Low C Tech
How we slashed household heating & hot water energy use by 73.5%

Borehole Drilling rig

My retired parents were planning to replace the 25 year old gas fired boiler back in September 2010 but I convinced them to consider installing a heat pump and a Solar PV (electricity) system instead.

The average UK temperature during the winter months is around 7C which is easily within the operational limits of air source and ground source heat pumps. Most modern heat pumps provide decent levels of heat output down to -20C

The thinking was that the PV system would provide the heat pump with around 40% of its electricity needs all CO2 free (after 2 years of PV operation to offset the embedded carbon in the manufacture of the panels of course) and the green electricity tariff would provide the remainder.

Once I made them aware of the Solar PV Feed in tariff (FIT) and the April 2011 launch of the Renewable Heat incentive (RHI) the decision became a bit of a no brainer especially after the RHI apparently survived the UK Coalition Governments October 2010 spending review unchanged. The FIT and RHI pay households for each unit of renewable electricity and heat they produce.

The contract was signed in early November 2010 and the 3.15 kWp Solar PV system was installed by Ice Energy in about 7 hours on the 14th of December 2010. In early January the gas hob and oven were replaced with an induction hob and an electric fan oven.

The 2 x 60 metre boreholes were drilled & the ground loop inserted in early February 2011 by Geocore over a period of about 2 weeks.

The Ice Energy supplied Swedish 7 kW IVT Ground Source Heat Pump (GSHP) was finally commissioned and connected to the existing heating system in late March / early April 2011 by H2O Renewable Energy Solutions. The boiler was removed and the house was disconnected from the natural gas grid on the same day. As a result my parents small 1979 4 bed detached (149 m2) self built property became a zero local CO2 and air pollution (aka no solid biomass) home.

Running Costs & Energy consumption comparison.

Approximately 12 months on from the date of commissioning (April 2012) the GSHP had consumed 6672 kWh (units) of electricity. The annual electricity consumption of the GSHP had been predicted to be around 5634 kWh for Space Heating & 1245 kWh for hot water production, bringing the total to 6879 kWh. Thus the GSHP installation managed to beat the prediction by 207 kWh or 3.1%.

The properties estimated average annual heat energy requirement is 21170 kWh which breaks down into a space heating figure (heat required to heat the house) of 17321 kWh & a hot water figure of 3849 kWh.

Thus the GSHP coefficient of performance (COP) can be worked out by 21170 / 6672 = COP of 3.17. This means for every 1 unit of electricity consumed the GSHP produces 3.17 units of usable heat energy extracted from the ground via the boreholes to provide space heating & hot water.

Using the Scottish Power “Green energy H2O” electricity tariff rate of 11.73p per kWh x 6672 kWh =  £783 a year to run the GSHP.

The old gas boiler rated at 55000 BTU (16.1 kW) consumed about 25200 kWh ( Annual Gas consumption of 28000 kWh – 10% for Gas cooking) which would cost about £927 a year based on a recent uSwitch search for gas only prices. So that’s a saving of £144 a year over the old boiler.

Now if we do a rough estimate for a condensing boiler. The old 1985 boiler was about 65% efficient and a typical modern condensing boiler is about 85%. Thus a 20% reduction in gas consumption from 25200 down to 20160 kWh, costing £762 a year. Thus a typical condensing boiler would cost £21 a year less.

If we look at the actual energy used to heat & provide hot water for the property. The GSHP consumes 6672 kWh which is 73.5% less kWh than the old gas boiler and 66.9% less kWh than a typical condensing gas boiler!

However a proportion of the electricity consumed by the GSHP will have been provided by the Solar PV system. This is a tricky area for me to estimate due to not having an export meter installed, the fact that GSHP uses the most energy during the winter months & the Solar PV outputs most of the low carbon electricity from Spring to Autumn (Fall). If you check out the graphs below you will gain a better understanding of what I mean.

The white lines are last years figures.

Note the Solar PV weekly credit figure is revenue from the Feed in Tariff for generating the electricity. It does not include any money saved on the electricity bill by reducing the amount of electricity imported from the grid at 11.73p per kWh (unit)

A rough estimate would be to take the weekly Solar PV generation figure of 51 kWh x 52 weeks = 2652 kWh per yeah and assume 30% of that figure is exported to the grid rather than the more usual 50% for a property with gas central heating.

Therefore a guesstimate of 1856 kWh of Solar PV supplied electricity was consumed by the GSHP or 27.8% of the annual electricity consumed. 

Thus 11.73p x 1856 = £218 saving on the electricity bill. Reducing the annual GSHP bill down to £783 - £218 = £565 per year 

Thus that’s a saving of £363 & £197 per year respectively compared to the old boiler and a typical condensing boiler when the Solar PV electricity is included.

If I also include the energy produced by PV system that was consumed by the GSHP and subtract that from the total consumed by the GSHP:- 6672 – 1856 = 4816 kWh imported from the electricity grid.

Therefore the GSHP + PV system have reduced the amount of imported energy by (100-(4816/25200 x 100)) = 80.89% compared to the old gas boiler or (100-(4816/20160 x 100)) = 76.1% compare to a a typical condensing boiler.

CO2 emission comparison.

If we look at the CO2 savings from the GSHP using the carbon trusts conversion factors we get

Old boiler 25200 x 0.1836 = 4626.72 kg CO2e

Condensing boiler 20160 x 0.1836 = 3701.38 kg CO2e

GSHP 6672 x 0.5246 = 3500.13 kg CO2e

A CO2 reduction of 1126.59 kg or 24.35% and 201.25 Kg or 5.43% CO2e respectively assuming non green grid electricity.

If we add in the 2652 kWh of Solar PV generated electricity using offset method as most kWh generated in summer least in winter then the GSHP CO2 output drops to 6672 – 2652 = 4020 x 0.5246 = 2108.89 kg CO2e per year

A saving of 2517.83 kg or 2.517 Metric tons or 54.42% and 1592.49 kg or 1.592 metric tons or 43% over the old boiler and a typical condensing boiler respectively. 

Finally if we assume that a green electricity tariff does actually deliver zero carbon electricity (offset of course) then the CO2 emissions of the heat pump drops to zero.

Future CO2 emissions

The UK has a target to decarbonise electricity generation by 2030 and thus as the carbon intensity per kWh decreases over the coming years to meet the UK climate change targets so will the emissions attributed to the operation of the heat pump.

The same can’t be said of a Gas boiler. I doubt biogas production could scale to provide gas for the UK’s 26 million domestic properties.

"Emissions from the power sector need to be reduced by around 40% by 2020, and by around 90% by 2030. 

This will require the carbon-intensity of the electricity we use to fall from around 500 gCO2/kWh today to around 50 gCO2/kWh in 2030.”  UK Committee on Climate Change. 

This would lower the heat pump emissions by 2030 to 6672 x 0.05 = 333.6 kg CO2e a year without Solar PV or 6672 – 2652 = 4020 x 0.05 = 201 kg CO2e a year with Solar PV.

That equates to a reduction in CO2 emissions of 92.8% & 91% without solar or 95.7% & 94.6% with Solar PV for the old gas boiler and a typical condensing boiler respectively by 2030.

Future potential energy / cost savings & heating system overview.

I suspect that the GSHP consumption is higher than it could be due to the fact that it was set to a very high slope setting of around 9.9 out of 10 for a few months to enable the generation of higher than normal flow temperatures to activate the prototype Aquair central blown warm air heater. Note the sharp reduction in the weekly GSHP consumption graph (above) in late September after a new control board was fitted to the Aquair unit allowing the GSHP slope setting to be reduced to 6.2.

The next several paragraphs provide a give a quick overview of how the GSHP works with the properties heating circuit / system and a brief explanation of slope settings.

The heating system that the GSHP is connected to comprises of 3 low temperature radiators ( bathroom, WC & Kitchen), 1 conservatory underfloor heating circuit and 1 water to air heat exchanger in the loft. The water to air heat exchanger provides warm blown air heating via ducting to all the rooms except the bathroom, WC & Kitchen of the house.

The GSHP modulates the heating circuit return temperature in reference to the outside air temperature and a predetermined GSHP slope setting. Thus in our heating system with a slope setting of 6.2 the heating circuit return temperature usually varies between 23 to 43C and switches to hot water only mode when the outside air temperature is 18C or above.

After several trips to different areas of the USA including visiting my American brother in laws in-laws, it seems central blown / forced air heating / cooling is the norm. In Europe however radiators seem to be the preference.

Blown air heating had a small domestic following in the UK in the 1970s and hence the reason why my father installed the system when he built the house in the late 70s.

The original 1979 era water to air heat exchanger blower unit was a Hi-Vee Heating Ltd 50,000 BTU (14.65 kW) high flow temperature (60 - 80C) design which I suspected would not produce enough heat output with the lower flow temperatures generated by the GSHP.

Old water to air heat exchanger 

Therefore I contacted Johnson & Starley the UKs leading provider of blown warm air systems. J&S unfortunately did not have any low temperature units in production but the R&D department did have a low temp prototype. Several emails, phone calls and one meeting later J&S agreed to supply and retrofit the prototype low temp Aquair 14kW unit free of charge in exchange for feedback on the performance of the unit. 

The prototype unit was installed on the 23rd of March 2011 before the commissioning of the GSHP to allow the unit to operate with the old boiler for comparison purposes. The Aquair prototype worked very well outperforming the old unit by some margin.

However once the old boiler was replaced with the GSHP in early April the Aquair unit’s fan failed to operate. It transpired that the minimum flow temp required to activate the units fan motor had been hard coded to 40C. Thus the GSHP had to be turned up to almost maximum to allow the Aquair unit to operate. A replacement control board was finally fitted by J&S in late September lowering the minimum fan activation flow temp down to 23C and thus reducing the consumption of the GSHP quite significantly.

Insulation levels

The GSHP consumption could be reduce further by increasing the properties insulation levels and hunting down and sealing any air leaks / drafts in the properties envelope.

The original 1979 wooden framed double glazing was replaced with uPVC units around 1993 and thus are well below (2x) the levels of insulation provided by modern A rated double glazing, especially considering some of the double glazing units seals have broken. A good short article covering the merits of modern double and triple glazing can be found here. Triple glazing with a u-value of 0.8 is definitely something that may be installed at a later date as funds permit.

The property had retrofit cavity wall insulation installed about 12 years ago which slashed the gas heating bill by one third at the time. In September 2010 we increased the loft insulation levels from 90mm to 270mm of mineral fibre. We also re-board the storage areas in the loft with B&Q insulated boards to maintain a continuous 270mm equivalent insulation level throughout the loft. 

Reducing equipment costs.

So lets get straight to the point, heat pumps are currently expensive to buy and install compared to a conventional condensing gas boiler.

However a Solar PV system now costs in May 2012 almost 50% of what it did back in October 2010 due to economies of scale (thanks to the Feed in Tariff), competition amongst Solar PV companies and the introduction of cheap Chinese produced panels.

If PV costs keep falling in-line with projections then Solar PV electricity will cost the same per kWh (unit of electricity) to buy / produce in the UK as that from a standard brown electricity supplier by 2016-2019. This point is called grid parity.

Hopefully when / if the domestic Renewable Heat Incentive is finally introduced in the summer of 2013 the same economies of scale and competition amongst companies / installers will drive down the cost of an installed heat pump.

The domestic RHI was delayed by the UK Government from April 2011 to October 2012 and was recently pushed back further to Summer 2013. There is also some doubt as to whether properties connected to the gas grid will now be eligible for the scheme. This of course has rather annoyed my father to put it mildly. At 71 he now may not live long enough to recoup the additional cost of the ground source heat pump from the reduction in energy bills alone.

It seems strange to me now that people can quite easily spend £10,000 to £20,000 on a new kitchen or bathroom yet they reject the idea of spending more than a couple of thousand on a new heating system especially considering it is the one item of equipment in a property that uses the most energy and subsequently generates the highest bills.

GSHP + PV System installation costs as of Q4 2010 - Q1 2011.

The installation cost of ground source heat pump would have been about £2000 to £3000 lower if we had not needed to use boreholes. However as can been seen in the previous pictures the property does not have a large enough front garden for the long trenches required to install slinky type ground loops.

As we purchased a Solar PV system & ground source heat pump from Ice Energy we managed to secure a 15% discount off their standard prices at the time.

All prices include VAT @ 5%

Ice Energy

3.15 kWp Solar PV system 14 x 225 Watt panels £12921.62 installed. 

IVT Greenline HT+ C7 Ground Source Heat Pump (Single Phase) + 2x 60 metre 32mm borehole loop + 120 litre buffer tank. Design, supply, commissioning and support of specified heat pump system £7027.55


Drilling of 2 x 60 metre x 150mm boreholes, insertion of the borehole loop, grouting up of boreholes and borehole loop pressure testing £5239.50

H2O renewable energy solutions

Installation of GSHP in garage, additional ground work / trenches to connect borehole loops to manifold chamber with single flow and return to garage. Connection of GSHP to heating system & hot water circuit. Supply + fit 2 x low temp radiators £3627.75

Solar PV system = £12921.62

GSHP Total = £15894.80 

Ice Energy administered CERT GSHP grant £1250

Total system cost £27566.42

As mentioned previously the cost of a similar sized PV system today in May 2012 has dropped by about 50%. Thus that would bring the price of the GSHP & PV system down to about £21105.61.

However the Government has reduced the FIT payment per unit of Solar PV electricity produced by 46.3% from 45.4p to 21p for PV systems installed after March 2012 to compensate for the large price drop. An export figure of 3.2p is also paid for 50% of units (kWh) generated per year for both systems. 

Alternative Air Source Heat Pump system costs.

We also obtained a quote for a Mitsubishi Ecodan 14 kW air source heat pump but my father thought that the noise of the evaporator fans might be annoying in the summer months when the bedroom windows could be open at night. The sound level at 1 metre is only 53dB or 46dB in quiet mode. A normal conversation is about 60dB.

Mitsubishi EcoDan W140 14kW Air Source Heat Pump (Single Phase) + 250 litre hot water tank, Design, supply, commissioning and support of specified heat pump system £7295.30

Installation of Air source heat pump £3360

Ice Energy administered CERT ASHP grant £850

Total ASHP cost £9805.30 installed.So that is a saving of £4839.50 over a borehole GSHP system. PV + ASHP total of £16266.11

Feed In Tariff & proposed Renewable Heat Incentive income.

We have received Solar PV FIT payments totalling £1525.67 since the 17 of January 2011 when the system was registered. FIT payments are paid for 25 years every quarter upon receipt of the Solar PV generation meter reading by the preferred energy supplier that the system is registered with.

£182.33 Jan 11 to April 11 @ 41.3p per unit + 50% of units @ 3p

£1272.84 April 11 to April 12 @ 43.3p per unit + 50% of units @ 3.1p

£70.50 1st April 12 to 16th April 12 @ 45.4p per unit + 50% of units @ 3.2p

As can be seen from the above break down, the FIT payments increase at the Retail Price Index rate of inflation every April.

Thus at the current FIT rate we can expect a payment of 2652 units x 47p (45.4 + 1.6) = £1246.44 per year. The level of sunlight received will vary slightly each year.

Thus assuming that the RPI drops to zero this year (unlikely) the Solar PV system will repay the initial investment of £12921.62 in 9 years 2 months with an annual return on investment of 9.64% excluding electricity bill savings or 7 years 9 months with an annual ROI of 11.33% including bill savings.

£12921.62 - £1525.67 = £11395.95 / 1246.44 = 9.14

£12921.62 - £1525.67 = £11395.95 / (1246.44 +217.76)  = 7.78

So there are roughly 23 years left of FIT payments. 23 - 9.14 = 13.86 years of profit without bill savings or 15.22 year with the bill savings.

13.86 x £1246.44 = £17275.66 FIT + 3018.09 bill savings

15.22 x £1246.44 = £18970.82 FIT3314.24 bill savings

Now lets look at the proposed Domestic RHI payments if and when they are ever finalised.

The RHI payments for ground source heat pumps looks likely to be set at around 7p per kWh (unit) of heat produced paid for 23 years. The GSHP in this blog has been predicted to produce 21102 kWh of heat per year. 

Thus 21102 x £0.07 = £1477.14 RHI income per year plus £144 in bill savings over the old boiler. Which equates to a payback period for the GSHP of 9 years and an annual ROI of 11%.

Final thoughts & some graphs.

I hope people find this blog useful and possibly interesting. This is my first ever blog and I would appreciate any constructive criticism on the layout, content, detail & topic etc. 

I thought I would finish this blog off with graphs illustrating the 12 months of Solar PV output in 2011 followed by a newer combined graph from the first several months of 2012 depicting the total household energy import, GSHP consumption and Solar PV generation.

Check them out below. You will see as the months progress the GSHP consumption decreases and the Solar PV out increases.

The Temperature sensor seems to have developed a fault from this point onwards. 

The End.