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Charlesworth, S.

Coupe, S.

Faraj-Lloyd, A.

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NOVATECH 2016 
1 
Combining Sustainable Drainage to manage 
stormwater with renewable heat from the ground; 
monitoring of the Hanson Ecohouse, Watford, UK 
Combiner la gestion durable des eaux pluviales et 
l'utilisation de la chaleur du sol ; suivi de la maison 
écologique Hanson, Watford, UK 
 
Charlesworth S.M.*, A. Faraj-Lloyd and S. Coupe 
 
Centre for Agroecology, Water and Resilience, Coventry University, CV1 5FB, 
UK  
* corresponding author, email: apx119@coventry.ac.uk; Tel: +442476887688 
 
RÉSUMÉ 
La recherche de la durabilité implique de considérer des technologies à bénéfices multiples et flexibles 
dans leur application. Les Systèmes de Drainage Durable remplissent ces critères et cet article traite 
de la possibilité de combiner le pavage perméable à réservoir permettant la récupération des eaux de 
pluie et un système de géothermie superficielle pour chauffer la Maison Ecologique Hanson, à 
Watford (GB). Ces deux techniques ne sont pas nouvelles, mais leur association est relativement 
récente. Le suivi des températures dans la maison, dans le réservoir du pavage perméable et dans 
l’air ambiant a montré que l’eau drainée gelait à certaines périodes, que l’air ambiant avait un impact 
sur les températures jusqu’en profondeur du réservoir et que la récupération de la chaleur menait à un 
déficit thermique qui perdurait jusqu’à la saison suivante, et que ce déficit pouvait s’accumuler dans le 
temps. Cependant, nos résultats indiquent que le système de pavage perméable a permis une gestion 
efficace des eaux de pluie, sans observation d’inondation, et que la qualité de l’eau était excellente. 
Les leçons tirées de cette étude innovante recommandent de restituer de la chaleur au sol pendant 
les mois les plus chauds et de construire le réservoir du pavage perméable plus profond. Ce système 
combiné a depuis été installé dans un bâtiment de bureau de 3 étages, dont le suivi est en cours. 
ABSTRACT 
Achieving sustainability requires multiple benefits and flexibility in application. Sustainable Drainage 
Systems have such credentials and this paper reports on the potential for a combination of tanked 
pervious paving and rainwater harvesting with ground source heat to heat the Hanson Ecohouse, 
Watford, UK. These techniques are not new, but the blend of energy and water is relatively recent. 
Monitoring temperatures in the house, the porous paving  tank and ambient air found that, at times, 
the harvested water froze, ambient air influenced temperatures even at depth in the tank and there 
were indications that harvesting heat led to a heat deficit carried over into the following season, with 
the possibility of this accumulating with time. However, it was found that the porous paving performed 
efficiently in terms of water management, with no flooding reported, and water quality was excellent. 
Lessons learnt from this first-of-its-kind study included recommendations to replace harvested heat in 
the warmer months and construct a deeper porous paving tank. The combined system has since been 
installed into a 3-floor office development, monitoring for which is ongoing. 
KEYWORDS 
Sustainable Drainage Systems, Ground Source Heat, Rainwater Harvesting, Pervious Paving 
SESSION 
2 
1 INTRODUCTION 
Achieving sustainability needs there to be multiple benefits from the technology used and also 
flexibility in its application. Whilst neither the harvesting of heat from the ground (Ground Source Heat, 
GSH) nor Sustainable Drainage systems (SUDs) are new, both having been used successfully for 
decades (Coupe et al., 2013), the blending of these 2 techniques to provide heat to domestic, or 
industrial buildings, is fairly new and innovative (Charlesworth et al., 2016). They would also provide 
the multiple benefits associated with both of their technologies in providing a clean and renewable 
source of heat, flood resilience and water quality improvements in one combined system. Laboratory-
scale testing has been undertaken by Tota-Maharaj et al., 2010, which showed the potential of this 
blend, but a true insight into its performance could only be gained by monitoring at the building scale. 
This paper highlights some of the lessons learnt from a 3-year study of combining GSH extraction and 
a tanked, or sealed, block paver Pervious Paving System (PPS) at the Hanson Ecohouse, Watford, 
UK, and a critical appraisal of its potential to provide heat to a domestic dwelling. 
2 METHODOLOGY 
Built on the Building Research Establishment (BRE) site at Watford, UK, the Hanson Ecohouse is a 3-
bedroom, detached, domestic dwelling completed in 2007 to demonstrate innovative construction 
techniques including the combined PPS/GSH extraction system. As such, therefore, it was not a true 
domestic dwelling since a family did not live there; the environment was therefore a challenging one to 
test new technologies in which groups of visitors were shown around the house with attendant 
problems associated with doors and windows being left open, equipment tampered with, including the 
thermostat and also siting of the building having to fit with other demonstrators. The PPS was sealed 
with an impervious membrane to provide a “tank” in which rainwater was allowed to collect. At 350mm, 
it was about 150 mm shallower than usual, with the coils for GSH extraction located at the bottom of 
the tank. Further details of the Ecohouse, the PPS/GSH combined system and the monitoring carried 
out can be found in Charlesworth et al., 2015. Whilst the combined system could potentially be used 
for both heating and cooling, only the former setting was used since it was considered that under the 
temperate UK climate it was likely that it would mainly be used for heating, unlike elsewhere where the 
ground may be used for both heat extraction (space heating) and rejection (space cooling). It was also 
an aim of this study to monitor the impacts on the ground temperatures of harvesting heat over a 3-
year period since previous studies found that if the heat absorbed from the ground annually is not 
equal to the heat rejected to it, reduction in the ground heat may result over time (Xi et al., 2011; Ooka 
et al., 2011; Yu et al., 2011). 
2.1 Temperature monitoring  
Temperature was monitored at 10-minute intervals over almost three years, at the 4 cardinal points 
around the house, both inside and out providing >1.3 million data points. For the combined PPS/GSH 
extraction system, temperature was measured at various depths in the tank: 60mm, 130mm, 200mm 
and 350mm, from 8th August, 2008 to 30th March, 2010. The ambient or overlying air temperature 
during this period was also measured at 1300mm above the surface of the PPS. There were a total of 
15 thermistors installed in and around the house. Further specific details of the instrumentation are 
given in Charlesworth et al., 2015. 
2.2 Water quality and incidence of flooding 
Water in the PPS subbase was collected for 2 years at 2-3 weekly intervals, yielding a total of 36 
samples. These were analysed for Zn, Cd, Cu, Ni, P, Na and Pb using ICP-AES. Incidence of flooding 
was by observation of the staff at the BRE site over the 3 year study. 
3 RESULTS AND DISCUSSION 
3.1 Temperature in the PPS and ambient air monitoring 
Descriptive statistics for the temperatures of the ambient air and four PPS tank depths are given in 
Table 1 which shows that the minimum temperatures at all depths in the tank were below zero (i.e. 
any stored rainwater was probably frozen). During very cold conditions, it was found that the surface 
of the rainwater in the reservoir froze, whilst at the bottom the harvested rainwater remained liquid. 
This finding was in contrast to studies concluding that freezing started around the extractor coils, e.g. 
Eslami-nejad and Bernier (2012). These different observations are probably related to the medium 
NOVATECH 2016 
3 
surrounding the coil, since it was surrounded by rainwater not soil; therefore during the freezing 
process, ice forms on the surface of a body of water, in contrast with the coil buried in soil where 
freezing of the ground begins around the GSH extractors. The highest maximum and lowest minimum 
temperatures were recorded at 60mm in the PPS tank; these characteristics reduced with reservoir 
depth. Table 1 shows that there was very little difference between mean temperatures recorded for 
each depth and that they were not significantly different from that of the ambient air.  
Depth (mm) Ambient  60mm 130mm 200mm 350mm 
Minimum   -3  -4.4  -3.1  -1.9  -1.1 
Maximum 22.5 26.2 24.7 21.2 20.0 
Average 10.0  9.7  9.5  8.8  9.6 
Median  10.8  9.5  9.2  9.1 11.0 
Standard Deviation   6.0  6.9  6.9  6.6  6.7 
Significant difference between depths 
and ambient air (p<0.001). 
  3.931  4.718  8.074 10.541 
Table 1 Ambient air and PPS tank temperatures (°C) throughout the monitoring period (n = 2,449) 
Table 2 shows the mean annual monthly temperature characteristics of each PPS depth, together with 
those for the ambient air; temperature differences ranged between 0.8°C and 7.4°C. Table 2 also 
shows how closely the temperatures in the tank follow those of ambient air, and hence the influence 
that it had on thermal distribution in the PPS reservoir. 
Depth  60 mm 130 mm 200 mm 350 mm ambient 
Jan 1.8 2.2 2.6 1.8 2.4 
Feb 1.8 0.6 0.1 -0.3 3.8 
Mar 5.2 3.5 0.3 0.4 7.7 
Apr 10 8.8 6.5 6.1 10.8 
May 14.9 14.6 13 12.4 13.6 
Jun 18.6 18.3 16.4 15.7 16.3 
Jul 19.2 19.1 17.9 17.5 17.4 
Aug 18 17.9 17.1 16.8 17.5 
Sep 14.5 14.6 14.4 14.5 14.3 
Oct 10.4 9.4 9.4 9.7 11.6 
Nov 8.3 8.6 9 9.5 9.1 
Dec 3.7 4.8 5.3 4.9 4.1 
Table 2 Mean monthly temperature measurements (oC) of the ground at several depths and the 
ambient air throughout the monitoring period (n = 2,449) 
During the period 27/01/2009 – 30/04/2009, heat was extracted more or less continuously. It was 
found that the temperature at the bottom of the reservoir was lower than both ambient air and the 
ground at 60mm and 130mm depth by an average of 5.5, 3.6, and 2.3°C respectively. The low 
temperature at 200mm and 350mm is, apart from the influence of the cold ambient air, most likely 
because of the GSH extracting coil removing heat (Gonzalez et al., 2012). In fact, Wu et al. (2010) 
concluded that the effect of heat extraction could be observed up to a distance of 900mm from the 
heat exchanger, and 250mm below the ground surface. The total depth of the PPS at the Ecohouse 
was 350mm, thus extraction of heat was likely to have contributed to the dropping of ground 
temperature - predominantly at the bottom of the reservoir but also in the top layers. Hence, extracting 
heat continuously from the ground could have contributed to an underground temperature drop. Also, 
given Britain’s climate and environmental conditions it is likely that if temperatures near the GHS 
extractors decreased over time, the ground may not have been able to completely recharge for the 
following season during the summer; hence this could lead to a reduction in GSH performance over 
time. At 350mm, the PPS was shallow, with the stored rainwater therefore closer to the surface than is 
usual and overly influenced by ambient air temperatures. Other studies, eg Singh et al., 2010 and Gan 
et al., 2007 used horizontal heat extractors, but these were buried in soil 0.5 and 1.8 m deep 
respectively, leading to less influence of ambient conditions.  
SESSION 
4 
3.2 Efficiency of the PPS: water quality and quantity 
Concentrations of Cd, Cu, Ni, P and Pb in the PPS tank were below the limits of detection, whilst the 
mean concentrations of Zn and Na were 2.01 and 7.69 mg -1 respectively and were thus of no 
concern. There was no reported incidence of flooding associated with the Ecohouse PPS, and thus 
the combination of GSH extraction with the PPS did not appear to have compromised the performance 
of the pavement from the water quality or quantity point of view.  
4. CONCLUSIONS 
It was found that temperatures in the bottom of the PPS reservoir were less than both the layers 
above and also ambient, overlying air. This could be due to two factors: firstly, by not replacing the 
heat extracted during the colder months by heat in the warmer ones, the temperatures in the bottom of 
the tanked PPS could gradually cool such that ground temperature at 60mm and 130mm were higher 
than those at depth since the top of the reservoir was less affected by the heat being harvested. By 
submerging the extraction coils in rainwater, harvesting of heat is more efficient, and therefore the 
effect of cooling at depth may have been more than in studies where the coils were buried in soil. It is 
therefore recommended that even in a temperate climate such as the UK, if a combined system is 
used during the warmer months heat is rejected into the ground in a cooling cycle in order to replenish 
the heat at depth.  
Secondly, temperatures with depth in the PPS reservoir were influenced by those of the overlying 
ambient air. This was likely due to the fact that the PPS tank was quite shallow and the coils were 
installed at a depth of 350mm. This was due to site conditions at BRE, but in future it would be 
recommended that the tank be at least 500mm in depth in order to reduce any effects of external 
environmental conditions. However, it is not possible to estimate the contribution of either of these 
factors to the heat loss found. 
Both of these recommendations have been carried out in a new build office block at Stewartby, 
Bedford, UK whereby the PPS tank was excavated to 500mm and the cooling cycle engaged during 
the summer months. Whilst monitoring of this system is on-going, as the only source of heat to the 
building, the combined system has been very successful. 
REFERENCES 
Charlesworth, S.M., Faraj-Lloyd, A.S., Coupe, S. (2015) Renewable energy and sustainable surface runoff 
management: combining Ground Source Heat and Porous Paving to heat buildings. Proceedings of Water 
Efficiency Conference, Exeter University, UK. Pp 206-215. Available at: http://tinyurl.com/nufkbnf (accessed 
14 October 2015).  
Charlesworth, S. A. Faraj-Lloyd, S. Coupe (2016) Combining sustainable drainage devices and sources of 
renewable heat: pervious paving and ground source heat. Renewable & Sustainable Energy Reviews. DOI: 
10.1016/j.rser.2016.02.019 
Coupe S.J. AS Faraj, EO Nnadi, SM Charlesworth (2013) Integrated Sustainable Urban Drainage Systems Book 
title: Water Efficiency in Buildings: Theory and Practice. K. Adeyeye. Wiley Blackwell Chapter 9, 147-163 
Eslami-nejad, P., Bernier, M. (2012). Freezing of geothermal borehole surroundings: A numerical and 
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Wu, Y., Gan, G., Verhoef, A., Vidale, P.L., Gonzalez, R.G. (2010). Experimental measurement and numerical 
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Xi, C., Lin, L., Hongxing, Y. (2011). Long term operation of a solar assisted ground coupled heat pump system for 
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Combiner la gestion durable des eaux pluviales et l'utilisation de la chaleur du sol ; suivi de la maison écologique Hanson, Watford, UK

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Combiner la gestion durable des eaux pluviales et l'utilisation de la chaleur du sol ; suivi de la maison écologique Hanson, Watford, UK

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