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Energy Efficiency E-modules - Guidance

Application of Decentralised Domestic Hot Water in the Public Sector

Application of Decentralised Domestic Hot Water in the Public Sector | 2

Contents

1 Introduction 3

2 Learning Objectives and Outcomes 3

2.1 Learning Objectives 3

2.2 Learning Outcomes 3

3 Overview of Principles of Hot Water Generation and Distribution 4

4 Opportunities for Decentralised Hot Water in the Public Sector 6

5 Technology and Application 9

6 Building a Business Case 11

6.1 Case Study 1 11

6.2 Case Study 2 12

6.3 Summary 12

7 Useful Links and References 14


1 Introduction

This guidance follows the format of the e-module “Application of Decentralised Hot Water

Systems in the Public Sector” and provides further details on the subjects covered in the

module.

Please note that module users working in a healthcare environment should always refer to

the relevant Scottish Health Technical Memorandum (SHTM) prior to considering installation

of the measures suggested in the module. The advice given in the SHTM may conflict with

the advice given in this module, as it has been developed for the wider public sector. The

relevant SHTM can be found on the Health Facilities Scotland website.

2 Learning Objectives and Outcomes

2.1 Learning Objectives

The learning objective from this module is to understand the benefits and applications of

decentralised domestic hot water systems.

2.2 Learning Outcomes

The learning outcomes from this module are to:

Describe the principles of hot water generation;

Identify where the opportunities for decentralised hot water exist in Scottish public

sector sites and buildings;

Illustrate the main decentralised hot water technologies along with their advantages

and disadvantages;

Prioritise the opportunities for applying decentralised hot water systems in public sector

buildings; and

Understand the key aspects in relation to decentralised hot water systems when

building a business case.


3 Overview of Principles of Hot Water Generation and Distribution

There are different uses for hot water in the public sector built environment. These comprise

wash hand basin outlets, sink outlets and showers. In some buildings there may be other

larger hot water users such as kitchens, blocks of showers and laundries.

There are circumstances under which some of these may or may not be suitable for

decentralised hot water supply. In order to assess this, the costs and practicalities

associated with hot water generation must be considered.

The first point to consider is the contribution of hot water to the energy use of a building. As

typically hot water is not metered in any way this is often difficult to measure accurately.

This means that often the only way to obtain an estimate of energy consumption for heating

water is by calculation based on first principles.

Sensible heating of a fluid (e.g. air or liquid water) can be described as follows:

Where:

q is the heat transfer

m is the mass

Cp is the specific heat capacity of the substance

dT is the temperature difference

Points to note are that this equation provides the energy required in kJ. For most practical

uses this must be converted to kWh (1 kWh = 3600 kJ). Specific heat capacity is the

energy needed to increase the temperature of 1 kg of fluid by 1°C. The specific heat for

water is 4.2 kJ/kg°C.

A worked example – a typical office building

The hot water consumption is not known in detail but an estimate can be made using rule of

thumb values (taken from the BSRIA Rules of Thumb 5th Edition). It is worth noting that

there can be huge variations in water use from building to building and that where possible

actual metered values should be obtained.

It is assumed that the hot water system is based on a domestic hot water vessel heated by

a water coil from the main gas fired boiler system (a calorifier). If the boiler has an

assumed efficiency of heat delivered to the calorifier of 85% then for every 100 kWh of fuel

consumed by the boiler, 85 kWh ends up at the calorifier. The gas cost is assumed to be 3.5

p/kWh. The storage temperature of the water is 60°C and incoming mains water

temperature will be around an average of 10°C. This makes the dT value 50°C.


Number of occupants 1,000 staff

Domestic hot water use per day 15,000 litres

Domestic hot water use per year 4,500,000 litres

Density of water 1 kg/litre

So using q=mCpdT/3600

m is 4,500,000 kg

Cp for water 4.2 kJ/kg°C

dT 50 °C

q (heat energy delivered for hot water) 262,500 kWh

Boiler system efficiency 85%

q (fossil fuel consumption related to hot water) 308,824 kWh

Cost £10,809

The annual gas consumption to produce hot water is 308,824 kWh at a cost of £10,809.

It is important to put this into the context of the whole building energy consumption. In our

example the office occupies 15,000 m² and has an annual fuel consumption of 1,800,000

kWh. This means that hot water accounts for just over 17% of total consumption.

Due to the inefficiencies associated with heating older buildings, benchmark figures suggest

that hot water might only account for 7% of fossil fuel consumption. This emphasises that

there can be very large variations in the proportions of energy consumption for hot water

depending on the building type, age, use and hours of operation. This puts into context the

magnitude of potential energy savings that are available from changes to the hot water

system. However, it is understood that in some instances moving to a centralised hot water system has advantages.


4 Opportunities for Decentralised Hot Water in the Public Sector

Domestic hot water systems can be split into two very broad categories:

Centralised systems; and

Decentralised systems.

Decentralised systems are characterised by the provision of water heaters at, or very

close to, the point of use. These can be gas fired, but are more commonly based on electric

water heating. While these systems can be very efficient they are best suited to discreet low

volume applications such as wash hand basins or single shower cubicles.

Centralised systems are better suited to larger applications. They use a flow pipe to

distribute the hot water from a central point to users. Larger systems also incorporate a

return connection to allow the hot water to circulate back to the heat raising plant. This

return connection will be very close to the point of use. This arrangement ensures that the

water delivered to users is at the correct temperature even under conditions of high

demand. Frequently water in this pipe work will flow at around 55-60°C though this may

well be controlled at the point of use by a thermostatic mixing valve to avoid scald risks.

Figure 4.1 illustrates the water connections in a typical shower room.

Figure 4.1 – Water Connections in a Typical Shower Room

The flow and return pipework for centralised systems can be extensive. Even when well

insulated the heat loss from this distribution pipework can add substantially to system

inefficiencies. Modern design standards dictate maximum flow and return lengths from the

hot water storage to the point of use, but in older buildings they may be beyond these

maximum lengths.

Additional layers of inefficiency come if several domestic hot water (DHW) storage vessels

are distributed around a large building or complex and are served by a large centralised

boiler system. This is particularly the case where very large boiler systems that are

designed for both heating and hot water operate at low load to serve small domestic hot

water loads during summer months.


NHS Scotland has specific requirements for domestic hot water which are detailed in

guidance such as the Scottish Health Technical Memorandums.

There is one other important consideration which has a bearing on the discussion of

domestic water systems. Due to the severity of the potential outcome this must supersede

any desire to reduce energy consumption. This is the risk to human health from the

bacterium Legionella pneumophila and related bacteria. The collective term for the range of

diseases caused by legionella bacteria is Legionellosis. These include potentially fatal

diseases. Although some people are particularly vulnerable, everyone is susceptible to

infection.

Legionella bacteria are common in all natural water systems. Storing or distributing water at

temperatures between 20°C and 45°C should specifically be avoided as these are the

temperatures at which the bacteria is most likely to proliferate. It is important to ensure

that any domestic water systems (hot and cold) avoid these temperature ranges. Typically

it is best to seek a margin of safety, for example a hot water storage temperature of 60°C

will greatly reduce the chance of the bacteria surviving.

There are many other considerations in terms of managing the legionella bacteria. It is

advisable to be familiar with the HSE’s Approved Code of Practice for Legionnaire’s Disease

L8 and HSG274 which details the measures necessary to comply with legal duties in relation

to legionella.

These points certainly have a bearing on the management of a hot water system, but the

question of why to consider this when the focus is on the benefits of decentralised water

systems versus centralised types is worth discussion.

Decentralised systems tend to include many of the preferred design features that can

minimise the risk from legionellosis. These include:

Low or no storage volumes;

Very short distribution runs; and

The minimisation of effective dead legs in the system (i.e. runs of pipe containing water

which will only be drained off infrequently).

This is not to say that care is not needed in relation to these risks when applying a

decentralised system, however, decentralised systems are often preferable from this

perspective.

Looking specifically at the efficiency of water heating clearly any inefficiency in the boiler

system will obviously be carried over to the DHW storage and distribution. Extensive runs of

distribution pipework from the boiler will also lead to significant losses. In buildings where

the summer hot water requirement is low, a major problem is that this low heat demand

leads to boilers operating inefficiently.

Some types of centralised systems overcome this problem by include dedicated gas or oil

fired domestic water heaters sized specifically to meet this heat load. These tend to have

higher basic efficiencies than comparable boiler and calorifier arrangements particularly if

they are condensing models which recover additional heat from the flue gas compared to

conventional water heaters.

The installation of local high efficiency gas water heaters to replace a complex boiler and

calorifier arrangement can be thought of as a potential decentralisation option which can

achieve good savings in the right circumstances.


When considering decentralised hot water systems there a range of different systems, all

with different features to consider. For instance, gas fired units need a flue and must have

sufficient ventilation. Electric units overcome this, but the cost and carbon content of

electricity is higher than gas. In both cases, hard water can also prove problematic.

As with centralised systems there are different levels of decentralisation. Some systems will

incorporate small amounts of storage, while others will have no storage incorporating an in-

line instantaneous heater.


5 Technology and Application

Small or no storage electric water heaters are often applied in decentralisation projects.

These can often be adapted to serve one or more proximate outlets, for example two or

three wash hand basins in a small toilet block.

These are available in unvented versions which are fed from the mains water supply. These

need to include some additional equipment to accommodate potential expansion and

pressure relief. Vented equivalents use low pressure tanked supplied water. Here, the feed

tanks provide the requirements for expansion and pressure relief.

There are also systems which incorporate no storage and are merely a small in-line electric

water heater. These would include electric shower units.

When selecting a system the requirement for pressure relief in storage vessels, even

vessels as small as 15 litres has to be taken into account. For unvented systems this will

involve a pressure relief kit and the requirement for discharge to a safe place. The typical

installation details for a small water heater, as dictated by the Scottish Building Standards,

are illustrated in Figure 5.1. The requirement for a safe discharge (usually to exterior) can

be difficult where heaters need to be located away from the building perimeter for example.

Figure 5.1 – Typical Installation Details for Small Water Heater

Each manufacturer will have their own installation requirements and it is important to be

aware of these.

The water pressure that is available can also be a determining factor in system selection

and design. In some cases it can be desirable to choose unvented systems as the high

pressure allows delivery of high volumes of hot water. Some review of the available

pressure and thus flow rate for a tanked supply should be undertaken if a vented system is

chosen.

The unit locations can be reasonably flexible and often they are located on adjacent wall

space either above or below the outlets served.


Due to the minimal storage volumes the electrical demand of instantaneous electrical water

heaters can be substantial. For example a shower unit may be 9 kW whilst a smaller heater

incorporating water storage would have a power rating of around 3 kW to 4.5 kW. When

looking at replacing a calorifier with a comparable number of local electric heaters, the

increase in building electrical load could be significant. This is particularly so in buildings

where the use of the DHW outlets is high and there is potential for all or most of the heaters

to be drawing power simultaneously. The site electrical capacity should be confirmed early

in any project of this type as should the proximity of the proposed heater locations to the

nearest suitable electrical supply board.

One benefit of electrically heated hot water systems is that they can be easily time

controlled to maximise energy reductions.

When considering converting outlets from tanked supplies to mains it’s worth considering

the instantaneous water requirement versus the actual site mains water capacity. Any

significant changes to a sites domestic water system should be cleared with the relevant

water authorities

As discussed in the previous section, another option is to decentralise using smaller local

heaters to replace a large network of boilers and calorifiers. There can be clear cost benefits

from this approach. For example a condensing gas fired water heater can achieve an

efficiency of 98% whereas a large steam boiler and distribution system could be as low as

65% efficient. Where hot water systems are used intensively this approach can be cost

effective. Gas fired condensing water heaters should be selected from the energy

technology list.

Because of safety requirements consideration should be given to transporting fuels through

a building. The combustion based heaters considered will generally have a larger storage

capacity and physical size in comparison to comparable electric units. The spaces in which

they can be located will also be more limited to achieve compliance with the standards on

combustion appliances. This means that in practice only dedicated plant rooms may be

suitable for them.

For fuel oil and LPG systems the price of fuel may be such that they offer no benefit over

smaller electric systems.


6 Building a Business Case

It is important to note that decentralisation is certainly not a solution for every building and

even where it could be considered as an option there may be alternatives. For example

where boiler and DHW systems are losing substantial heat from poor pipe insulation, it may

be cheaper to look at re-insulating. System inefficiencies could be reduced by no or low cost

measures, for example implementing improved time control.

Where hot water use in a building is high then it might be worth investigating if there are

any problems causing this and resolving them. For example there may be leaks in the

system or it may be that the staff use of water could be greatly reduced by installing water

efficient fittings i.e. percussion taps or low flow fittings. Where reductions can be achieved,

the justification for substantial alteration to the DHW systems will be reduced.

There may be costs for decentralisation that at first are not obvious. The necessary

electrical works have been mentioned but it is worth noting that projects of this type

frequently involve the removal of older services and the creation of wall penetrations for

pipe and cabling. Particularly in older buildings these activities can run the risk of disturbing

asbestos. It is always worth considering this at an early stage where substantial alterations

are proposed.

Finally, the relative price of the fuels cannot be ignored. Many decentralisation projects

involve the change from a centralised gas system to localised electric heaters. Currently,

electricity is 2.5 to 3 times more expensive than gas and if the project is to be based on

energy cost reduction alone then the improvements in efficiency would have to be very

substantial (i.e. the existing gas fired system would have to be very poor). It should also be

remembered that the carbon intensity of grid supplied electricity is far higher than for gas

on a kWh basis so switching from gas to electricity has the potential to increase carbon

emissions.

In some cases a partial decentralisation is achieved by switching off the main boiler plant in

larger buildings outside of the heating season and using local electric immersion heaters

fitted to existing calorifiers to generate the necessary hot water. This can be a good solution

in buildings with a large steam boiler plant serving a multitude of smaller calorifiers.

6.1 Case Study 1

This project involved two similar sites where a full heating and hot water decentralisation

project was undertaken. Each site involved a large boiler plant serving the heating and hot

water needs for multiple buildings. At site 1 this was a large steam system serving heating

and DHW calorifiers at each building. Site 2 was similar, although the heating medium was

low temperature hot water (LTHW) rather than steam.

The solution combined the use of small unvented local electrical storage heaters or point of

use units in some of the core areas where DHW demand had reduced drastically. In addition

localised condensing gas fired water heaters were applied in some of the high DHW use

areas.

This project worked well because:

The central boiler systems and distribution systems in both these sites were inefficient

and of an age and type that would have made gradual energy efficiency improvements

difficult and costly;


The actual hot water demand had reduced significantly in many areas of the building

meaning the equipment and storage was oversized; and

The entire heating and DHW network in each site had to be active through the year on

a 24/7 basis to ensure that DHW could be provided in specific small areas. The new

system allowed much better time control of individual spaces and buildings. The result

was that economies of scale could be achieved because the DHW decentralisation was

being combined with heating decentralisation and it also allowed a reduction in stored

water at the site, thus reducing the legionella risk.

6.2 Case Study 2

This was a multi-storey emergency services HQ where an existing LTHW boiler and calorifier

arrangement was to be upgraded. The management of the boiler fuel (fuel oil) was

becoming difficult and the proposed solution included:

A new natural gas supply to the building; and

New condensing gas fired water heaters to replace calorifiers and allow switch off of

heating boilers in summer months.

The project worked well because:

It was combined with another series of improvements, to include the removal of

hazardous oil;

It allowed better control of the system boilers during summer months;

The fuel change greatly reduced the running costs for the system; and

The efficiency of hot water generation was greatly improved.

6.3 Summary

These are two examples of where varying degrees of decentralisation of DHW been

successfully applied. As with any other type of improvement there is no single solution.

Instead, there are general things to look out for with decentralisation. Just by considering

relative fuel prices, replacing gas fired water heaters with electric storage or non-storage

units will seldom stack up on its own basis. Usually, these projects can be justified more

easily where hot water and heating are provided by the same boiler system and where this

boiler system is inefficient. Many of the losses associated with steam DHW raising plant can

be eliminated by a change to a LTHW or direct gas fired DHW system.

It is also important to look for extensive runs of pipe and to see if there are ways in which

these can be avoided. Table 6.1 may prove useful in estimating the heat loss from both

insulated and uninsulated runs of pipe. The 75°C temperature is a typical average

temperature for many heating systems, the 50°C will be in the range that many DHW

circulation systems operate. The heat loss figures assume that the surrounding air is still

and at 20°C.


Table 6.1 – Heat Loss from Insulated and Uninsulated Pipes

Pipe Size Insulation Temperature = 50C

Thickness Bare Pipe Insulation Savings

Inches Millimetres used (mm) W/m W/m W/m

3/8 10 19 25 5.5 19.5

1/2 15 19 30 6.4 23.6

3/4 20 19 36 7.2 28.8

1 25 25 44 7.1 36.9

1 1/4 32 25 54 8.3 45.7

1 1/2 40 32 60 7.8 52.2

2 50 32 74 9 65

2 1/2 65 38 90 9.6 80.4

3 80 38 102 10.08 91.92

4 100 50 127 10.08 116.92

Pipe Size Insulation Temperature = 75C

Thickness Bare Pipe Insulation Savings

Inches Millimetres used (mm) W/m W/m W/m

3/8 10 19 50 10 40

1/2 15 19 63 11.8 51.2

3/4 20 19 76 13.3 62.7

1 25 25 91 13.3 77.7

1 1/4 32 25 111 15.4 95.6

1 1/2 40 32 125 14.8 110.2

2 50 32 155 17 138

2 1/2 65 38 190 15 175

3 80 38 240 16.8 223.2

4 100 50 270 17 253

This provides losses in W/m length of pipe. To convert this to kW/m, divide by 1,000.

Finally, it is also worth reviewing changing uses of water in buildings and to identify if there

is benefit from using more water efficient taps. Clearly any investment proposals are best

based on real data where possible. Where this is absent it may be best to undertake some

limited monitoring of DHW use.


7 Useful Links and References

Title Source Link

Zero Waste Scotland General www.zerowastescotland.org.uk

How to implement decentralised hot

water systems (CTL146) The Carbon Trust

www.carbontrust.com/media/147163/j8053_ctl146_how_to_implement

_decentralised_hot_water_systems_aw.pdf

CIBSE Guide F Energy Efficiency in

Buildings, chapter 10

Chartered Institute

of Building Services

Engineers

www.cibse.org

Building Standards Handbook 2013

Non-Domestic Handbook Section - 4.9 Scottish Government www.scotland.gov.uk/Resource/0043/00435261.pdf

http://www.zerowastescotland.org.uk/

http://www.carbontrust.com/media/147163/j8053_ctl146_how_to_implement_decentralised_hot_water_systems_aw.pdf

http://www.carbontrust.com/media/147163/j8053_ctl146_how_to_implement_decentralised_hot_water_systems_aw.pdf

http://www.cibse.org/

http://www.scotland.gov.uk/Resource/0043/00435261.pdf

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