Development of an integrated Ground / Air / Solar Heat Pump for plug in retrofit

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The retrofit will be carried out to an occupied house owned by Penwith Housing Association. It addresses the whole house solution by applying a comprehensive package of measures including fabric heat losses, heating and ventilating technology, renewable energy and lighting and appliance usage. It includes an innovative ground / air / solar source heat pump and the HeatPod a small prefabricated multi-purpose glazed porch / conservatory which contains much of the equipment required for reducing the carbon emissions of the dwelling. The complete package of measures builds on Penwith HAs practical knowledge of what can realistically be achieved in social housing homes and is designed to be highly replicable.

Retrofit for the future ZA148E
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Development of an integrated Ground / Air / Solar Heat Pump for plug in retrofit : Project images

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CO2 emissionsPrimary energy requirement
Energy target
Retrofit for the Future

Energy and fuel use

Fuel use by type
Primary energy requirement
CO2 emissions

Measured data from renewable generation is not yet available.

Fuel use

Electricity use 2640 kWh/yr 2737 kWh/yr -
Natural gas use- - -
Oil use- - -
LPG use- - -
Wood use- - -
wood pellets (secondary heating) 22946 kWh/yr - -
Primary energy requirement 498 kWh/m².yr 107 kWh/m².yr -
Annual CO₂ emissions 38 kg CO₂/m².yr 25 kg CO₂/m².yr -
Annual space heat demand - 64.6 kWh/m².yr -

Renewable energy

Electricity generationForecastMeasured
Direct coupled 12v PV160 kWh/yr -
Other Renewables Tech--
Electricity consumed by generation --
Primary energy requirement
offset by renewable generation
101 kWh/m².yr -
Annual CO₂ emissions
offset by renewable generation
24 kg CO₂/m².yr -

Calculation and targets

Whole house energy calculation method OTHER
Other whole house calculation methodSAP2005 and SAP Extension supplemented by full load duration analysis as typified by BS EN15316-2-4 methodology. Final design h
Energy target Retrofit for the Future
Other energy targets-
Forecast heating load 33.15 W/m² demand


Pre-development air permeability test23 March 20105.57m³/m².hr @ 50 Pascals
Final air permeability test--

Project description

Start date05 April 2010
Occupation date
Location Sennen Cornwall  England
Build typeRefurbishment
Building sectorPublic Residential
Property typeEnd Terrace
Construction typeMasonry Cavity
Other construction typeThe 50mm cavity has been filled with blown mineral wool
Party wall constructionMasonry cavity wall
Floor area 64
Floor area calculation method Actual Floor Area (SAP)
Building certification

Project Team

OrganisationPenwith Housing Association Ltd
Project lead personDenys Stephens BA(Hons) MCIAT CEnv, Sustainability Manager, Penwith HA
Landlord or ClientPenwith Housing Association
ArchitectLead Designer (not architects) - Penwith HA Property Services Department
Mechanical & electrical consultant John Parker B.Eng C.Eng MCIBSE FRSA, Earth Energy Engineering Ltd.
Energy consultantJohn Parker B.Eng C.Eng MCIBSE FRSA, Earth Energy Engineering Ltd.
Structural engineerMassie,Ludnow & Jenkins.
Quantity surveyorPenwith HA Property Services Department
ConsultantMonitoring: Gerry Hargreaves
ContractorMears Ltd.

Design strategies

Planned occupancyThe house is occupied by a married couple and their grown up daughter. It is expected that they will remain in occupation throughout the retrofit work and the project has been designed to make this possible.
Space heating strategyHigh efficiency ground source heat pump serving low temperature (45/40oC) high thermal mass two-pipe radiators. Control by progammable timeclock and zone thermostat. TRVs on bedrooms and kitchen only.
Water heating strategyHigh efficiency ground source heat pump serving primaries of indirect mains pressure factory insulated hot water cylinder. Timeclock controlled charging periods and storage temperature thermostat set point at 60oC. Electric immersion heater fitted for emergency use only.
Fuel strategyThis project uses mains electricity as its main fuel with some further electricity from a PV system. Key drivers for the use of electricity are the absence of main gas in this rural location and the cost and carbon consequences of the fossil fuel alternatives oil and coal. Penwith HA has previously demonstrated in its pioneering work with ground source heat pumps that they can provide very cost effective space and water heating with very low CO2 emissions in off gas areas.
Renewable energy strategyDespite objections from some purists it is now widely accepted (including by government agencies) that ground source heat pumps are renewable energy systems. The key renewable component is the solar energy retrieved from the ground by the ground loop. The key feature of this project is it's innovative ground / air / solar source heat pump which achieves a 25% improvement on the performance of traditional GSHPs. The design also includes a 0.25kWp PV array for direct coupling to 12v DC fans and pumps only.
Passive Solar strategyHeatPod solar porch tempers incoming fresh air supply for mech. vent system. Balancing mechanical extract recovers solar gain using coil in low temp. ground loop circuit to supplement stored solar energy in ground .
Space cooling strategyMechanical vent system can be boosted to increase summer time flow rate, otherwise windows are openable.
Daylighting strategyThis project does not propose to alter the existing house in this respect. It is planned to replace the existing windows with new triple glazed high performance units, but these will retain patterns similar to the existing windows to avoid radical change in the appearance of the property. Owing to the relatively small size of the house it is well served by traditional windows for daylighting purposes.
Ventilation strategyControlled balanced supply and extract provides fresh air ventilation .
Airtightness strategy The project includes provision for a programme of pre and post retrofit pressure testing. From past experience of pressure testing Penwith HA properties of this era it has been found that their air tightness is better than might be expected owing to the original use of render and plaster finishes to concrete floors and block walls. The retrofit's inclusion of external wall insulation and high performance windows and doors (and careful sealing of the joints between these elements) is expected to significantly improve air tightness of the main envelope. It is intended to enhance air tightness between walls and first floor ceilings by the addition of sealed coving. There will then be further joint sealing carried out to any areas of weakness revealed by the first post-retrofit pressure test. By these means it is planned to reduce uncontrolled ventilation losses to 3 ACPH.
Strategy for minimising thermal bridges There are three main areas of weakness in terms of thermal bridging: Window and door openings, the solid concrete ground floor and the Finlock gutters. (These consist of a pre-cast concrete cavity closer with an integral concrete gutter. They were widely used in the 1950s). In the retrofit the window and door opening detail will be changed so that window & door frames span the junction between the external insulation and external wall, eliminating cold bridging at that point. To reduce heat losses from the ground floor the external wall insulation will extend 600mm below dpc level. The thermal bridging at the Finlock gutters will be overcome by fixing a layer of insulation board with a coving internally at first floor ceiling level. With careful detailing and matching decorations this will not be intrusive and will avoid radical alteration of the Finlock gutters.
Modelling strategyLoad and energy modelling carried out by NHER/SAP2005 and SAP Extension plus various load duration analysis techniques (ESIBEEP and EN15316-2-4:2008) to deliver full multiple source energy matching as well as the monthly energy summaries necessary for any standard gshp design.
Insulation strategyPenwith HA has very substantial practical experience of external wall insulation which has been used very effectively on approximately 1000 houses in its stock over a 25 year period. It is very effective in enclosing the thermal mass of a dwelling, tempering the fluctuation of internal temperatures, and when used with replacement windows and doors it significantly reduces air leakage. The innovations here are in applying external insulation to a previously cavity filled wall, doubling the traditional thickness of insulation and use of the system below ground to reduce heat losses from the solid ground floor. The system will include 100mm thick phenolic board external wall and ground floor edge insulation. The wall insulation will be complimented by high performance triple glazed windows and doors. This package will be completed with 400mm loft insulation and air pressure testing and and sealing areas of uncontrolled ventilation.
Other relevant retrofit strategiesIt is an important feature of this project that it is designed to be applied to an occupied dwelling. It builds on Penwith HAs long experience of refurbishing and retrofitting existing homes and the knowledge that most tenants can cope with work to the outside of their home. Hence the external wall insulation, window & door replacement and the design of the HeatPod placing M&E equipment outside the dwelling. Although there will be internal work, it is kept to a minimum. This approach presents a much more achievable retrofit for large scale roll out than designs which require substantial internal alteration.
Contextual informationThis project is located in rural Cornwall only a few miles from Lands End, the most Westerly point in England. Despite being in an area of outstanding natural beauty, low incomes, unemployment and fuel poverty are common. The site of this project, in common with much of Penwith, does not have mains gas and can experience very severe weather in winter. The retrofit solution for this project, whilst entirely appropriate for replication in less demanding areas, is particularly suited to this type of environment.

Building services

Occupancy3 adults
Space heatingPrototype ground / air / solar source heat pump. The system utilises a Calorex ground source heat pump combined with a NIBE ventilation & heat recovery unit.. The gshp is connected to a 60m vertical ground loop installed in the rear garden. The heating system provides space heating via a wet radiator system.
Hot waterMains pressure hot water cylinder heated by the main central heating system and providing all domestic hot water.
VentilationWhole house ventilation system using a NIBE FLM ventilation & heat recovery unit. The system provides passive air supply to habitable rooms with mechanical extraction from the kitchen and bathroom.
ControlsStandard 2 channel programmer for the space heating and hot water system.
CookingBuilt in induction hob and electric oven.
LightingCFLs fitted throughout the house.
AppliancesA++ rated fridge freezer & washing machine and LED television all supplied by Currys.
Renewable energy generation system1.15 kW roof mounted Solar PV system and Ground / Air / Solar Source heat pump
Strategy for minimising thermal bridges100mm external wall insulation carefully detailed in relation to window & door openings to minimise thermal bridging. External floor edge insulation provided below ground level to reduce heat loss from concrete ground floor. Concrete 'Finlock' gutters insulated internally.

Building construction

Storeys 2
Volume 164.35
Thermal fabric area 148
Roof description Traditional 'cut' timber roof finished with concrete plain tiles insulated internally at ceiling level with 400mm mineral wool. Roof void ventilated with previously retrofitted tile vents.
Roof U-value 0.13 W/m² K
Walls description Traditional cavity wall with two leaves of 100mm concrete blockwork and 50mm mineral wool filled cavity plastered internally and rendered externally. Retrofitted under this project with Knauf Marmorit Slimtherm 100mm external wall insulation with self coloured textured render finish.
Walls U-value 0.15 W/m² K
Party walls description Traditional cavity wall with two leaves of 100mm concrete blockwork and 50mm cavity plastered on both sides.
Party walls U-value 0.00 W/m² K
Floor description Existing concrete floor slab with t&g boarded finish and 100mm EPS floor edge insulation 500mm deep below dpc level.
Floor U-value 0.54 W/m² K
Glazed doors description Main entrance doors (front & rear) Homesafe Regency range 'Amersham' GRP composite doors with ECO Packs 1 & 2. Note: these doors have small double glazed panels.
Glazed doors U-value 1.10 W/m² K installed
Opaque doors description None fitted.
Opaque doors U-value 0.00 W/m² K -
Windows description JELD-WEN 'Dreamvu' softwood triple glazed windows with JELD-WEN colour coated aluminium sub-sills.
Windows U-value 0.70 W/m² K -
Windows energy transmittance (G-value) -
Windows light transmittance -
Rooflights description None installed
Rooflights light transmittance -
Rooflights U-value 0.00 W/m² K