Case study West Grimstead

West Grimstead

Wiltshire, United Kingdom - Ground-source heat pump

Summary
This large, well-insulated detached single-family house in West Grimstead consists of a two-storey main house and a linked single-storey annex. A horizontal ground-source heat pump installed in January 1998 provides space heating and DHW. The annual performance factor is 3.16 and when mostly used for DHW in the summer it is 2.5. The operational experience of this low maintenance installation have been positive.

A single-family house in West Grimstead near Salisbury.

The results of the first year’s operation of the heat pump are presented in the following.

Building and design values

Building type: Single-family house

Location: West Grimstead, near Salisbury, UK

Year of construction: 1997

Number of storeys: 2 in the main house, 1 in the annex

Heated floor area (m2): 288

% of total floor area (%): 100

Design outdoor temperature (ºC) Heating: -1 Cooling: -

Design indoor temperature (ºC) Heating: 18 Cooling: -

Degree days Heating: 2 263 Cooling: -

Base temperature for degree days (ºC) Heating: 15 Cooling: -

Heating and cooling

Application: Space heating, DHW¹

Heat pump type(s): Water-to-water

Heat pump installed capacity (kW) Heating: 3.96 Cooling: -

Refrigerant: R407C

Heat source Ground

Details (circulating pumps): Distribution pump (within the heat pump) 86.7 W

a) Type of ground heat exchanger: Horizontal/single loop (PE 32/28 mm)

b) Borehole depth (m): Trench depth 1

c) Pipe length (m): 200

d) Heat transfer fluid: Water/antifreeze

e) Flow rate (l/h): 828 (in the ground coil)

Distribution system(s): Underfloor

Supply and return temperature (ºC) Heating: 35/28² Cooling: -

Auxiliary system: Heater; an additional time switch is used in this installation to limit the use of auxiliary heater to the “Economy 7”³ tariff period

a) Size (kW): 2 x 2

b) Fuel: Electricity

c) Operating regime: -

Heat pump design: 50% of the design heat load

Supplementary system: Wood-burning stoves for additional space heating in the living room and the kitchen. Contribution to the annual energy consumption is negligible..

Heat pump system completion date: January 1998

¹ DHW is heated via an exchange coil in a mains pressure cylinder (capacity 210 l), which has a 3 kW electric immersion heater as a back-up. DHW usage was designed to be 250 l/d. DHW usage was not measured, but it was very variable.
² Since July 1998 the operation of the space heating system has been based on an outdoor temperature sensor instead of the temperature sensor in the dining room. The return temperature to the heat pump was adjusted to an outdoor temperature sensor control curve.
³ Low-cost storage heating with electricity during the night time (7 hours).

Performance

Heating energy Heat pump¹ Aux. heating system Auxiliaries

Energy input (kWh/year): 5 025 1 270 Included in “heat pump”

Energy output (kWh/year)²: 13 985 1 270 n/a

Energy cost (GBP/year)³: 419 - -

Cost tariff (GBP/kWh)³: 0.0804, 0.0284 0.0804, 0.0284 -

¹ Including DHW and auxiliaries
² Measured energy output.
³ The price has been calculated using three-year average values of the Government’s Standard Assessment Procedure for Energy Rating of Dwellings (SAP) 1998. The annual running costs for space heating and DHW include on-peak consumption of 3,518 kWh at GBP 0.0804/kWh and off-peak consumption of 4 307 kWh at GBP 0.0284/kWh. An annual standing charge of GBP 14 of off-peak use has been added to the total price.

Cooling energy

Not applied

Energy for DHW Heat pump¹ Aux. heating system Auxiliaries

Energy input (kWh/year): Incl. in heating 1 530 -

Energy output (kWh/year)²: 1 895 1 530 n/a

Energy cost (GBP/year)³: Incl. in heating - -

Cost tariff (GBP/kWh)³: 0.0804, 0.0284 0.0804, 0.0284 -

¹ Including DHW and auxiliaries
² Measured energy output.
³ The price has been calculated using three-year average values of the Government’s Standard Assessment Procedure for Energy Rating of Dwellings (SAP) 1998. The annual running costs for space heating and DHW include on-peak consumption of 3 518 kWh at GBP 0.0804/kWh and off-peak consumption of 4 307 kWh at GBP 0.0284/kWh. An annual standing charge of GBP 14 of off-peak use has been added to the total price.

Additional notes

The estimate of the energy consumption for DHW was based on a daily usage of 250 l, a constant mains supply temperature of 8ºC, and a constant output temperature of 50ºC.
The heat pump provided 10,855 kWh/year of “free” energy from the ground.

Coefficient of performance (COP)

Heating: 3.16 overall (November-April), 2.5 in summer when predominately for DHW, see section 4

Test conditions: Seasonal performance factor

Cooling: -

Test conditions: -

Heat pump cost breakdown

Heat pump only (GBP): 1 108 including controls

Installation (GBP): GBP 30 (done by the owner), cold loop GBP 78 (marginal cost estimate for excavator time)

Capital cost (excluding heat pump) (GBP): 662 excluding the underfloor heating system and the hot water cylinder

Maintenance: No data

Alternative system (if has been considered) The cost of an oil-fired heating system was estimated to be about GBP 1,278 (see breakdown below). There was no mains gas supply available to the property. This system would have an energy consumption of 26 686 kWh/year. Cost breakdown - Boiler and ancillaries: boiler GBP 660, controller GBP 60, flue lining and flue installation GBP 180, installation GBP 45. Oil tank and feed: ground pipe about GBP 20, a 955 litre (210 gallon) tank GBP 180, foundation about GBP 280, installation about GBP 13.

Fuel cost Electricity: on-peak GBP 0.0804/kWh (standing charge not heating-dependent), off-peak GBP 0.0284/kWh (standing charge GBP 14/year).
Oil GBP 0.0143/kWh (no standing charge).
Gas GBP 0.0149/kWh (standing charge GBP 36/year).
Annual fuel costs (based on SAP 1998) are about 10% higher than those for a gas condensing boiler and about 20% higher than those for a new regular oil boiler. The servicing costs of the heat pump are possibly lower. The running costs of the heat pump are significantly cheaper than those for an all-electric heating system (from GBP 545 to GBP 1,100/year). The running costs given do not include annual servicing

Payback No data.

CO₂ emissions The CO2 emissions of the heat pump and alternatives are:

Ground-source heat pump: 3 600 kg CO2/year

All electric (efficiency 100%): 8 590 kg CO2/year

Regular oil-fired boiler, pre-1998 (efficiency 70%): 7 210 kg CO2/year

Regular oil-fired boiler, 1998 or later (efficiency 79%): 6 390 kg CO2/year

Gas-fired condensing boiler (efficiency 85%): 4 260 kg CO2/year

Assumptions: Assumed CO2 emission: oil 0.27 kg/kWh; gas 0.194 kg/kWh; electricity 0.46 kg/kWh.
For the oil-fired boilers, default values from SAP 1998 are adjusted for underfloor heating.

Operational experience and other comments

At the time of the installation suitable products were not available in the UK and hence the heat pump was imported from Sweden. During the installation it would have been convenient to have the instructions in English. Another drawback is the possible difficulty in obtaining spare parts. The lifetime of the heat pump system is expected to be at least 20 years.

A schematic of the heat pump system.

The system has been reliable and the performance has been good up to date. Over its first year of operation the ground-source heat pump provided 91.7% of the total heating requirement of the building and 55.3% of the DHW heating requirement although it was sized to meet only 50% of the design heating capacity. The energy consumption was lower than expected during the monitoring period possibly due to the warmer weather (degree day total 1,899) than during an average year.

The occupants have been pleased with the comfort levels achieved and found the system quiet and unobtrusive. In the first year of operation daily average indoor temperature of around 18-23ºC was maintained during the heating season from November to April.

Detailed analysis has shown that the difference in efficiency between winter and summer is largely due to the heat pump distribution pump, which runs continuously. If this was run only when the heat pump supplies heat, energy consumption would be reduced and it is estimated that the winter and summer average system efficiencies would be 3.42 and 3.44 respectively.

This is a low maintenance installation. It is recommended that the filter is cleaned annually and the sight glass inside the heat pump is checked every 3 months. Up to date the heat pump has not required any other maintenance apart from filter cleaning.

References
This case study is from General Information Report 72, Energy Efficiency Best Practice Programme, UK, March 2000. The publication is available from:
BRECSU Enquiries Bureau, BRE, Garston, Watford, WD2 7JR, UK.
Tel. + 44 1923 664 258, Fax. + 44 1923 664 787.

Last updated: 1 March 2004

The world's leading information centre on heat pumps

Building type:	Single-family house
Location:	West Grimstead, near Salisbury, UK
Year of construction:	1997
Number of storeys:	2 in the main house, 1 in the annex
Heated floor area (m2):	288
% of total floor area (%):	100
Design outdoor temperature (ºC)	Heating:	-1	Cooling:	-
Design indoor temperature (ºC)	Heating:	18	Cooling:	-
Degree days	Heating:	2 263	Cooling:	-
Base temperature for degree days (ºC)	Heating:	15	Cooling:	-

Application:	Space heating, DHW¹
Heat pump type(s):	Water-to-water
Heat pump installed capacity (kW)	Heating:	3.96	Cooling:	-
Refrigerant:	R407C
Heat source	Ground
Details (circulating pumps):	Distribution pump (within the heat pump) 86.7 W
a) Type of ground heat exchanger:	Horizontal/single loop (PE 32/28 mm)
b) Borehole depth (m):	Trench depth 1
c) Pipe length (m):	200
d) Heat transfer fluid:	Water/antifreeze
e) Flow rate (l/h):	828 (in the ground coil)
Distribution system(s):	Underfloor
Supply and return temperature (ºC)	Heating:	35/28²	Cooling:	-
Auxiliary system:	Heater; an additional time switch is used in this installation to limit the use of auxiliary heater to the “Economy 7”³ tariff period
a) Size (kW):	2 x 2
b) Fuel:	Electricity
c) Operating regime:	-
Heat pump design:	50% of the design heat load
Supplementary system:	Wood-burning stoves for additional space heating in the living room and the kitchen. Contribution to the annual energy consumption is negligible..
Heat pump system completion date:	January 1998

Heating energy	Heat pump¹	Aux. heating system	Auxiliaries
Energy input (kWh/year):	5 025	1 270	Included in “heat pump”
Energy output (kWh/year)²:	13 985	1 270	n/a
Energy cost (GBP/year)³:	419	-	-
Cost tariff (GBP/kWh)³:	0.0804, 0.0284	0.0804, 0.0284	-

Energy for DHW	Heat pump¹	Aux. heating system	Auxiliaries
Energy input (kWh/year):	Incl. in heating	1 530	-
Energy output (kWh/year)²:	1 895	1 530	n/a
Energy cost (GBP/year)³:	Incl. in heating	-	-
Cost tariff (GBP/kWh)³:	0.0804, 0.0284	0.0804, 0.0284	-

Coefficient of performance (COP)
Heating:	3.16 overall (November-April), 2.5 in summer when predominately for DHW, see section 4
Test conditions:	Seasonal performance factor
Cooling:	-
Test conditions:	-

Heat pump cost breakdown
Heat pump only (GBP):	1 108 including controls
Installation (GBP):	GBP 30 (done by the owner), cold loop GBP 78 (marginal cost estimate for excavator time)
Capital cost (excluding heat pump) (GBP):	662 excluding the underfloor heating system and the hot water cylinder
Maintenance:	No data

CO₂ emissions	The CO2 emissions of the heat pump and alternatives are: Ground-source heat pump: 3 600 kg CO2/year All electric (efficiency 100%): 8 590 kg CO2/year Regular oil-fired boiler, pre-1998 (efficiency 70%): 7 210 kg CO2/year Regular oil-fired boiler, 1998 or later (efficiency 79%): 6 390 kg CO2/year Gas-fired condensing boiler (efficiency 85%): 4 260 kg CO2/year
Assumptions:	Assumed CO2 emission: oil 0.27 kg/kWh; gas 0.194 kg/kWh; electricity 0.46 kg/kWh. For the oil-fired boilers, default values from SAP 1998 are adjusted for underfloor heating.