Plumbing TRD Level 3 Sample Pages - 3

3.1 Introduction to unvented hot-water systems

Tutor guidance notes

Aims and objectives

At the end of these activity sheets learners should be able to:

• recognise the key differences between vented and unvented systems

• identify why safety is an important feature of unvented system design

• identify the advantages and disadvantages of unvented systems

• state the various types of unvented system and appliances.

The questions and activities are not intended as formal assessment. However, the answers should be debated with all the learners in a group discussion to ensure full comprehension of the subject.

Individual teachers will wish to use these questions and activities in different ways: at the end of a session as a recap; individually during the session as a ‘tothe-point’ learning tool; or as homework. Exercise 3.1: Introduction to unvented hot-water systems could be used when the subject is revisited at the beginning of a subsequent lesson to link prior learning. Whichever method is chosen, it is strongly recommend that a completion date is set and agreed.

Plumbing: NVQ and Technical Certificate Level 3 Tutor Resource Disk 1 of 39

Candidate name: ______________________________________ Date: ______________________

3.1 Introduction to unvented hot-water systems

Chapter 3: Unvented hot-water systems


At the end of these activity sheets you should be able to:

• recognise the key differences between vented and unvented systems

• identify why safety is an important feature of unvented system design

• identify the advantages and disadvantages of unvented systems

• state the various types of unvented system and appliances.

1 Draw a vented hot-water system including a cold-water storage cistern

(CWSC) connected by a cold feed pipe to a hot-water storage cylinder

(HWSC). The system must include a vent pipe.


2 Explain why the vented hot-water system is very safe.

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3 Complete the following paragraphs:

Below __________°C water __________ as it cools; this means that water in an enclosed space (such as a pipe) increases in volume when it __________.

With __________ for the frozen __________ to go, the pipe __________.

Similarly, when water is heated from __________°C to __________°C it

__________. This expansion has to be __________ for in any hot-water system, to stop __________ build-up and __________ damage to the components.

A typical __________ in volume for water heated in a 120-litre hot-water cylinder is:

120 x __________ (4 per cent) = __________ litre increase in volume

Water __________ by up to __________ per cent under normal system heating __________. This must be catered for __________ in the system!

The __________ point of water increases as the __________ in a system increases. For example, water at 2 bar __________ has a boiling point of

133.5°C. If the temperature in a __________ vessel is increased, then the

__________ in that vessel is increased.


The conclusions that we need to draw from this are that it is extremely important that both the temperature and pressure are controlled in hot water systems.

4 Why are unvented hot-water systems becoming more popular?

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5 Label this unvented system:

6 Describe what would happen if the unvented system were not controlled properly.

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7 Eventually in an uncontrolled scenario either of the following happens:

• the pressure in the system increases to a point at which a component bursts or ruptures and the pressure in the system decreases

• someone opens a tap in the property and begins to reduce the pressure.

What is the likely outcome of a reduction of pressure in the system?

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8 Fill in the missing information:

A __________ litre cylinder of __________ expanding up to __________ times its volume would need to accommodate:

__________ x __________ = __________ litres of steam

The __________ effects of a defective __________ __________ as it

__________ to __________ are that buildings can be __________ in much the same way as a __________ __________.

9 Explain these advantages of unvented hot-water systems.

Advantages a High pressure operation

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b Balanced pressure at outlets

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_______________________________________________________________ c A reduction in pipe diameter

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_______________________________________________________________ d Quicker to install

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_______________________________________________________________ e Easier siting

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f Reduced water contamination risk

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10 Explain these disadvantages of unvented hot-water systems.

Disadvantages a Poor supply flow rate

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_______________________________________________________________ b No stored water

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_______________________________________________________________ c Discharge pipe required

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_______________________________________________________________ d Space requirements

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_______________________________________________________________ e Purchase price

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11 Name the following four types of unvented heaters.

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12 Name and label this hot-water heater

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Exercise 3.1: Introduction to unvented hot-water systems

1 By how much does water usually expand when heated under normal operating conditions?

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2 State two advantages of an unvented hot-water system.

• ______________________________________________________________

• ______________________________________________________________

3 What type of unvented system is shown in this diagram?

4 a Is the following statement true or false?

Vented systems usually provide a higher flow rate at taps than unvented systems. b Explain the reason for your answer.

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3.2 Unvented system components and operation




• identify the positioning requirements of components within unvented systems

• state the purpose, and operating principles, of unvented system control components:

– safety controls

– functional controls.


Individual teachers will wish to use these questions and activities in different ways: at the end of a session as a recap; individually during the session as a ‘tothe-point’ learning tool; or as homework. Exercise 3.2: Unvented system components and operation could be used when the subject is revisited at the beginning of a subsequent lesson to link prior learning. Whichever method is chosen, it is strongly recommend that a completion date is set and agreed.



3.2 Unvented system components and operation




• identify the positioning requirements of components within unvented systems

• state the purpose, and operating principles, of unvented system control components:

– safety controls

– functional controls.

1 a On the diagram, mark the correct installation position of each component:

• a line strainer • expansion device

• expansion valve • pressure reducing or limiting valve

• single check valve • temperature relief valve.


b State what each component does:

• a line strainer

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• pressure reducing or limiting valve

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• single check valve

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• expansion device

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• expansion valve

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• temperature relief valve

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2 There are three safety controls (components) sometimes referred to as ‘three tier safety protection’. What are they and how do they work?

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• ______________________________________________________________

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3 With the aid of a sketch, explain what a tundish is and how it works.


4 Explain an alternative to the expansion device in Question 1.

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5 How does a composite control valve differ to the components listed in

Question 1?

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6 a How do installations of unvented hot-water heaters with a capacity below

15 litres differ from those above that volume?

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_______________________________________________________________ b Why do the manufacturers of heaters below 15 litres require that the supply pipe right back to the mains does not contain check valves, stop valves with loose jumpers or fittings that prevent reverse flow?

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Exercise 3.2: Unvented system components and operation

1 Which component comes first, the line strainer or the pressure reducing valve?

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2 Can an unvented appliance with a capacity of less than 15 litres expand water back into the supply pipe if a loose jumper to the stop valve is fitted?

Explain your answer.

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3 State two methods of accommodating expanded water in an unvented system.

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4 At what temperature will an overheat thermostat on an unvented system usually operate?

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5 What components perform a three tier level of protection on an unvented system?

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6 Is the following statement true or false?

The temperature relief valve provides protection in the event that the expansion vessel fails.

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7 State two methods of preventing the growth of bacteria in expansion vessels.

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8 What component prevents potential backflow into the supply pipe in an unvented system?

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3.3 Unvented hot-water systems legislation




• state the requirements of legislation relating to the installation of unvented hot-water systems:

– Building Regulations

– Water Regulations.


Individual teachers will wish to use these questions and activities in different ways: at the end of a session as a recap; individually during the session as a ‘tothe-point’ learning tool; or as homework. Exercise 3.3: Unvented hot-water systems legislation could be used when the subject is revisited at the beginning of a subsequent lesson to link prior learning. Whichever method is chosen, it is strongly recommend that a completion date is set and agreed.



3.3 Unvented hot-water systems legislation




• state the requirements of legislation relating to the installation of unvented hot-water systems:

– Building Regulations

– Water Regulations.

1 What are the two main items of legislation that cover the installation of unvented hot-water systems?

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2 State the formal description in the legislation for a unvented hot-water system.

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3 a What size storage heaters are covered by G3?

_______________________________________________________________ b What other types of sealed system are not covered by G3?

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4 A quick recap (you may need your tutors help here): Why do the Regulations prevent the temperature of stored water exceeding 100°C?

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5 Describe the differences between a package and a unit.

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6 a In a package, what components and controls do the manufacturers supply?

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_______________________________________________________________ b In a package, what components and controls are factory-fitted?

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7 a In a unit, what components and controls do the manufacturers supply?

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_______________________________________________________________ b In a unit, what components and controls are factory-fitted?

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8 Why is it the law that only approved products be used for initial installation or when replacing faulty components?

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9 Who can approve unvented hot-water products?

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10 G3 states that unvented hot-water units or packages have a minimum of two temperature-activated safety devices operating in sequence. a What are they?

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_______________________________________________________________ b What makes up the third tier of three-tier protection?

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11 Complete the following Regulation requirements relating to directly heated systems:

In both __________ and __________:

• The __________-__________ __________(s) should be located directly on the storage vessel such that the stored water does not exceed _________°C.

• The valve(s) must be __________ sized (this one is for the manufacturer) in accordance with the requirements of BS __________.

• The valves __________ __________ be disconnected other than for

__________ or __________ (so temporarily capping off a __________ valve on a working system while a new one is obtained is absolute __________ allowed).

• Each __________ should __________ via a short length of __________ pipe (this is referred to as __________ and you’ll encounter this later) of a size not less than the __________ __________ of the temperature-relief


valve, through an __________ break over a __________ located

__________ as near as possible to the valve(s).

12 Label the missing safety features on the immersion heater shown.

13 Complete the following Regulation requirements relating to indirectly heated systems:

• The non __________-__________ thermal cut-out should be wired up to a

__________ __________ or some other suitable device to __________

__________ the __________ to the primary heater.

• If the unit incorporates a __________, the thermal cut-out may be

__________ the boiler. There are a limited number of manufacturers that provide unvented __________ and __________ __________ that

__________ be fitted together – these take advantage of __________ the number of __________ by only having one __________ cut-out on the boiler.

• The temperature-relief valve should be __________ and __________, and the discharge pipe provided, as described for __________ __________.


• Where an indirect unit or package has any __________ direct method of water heating fitted, e.g. an __________ __________ ,a non-self-resetting

__________ cut-out device will also be needed on the __________ source(s).

14 a Unvented hot-water units and packages should only be installed by a competent person. How do you know who is competent?

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_______________________________________________________________ b Who can say a person is competent to install unvented hot-water systems?

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15 The local building control office must give permission prior to installation of unvented hot-water systems covered by Building Regulations G3. How is an application made and what actions may the building control officer take?

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16 a Label the missing safety features on the unvented hot-water discharge pipework shown – it includes D1 and D2. b Why should all the discharge pipework continuously fall?

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_______________________________________________________________ c Where should the tundish be located and how close to the relief and expansion valves?

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17 What are the requirements for pipework D2?

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18 Use the discharge pipe requirements chart on page 145 of Plumbing NVQ

and Technical Certificate Level 3 to answer the following questions. a Will a 28 mm discharge pipe (D2) connected to a G3/4 valve be of sufficient size if it is 6 metres long and contains two elbows?

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_______________________________________________________________ b Will a 22 mm discharge pipe (D2) connected to a G1/2 valve be of sufficient size if it is 7 metres long and contains three elbows?

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19 Why do the Water Supply (Water Fittings) Regulations 1999 cover unvented hot-water systems?



Exercise 3.3: Unvented hot-water systems legislation

1 Is the discharge pipe termination shown in this diagram acceptable? Explain your reasons for your answer.

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2 What is the maximum length of the D1 discharge pipe?

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3 Is it acceptable to connect a 28 mm D1 discharge pipe to a G3/4 temperature relief valve outlet?

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4 What is different about the immersion heater fitted to an unvented cylinder?

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5 Using the discharge pipe requirements chart on page 145 of Plumbing NVQ

and Technical Certificate Level 3, will a 22 mm discharge pipe connected to a G3/4 valve be of sufficient size if it is 6 metres long and contains two elbows?

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6 An expansion valve discharge pipe, rises up from the valve outlet to connect into the pipework from the temperature relief valve discharge pipe. Is this acceptable? Explain your reasons for your answer.

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7 The Building Regulations identify that suitable controls should be provided to guard against a certain temperature being reached. What is this temperature?

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3.4 Unvented systems: install and commission




• identify the key installation requirements of unvented hot-water systems

• state the commissioning requirements for unvented hot-water systems.


Individual teachers will wish to use these questions and activities in different ways: at the end of a session as a recap; individually during the session as a ‘tothe-point’ learning tool; or as homework. Exercise 3.4: Unvented systems: install and commission could be used when the subject is revisited at the beginning of a subsequent lesson to link prior learning. Whichever method is chosen, it is strongly recommend that a completion date is set and agreed.



3.4 Unvented systems: install and commission




• identify the key installation requirements of unvented hot-water systems

• state the commissioning requirements for unvented hot-water systems.

1 What are the two considerations to take into account when choosing a location for an unvented hot-water cylinder?

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• ______________________________________________________________

2 While you are installing an unvented hot-water cylinder the customer asks you to connect in a new solar panel he has fitted to his roof. Write a short report to your boss stating why this is not permissible.

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3 Your company has been asked to install a number of unvented hot-water cylinders on an estate at an army barracks. During the day the water pressure is 1.5 bars but at peak times heavy demand occurs and the flow rate seems to drop remarkably. What checks should be carried out on the supply before installation commences and when should these checks be carried out?

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4 List the equipment used for testing and write a logical explanation of how to carry out the tests.

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5 What size cylinder would you recommend for each of these installations? a An indirect installation in a three-bedroom house

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_______________________________________________________________ b A direct installation in a single-bedroom flat.

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6 Why does supply pipework need to be correctly sized for these units?

<authors to clarify: what units are you referring to?>

7 a What do we mean by an unbalanced supply?

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_______________________________________________________________ b How can unbalanced supplies and mixer valves cause a number of problems?

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8 Expansion vessels and cylinder air gaps have been discussed. Why is it necessary to take special care with regard to expansion on large systems or those that contain secondary circuits?

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9 Your tutor will show you either a thermostatic mixer valve or an installation containing one. Because of the high flow temperatures at outlets, thermostatic mixer valves may be used. What are they and how do they work?

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10 Fill in the missing information.

Indirectly heated systems must only be __________ to fully pumped heating systems. Indirectly heated __________ systems for use in __________ properties require the installation of a __________ __________ (usually supplied with the cylinder) to __________ the __________ of primary water entering the system under __________ operating conditions and to

__________ the primary heat supply to the cylinder in __________ situations.

The control system can therefore be based on either:

• a 2-two port motorised valve configuration – ‘S Plan’, or

• a 3-port motorised valve configuration – ‘Y Plan’.

As the __________-port valve does not provide full isolation of the hot-water port in all __________ conditions, when installing a Y-Plan configuration

(unless advised by the manufacturer) you will be required to __________ both a __________-port valve and the __________-port valve supplied by the __________, to ensure that there is effective __________ of the cylinder in an __________ situation. You may also have to order an optional

__________ __________ to wire both valves.

For new installations it is better to specify the 2-port valve __________, as it is cheaper and __________ to install.


11 The diagram shows a typical dual thermostat provided with most modern unvented hot-water storage systems where, for ease of wiring, both the control thermostat and the thermal cut-out device (high limit thermostat) are housed in one unit with a simple 3-wire connection (live in from the programmer, live out to the motorised valve and an earth terminal). Label the drawing.

A/W 023C_PL3TRD – to come

12 Write a commissioning list of operations numbering each procedure from

1 to 10.

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Exercise 3.4: Unvented systems: install and commission

1 A balanced system is preferable to an unbalanced system. True or false?

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2 What component is checked with a tyre foot-pump on commissioning?

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3 What is the preferred incoming supply pipe size for an unvented system?

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4 When installing an unvented system with primary flow provided by a mid position valve, what additional control component is required?

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5 What is considered to be an acceptable minimum water flow rate to an unvented system?

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6 How is the operation of the expansion valve checked on system commissioning?

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3.5 Unvented systems: service and maintenance




• state the service and maintenance procedure for unvented systems

• diagnose a range of common faults on unvented systems.


Individual teachers will wish to use these questions and activities in different ways: at the end of a session as a recap; individually during the session as a ‘tothe-point’ learning tool; or as homework. Exercise 3.5: Unvented systems: service and maintenance could be used when the subject is revisited at the beginning of a subsequent lesson to link prior learning. Whichever method is chosen, it is strongly recommend that a completion date is set and agreed.



3.5 Unvented systems: service and maintenance




• state the service and maintenance procedure for unvented systems

• diagnose a range of common faults on unvented systems.

1 How often should maintenance periods be for boilers and unvented hot-water systems?

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2 Fill in the missing information for this typical maintenance procedure:

• Check that all __________ __________ are still fitted and are unobstructed. Check to see if all __________ are still in position and all

__________ are properly wired. Be careful with the __________: it can be common for __________ heaters to fail and to be __________ with a

__________ immersion __________ rather than one using the proper thermostats.

• __________ for evidence of recent water __________ from the

__________ __________, visually and by questioning the customer if possible.


• __________ check the temperature-relief __________ – lift the gear or

__________ the top on the integral test device on the relief valve for about

__________ seconds to remove any __________ that may have collected on the __________ __________. Check that it reseats and __________.

• Manually check the __________ valve – lift gear or __________ on the integral test device on the relief valve for about 30 seconds to remove any

__________ that may have collected on the valve __________. Check that it

__________ – and reseals.

• Check __________ pipes from both __________ and __________-relief

__________ for __________ and that their __________ points have not been obstructed or had building work carried out around them.

• Check the __________ operating __________ setting (e.g. 60–65°).

• Isolate __________ and __________ supplies to the heating __________, turn off __________ supply and __________ water __________ by opening taps:

– __________ inlet pipework where necessary to __________,

__________ or __________ the line strainer __________ (this may be part of the pressure-reducing valve assembly).

– Check __________ in __________ vessel and top up as necessary while the system is empty or __________. Use a __________ gauge and a pump to recharge to __________ pressure. If the __________ includes an

__________ gap as the expansion device, __________ the air gap as part of the procedure in line with manufacturer’s instructions.


• __________ water, electricity and gas supply. __________ the system up to temperature and __________ that control __________ are __________ effectively and relief __________ are not __________ water.

• __________ any maintenance __________ for the system.

3 Using the fault finding flow chart, Figure 3.34 on page 156 of Plumbing NVQ

and Technical Certificate Level 3, state the diagnostic and remedial actions required for the following fault.

Water is discharging from the temperature relief valve and on investigation you find the cylinder-operating thermostat setting is incorrect.

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4 Using the poor flow rate at taps or outlets chart, Figure 3.35 on page 157 of

Plumbing NVQ and Technical Certificate Level 3, state the diagnostic and remedial actions required for the following fault.

There is an inadequate flow of water from the hot taps due to grit in the in line strainer.

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5 Using the water discharging from the expansion valve chart, Figure 3.36 on page 158 of Plumbing NVQ and Technical Certificate Level 3, state the diagnostic and remedial actions required for the following fault

Water is discharging from the expansion valve and on investigation you find insufficient pressure in the expansion vessel.

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Exercise 3.5: Unvented systems: service and maintenance

1 The expansion valve from an unvented cylinder with bubble-top arrangement, intermittently discharges water. What would be the first action you would try to remedy the problem?

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2 On servicing an unvented system you find that the overheat thermostat has been ‘bridged out of the circuit’ and you do not have any spares. What do you do?

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3 A system is suffering from a reduced flow rate at all taps. What are the two most likely causes?

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