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Proceedings of Conference: TSBE EngD Conference, TSBE Centre, University of

Reading, Whiteknights Campus, RG6 6AF, 6th July 2010. http://www.reading.ac.uk/tsbe/

A review of domestic hot water demand calculation methodologies and their

suitability for estimation of the demand for Zero Carbon houses.

R. Burzynski1*, M. Crane2 and R. Yao3

1 Technologies for Sustainable Built Environments, University of Reading, UK

2 SSE Utility Solutions, Thatcham, UK

3 School of Construction Management and Engineering, University of Reading, UK

* Corresponding author: r.burzynski@student.reading.ac.uk

ABSTRACT

In 2006 a typical UK household used about 26% of its total energy consumption for

hot water preparation. Zero Carbon houses, which are to become a mandatory

standard from 2016, are characterised by a very high level of thermal insulation,

significantly reducing their space heating requirements and bringing the

proportion of hot water energy to a much higher level. Therefore, for such

buildings the accuracy of hot water demand estimations becomes much more

important than for a typical residential building. This paper presents results of a

review of methodologies used to estimate hot water demand in the UK dwellings.

Special attention is given to the suitability of the methodologies for the demand

estimation in houses built to the Zero Carbon standard. The paper also presents an

outline of the Greenwatt Way Zero Carbon housing development with its energy

performance monitoring programme. The monitoring will help to verify practically

the suitability of the existing hot water demand estimation methodologies for

modern houses.

Keywords:

Domestic Hot Water, Water Efficiency, Sustainable Solutions, Sustainable Homes

1. INTRODUCTION

In October 2008 the UK government announced very ambitious targets to reduce

greenhouse gas emissions by at least 34% by 2020 and 80% by 2050 against a 1990

baseline [1]. This commitment is spread across all industries including the housing

sector. In 2008 final energy consumption in the UK domestic sector increased by 3%

compared to 2007 and by 15% since 1990 [2]. According to DEFRA’s statistics [3]

energy consumption by end user in the residential sector accounted for 28% of

carbon dioxide emissions in 2006. Space heating and hot water alone in residential

buildings accounted for 13% of the UK’s greenhouse gas emissions. The UK Low

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Carbon Transition Plan [4] envisages that by 2050 these emissions are to be reduced

to almost zero by improving energy efficiency and utilising more low carbon energy

solutions.

According to the DECC’s statistics [5] energy used for hot water preparation

constituted about 30 % of the total domestic heat consumption in 2007. These

statistics have been derived from data collected from all the UK households;

therefore they are not necessarily applicable to modern houses built to the Zero

Carbon standard, which will become a mandatory requirement from 2016. Zero

Carbon houses are characterised by a very high level of thermal insulation with

significantly reduced space heating requirements. Therefore, the proportion of

energy used for hot water preparation out of total dwelling heat demand is

expected to be close to 60%. Resultantly, the accuracy of the hot water demand

estimations becomes more important for the design of an efficient heating system.

There are a few methodologies commonly used for estimation of hot water demand.

Unfortunately, none of them has been practically verified for houses built to Zero

Carbon standard yet.

2. REVIEW OF HOT WATER DEMAND ESTIMATION METHODOLOGIES

Domestic hot water consumption is a key variable for the design and planning of a

heating system. However, it is not possible to precisely calculate the consumption

as in practice it can significantly vary. Two similar families living in identical

neighbouring homes could use significantly different amounts of hot water.

Another important parameter of hot water consumption is the rate at which water

is drawn from the heating system. This is usually presented as a histogram of the

consumption on a typical day (working and weekend day). Figure 1 and Figure 2

present patterns of such demand from monitoring projects first in UK and second

in USA.

Figure 1 Average daily hot water consumption in UK [6].

Figure 2 Average weekday/weekend daily hot water consumption profiles for 15-unit building in USA [7].

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In both figures it is clearly visible that the hot water demand has two peaks. For UK

first peaks is at about 9 am and the second one lower than the first one at about 6

pm. For the example from USA first peak on a working day occurs around 8 am and

the second one, higher than the first one at around 9 pm.

Some good practice guides provide rough estimations of the amount of hot water

required by a household. For example in BSRIA’s Rules of Thumb handbook [8] it is

recommended to estimate daily consumption based on number of bedrooms.

According to this book for a single bedroom, two bedroom and three or more

bedroom dwellings the amount of hot water should be estimated at 115 litres, 75

litres and 55 litres per bedroom respectively. Alternatively, BS6700 [9] recommends

that hot water (60°C) consumption of a dwelling should be estimated between 35

litres and 45 litres per person per day. Yao and Steemers [10], based on data

provided by Marsh [11], envisage that the energy consumption breakdown of a

typical UK household will comprise of bathing/shower - 16%, washing hand in a

basin - 21%, dish washing - 34% and clothes washing - 29%. In contrast the

breakdown of the energy consumption in typical American family as reported by

Harvey [12] reveals that 51% of total hot water consumption is used for showers, 23

% for baths, 10 % for dishwashers and 16 % by washing machines (excluding system

standing and distribution losses). Harvey also concludes that even if showering and

washing habits of people living in sustainable houses do not change the hot water

consumption for showering and washing can be halved if water efficient fixtures

replace standard ones.

However, the most commonly used methodology for estimating domestic hot water

demand has been defined in BRE Domestic Energy Model (BREDEM) [13].

This methodology was also used to establish the Government’s Standard

Assessment Procedure for Energy Rating of Dwellings (SAP) which is enforced by

Building Regulations to assess energy and carbon (CO2) performance of new and

existing domestic buildings.

In the BREDEM the estimation of hot water demand and related energy demand is

based on the expected number of occupants (N) which is in turn related to the total

floor area (TFA) of a dwelling. However, as the authors of BREDEM indicate that this

relationship is only a rough indicator, as there is a large variability in practice. In

the most recent version of BREDEM 12 (updated in 2001) the standard number of

occupants, N is given by Equation 1.

if TFA ≤ 450 N = 0.0365 TFA - 0.00004145 x TFA2,

if TFA > 450 N = 9/(1+54.3/TFA)

(Equation 1)

Where: N is the assumed number of occupants and TFA is the total floor area of the

dwelling in m2.

Furthermore, the annual, daily hot water usage (Vd,average) is defined by Equation 2.

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Finally, assuming a 50°C temperature rise (from 10°C of mains water to 60°C within

a cylinder and a 15% loss of energy between the tank and tap), the hot water energy

at the tap Qu is given by Equation 3.

The authors of BREDEM state that the above demand function applies to an average

household, but the following adjustments to Qu can be made to account for

different levels of usage: above average +20%; below average -20%; well below

average -40%.

The aforementioned equations were slightly adjusted when implemented to SAP

2005 methodology. Equation 4 from SAP 2005 revision 3 allows calculating the

number of occupants.

if TFA ≤ 420 N = 0.035 x TFA - 0.000038 x TFA2,

if TFA > 420 N = 8 (Equation 4)

The annual, daily hot water usage (Vd,average) is defined by Equation 5.

Hot water energy (Qu) at the tap is given by Equation 6.

The Energy Saving Trust report [14] on the field monitoring of over a hundred

domestic hot water systems confirmed that the current BREDEM/SAP model of the

consumption (based on the number of occupants in a dwelling) is appropriate.

However, the assumption of a 50°C temperature rise of hot water in the cylinder

was found to be incorrect. The monitoring data shows that the average temperature

rise of water in the cylinder was about 36.7°C, which is significantly lower than the

one assumed in BREEDEM. This was partly due to a higher than assumed cold water

feed temperature (mean value 15.2°C) and a lower than assumed hot water

temperature (mean value 51.9°C).

Vd,average = 25 x N + 38 [litre/day] (Equation 2)

Qu= [(52 x N) +78] x 8.76 [kWh/year] (Equation 3)

Vd,average = (25 x N) + 38 [litre/day] (Equation 5)

Qu = [(61 x N) + 92] x 0.85 x 8.76 [kWh/year] (Equation 6)

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It is worth mentioning that the 10°C difference in water temperature results in 20%

energy savings in. Hot water consumption of the dwellings monitored in the EST

project would be over-predicted by BREDEM by approximately 35% [14].

Further investigation has also been carried out of the relationship between the

number of occupants and the floor area using data from English House Condition

Survey [15].

All aforementioned findings led to further changes of SAP. The recently introduced

2009 version of SAP has improved algorithms for all three parameters: number of

occupants, daily hot water demand and the hot water energy.

The algorithm for the number of occupants N is currently more sophisticated and is

expressed by Equation 7.

if TFA > 13.9: -exp (- -13.9)²

)] +

-13.9)

if TFA ≤ N = 1

(Equation 7)

Where: N is the assumed number of occupants and TFA is the total floor area of the

dwelling in m2.

Annual, average, daily hot water usage Vd,average has also been slightly adjusted by

reducing the fixed consumption by 2 litres. Current algorithm is presented by

Equation 8. Monthly variation of hot water demand may be calculated using factors

from Table 1.

Finally, hot water energy (Qu) at the tap is given by Equation 9.

Where: nm is a number of days in month m1, Vd,m is a daily use of hot water

adjusted by factor from Table 1 and ΔTm is the temperature rise for month m from

Table 2.

1 For February the number of days is fixed to 28.

Vd,average = (25 x N) + 36 [litres/day] (Equation 8)

3600/19.412

1, mm

mmdu TnQ V

[kWh/month] (Equation 9)

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Table 1 Monthly factors for hot water use

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Annua

l

1.10 1.0

6

1.02 0.98 0.94 0.90 0.90 0.94 0.98 1.02 1.06 1.10 1.00

Table 2 Temperature rise of hot water drawn off (ΔTm, in C)

Jan Feb Mar Apr May Jun Jul Aug Sept Oct Nov Dec Annua

l

41.2 41.

4

40.1 37.6 36.4 33.9 30.4 33.4 33.5 36.3 39.4 39.9 37.0

SAP 2009 also introduced a provision for reducing annual hot water usage by 5% in

cases where the dwelling is designed to achieve a water use target of not more that

125 litres per person per day (all water use, hot and cold) [16]. However, this

provision will always have to be used since the new Approved document G [17]

requires all new dwellings to have wholesome water consumption not greater than

125 litres per person par day. In addition to that, some boroughs, especially in

London, require from the developers to build new houses to a minimum of Code

Level 3 of the Code for Sustainable Homes (CSH) with Wales and Northern Ireland

also making this obligatory for all new housing supported by public funding [18].

Such houses should be designed and built in such a way that the water

requirements should not exceed 80 litres per person per day. This is often achieved

by installing grey and rain water recycling systems along with low flow water

fixtures. Some developers have even greater aspirations than Code Level 3 and have

started building houses to the Code Level 5 and Code Level 6 (Zero Carbon).

The impact of all of the aforementioned changes to the BREDEM/SAP

methodologies of the hot water energy demand of dwellings of total floor areas up

to 150 m2 have been presented in Figure 3 and Figure 4. Figure 3 clearly shows that

there is quite significant difference in the results of calculation of occupancy for

dwellings of total floor area more than 100 m2. It is also surprising to see that the

occupancy seems to be limited to about three occupants even for very large

dwellings. The second chart shows that even for small dwellings there is noticeable

reduction in estimations of hot water energy demand calculated using BREDEM

12/SAP 2005 and SAP 2009 methodologies.

However, it is rather difficult to evaluate whether the new algorithms and

additional provision of a 5% reduction of “standard” hot water demand would be

sufficient to reflect a potential reduction of hot water demand in houses build to

high level of the CSH.

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Figure 3 Changes in estimations of occupancy as function of total floor area for discussed methodologies.

Figure 4 Changes of hot water energy demand estimations as function of total floor area for discussed methodologies.

3. MONITORING OF ENERGY PERFORMANCE OF GREENWATT WAY THE ESPERIMENTAL ZERO CARBON DEVELOPMENT

Expecting significant changes in energy consumption of new houses that can affect

energy supply business in UK, SSE, one of the UK’s major energy utilities, has

developed a Zero Carbon housing project called Greenwatt Way. The main aim of

the project is to study energy usage and individual occupant’s interaction with

energy efficient Zero Carbon homes. As part of this study, the hot water demand

will be monitored and the results will be used to verify practically the suitability of

the existing hot water demand estimation methodologies for modern Zero

Carbon/Sustainable houses.

The development is located in Slough, about 20 miles west of London and is shown

in Figure 5. The site consists of ten dwellings; two 1 bed apartments (45 m2 each), a

terrace of three 2 bed houses (80 m2 each), a terrace of three 3 bed houses and two

3 bed detached houses (94 m2 each). There is also a renewable Energy Centre and an

Information Centre. The project partners combined conservative architectural

design with the latest construction methods, technologies and sustainable features

available in order to deliver Zero Carbon housing to Level 6 of the Code for

Sustainable Homes.

Occupancy per TFA

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

15 30 45 60 75 90 105 120 135 150

Total Floor Are of Dwelling [m2]

Occu

pan

cy

N-BREDEM-12 N 2005 N 2009

Hot Water Energy Demand

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

15 30 45 60 75 90 105 120 135 150

Total Floor Are of Dwelling [m2]

En

erg

y [

MW

h/y

ear]

QBREDEM-12 QBREDEM-12 -40%

Q2005 Q-2009-5%

±20%

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Figure 5 Aerial view of Zero Carbon Housing project in Slough.

Figure 6 Integrated renewable energy centre with district heating scheme.

The homes are equipped with modern hydraulic interface units (HIU) which provide

energy for space heating and hot water. The schematic of the HIU and its key

components is presented in Figure 7. Low carbon heat is supplied to each HIU from

the site’s renewable Energy Centre (Figure 6) via a low temperature district heating

(DH) scheme. The district heating scheme is built with a pre-insulated twin pipe

system which aims to reduce heat loses.

The district heating scheme operates at a flow temperature of 55°C and the domestic

hot water is supplied at 43°C via an on-demand heat exchanger in each house. The

radiators and hot water heat exchanger in all homes are directly connected to the DH.

The heat loads in the house are designed to achieve the lowest possible DH return

temperature to minimise heat losses and maximise the heat pumps coefficient of

performance.

40 kW

10 kW District Heating

Space Heating

Hot Water

10°C

43°C

55°C

35°C

55°C

20°C

Figure 7 Key parameters and schematic of Hydraulic Interface Unit (HIU).

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The research programme includes several work streams with an initial monitoring

programme of two years and includes:

Modelling and monitoring of the energy performance of the renewable energy centre, district heating scheme and domestic heat and power demand.

A post occupancy evaluation of the tenants.

An evaluation of the whole house mechanical ventilation with heat recovery system (MVHR).

A demonstration of hot fill washing appliances and energy efficient smart kit.

An electric vehicle car share scheme for residents.

Monitoring of water usage.

4. CONCLUSIONS

The review of methodologies used to estimate hot water energy demand of the UK

dwellings shows that there is a limited number of methods used for this purpose.

The most advanced one was derived from BREDEM model. The methodology has

been recently verified and updated using data from the hot water monitoring

project from more than 100 UK dwellings. Generally the update resulted in

significant decrease of hot water demand estimations per square meter of dwelling.

However, the data collected during the monitoring project did not cover CSH Level

3 and higher Code Levels houses. Therefore, it is still some uncertainty whether

currently used models are accurate enough to model hot water demand in Zero

Carbon houses. The monitoring programme of the Greenwatt Way project should

help to verify and improve the suitability of the methodologies for modern Zero

Carbon/Sustainable houses.

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5. REFERENCES

[1] DECC, Climate Change Act 2008, DECC, Ed., ed. London, 2008.

[2] DECC, UK Energy in Brief 2009, DECC, Ed., ed. London: National Statistics,

2009.

[3] DEFRA, The environment in your pocket 2008, DEFRA, Ed., ed. London:

National Statistics, 2008.

[4] H. Government, The UK Low Carbon Transition Plan, ed. London: The

Stationery Office, 2009.

[5] DECC, Energy Consumption in the UK. Domestic Data Tables, 2009 Update ed:

A National Statistics Publication, 2009.

[6] Energy Monitoring Company, Measurement of Domestic Hot Water

Consumption in Dwellings, DEFRA 2008.

[7] E. Vine, et al., Domestic hot water consumption in four low-income apartment

buildings, Energy, vol. 12, pp. 459-467, 1987.

[8] K. Pennycook, Rules of Thumb, 4th Edition ed.: BSRIA, 2003.

[9] British Standard, Design, installation, testing and maintenance of services

supplying water for domestic use within buildings and their curtilages

Specification, in BS 6700:2006+A1:2009, ed: BSI, 2009.

[10] R. Yao and K. Steemers, A method of formulating energy load profile for

domestic buildings in the UK, Energy and Buildings, vol. 37, pp. 663-671, 2005.

[11] R. Marsh, Sustainable housing design: an integrated approach, Ph.D thesis,

University of Cambridge, 1996.

[12] L. Harvey, A handbook on low-energy buildings and district-energy systems:

fundamentals, techniques and examples: Earthscan, 2006.

[13] B.R. Anderson, et al., BREDEM-12 Model description, 2001 update: IHS, BRE

Press, 2002.

[14] EST, Measurement of Domestic Hot Water Consumption in Dwellings, DEFRA

2008.

[15] BRE, A review of the relationship between floor area and occupancy in SAP,

Building Research Establishment 2009.

[16] DECC, The Government’s Standard Assessment Procedure for Energy Rating of

Dwellings, DECC, Ed., Version 9.90 ed. Garston: BRE, 2010.

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[17] Secretary of State, Building Regulations, Approved Document Part G - Sanitation,

hot water safety and water efficiency, UK Government, Ed., ed: NBS, 2010.

[18] DCLG, Code for Sustainable Homes - Technical Guide, DCLG, Ed., May 2009 ed,

2009.