November 28th 2011

"A ground source heat pump in practice"

       Dr David Wildon
University of East Anglia (rtd).

To a packed audience of over a 100 members and guests, Dr David Wildon opened his talk by informing us that the main motivation why he and his wife Gilly embarked on this heat pump project was energy sustainability. By this he meant that whatever they did by way of energy removal from their local environment, this environment would eventually return to its pre extraction energy state if the removal process were to be halted.

Before looking at the details of their study, David described the nature of the property he and Gilly live in. The basic house is a thatched 1660s cottage constructed using brick and timber frames with sections built using clay lump. 5

The property has had several modifications including a 19th(?) extension that is brick built with a pantile roof and thin concrete floor, a flat roofed brick 1960s built kitchen and utility room also with a thin concrete floor and finally a study and double garage was added in 1986. The latter was constructed with concrete blockwork and had a matching pantile roof and a steel mesh reinforced suspended concrete floor. One of the most important constraints on whatever they wished to do is that the whole structure is grade II listed.

David and Gilly had briefly thought about solar panels but given the 'listing' nature of the property had decided not to go down that route but look at heat pump possibilities. To use this method a reasonably substantial amount of land area is required. After a reasonably quick investigation David and Gilly came to the conclusion that they could easily fit in a heat pump system into their property.

As David explained to his audience a heat pump has four elements; a heat source that expands the working fluid, a compressor whose function is obvious, a condenser that acts as a heat sink by giving up latent heat of condensation to its surroundings and an expansion valve that returns the working fluid to the first stage. David pointed out this is exactly the same set up as a domestic refrigerator; in the case of heating applications instead of trying to remove heat from a fixed small volume and dumping it into an effectively infinite surroundings as is the case in a refrigerator heat was extracted from an effectively infinite source and dumping it into well insulated fixed relatively small volume.

Heat pumps are not new. Indeed a very succesful one was operational at the Duke Street Offices of the local electricity board for little more than a decade from the early 40s; for details see the book by the engineer involved - Domestic Heat Pumps (Prism Press, Dorchester, 1976) by John Sumner.

David conjectured that it was decommissioned because it was too successful in that it didn't use enough electricity! The heat environment for this system was the waters of the nearby River Wensum. David points out that John Sumner's book is a very interesting historical document as it demonstrates that Sumner thought long and hard about fossil fuel sustainability.

Having decided to go ahead with the idea of installing a heat pump based system the Wildons had to decide whether to use a ground source or to use a borehole heat extraction method.


In the case of a ground source system a horizontal array of pipes is normally buried between one and two metres underground whereas in the borehole method heat is exchanged between a vertical tube and its return pipe the local surroundings. However, as David explained, before making a decision about which one to opt for he and Gilly thought it is important to have some idea about what was going on in terms of heat flows and consequential temperature distribution in the Earth's surface layers within the scope of either method.

David then set about describing what he thought was going on. As a professional plant biologist he was well aware that one of the important parameters in plant growth is soil temperature and because of this the Rothamsted Research Centre that specialises in studying plant development had been measuring the soil temperature distribution for many years in the UK. David and Gilly had accessed this data and discovered that, not surprisingly, there was on average a cyclical variation over the year close to the the surface with the peak in the summer months and a trough in the winter. They also found out that the deeper one went in the soil the smaller the cyclical variation became. However the peak in the variation moved to later months in the year. The Wildons also discovered that to all intents and purposes that by a few metres down into the ground in the UK the variation had become very small with the temperature tending to an average of about 10 degC. In terms of heat flow the Wildons estimated, from the data available, that the peak heat influx is roughly between 40 to 80 Watts per sq metre at the height of summer in Norfolk with roughly the same heat outflow in the depths of winter.

David also told us that in there is a geothermal heat flux of about 0.06 Watts per sq metre coming from the Earth's core. Owing to the relative stability of the production of heat within the core this flow is roughly the same all year round. The nett effects of balancing this heat flow with the variable solar heat flow produces the observed average temperature of 10 degC a few metres below the surface with a very slightly positive gradient ( of aboout 0.02 degC per metre) as one moves towards the centre of the earth.

Armed with these basic facts in 2008 David and Gilly consulted with experts in the field ( in particular Dr Keith Tovey of UEA) and with commercial concerns working in this field. With the assistance of an architect they drew up a brief and put it out for consideration by a couple of organisations who advertised expertise in installing heat pumps in domestic properties. On receiving replies from both companys they settled on RenEnergy of Blofield Heath(see the NES talk on 04/10/2011). It transpires that the Wildons had sufficent surface land area attached to their property that either type of heat extraction could be used.

In the end they settled for boreholes rather than ground source because even though the latter was cheaper to install they didn't have sufficient area to install such a system without completely removing a very old and productive vegetable/fruit garden. David, as an expert on plant growth, knew that however careful the installers might be in physically returning the garden material to its initial position the organic micro balance would take many years to recover.

David told his audience that before drilling could commence several important decisions had to be made about how and where the extracted heat was going to be employed in the house. Ideally the Wildons would like to have had underfloor heating throughout but because of the age and structure of the building the they rapidly came to the conclusion that this was not feasible in upstairs. Installing it downstairs would also potentially be difficult because the records of the house, with the exception of the last were vague as the nature of the under flooring construction.

After much thought they decided that they would plan to install underfloor heating in the whole of the ground floor living area including the kitchen area. Installing this type of heating in the kitchen brought with it the need to completely refurbish/modernise that area; something the Wildons had been wanting to do for a little time!

They were also advised that wall/roof insulation needed to be improved particularly in the flat roofed kitchen. Because of the thatched nature of the roof, improving the insulation above bedrooms had to be done carefully because airflow to the main roof space had to be maintained to avoid thatch rot.

Having decided in outline what needed to be done David and Gilly, with the assistance of an architect and REN energy set up a project timeline and then started work on clearing access to the garden for the borehole team and then in early 2009 applied for permission for work to start under the appropriate listed buildings regulations. This having been granted by June 2009 and the potential disruption discussed and debated with neighbours, the borehole team arrived in November to commence drilling.

As David showed us on a couple of slides, even though the final boreholes would have minimal impact on their grassy areas, during the drilling process the whole area was covered with equipment. Of course there was the drilling rig but also all the ancillary equipment that went with it including the power unit for the rig and all the borehole casing and the heat extraction piping to be installed when the drillers had reached the required depth.

To minimise disturbance to the neighbours, particularly noise and vehicular movement on and off the site drilling was confined to normal working hours; the power unit was by far the largest source of noise. Drilling in Norfolk does not come all that easily because of the variation in strata over even a 100m from the surface. Of particular importance are the positions of the chalk layer interface and the water table. Chalk is particularly important because the drilling bore bit has to be reduced in size to prevent bore collapse.

As David illustrated in one of his slides actually detecting chalk was done very simply by examining the outflow from the drilling fluids using a simple kitchen sieve on a pole; much time was spent looking at the contents of the sieve! As the drill bit descended bore casing was inserted to prevent collapse; wider at the top narrowing down within the chalk strata.

Once the required depth had been reached the heat extraction pipes were preassembled with a purpose fabricated U bend at the very bottom. David told us that getting the pipes to the bottom of the bore was not as simple as it appeared on paper. A specially designed weight had to be attached to assist in the process.

Once the pipes were in position the bore hole was back filled with a gravel mix. In the chalk region this filling had to be done carefully and bore casing removed section by section so that voids did not occur between the pipes and their surroundings; if voids occurred this would very much reduce heat exchange to the local environment. Above the aquifer the space between the pipes and the bore sides was filled with bentonite pellets which is a clay like material that swells in the presence of water and forms a good seal preventing surface water draining easily into the aquifer. This is important because the aquifer is the main source of drinking water for the region.

About a metre below the surface the bore-hole pipes were attached to the feeder pipes to the compressor/expansion apparatus that was located in the back wall of the garage. In the event this connection was slightly more complicated than David had expected initially. The reason for this was straightforward in that quite early on the Wildons were advised that it would be a good idea to put in two boreholes of about 90 metres in length rather than one of 180 metres which was the length that had been calculated to be the required to obtain the necessary heat exchange rates to supply their maximum needs. The two boreholes were connected in parallel to ensure maximum heat transfer to the working fluid. In a series connection the second borehole would be less effective because the fluid passing through it from the first would already have picked up some heat so the temperature differences would be smaller in the second borehole hence less heat transfer.

Once the boreholes had been completed and the piping connected building work commenced on the house. Extra insulation was put in the loft space making sure that sufficient draughting was present to maintain the thatching in good condition. 100mm of insulation was put in the kitchen and 19th century extension roof spaces and 100mm of insulation was added externally to the kitchen exterior walls.

Work also started on installing the underfloor heating pipes and connection manifolds. Each under floor pipe is jointless thus minimising the possibility of leaks. The original intention had been to have underfloor heating throughout the whole of the groundfloor but on investigation in one room at least this turned out to be very difficult (and hence expensive!!) to do so quite early on it was decided that this room was going to be heated using low temperature radiators.

However there were some positive paybacks from installing the heating in that in one area the contractors discovered some (unknown to the Wildons) very old flagstones. A sufficent number of these were recovered in good condition to be reused as the main flooring in the kitchen area.

By the end of March 2010 all the building work had been completed and all the basic equipment for the heating system installed including a 3 phase electrical supply. As David indicated the latter was strictly not necessary for the power requirements that their system would need but it gave them a bigger margin for any future needs.

The system was system was then ready for commissioning. The heat exhanger pipes were filled with their working 'brine'; as David pointed out this is a complete 'misnomer' as no salt is allowed anywhere near the pipes. The actual fluid is 35% ethylene glycol in water; basically an antifreeze mixture. As care had been taken to check for leaks during installation the system became operational in the middle of April 2010.

Over the past year David and Gilly have been monitoring the system performance and have discovered that the coefficient of performance, the ratio of heat output to electrical power input is about 3.3; this means that the system is indeed using 3 times less input power to heat their house than they would if they had used the electrical power as the direct heating agent.

David concluded his talk by offering his advice about installing heat pumps. Clearly adequate land is needed for heat extraction and an electrical supply that is adequate for the needs of the sytem must be present. The property needs to be properly insulated and ideally the system put in as part of a new build or perhaps a new barn conversion; retrofitting as they did it is a very expensive business. Indeed his opinion was that as far as their property was concerned they would not get payback in their lifetime. However as far as the Wildons are concerned the important feature is that the system they have is sustainable.

There then followed a long an interesting Q&A session. In answer to one of the questions that was concerned with savings on running costs, stating that although comparisons were difficult to make because of all the extra insulation that had been included in the rebuild, David estimated that the new system was costing about 35% less in cash terms compared to previous years.

After the formal vote of thanks the audience showed their appreciation in the normal manner for the amount of effort, clarity of exposition and excellent presentation skills that David had demonstrated during his talk.

Richard Aldridge           November 2011