David NicholsonAt the moment in the UK it’s not necessarily a given that an air source heat pump will be cheaper than a gas boiler. On my standard tariff my electricity cost more than 4 times as much per kWh than gas. As a result my heat pump has been costing more than my old gas boiler. Ironically this price became higher because UK electricity prices were driven up by a spike in…gas prices. Still, heat pumps can become even more efficient if you optimise them, and new electricity tariffs do help you lower bills. Will it be enough?
After your shiny new air source heat pump has been commissioned and the installers van has trundled off down the road how much do you know about it? The manual looks thick, unfamiliar and hard to follow. The installer has said you shouldn’t need to touch anything. So that’s all sorted. All perfect. Don’t touch anything, right?
Frankly that would be a bit like buying a new car and the dealer setting the climate control permanently to say 20C. One day you will probably want to adjust it , 5 mins, an hour, a month, or a year later.
Having taken the plunge to learn more about my Heat pump, my Victorian house is now at a more comfortable temperature and my heat pump is running more quietly and saving a lot of energy giving me lower bills.
I’m an owner not an expert but I finally took the plunge in September 2024 to learn more about my Heat pump that was commissioned in April 2021. See link for how replacing a gas boiler with a heat pump went.
Some reasons you might want to know more
- your energy bills are higher than you expected
- The weather changes and your home becomes too hot or too cold
- your heat pump is switching off at odd times then working hard and noisily when it comes back on
- your heat pump seems to be working very hard even in mild weather
- you aren’t getting enough hot water
- you switch to a tariff with low off peak rates that you want to make the most of
Some reasons you won’t want to touch anything!
- the control panel or app may look new, complicated and intimidating
- Heat pumps have sensors that adjust your heating as outside temperatures change so it should work perfectly whether it’s 14C or -14C.
- you don’t want to mess anything up
- you don’t know who to call if you mess something up!
Top Tip – this is my own experience and not advice. If you do decide to change any settings then take a screenshot of your control panel before and after you change settings so you know how to go back to the old setting if you need to.
Before you do anything…
- Take a look around the app or control system without changing any settings.
- Every heat pump will be different but you should find menu options for heating and hot water, including temperature and operating times. There should also be information about the heat pump and what it’s doing at that moment.
- If you’re unsure think about what you plan to do and ring your manufacturer or installer and ask if it makes sense. Somewhat to my surprise NIBE tech support line were super helpful which reassured me I was on the right track and not about to shut it all down by accident!
- you should also keep an eye on your energy consumption using the app or website from your electricity provider…Let’s look at that first.
1 Looking at your energy consumption and bills
Your Coefficient of Performance or COP
One thing you will hear about heat pumps is that they are fantastically efficient. The good news is this is true. All heat pumps are tested to show what coefficient of performance or COP it can manage. Your COP essentially shows how much heat it produces for a single kWh of electricity. Generally a good result is around 4kWh of heat from 1kWh of electricity or a COP of 4. Higher than 4 would be even more efficient but 3 would be considered on the low side. Essentially this is rather like a miles per gallon figure on an old fuel burning vehicle.
Unfortunately, although it can be measured with additional equipment, most heat pump’s don’t allow you to see what COP you are actually getting at your home. But looking at your energy consumption before and after the heat pump was installed can give you a pretty good idea.
How much less energy does a heat pump use compared to gas?
Let’s have a quick look at some stats for my installation showing the total power consumption of my home (4 bed Victorian terrace) over 2 full years before the heat pump (when I had a gas condensing boiler) in 2016 and 2017 versus 2023 with the air source heat pump..
So what happened? As you can see electricity consumption before the heat pump was pretty steady at around 5,250 kWh while the gas boiler burnt through around another 21,600 kWh to provide central heating and hot water. So a total of 27,410 kWh for electricity and gas combined.
By comparison in 2023 our hob became our only gas appliance using 471kWh per year and our total energy consumption was 11,693kWh. So by switching to a heat pump our total kWh of energy consumption was 57.6% smaller. You can see this in the graph below.
That total figure still isn’t what we need though because we just want to compare the heating and water. So we need to remove 471kWh from the hob for the gas numbers.. and 5,250kWh from the electricity to remove all lighting and other appliances.
And that gives us 21,130kWh of energy used by the gas burning boiler versus 5,972 kWh for the heat pump in 2023 before I optimised.
So the gas boiler uses 3.53 times more energy than the 2023 heat pump.. or expressed the other way the heat pump uses 72% less energy. Pretty amazing isn’t it. In many countries that would lower your bills...
While this is a reasonable proxy for my COP we will get closer in section 3.
Does a heat pump that uses 3.53 times less energy save you money in the UK at standard rates?
Unfortunately I live in the UK where the electricity market has been set up to make domestic gas as cheap as possible and electricity as expensive as possible. This favours the profits of multinational oil and gas companies who donate generously to politicians and UK political parties. This may change but my standard tariff is 25.68p per kWh while gas is just 6.31p per kWh. So a heat pump in the UK at the time of writing (Dec 2024) to be 4.06 times more efficient than gas just to break even. That is better than this summer when it was 4.24 times!
In fact in this research above, the UK at the time had the highest electricity price in Europe relative to the cost of gas at 4.6 times more while only Belgium was over 4 times. In Germany the worst other major economy it was 3.6 while in France, Netherlands and Ireland it’s around 2 times and in Norway and Sweden electricity and gas cost nearly the same. The UK should eventually improve but only when the UK electricity market is reformed to bring prices down.
2 Saving money with a special electricity tariff
Switching tariffs can work well but some of the best options are if you have a driveway where you charge your electric car. I have an electric car but can’t charge it at home so don’t qualify for an EV tariff. Also some tariffs only work well if you can use a lot of off peak power because they charge you a big supplement at peak times. Some are best for households who can suck cheap off peak power into a home battery to use later.
What I have done is recently gone for a Heat Pump Tracker with my existing supplier, EDF, which does not have any lock in period. Essentially this is simple.. it keeps your standard rate the same but gives you an off peak discounted rate for 6 hours a day.
- standard rates for peak periods of 25.68p per kWh
- off peak at 15.68p per kWh between 4am and 7am and 1pm and 4pm
- electricity standing charge is 40.78p per day or £148.84 annual
Many recommend Octopus and but without an EV charging on a driveway they suggest their “cosy rate”. This offers even lower off peak rates for set times 8 hours a day and a slightly lower standing charge but this will be largely or completely offset by a small and rather large price supplement at all other times.
- Standard rate of 26.3p
- A high peak rate of 38.14p per kWh between 4pm and 7pm
- Off peak rate price of just 12.89p per kWh between 4am and 7am and 1pm and 4pm and 10pm and midnight
- standing charge 39.54p or £144.84 annual.
If you have a home battery or cook things after 7pm in a microwave you might be able to time shift enough to get a discount.. if you like slow cooked casseroles from the oven then this might not work for you! Also if you have a home battery you can probably find an off peak overnight only tariff for half the rate here.
Shifting your electricity usage to off peak
The key to both tariffs is to gulp electricity during off peak times and sip electricity at the highest rates. I’m sure many owners have gone a lot further but during off peak rates times (4am and 7am and 1pm and 4pm) we have done the following
- Washing machine used the timer on the washing machine to run it in both off peak periods.
- Dishwasher – sadly this has no timer so we run it where we can at the lunchtime off peak rate. That extra time slot from Octopus at 10pm would definitely make life easier for this.
- Heat pump –
we run all the hot water during off peak periods.
We lower the radiator flow temperature by 1C prior to both off peak times (in the morning from 9am to 1pm and at night between 11.30pm to 4am) then increase to full temp during the off peak period and after it for a while.
Did time shifting our usage work?
It seems so. In November 2023 we were paying 27.07p per kWh for electricity. By comparison in the:
- 3 weeks to 30th September we were able to use 32.8% of our power at off peak rates with an average price of 19.27p per kWh. Standard rate was 22.4p.
- In the month to 6th November we were able to use 32% of our electricity during our off peak periods giving us an overall average price of 21.33p per kWh. Standard rate was 24.46p.
- For December standard rates have now gone up for winter to a 24.46p standard rate and 14.93p off peak. We kept 32% of consumption at the lower rate and paid an blended price of 21.39p.
- In the first week on January 2025 rates when to 24.816p peak / 15.292p off peak with a blend of 21.93p based on 30% off peak usage.
Looking at the December tariffs after benefiting from off peak rates 22.48p is 3.56 times the price of gas.. so we’ve moved that spark gap to about the same as Germany. And remember my heat pump is using 3.53 times less energy than a gas boiler so they are nearly even. If only we could squeeze a little more efficiency out of our heat pump it would be cheaper than gas.
3 Signs that you may want to optimise
Optimising your heat pump for best performance and efficiency
Sometimes your installer doesn’t leave your heat pump set up to perform at its best or its most efficient. Sometimes client’s ask for a change to performance without being aware of what this will do to efficiency..mea culpa. Optimising is about realising this and taking some action instead of writing some absurd article in a climate hostile media outlet.
If your Coefficient of Performance is not the best..My 2023 COP
So we’ve looked at the energy consumption and tariffs. We also have an idea that my COP figure is ok but not great. My old boiler ( as an older condensing boiler) was probably at best 90% efficient. So my actual COP on a NIBE 2040-12 is probably around 3.17 versus a rated SCOP of 3.44. If it was closer to 4 and the house was warm I would probably have stopped right there but I felt there was room for improvement.
If your radiators are reassuringly hot even in mild weather
Heat pumps run on one basic principle. If you run at a lower flow temperature you get more efficiency and by continuously running cool or lukewarm radiators. If you run it with piping hot radiators that are then switched off by the room thermostats even in mild weather you get less efficiency.
With a heat pump if it’s 14c or 15c outside and the radiators seem to be barely on but the room is cosy your heat pump is working perfectly.
- If you live in a medieval castle your radiators might need to run hotter
- If your radiators are on the small side you may need to run hotter.
- If your radiators are running in hot and cold cycles instead of a steady temperature your radiators would need to run hotter to keep you warm but you should instead think about optimising.
- If your heat pump is running at high and noisy output then stopping several times an hour then you should look to optimise.
3 Issues I found with my heat pump settings
Heating curves
Most heat pumps will have an external thermostat to measure the outdoor temperature. As it gets colder outside the heat pump will increase the temperature in your radiators.
Essentially it’s a graph. For every outdoor temperature running from +20C to -40c there is a corresponding flow temperature which is the temperature your radiators will be.
- 25C flow when it’s +15c outside
- 37c is the temperature of human body
- 41c flow when it’s +5c outside
- 60c when it’s -10c outside
- 60c flow is also an optimal temperature for many gas boilers although they can be set between 50c and 70c
Use the side arrows below to see some examples
What I discovered was that our heat pump curve and flow temperature was too high.
I have optimised to a steep curve which reaches maximum flow temperature at -10c ( when our Victorian house will be losing a lot of heat) but has a low flow temperature in mild weather. On this curve I suspect I am running my heat very efficiently until outside temperature is between 2c and 5c. It’s worth remembering that on many UK winter days this means daytime still will be very efficient but a heavy frost at night will be less so.
Noise
Efficiency and noise run hand in hand. If your heat pump is producing a high flow temperature it will be louder. This also makes them louder at night in cold weather. At the same time in warmer months when people might have a window open they are at their quietest which is how you would you want it.
Overall my heat pump seems to be noisier than some others and it is something I will pay a lot of attention to next time but is acceptable. Worth remembering gas boilers are also often noisy and unlike them heat pumps don’t expel toxic gases out of a flu pipe and will never cause an catastrophic explosion in your home. So a bit of a hum is fine with me.
Hourly priority cycles and hot water
The other thing is noise is more annoying when it is continuously changes. If a heat pump is running steadily it’s more like a distant train or the background hum of a road. If its stopping at starting then it’s more like a motorbike with a loud exhaust or an airliner flying low.
I can vaguely remember my installer saying he would set my heat pump to share each hour of it’s time between heating and hot water. I remember nodding. Perhaps there is some reason to do this but the result was:
- For 30 mins in every hour our heat pump was ignoring the radiators, which meant they got cold
- Because the radiators were getting cold for half of every hour we needed a higher flow temperature for the radiators in the 30 mins it was in heating mode. This was probably why we had to choose a hotter heat curve.. essentially we were not warm enough.
- Obviously unless you spend hours every day under a power shower you don’t need your heat pump to spend half of every day heating hot water. Essentially the setting meant that for half of each hour our heat pump was having a tea break followed by half an hour flat out in a spin class!
- When we removed the hourly priority cycles altogether we were were able to move to a more efficient heat curve and produce all our hot water in 2 hours a day (at a set time each day) instead of 12! We may need to step this up this a little on sundays when we have more baths but literally half an hour extra will do it.
Other factors – Insulation
It’s not rocket science but more insulation will reduce the amount of heating you need. We have slowly swapped out our old single glazed windows ( 2 left to do) and this year upgraded some older double glazed units as well. This not only makes the house use less heat, it also cuts out the draughts that make you feel colder at the same temperature.
Adding double glazing does cut bills and can be done over time. If it will take you some time to make all windows double glazed in a terraced or semi detached property you may find that you get less of a draught by completing all the windows on one side of the building first. After all a draught is travelling from one side of a house to the other. If one is a double glazed barrier then even the single glazed side should get less cold air through it.
A particular win insulation wise is we finally replaced a single glazed window that had old fashioned swivelling ventilation slats with triple glazing. It has made a big difference on cold days and meant that so far the bathroom under floor heating hasn’t been required so far.
So has this tinkering affected efficiency and cut bills?
Well we have a lot of simultaneously spinning plates here. Lower electricity prices and a tariff with off peak prices, more double glazed windows, a new more efficient heat curve and ditching those terrible hourly priority cycles. And don’t forget weather varies each year! Most of this tinkering has been since September 2024 so it’s early days.
On the graph below we have isolated the electricity for the heat pump after deducting our typical household demand for everything else.
Over the 3 months the heat pump used 29% less energy compared to the same months in 2023. Weather may have played a part but doesn’t seem to be radically different. The weatherspark website shows actual and average min and max temperatures here if you want to check.
If we look at the 2024 full year
- the final 2024 electricity numbers came in at 10,026 kWh or around 10% down on 2023 while 2024 the gas hob was steady at 468kWh.
- After deducting the 5,250kWh for general household electricity estimate is the heatpump used 4,776 kWh in 2024. That’s 20% down on 2023 and 4.42 times less energy than my old gas boiler
- This means that with a partial year of optimising from Sept to Dec in 2024 my estimated COP improved to 3.98 compared to just 3.17 in 2023
Total Electricity bills from Sept to December 2024
- Sept 2024 473kWh (397kWh)
Cost £96.40( (£123.79) - Oct 2024 651kWh (740kWh)
Cost £140.78 (£210.25) - Nov 2024 1,048KwH (1360kWh)
Cost 236.58 (£386.28) - Dec 2024 1172kWh (1391kWh)
£263.05 (395.28) - 3 months Q4 all electricity 2871kWh (3,489) 18% less
£640.41 (£991.81) £36% less - or net to Heat Pump 1551kWh (2,171) 29% less
Net cost for heat pump after fall in usage and unit cost £345 (£614) 44% less
(2023 numbers are in brackets)
Conclusion
Based on the latest heat pump tariff we estimate that even with no reduction in energy use compared to 2023 the annual cost of running the heat pump was £1,232 versus £1,356 if we were still running our gas boiler.
For the full year 2024 consumption for the heat pump priced at my January 2025 average electricity price of 21.93p per kWh came in £1,047. This represents a £309 annual saving off the current price of gas from my old boiler which will likely grow as the price differential between gas and electricity continues to close.
