Background

In October 2024 we finally replaced our Worcester Greenstar combi boiler with an Octopus Heatpump installation of a Daikin Altherma 3 monoblock Airsourced Heat-pump and 180 litre hot water tank.

So two months on how well does it work?

• Is it keeping the house warm?
• Is it costing more or less money to run?
• Is it using less energy and saving carbon?

If you want to see the background of our house retrofit leading to this point and a tour just after the install please watch these two videos first…

Running Costs

So how does it compare with our running costs last year?

Well we have a couple of differences to factor in. The insulation, solar, and battery are largely the same. The main difference is we no longer have gas and also cook with an induction hob.

November 2024 costs

Our total Electricity usage in November 2024 came to 699kWh.

This was charged at £107.17+standing charge, so £125.84 in total.

November 2023 Energy usage.

November 2023 usage is split into gas for heating and the gas hob, and electricity usage. As in November 2024 we also charged our car over night.

We used a total of 525kWh of electricity in November 2023.

We used a total of 1172kWh of Gas in November 2023.

Total energy usage for November 2023 = 1697kWh

Nearly 60% Energy Saving

So our switch away from gas has meant we’ve only used 41% of the energy in November 2024 used in November 2023!

1697kWh in 2023 down to 699kWh in 2024!

In cost savings:

November 2023 £194.25

November 2024 £125.84

So 64% of the cost of November 2023! a £68 saving!

Weather Differences?

Looking at historical data from the same source the average temperature in November 2024 is 8 degrees C as per  November 2023. So no discernible difference.

November 2023 High 14 deg C, low -3 deg C, Average 8 deg C

November 2024 High 16 Deg C, low -1 deg C, Average 8 deg C

Other Differences

So having removed our gas meter we have lost the standing charge for that meter.

However our Electricity standing charge was 45p per day in Nov 2023 and is 62p/day in 2024, negating this difference. Our electricity unit charges are fractionally lower.

Our charging usage is appoximately the same – we don’t use our car very often, it is charged overnight.

December 2024 Costs

In December 2024 we used 848 kWh of electricity at a total cost of £156.90

This compares with December 2023 where we used 579kWh of electricity and 1454kWh of Gas, so a total of 2033kWh of energy leading to a combined bill of £217.57

Nearly 60% energy saving in December 2024!

So December 2024 mirrored November 2024 in that we only used 41% of the energy we used in December 2023 and saved £60 over our December 2023 bill.

60% energy saving and our bill was £60 less than December 2023!

Other Differences

Well the saving is actually all the more remarkable given for December 2024 we had AirbnB guests staying, something new for 2024. We also had full family occupancy at Christmas. So at weekends in December our house had five to six people every weekend.

Efficiency of the Heat Pump

To measure the Coefficient of performance of the heat pump we need to divide the Heat Output generated in kWh by the input Electricity used in kWh. This is achieved by noting the displays on the Daikin MMI.

November 2024

For November for combined Heating and Hot water this gives 924kWh Heat output / 282kWh electricity input = 3.27 COP.

For heating alone it is 720kWh / 192kWh = 3.75 COP.

This isn’t surprising as the heat pump is working harder to heat the hot water to 50 degrees C so is slightly less efficient, plus there is the weekly anti-legionella cycle. But as hot water heating runs at night when we are paying just under 7p/kWh of electricity. If you consider our gas rate was 6.6p / kWh and we’re running at 3x the efficiency, it’s costing us three times less for our daily hot water than a year ago.

December 2024

For December COP

Hot water = 240kWh Heat output / 98kWh electricity input = 2.44

Heating = 1028kWh heat output / 244 kWh electricity input = 4.21

Total combined COP = 3.7

As before our heating is more efficient than our hot water. The heating runs at night when it is colder and runs and anti legionella cycle once per week.

Conclusions

So far through two of the winter months that really test the heat pump it’s saved me £128 from 2023 bills and run consistently at 41% of the energy usage of 2023.

£128 saved in Nov/Dec and 41% less energy used with a heat pump

For heating we’re running at an efficiency of over 400% which at a daily rate of 24p/kWh means we’re at the equivalent of under 7p/kWh of heating. But if you consider a gas boiler is at best 90% efficient, then even at this electricity rate there is a small saving over gas for heating.

The bigger saving is made from heating hot water at night. Whilst the December COP for hot water was only 2.44, if you consider this is charged at a rate of 6.67p/kWh then we are only paying 6.67/2.44 = 2.73p/kWh for hot water.

So it is incredibly cheap for hot water due to our night time tariff.

I have to admit one of the biggest barriers from moving from a combi boiler to a heat pump was finding the space for the internal hot water tank. But now I view the hot water tank as a giant thermal battery that allows me to save money by heating water at night. Whilst the hot water loses temperature gradually over the day, it’s still hot enough to have a bath in the evening. We don’t reheat the water during the day.

The Hot water tank is a giant Thermal Battery allowing me to heat at night for incredibly cheap hot water at 2.73p/kWh of hot water

Whilst we do have Solar Panels and a 5kWh battery this doesn’t factor at all into these savings. The reason being we had the same solar and battery last December and it wouldn’t last the whole day ever in December with our electricity usage then. If we had a much bigger battery then we clearly could save more money by charging at night and running the heat pump on battery, but this is why I see the hot water tank as a ‘thermal battery’ achieving effectively the same thing.

What about carbon savings?

Well if you consider the grid in the winter period perhaps runs at 50% on gas fired generators, and, the heat pump is roughly 4x more efficient than a gas boiler, then as a very quick estimation we’re producing 1/8 of the carbon emissions we would with a gas combi boiler. Can we look at this in more detail?

Worcester Greenstar Combi November to December 2023

So if we consider Carbon emissions for November and December 2023 with a gas boiler.

2626kWh of gas at 0.185 kg/kWh = 485 kg of CO2. If we allocate 1/10th of this to the gas hob then 436 kg CO2 attributable to heating and hot water

Daikin Altherma 3 8kW Air source Heat Pump November to December 2024

With a heat pump…

Approximately 640kWh of electricity in November and December 2024 attributable to heating and hot water (taken from Daikin mmi)

Average Carbon intensity approx 200gCO2eq/kWh (taken from electricity local carbon intensity for December 2024). This varies and in theory we’re on a 100% renewable energy tariff so in theory it would be zero, but this is a more realistic estimate of local grid emissions at the heaviest usage time of the year..

640kWh x 0.2 kg/kWh = 128 kg of CO2e 

In the summer months with our solar and battery emissions will be much less.

128/485 x100 = 26% of the emissions of our gas combi-boiler in November and December 2024 or a saving of 308kg of CO2e.

This is a very rough calculation and doesn’t take into account load shifting for hot water, some solar power in the middle of the day, and a small amount of load shifting with our battery.

But most importantly there is a significant operational carbon saving.

The embodied emission in the manufacture and installation of a heat pump in a UK home has been calculated by Jan Rosenow as 1,536 kg CO2e. So our heat pump will likely pay back its embodied carbon with the savings made over a gas combi-boiler in 1.5 years.