Scroll down for the diary of our heat pump install from Octopus Energy Services.

Since buying a “fixer upper” of a house in 2011 we’ve been … well, fixing it up. We self-built an extension and replaced all the windows and doors. Insulation of the older part of the house is not yet finished (hung tiles makes it tricky in places). We added PV in 2015, remodelled the downstairs of the original house and even moved the staircase. It was a busy few years.


We’ve ended up with a pretty well insulated home, but one that still relies on gas central heating. In the UK over 80% of homes enjoy a mains natural gas connection so the default heating method is gas boilers connected to a wet central heating system and “radiators” that work as convector heaters. It also happens that the electricity network is largely powered by gas fired turbines. Much of this gas comes from LNG tankers from Qatar and other far-off places.

We also have 4kW of solar PV; no battery storage, but store spare solar as hot water in a 190 litre tank. This can store up to ~10kWh, with our typical water use. This means that for at least half the year, we burn no gas.

We’ve enjoyed electric cars since 2016, the focus naturally comes to other carbon intensive activities; and home heating is a big one. How much energy do we use? How much heating power is needed on the coldest days? Can we use renewables to heat our home? What will it cost? What changes might be required?

Fortunately we have a smart meter for both electricity and gas, so now have an detailed picture of how much energy we need and what the use per day, and use per half hour period is.

We have used 52kWh of gas on the coldest day, and winter 20-21 was colder in the UK than average. In total we used 6000kWh over the year. How does this compare with other houses? By normalising by floor area we can compare houses of any size. In terms of energy use per square metre per year – 38kWh/m2/yr. Passivhaus in the UK averages 10.8 and the average UK house is 145. The average new-build is 50. (Mitchell and Natarajan, 2020) So we seem to be a bit better than a new build and use nearly four times the energy of Passivhaus standards.

Another way to characterise energy use is to measure the energy consumed per day at a range of outside temperatures, to enable correct sizing of a heating system. We can then take the coldest temperature that is likely, and get an accurate idea of the power that would be required. A smart meter provides the power consumption and a local weather station provides an accurate temperature.

In terms of cost, in 2021 we are spending £180 per year on gas, or on a monthly bill, £15 per month. (I know, in mid-2022 this seems almost unbelievable; how reliant we became on cheap gas at 3p/kWh!) This would be more like £650 in winter 22-23.

£15 a month isn’t high in terms of money. In terms of carbon however, we’re emitting 1.1 tonnes per year.

Is a heat pump worth installing?

Whether the alternative to gas – a heat pump – is lower carbon or cheaper, depends on the cost and carbon content of UK electricity. It also depends on the COP of the heat pump; how much heat energy you get per unit of electricity. In 2020 UK electricity was 180gCo2/kWh, exactly the same as gas. However a heat pump achieves a COP of 3 or more (and gas burning isn’t even 100% efficient) so a move to HP could cut the carbon content to a third of its previous. (A COP relates to the efficiency; a cop of 3 means 3 units of heat for 1 of electricity. A gas boiler has a cop of around 0.9 or 90% efficiency)

What about your solar panels?

How much solar PV can we actually use to help heat the house? A lot of our solar PV is already used to heat water resistively (with an electric “immersion heater”), on average 6kWh per day or 2190kWh per year. Using the heat pump this could be reduced by roughly half. However during the heating season, solar PV is at its lowest so a far smaller amount is available.

Nov ’20 -Feb ’21 generated 450kWh. Most of that was used to resistively heat water. A move to HP would multiply that by 3 and so represent about a quarter of the electricity required by the HP. Maybe we could use some resistive heating to boost the water tank temperature?

Overall 6000kWh heat needs (2000-450) kWh of imported electricity = 1550kWh and 279kg carbon. That’s a heck of a lot less carbon than 6000kWh of gas generates; (around 1260kg). So we’d save around a ton of Co2 per year. Cost is a hard one since we’re seeing unprecedented rises, but for this winter at least we’re on a fixed tariff. Octopus are good at looking after high off-peak electricity users and we’ve benefitted from a low tariff for 5 hours each night for EV charging. This could be used to at least provide hot water in winter on days when the sun didn’t shine, charge up the buffer tank, and heat the house for the rest of those hours. Clearly the rest of the day would be on at the peak rate with solar PV helping on the sunny days. Even so, help is needed to overcome the investment hurdle that heat pumps represent. Enter the “Boiler Upgrade Scheme” or BUS to help fund heat pumps…

So the overall heat need of a house is one thing, how that heat is delivered is quite another. Watching Heat Geek on YouTube it’s clear that the UK has not been running its gas boilers as efficiently as it could, and particularly condensing boilers are often not running in a condensing mode. I’d recommend watching Heat Geek even if you just want to save on your gas bill. Much of the advice for getting the best from a condensing gas boiler also applies to heat pumps, and even more so. These are low flow temperatures, balancing and sizing the boiler to the heat load. Unfortunately most boilers are grossly oversized for steady-state operation, for example a well insulated house like ours might need only 2kW once warmed up in autumn, but most gas boilers won’t run at this low output. So they switch on and off, an inefficient way of running.

Other interesting points I picked up are about how best to run a heat pump; to get the highest efficiency they need to run for long periods rather than short bursts (the same applies to a gas boiler, but less pronounced). Best results are obtained heating all rooms all the time, with as low a flow temperature as possible. Even better if the heat pump has weather compensation; reducing the flow temperature as outside temperature increases. It turns out this is almost standard for heat pumps and can be fitted to gas boilers. In this way up to 5 units of heat can be delivered for 1 unit of electricity. I don’t think this “COP” concept is widely appreciated; 500% efficiency sounds too good to be true!

The most important issues to address for an effective and low cost heat pump install seem to be:

-Large enough radiators (or underfloor heating) to deliver the heat at low temperatures;

-Generously sized pipes to distribute the heat

-A suitable outside site for the external unit, which has straightforward access to power, heating circuit and a drain. There are also acoustic considerations regarding distance to any neighbours windows.

-A large enough house fuse (the electricity network operator, the DNO, are obliged to upgrade here for free)

-Space for internal hot water tank and a buffer tank, and associated plumbing

In late 2022 I upgraded our radiators while running with the gas boiler. By upgrading I mean changing single rads for doubles, and adding small radiators to the bathrooms. It turns out that towel radiators are not very good at heating (particularly when covered in towels!) This enabled us to reduce the boiler flow temperature. It was a mild autumn but even so we burn a lot less gas than in previous years and i expect running cooler helped. It was also quite striking how much more comfortable rooms were even with the lower flow temperatures.

2022 Update: our Heat Pump story

Heat pumps now attract a £5000 grant in England which on average will halve the installation price from ~10k to ~5k. In addition heat pumps installs are VAT-free, saving 20%. However only a fixed pot of funding is available initially, enough for 90,000 homes over 3 years. This has led to a spike in demand and it was looking difficult to find a company, until Octopus contacted us, resulting from an enquiry we lodged last year.

Because we have a suitable level of insulation, and mostly generously sized radiators, I thought we would qualify, and an EPC survey confirmed this.

Octopus then sent a surveyor for the heat pump install. This needs a space outside for the heat pump itself, which is close to the hot water tank and not too close to any neighbouring windows, for acoustic considerations. At the moment Octopus fit Daikin monobloc units – which means that the refrigeration parts are self-contained in the one heat pump box, but has the downside that the external box is fairly large. There was only one location that was suitable, and it takes up ~1m2 of the patio. In addition a drain is needed for a relatively small amount of water from condensation or ice build-up. In our case we plan to take up the relevant patio stones and put down pea shingle. When you add power and heating water pipe connections into the considerations, we were left with one only location.

The survey identified that many radiators were undersized, and we had a couple of cheap towel rails already showing signs of corrosion after 10 years, so it was clear I’d be getting my plumbing toolbox out again. Small towel rails really don’t put out much heat at the best of times, they just don’t have enough surface area available. In general for any radiator, at 50C the output drops to a half of that at 70C flow temperature. There’s a gap in the market for a modern towel radiator design with extra finned area to get more than ~400W at 50C flow. Maybe something fan-assisted is needed. All that seems to be available is a Victorian design with a few rails added. It’s also not possible to add another radiator to the room, as the Octopus software used by the surveyor had no such provision.

A new radiator replaced an imperial-sized, 40 year old rad, so the pipe spacings differ. To save pulling the floor up, a chrome extension piece was used to make up the difference. I’m sure it will increase the output a bit 🙂

Weekend spent swapping old bedroom radiators and aligning imperial pipe spacings with metric radiator sizes. Another one swapping towel rads, and working under the chipboard floor to replace 10mm microbore with 15mm plastic. Testing for leaks, cleaning , flushing and finally refilling with inhibitor took another couple of evenings. Apart from sealing the towel rail connections, which everyone seems to struggle with, and one poorly assembled compression fitting, all was fine. Having an Adey mag filter pot in the system makes dosing with cleaner or inhibitor quite easy too.

Waiting for Octopus to come back with a date; a cryptic email from the DNO, the company who looks after the supply into the house, not to be confused with the company you pay for electricity and gas!

25 July – DNO checked the “fuse” and apparently our 1962 vintage kit is a bit old (no surprise!). They will come back with a date for a new connection that can handle a larger than 60A fuse.

28 July – email from SSE – do we have your permission to develop a quotation for the necessary electrical upgrades? Of course I replied “yes”. (Seems odd that we could fit a 32A EV charger and an induction hob without any upgrade work, but adding a heat pump that would average 6A requires upgrades!)

13 September – after our holiday I chased up Octopus, who chased up SSE – miraculously a £0 quote for the fuse upgrade appeared the same day. Apparently this could take 8 weeks!

6 October – fuse upgrade. SSE fitted a 100A fuse as an upgrade from our 60A. No charge was made, I understand the DNO is obliged to do low-carbon “enabling works” like this.

12 December – Heat pump install day! A team of 3 arrived, Josh the lead installer, Gav a plumber and Matt, an electrician. The weather was not exactly ideal, hovering around zero with a very cold week in prospect.

Our existing hot water tank was removed, along with the ancillary valves, filter and expansion vessel, and of course the boiler and flue. In went a 250l Daikin tank, expansion vessel and a 20 litre buffer tank to increase the volume of the central heating system, to aid defrosts. We were to be very familiar with defrosts by the end of the week.

On Tuesday the heat pump was in place, much of the primary system pipework was installed and most of the electrics. A separate small consumer unit was added to feed the 3 circuits for the heat pump. We kept the house warm (well, a couple of rooms!) with electric heaters. We had hot water again but decided to use the showers at the gym, as it was a bit warmer there!

By Wednesday the temperature had fallen to -4C, so many cups of coffee and some biscuits were supplied. I was working from home in the warm, which still feels odd and a little guilty after years of selfbuilding. The system was all in place and ready for a switch-on Thursday morning.

Thursday: (-8C) Commissioning went well, and we soon had 45C water to the radiators. The heat pump quickly frosted up with such low ambient air temperature and was defrosting about 3 times an hour. The defrost is quite dramatic and results in a lot of water, good drainage under the pump is certainly needed. After creating a garden’s worth of fog, the heat production cycle continues.

Getting a heat pump at this time of year reminds me of getting our first Ioniq EV: you really appreciate the benefits of milder weather! Today it’s 5C and the pump has kept a steady output at 38C for over 6 hours without a defrost, drawing about 1000W. From the rudimentary energy stats on the Daikin system, that’s a COP of ~3, or about 300% efficiency, which makes it roughly the same cost to run as a mains gas boiler right now since mains gas is about a third the price of electricity.

However we’ve been able to use 3kWh of solar PV to reduce the grid energy used, even on a cloudy day in mid-December. That halves the grid energy, meaning we’ll need to buy only 3kWh for 6 hours of heating.

Once the house had warmed up following the unusually cold weather during the week of the install, we noticed that the quality of “low temperature heating” was much better than previously. The building all warms up, rather than just the radiators and the air around them. This is taking around 7 to 10kWh per day, including hot water.

Setting up the system

Heat pump controls are much more involved than gas heating. Another way of looking at that is that gas heating controls are stuck in the past with no weather compensation or WC (this just means lower flow temperatures in warmer weather). So setting the “WC curve” becomes a trial and error process and we found it’s easy to overheat the home until this is set up. Balancing the radiators is somewhat more important too, since there isn’t the excess of power we became used to with gas.

Over the first month, mid December- mid January, we’ve seen a COP of 2.93, that is, 2.93 units of heat per unit of electricity. That puts it on a par with gas in terms of running costs. As the weather warms and we optimise our use of the system, the running costs should drop.

We’ve kept the heating on for 18 hours a day, most days. It’s scheduled to be off from midnight to 6am. This means that the house has heated up and stays warm. The quality of the heat is much better than it was on gas- rather than a room being warm by the radiator and cool on the other side of the room, it’s warm throughout, even if the radiator may only see 30C water temperature and not feel hot at all.


I extended our existing Open Energy Monitor system by resurrecting an Arduino-based “emonTX” unit. This gathers pipe temperature and electricity power being used, and results in charts like this:

From the left, the small blips are the water pump circulating water to prevent any icing. At 6am a fairly high power peak warms the water. The later cycles heat the central heating between 29 and 38C, drawing around 1000W (blue line) and delivering around 4000W of heat (red shaded area). (This output power is simulated here, we don’t have a heat meter) About 170W is the pump, and around 700W appears to be the lowest setting for the heat pump compressor. At 1pm the water heating cycle starts, giving a sawtooth shaped peak. Once the tank (and return water, green line) reaches 48C it reverts to central heating mode. There are 11 cycles of the heating. The water temperature gradually rises and then the compressor stops. This isn’t perfectly ideal but also isn’t an issue. You could argue that the 9kW heat pump is oversized, but that power has been required on the coldest nights. Experts such as John Cantor have some interesting YouTube videos on the subject.

Overall the (simulated) COP of 3.88, or 388% “efficiency” is resonable and we certainly won’t be paying any more than we would with gas.

Using my redundant Wiser radiator thermostats I can also see room temperatures, giving an idea of how rooms compare. This has been very useful to help balance the outputs of the radiators by adjusting the valves. The old part of the house clearly has greater heat loss than the newer extension:

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