Required volume of an open expansion tank can be expressed as

V et = k V w [(v1 / v0 ) - 1] (2)

V et = required expansion tank volume (gallon, liter)

k = safety factor (approximately 2 is common)

V w = water volume in the system (gallon, liter)

v0 = specific volume of water at initial (cold) temperature (ft3 /lb, m3 /kg)

v1 = specific volume of water at operating (hot) temperature (ft3 /lb, m3 /kg)

Note that in an open expansion tank fresh air is constantly absorbed in the water and tends to corrode the system. Open expansion tanks must also be located above the highest heating element, in general on the top of buildings, where they may be exposed to freezing.

Hot Water Expansion Tanks - Sizing (3)

Closed Compression Expansion Tanks

Closed compression tanks can be designed as

  • adjustable expansion tanks - air is evacuated or injected by automatic valves to the tanks to control system pressures when temperature and expansion of water rises or falls
  • pump-pressured cushion tanks - water is evacuated or injected to the systems to compensate for temperature rise or fall
  • compression tanks with closed gas volumes - the tanks contains specific volumes of gas which is compressed when temperatures and system volumes increase

Hot Water Expansion Tanks - Sizing (4)

Required volume in a closed expansion tank

V et = k V w [( v1 / v0 ) - 1] / [( p a / p0 ) - ( p a / p1 )] (3)

where

p a = atmospheric pressure - 14.7 (psia)

p0 = system initial pressure - cold pressure (psia)

p1 = system operating pressure - hot pressure (psia)

Hot Water Expansion Tanks - Sizing (5)

  • initial temperature 50 oF
  • initial pressure 10 psig
  • maximum operating pressure 30 psig

Diaphragm Expansion Tanks

Required volume in a diaphragm expansion tank

V et = k V w [( v1 / v0 ) - 1] / [1 - ( p0 / p1 )] (4)

Hot Water Expansion Tanks - Sizing (6)

  • initial temperature 50 oF
  • initial pressure 10 psig
  • maximum operating pressure 30 psig
  • safety factor aprox. 2
  • acceptance factor aprox. 0.5

Example - Volume in Open Expansion Tank

A system with 1000 gallons of water is heated from 68 oF to 176 oF .

Minimum expansion volume in an open expansion tank with a safety factor 2 can be calculated as

V et = 2 (1000 gallons) [((0.01651 ft3 /lb) / (0.01605 ft3 /lb)) - 1]

= 57 (gallons)

Related Topics

  • Heating Systems

    Design of heating systems - capacities and design of boilers, pipelines, heat exchangers, expansion systems and more.

Related Documents

  • Closed Expansion Tanks - Volume vs. Temperature and Pressure

    Sizing of low temperature closed expansion tanks.
  • Diaphragm Expansion Tanks

    Low temperature diaphragm expansion tank sizing - calculate tank volume and acceptance volume.
  • Ethylene Glycol Heat-Transfer Fluid Properties

    Properties like freezing point, viscosity, specific gravity and specific heat of ethylene glycol based heat-transfer fluids, or brines.
  • Heat Loss from Buildings

    Overall heat transfer loss from buildings - transmission, ventilation and infiltration.
  • Heating System Flow Rates

    Calculate flow rates in heating systems.
  • Hot Water Heating System - Design Procedure

    Hot water heating system design procedure with heat loss, boiler rating, heater units and more.
  • Hot Water Heating Systems - Flow Temperatures vs. Outside Temperatures

    Seasonal effects on hot-water heating systems flow temperatures.
  • Hot Water Heating Systems - Online Design Application

    Free online design tool for designing hot water heating systems - metric units.
  • Hot Water Heating Systems - Online Design Application, Imperial Units

    Online design tool for hot water heating systems.
  • Hot Water Heating Systems - Pressure Loss in Steel Pipes

    Pressure loss nomogram for hot water steel pipes.
  • Rectangular Tanks - Volumes

    Tank volume per foot depth.
  • Safety Relief Valves - Capacity vs. Pressure

    Maximum safety valve free air relief capacity.
  • Volumetric (Cubic) Thermal Expansion

    Volumetric temperature expansion calculator.

Search

Search is the most efficient way to navigate the Engineering ToolBox.

  • Popular internal searches in the Engineering ToolBox

Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with older versions of the amazing SketchUp Make and the newer "up to date" SketchUp Pro. Add the Engineering ToolBox extension to your SketchUp Make/Pro from the Extension Warehouse!

Translate this Page

Translate this page to Your Own Language.

About the Engineering ToolBox!

Privacy Policy

We don't collect information from our users. More about

  • the Engineering ToolBox Privacy Policy

We use a third-party to provide monetization technologies for our site. You can review their privacy and cookie policy here.

You can change your privacy settings by clicking the following button: .

Advertise in the ToolBox

If you want to promote your products or services in the Engineering ToolBox - please use Google Adwords. You can target the Engineering ToolBox by using AdWords Managed Placements.

Citation

This page can be cited as

  • The Engineering ToolBox (2005). Hot Water Expansion Tanks - Sizing. [online] Available at: https://www.engineeringtoolbox.com/expansion-tanks-d_885.html [Accessed Day Month Year].

Modify the access date according your visit.

Hot Water Expansion Tanks - Sizing (2024)
Top Articles
Latest Posts
Recommended Articles
Article information

Author: Tyson Zemlak

Last Updated:

Views: 6416

Rating: 4.2 / 5 (43 voted)

Reviews: 90% of readers found this page helpful

Author information

Name: Tyson Zemlak

Birthday: 1992-03-17

Address: Apt. 662 96191 Quigley Dam, Kubview, MA 42013

Phone: +441678032891

Job: Community-Services Orchestrator

Hobby: Coffee roasting, Calligraphy, Metalworking, Fashion, Vehicle restoration, Shopping, Photography

Introduction: My name is Tyson Zemlak, I am a excited, light, sparkling, super, open, fair, magnificent person who loves writing and wants to share my knowledge and understanding with you.