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 better than a new build and nearly four times 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’ve become on cheap gas!)

£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

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. 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!

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