Our bivalent heat pump and HVO biofuel – are we still green?

by Mars
Bivalent heat pump and HVO

How much things have changed in 12 months. In January and February 2021 we were going through a cold winter and our air source heat pump was cost-effectively keeping us warm. Things then started to unravel in the energy world, and over the course of 2021 our electricity tariff has gone up from 11p/kWh to 22p/kWh, and things could get even worse come April when the price cap is revisited. This tariff explosion cannot be overstated because it has taken heat pumps from being cost-effective to being way more expensive to run than oil boilers, raising fresh issues about sustainability.

Let’s get some context before we start. In January 2021, we consumed 2,231 kWh of electricity (just for the air source heat pump) which is a staggering average of 72kWh per day. At 11p/kWh (which is what we were paying at the time) our heating cost us £245 that month. At 22p/kWh we would have paid £490 – for us, this is not financially sustainable so we’ve started looking at alternatives.

Going bivalent: air source heat pump and HVO

Since announcing that we are trialling a biofuel called HVO (hydrotreated vegetable oil) to assist our heat pump this winter we’ve received some interesting comments, emails and messages from people questioning our decision. Apart from the merits of converting waste cooking oil and fats into a heating fuel source that cuts emissions by 88% over kerosene, let’s break things down further.

HVO derived from waste cooking oil and fat has a verified carbon emission factor of 0.036kg/kWh, compared to 0.298kg/kWh for kerosene. This is based on the latest edition of the government’s Standard Assessment Procedure for Energy Rating of Dwellings (SAP 10.2).

So let’s compare heating apples with apples – as I write and construct this post I have no idea what the outcome of this exercise will be.

I started by pulling the CO2 data from the national grid for where we live for January 2 and 4, 2022. Our electricity usage, per kWh, on January 2 was emitting 182g CO2/kWh. The second of January was an extremely windy and blustery day which is reflected in the graphic below (the one on the left).

Looking at the data for January 4 (the image on the right), the carbon intensity was 331g CO2/kWh. This probably had something to do with people going back to work after the festive season break and the fact that there was very little wind contribution with the grid leaning heavily on gas.

So I wanted to see how our bivalent system running the air source heat pump and HVO boiler compared to just running our heat pump by itself.

On January 2, 2022, our average ambient temperature was 8.4C – our air source heat pump consumed 9.70kWh of electricity and our ‘booster’ used 43.20kWh (more on that in a minute). To try and match things up, I time travelled to a similar day in terms of outdoor temperature. On April 16, 2021, our average ambient temperate was 8.5C and our air source heat pump used 39kWh of electricity to heat our property.

Based on this data, I assumed that on a day like April 16, 2021, where just the heat pump was used, we consumed 39kWh of electricity for our central heating. On a similar day in January 2, 2022, we used 9.7kWh for our heat pump, with the rest supplemented by the HVO boiler. For all intents and purposes this is a like-for-like comparison.

To work out our HVO usage, there’s a bit of a complicated calculation which we won’t get into here, but ultimately the 43.20kWh consumed by our ‘booster’ equates to 149kWh of HVO generated heat. This released 5.3kg of CO2. This figure, plus the 9.7kWh in electricity means that on January 2, our CO2 contribution was about 7kg.

As nothing more than an interesting comparison, if we’d been using kerosene instead of the HVO that would have emitted a staggering 44.5kg of CO2.

By comparison, the 39kWh used by our heat pump emitted approximately 7.8kg of CO2 from grid electricity back on April 16, 2021. On a windy day, such as January 2, 2022, that would have equated to around 7kg of CO2 emitted and a whopping 13kg on a day with little or no wind like January 4, 2022.

As I’ve been typing and working things, an interesting picture has been unfolding. On days when there’s an abundance of wind, our air source heat pump running solo emits pretty much the same amount of CO2 as our bivalent system (running HVO). We describe how we set things up and talk about the HVO in more detail in this YouTube video.

Purely from a CO2 emissions perspective, one thing has become apparent relating to our system. When there’s a lot of wind and solar, the bivalent system matches CO2 emissions against a heat pump running by itself. But when renewable energy on the grid drops below 50% (which is far more often than I thought) and gas has to prop it up, our bivalent system releases fewer CO2 emissions.

As an additional point on CO2, we also generated 6.7kWh and 9.7kWh in solar PV for January 2 and 4, respectively, offsetting our daytime electricity consumption for general usage.

Running costs

Now that we have a better idea about CO2 emissions and environmental sustainability, let’s take a look at financial sustainability, using the same data.

Given the state of energy prices in the UK, the tariff for our region is 22p/kWh with E.ON Next, up from 11p/kWh this time last year with Symbio. 39kWh, running the heat pump exclusively, at this new rate would cost us around £8.58/day to heat our property.

Using our bivalent system, we would pay £2.10/day for the electricity (9.7kWh). Prior to the energy crisis, HVO was priced at £1/litre, and this price has risen to £1.30/litre. The rate is loaded with government levies, duties and taxes. At £1.30, we’d be paying around £18 for the fuel per day, so the heating would be around £20/day, which is just crazy, making it much more expensive to run than the heat pump.

If the government was to drop related duties and levies, and bring the HVO more in line with the price of kerosene (which is a massive emitter of CO2 as we pointed out above) we should be paying around 50p/litre. What this would mean is that the price should drop to around £7/day, a total of £9/day, which again is very close to being in line with the electricity option at 22p/kWh to run the heat pump.

The scary thing, however, is that all signs are pointing to electricity tariffs going up again in April, with some experts hinting that tariffs will hit 35p/kWh – at those kinds of levels, running heat pumps is going to be insanely expensive and unaffordable for us as a household. Using our data in this post, the pure heat pump option at 35p/KwH would now cost £13.65/pay for 39kWh of heat.

I probably don’t need to point out that I’ve used a mild day (8.5C) for our example where we’re not using huge amounts of electricity. On February 11, 2021, when we had our coldest day hitting -6C and averaging -2.6C, we consumed 94.7kWh of electricity. At the time (at 11p/kWh) we paid £10.50 for our heating that day. Today, at 22p/Kw that would have cost us £21 for one day, and if tariffs hit 35p/kWh, it would have cost us £33.

Let me repeat that and let it sink it. £33 for heating, for one day. For us, that’s food and groceries for at least five days. It’s madness. We had six days in February 2021 where we exceed the 90kWh/day mark. What happens if we have a freezing cold, sub-zero winter? Are we really prepared to pay around £1,000 for heating for just one month? We certainly aren’t because we can’t afford that, and the energy we’re pulling from the grid isn’t even that green to justify the costs.

This is not an anti heat pump posts. We’re just sharing our experience and concerns, and as consumers and homeowners we need options. For us, that may just have to be juggling solar PV (which is heating our hot water during the day via the iBoost), our air source heat pump, a biofuel like HVO and our wood burning stoves, and figuring out what’s most cost-effective and environmentally-friendly to do.

Quite honestly, it’s an extremely worrying time, and we’re really hoping that the government can get its act together and figure out how they can protect households from having to chose between heating and eating, and they need to accelerate the rollout of more green energy sources.

Please leave a comment below letting us know how you’re heating your home this winter, what it’s costing you and whether you have any concerns.


Eddy Winko 13 January 2022 - 06:49

The way its going with energy prices I’m glad we stuck to burning wood and planting trees 🙂 I will mention our solar water heater again though as this can provide 300 litres of 50c water in the winter if the sun is shining even in sub zero temperatures, and looking at your figures you are spending a lot of money heating water up.

Mars 13 January 2022 - 08:45

We’ve got an iBoost on our system which does divert surplus solar production to heat our hot water. For us, our heating is going to have to become a balance of ASHP, HVO, solar PV and wood burning stoves.

Eddy Winko 17 January 2022 - 07:05

I was intrigued how much power is required to heat 300 litters of water from 15-60c, and you would need a little over 15Kwh. Whilst I know you are getting some heat from your ASHP so it wouldn’t be so much, it still means that a solar water heater would probably pay for itself in 2 or 3 years and then contribute to reducing costs.
When i made enquiries locally about ASHP the suppliers almost insist that you have solar water installed as part f the package, so I’m guessing there must be something in it. It might be worth a closer look.

Mars 17 January 2022 - 21:45

That’s a fair point Eddy. We do about 10-12kWh of electricity per day to heat our hot water using the heat pump by itself, which is aided by the iBoost when there’s sunshine around. I’ve made a mental note to look into solar water heaters this summer because it’ll definitely help, and we can divert the iBoost to the buffer store on the central heating. This’ll be my summer heating project and I also want to look into small, home wind turbines.

Tabula Rasa 13 January 2022 - 07:14

I’ve always pondered a heat source pump but when you compare the cost of my gas heating of roughly £4 a day on a very cold day versus your potential £33 it is hard to stack up without solar and or wind power to reduce the cost and keep it green. Even if you switched to a completely green tarif it is still eye watering!

Mars 13 January 2022 - 08:51

Thanks for your comments. Electricity and gas tariffs are intertwined and you may see a significant increase in your gas bill when your existing plan comes to an end.

The irony about green tariffs is that the electricity you’ll use still won’t be green and much of it will sill come from gas power stations when the wind in the UK isn’t blowing – green tariffs are certificates/tokens electricity providers buy to certify the ‘greenness’ of energy. Jon Fletcher from Big Clean Switch explains this very well in this interview: https://renewableheatinghub.co.uk/explaining-the-2021-energy-crisis-in-the-uk

Iain 13 January 2022 - 11:14

Morning MARS. Ouch, feeling yor pain re the astronomical cost of your power.

Regarding some of your comments on the amount of renewable power being generated I’ve attached a link to the N.G . Demand figures which in the last 3 months shows that yes 50% renewable is a struggle, but as can be seen from the current generation / supply data most winter days struggle to achieve 20% and then look at at how much coal is being used!!!!!!

You’ve previously mentioned your average COP, but do you know what the winter day COP is. Looking at your heat demand, it would seem to imply that it’s probably a good deal below 2?

Just had our December bill in and we are averaging around £5.80 per day for heating/ hot water and general electricity usage.

It’s true gas is likely to increase come the April CAP adjustment, but even a 50% increase will still see us only paying 6p kwh.

I worked in the Gas Industry in the distribution side of the business for 38 years and its become increasingly obvious that the countries energy policy is broken.

The desire to switch to a largely electricity powered system just will not work. Aside from the lack constraints on generation ( only likely to worsen as more nuclear power plants close) there are constraints on the transmission and distribution networks. The proposed switch would require the vast majority of the cables to be upgraded.

Keep the posts coming. It’ll be interesting the situation in a couple of weeks if we get some real winter weather and how your system copes then.

Mars 16 January 2022 - 16:36

Thanks for taking the time to leave such an interesting comment Iain, and we’ll definitely keep the updates coming. Thanks for the support.

Jez 25 January 2022 - 09:11

I agree about the power infrastructure. When we moved into our old farm house, I took the rather simplistic view that we would bin our ancient oil boiler and move to electric flow boilers running on a green tariff. When I did the basic calculations, I realised that our 100A supply was inadequate. When I contacted UKPN to ask about an upgrade to a 3-phase supply, they said that the nearby transformer was too old and I would have to pay the £15k to have it replaced. When I then asked about a second 100A cable instead, they simply said it “wasn’t policy” to install a second power cable to one property. Looking back, I think we dodged a bullet, as the costs of running such a simple electric heating system would have been astronomical. However the limitations of the existing electricity network remain.

Bob Bazley 13 January 2022 - 10:45

All of us on this site are “doing our bit” by trying to minimise our impact and use greener fuels to supplement our requirements for heat and electricity but even us with our knowledge are finding it increasingly hard to justify the costs and then what really makes me uncertain for the future is how is the entire country going to replace those fossil fuels ? unless Billions upon billions is invested now and continuing way into the future I cant see how it can ever work ! Im not a defeatist Im just a realist !

Bob Bazley 13 January 2022 - 08:26

Mars, Great post and exactly the reason why I decided to postpone my heatpump install, I simply could not make the numbers work, not only for the actual install of a heatpump but also the cost of matching the heatpump up to an efficient heating system which for heat pumps the only viable option is underfloor heating. That cost alone is large and then the actual cost of running it is also large, now you can potentially offset that by having solar PV to provide some of the electric needed to power it especially in the cold months but thats mostly in the time when their is the least amount of solar production. A ground source heat pump may make it a tad more viable but air source heat pumps are in the real world just going to cost a lot of money in electrical use. Heatpump companies keep going on about how efficient they are with a COP of 3.5 but unless you match it up to slightly larger radiators or underfloor heating and then ensure its balanced to give you that optimal rating it really is a costly exercise. As you clearly state, hoping that the Government get more involved with this is really the best option but they have completely failed in that process and I dont see them coming up with any viable options. What is amazing to see is if the entire nation turns towards these more environmentally green options rather than gas or oil and everyone gets a Heatpump, its is going to put a HUGE demand on our electricity requirement for the UK and we currently barely meet our existing requirement !! The numbers dont match up, the solutions being pushed dont match up and their virtually is no incentive to move away from traditional heating options when the greener options would require an individual households upfront budget spend in the thousands if not tens of thousands for potentially an then even higher ongoing monthly spends on electricity usage. The payback for these options would begin to run into the rates much higher than what is suggested and potentially far in excess of 25 years ! In a summary what is being done by the UK government to promote green energy use is completely flawed and if its not improved when it comes to all those cut of dates for using fossil fuels they are going to have to back track as where is the replacement option and do they think people are just going to put up with almost their entire household budget being used just to keep their house warm !!

Mars 16 January 2022 - 16:38

Great to hear from you again Bob, and I hope that you’ve had a good start to the year. There are no easy decisions to be made when it comes to renewable heating, with a lot to deliberate and factor into the decision-making process. Hopefully, there will be further advancements in the months and years to come that will make it easier and more affordable.

Iain 13 January 2022 - 11:31

Oops hit send too quick. Attached are the live stats.


Di 14 January 2022 - 10:23

First of all, thank you to both of you for your posts, which I’ve been reading over the last year or so with great interest. I think I came across you while we were researching heat pumps and really needed more real-life detail, rather than just the data, which of course is very important. I’ve been struck by the relevance to our experience of your posts on such a wide range of topics. We’re in our late 60s now and our aim is to live in as environmentally sustainable a way as possible.
We moved into our 19th century stone house last November, just as another lockdown started. It’s a sprawling building consisting of the original house, later linked to a converted barn, both with later 20th century additions. The state of insulation is poor to non-existent and most ceilings are vaulted. Over the years I’d looked into heat pumps and always noticed the brief mention of the fact that there’d be an increase in electricity consumption, so once we’d moved into a property fuelled by oil, I wanted to find out how much it would increase – you can see why I’ve found your posts so interesting!
Personal circumstances led to being able to fund the following:-
17.5KW ASHP, 6.2KW solar pv and just recently, a 13.5KW battery. As you can imagine, we are in those early throes of enjoying watching readings on all the apps. There are also 2 multi-fuel stoves for the rooms we’d sit in.
We have been with a fixed tariff with Octopus for 10 months, which enables us to charge the battery for 4 hours at 5p between 00.30 and 04.30, which gives us a few hours of heating, sometimes this is used up by 08.30 but today, the sun is shining and at 9.21 the solar pv is already generating 1.2KW, so we are using nothing from the grid and simultaneously the battery is also recharging at 0.9KW and is now 25% recharged. Unfortunately we haven’t had many days of clear weather since the battery was installed mid-December, but it is interesting to get a sense of the potential of the set-up as a whole and we look forward to the next 9 months or so. Hoping that the extremely high cost on many cloudy, cold winter days will be offset by the solar pv and battery most of the rest of the year.

Like you Mars, we kept the oil boiler just because it was only three years old and we intended to use it in case the heat pump broke down or wasn’t working well enough, but annoyingly the system has been set to go over to the oil boiler at 0 degrees C , which clearly is not unusual 750 feet up in the hills, so we’ve got to sort that out so that we can decide when to use it.

As far as increased electricity costs go, I think we’ll just have to cut our coats according to the cloth, if that’s the right saying. Once we’d experienced no power at all for a few days during storm Arwen it helped us focus on what’s absolutely necessary – and having one warm room and a tank of hot water would have been luxury then – and maybe a camping stove to boil a kettle!

In the meantime, on with the attempts to improve the insulation – the ceiling of the largest room has been done, but it took a year to get to that point, so it’s going to be a slow job.

Mars 16 January 2022 - 16:40

Thanks for the detailed comment Di, and it certainly sounds like you’re making some bold and sensible decisions. We would love it if you could join our forums and share your progress which will hopefully motivate other homeowners to go down a similar route: https://renewableheatinghub.co.uk/forums

Mark Brice 14 January 2022 - 15:57

Over 7 years ago we had to ponder whether to move away from an oil boiler to an ASHP, unfortunately no gas in our road . After works carried out on our bungalow we were caught with having to re site and replace the oil storage tank we had as it did not comply with the regulations of the time. With oil on an upwards climb, the high cost of a new bunded tank and ground works/foundations for the tank it was found to be cheaper to install a 16Kwh ASHP. The quarterly payment from the government for the ASHP also helped the decision at the time.

If we were having to make the same decision now, with the increase in electric prices to date and threatened increases for the future, we would probably have stayed with oil.

Hopefully electric prices will be brought down as fast as gas prices when this gas shortage has sorted itself out.

Mars 14 January 2022 - 21:47

Thanks for sharing Mark and you’re definitely not along – a lot of homeowners with heat pumps share your sentiments, and we’re all hoping that the tariffs go down next year.


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