Building a computer controlled ventilation system with HEPA filter to improve our indoor air quality

Introduction - why do we need this ?
In general, the Netherlands offers a reasonably good quality of life. Residential areas are well looked after, noise pollution is greatly reduced by quiet asphalt on roads, air pollution is fairly well controlled in most of the country (but not everywhere: certainly not for those living near the steelworks on the west coast). But there are three things which regularly cause ludicrous amounts of air pollution in this country: fireworks on New Year's Eve, Easter fires, and the idea that people have that it's especially "gezellig" to have a fireplace or stove burning wood in their living rooms. Luckily, none of our neighbours are especially keen on barbecues so the summer air quality is usually OK.

The government advises people not to burn wood for heating when we have weather conditions which will cause the pollution to accumulate, but it seems that almost no-one who burns wood for heating ever takes any notice of this advice. At the time of writing, the "stookwijzer" website advises people in our area that burning wood will cause nuisance to your neighbours and air pollution. From six o'clock this evening they advise that people don't use their wood burners at all because the smoke produced will cause health problems, especially for those with lung problems. But I expect that no-one will take any notice of this and our outside air will be thick with smoke this evening.

What I find especially absurd is that it's still September and we've not even had any cold days yet. Outdoor temperatures have not yet gone under 8 C. We've not yet used any form of heating at all and the lowest indoor temperature that we've seen so far was 19.5 C, in the middle of the night. Why buy firewood and spend weeks in the garden chopping it up (as some of our neighbours do), and then set fire to it when you could insulate your home instead and it'll be warm without any effort being required ?

Air filters
Initially we used air filters quite successfully to improve indoor air quality. They're also good to remove viral particles which visitors might bring with them. Though it's popular to pretend that Covid is "over", it's actually still killing about 200 people each week in the Netherlands, and it's similarly deadly elsewhere.

We have two Philips 800 series air filters. These adjust their speed and run quite quietly unless the air is full of particles, so you can sleep in a room with one active. Other companies produce good products as well, of course. The light on the top is purple because I opened our windows to let in fresh air and the neighbours were burning wood again.

A couple of years ago I also made this super inexpensive air filter. It looks like a cardboard box with a vacuum cleaner HEPA filter on one side and an old computer fan on the other because that's exactly what it is. It doesn't have the same throughput as the Philips filter, but it does clear smoke from the air in a small room surprisingly quickly. The 12 V fan is run from a 9 V "wall wart" power supply, so it runs slowly and reasonably quietly.

A house ventilation system
Free-standing air filters are good at removing smoke from the air, but they can't do much if you open a window for ventilation. Without ventilation, the CO2 level in a room rises rapidly due to nothing other than the occupants breathing. So we needed something else.

For the last couple of years I've been working on improving our indoor air quality. As we no no longer use gas the air inlet in the upstairs (2nd floor - two floors above ground) boiler room was not required for the heating system any more so I now use it to bring fresh air into our home. It opens into the boiler room, allowing air to mix and warm up if the sun is shining, but the air needs filtering before it comes into the rest of our home.

A fan which pulls air through a HEPA filter in the boiler room is controlled by a small computer. The fan is switched on if the downstairs controller indicates that the CO2 level is high. The computer also measures the temperature in the boiler room as well as on the upstairs landing so it can cool or heat the house (slightly) by bringing in cooler or warmer air when that's possible. It also monitors the temperature of the solar power inverter and switches on an external cooling fan when it gets warm. I already had to repair that inverter and I hope that this will extend its lifespan. The computer also can switch on a small heater in the boiler room if the temperature in the room is too low. This has not yet happened.

The wall outside the boiler room: An Arduino mounted on the wall next to the inlet for the incoming air. The green LEDs indicate when the fan is active (there are two speeds). The yellow LED indicates that the cooling fan for the inverter is active. The red LED indicates that the heater has been switched on. I'll cover it up so that it's less ugly some time soon...

Inside the boiler room. No-one usually comes in here, but I still need to cover up the circuitry of course. The HEPA filter is the same as the type used in the Philips air filter. The fan is an inline "quietline" fan from Hornbach, which seems to be one of the quietest inline fans available. The three relays are used to switch the two speeds of the fan and the small electric heater. I recovered them from the old gas boiler.

The other half of the system is downstairs. There's a controller on the wall in the living room where our central heating thermostat used to be. This measures the CO2 concentration and switches on the extractor fan in the kitchen as well as the incoming air fan upstairs (to attempt to create a balanced flow of air). Communication between the two devices works via a serial link using the same cable as linked the old thermostat to the gas boiler.

The downstairs controller measures CO2 concentration and will automatically switch on ventilation if the level is too high. Buttons allow selection of continuous low fan mode, 'visitor mode' which attempt sot keep an even lower CO2 concentration, and off mode which turns off the downstairs and upstairs fans. The latter is intended to be used if we find that the system is somehow overloaded and is bringing in smoke. The controller is also linked to the cooker extractor in the kitchen so that it knows to switch off the house ventilator fan when that is operating, and even if it was not otherwise required it will switch on the upstairs fan to attempt to approximately balance with the cooker extractor. The downstairs controller also has a light sensor so that it can automatically switch to nighttime mode when we go to bed.

We exhaust air from the kitchen on the ground floor, using the same duct as the extractor fan over our oven. Only one fan can operate at a time. Having air travel slowly from upstairs down to the kitchen helps keep cooking smells and condensation out of the rest of our home even after we've stopped cooking

Downstairs controller. This is also Arduino based, using a Nano this time to make the whole thing more compact. Sensors measure CO2 concentration, temperature and humidity. They agree quite well with my Aranet 4. The buttons on the front allow selection of constant low fan mode, visitor mode and no fan mode. The green LED indicates low fan setting, yellow indicates high and red indicates that the fan is currently off because we have a low enough CO2 concentration. The Aranet 4 made me realise that the concentration of CO2 in a house rises quite rapidly just through breathing. We saw the reading rise quite quickly to over 2000 ppm on winter evenings with the windows closed, but that no longer happens due to the ventilation system.

In the kitchen I put a a t-junction in the ducting from the cooker hood on the left of this picture so that an inline fan under the wooden cover on the right can also send air into the duct. It feeds from another duct on the right side of the photo which reaches nearly to the floor next to the refrigerator so that we take air out from a relatively low level in the room where it is cooler. Non return valves are used to prevent one fan blowing air back out the other. It's a bit ugly and I still need to cover the silver tube up. In this location I used a 125 mm version of the same inline fan as upstairs.

How well does it work ?
We now have fresh smelling air indoors even when the neighbours are doing their best to pollute. That was the intention and it's worked out well.

Though the design of the system as a whole takes advantage of latent heat in the boiler room, the upstairs landing and even of heat leaked out by our water heater, it makes no attempt at all to recover heat from the air which we extract from downstairs. This probably results in a slightly higher heating bill than we would have in a completely closed house, but the air quality difference is enormous. In any case, our heating bill is absolutely tiny so there isn't much to save. I may end up building a heat exchanger downstairs as that seems like a fun thing to try to do, but I doubt we'll be able to measure much of an effect.

We went through last winter with an incomplete prototype of the system. I initially had an old computer fan for the air inlet upstairs, but it wasn't adequate to keep up with the downstairs fan. It also initially just switched on and off with a timer and there was no attempt to even try to balance flow with the downstairs fan. A roughly balanced flow of air does seem to create a better result overall.

Something that surprised me is how poorly CO2 travels between rooms. With no open windows but the door between our bedroom and the hallway open, the CO2 level can reach over 1300 ppm in our bedroom despite the hallway being around 700 ppm. I was surprised that diffusion didn't do a better job of lowering the CO2 concentration in our bedroom. Running one of the Philips air filters in the room moves air around enough to lower the concentration by about 200 ppm. But to achieve a really low CO2 concentration around our heads I think we would need a fan blowing air through the doorway.

Of course in the past no-one had a CO2 meter so we could never measure these things. When we first had the Aranet 4 but we didn't have any kind of ventilation system we'd see the CO2 ppm race up and beyond 2000 ppm in the evening in our living room just from our breath. It also headed in the same direction in the bedroom overnight. I suspect that we've unknowingly slept with very high levels of CO2 in past years without realising it and I think the same is true for most people. CO2 rises very quickly in any room with closed windows and doors, and using candles also rapidly increases the CO2 concentration.

I think a ventilation system has much to recommend it. It's a step beyond what can be achieved by air filters on their own. This system works. If there's interest I'll put the Arduino code somewhere that it can be downloaded.

Saving energy with new appliances ?

I've always been a bit skeptical about the idea of replacing appliances early in order to save energy. The embedded emissions in creating large appliances are not small, so you'd have to make a pretty big saving in ongoing emissions for it to be worth scrapping a device early to install another in its place. Generally speaking, I prefer to repair things and keep everything working for as long as I possibly can. But sometimes it does make sense to buy something new.

Since we moved into our home in the Netherlands 17 years ago we've done a lot to make our home more energy efficient. We've improved the insulation, replaced the windows, built a ventilation system (which I'll write about in due course), removed the gas supply altogether, and installed solar panels to generate more electricity than we consume. But the inside of our home hasn't seen so much work.

The kitchen was a bit tired looking when we moved in, and it's not got any better with almost two decades of use, so this year I've been working on improving things.

The kitchen cabinets were mostly actually still OK but the doors mounted on them, which were made of thin white plastic coated chipboard, were falling apart and they looked super ugly. Rather than throw everything away I've made new doors from scratch to fit onto the old cabinets, and also constructed new cabinets of plywood where they were needed. This is also much cheaper than a complete new kitchen, so it's a better fit with our limited finances. We then had the question of the refrigerator. The old built-in refrigerator was in the house when we moved in and I always had the idea that it consumed at lot of electricity because it hummed almost constantly. Unfortunately, until I started taking the kitchen apart I couldn't reach the socket where it was plugged in and measure it.

Measuring refrigerator/freezer consumption
The old (late 1990s / early 2000s) refrigerator turned out to use a whopping 500 kWh of electricity per year. That's five times as high as some newer models of the same size, small refrigerators without freezer compartments. We also had a freezer which we'd brought with us from the UK. This was a Liebherr unit which was a well rated model when we bought it. I'd looked around and found one of the first devices available which did not use CFC style refrigerants. But I measured the freezer as consuming 250 kWh per year, which is actually not more than some new comparable models. But 750 kWh per year in total for refrigeration is ridiculous.

New A-rated fridge/freezers with a similar capacity to our old fridge and freezer combined are rated as consuming about 110 kWh per year. None of the A rated devices are built in types so we had to choose between free-standing fridge freezers. We chose an Inventum KV2010B as this company gives a standard 5 year guarantee and provides reasonably economically priced parts for repairs.

New and slightly imposing fridge/freezer in our kitchen, next to kitchen units with newly constructed home made doors. For some reason all the A-rated fridge freezers are black. Whatever happened to "white goods" ?

The real world consumption of the KV2010B in our first week works out as equivalent to 190 kWh per year. That's above the rated 113 kWh / year according to the manufacturer. However the standardized tests are carried out, the conditions are clearly not the same as ours over the last week. Of course a week at the end of June is warmer than the annual average and I expect this appliance will use a bit less electricity in colder months (our old devices also consumed less in winter than in summer).

Energy rating certificate for our new fridge/freezer.

The same refrigerant, R600a, or isobutane, is used in the new fridge freezer as in our old freezer. I don't know what was used in the old refrigerator which we didn't choose, but it was probably something horrible. When we bought the freezer this was an unusual refrigerant but it seems to be commonplace now. That's quite an improvement. R600a has a very low GWP (global warming potential) of 4 compared with CFC refrigerants which can be in the thousands. It's not quite so low as the R290 of our heatpump, though.

The value of the electricity saved
We expect to save almost 600 kWh per year with this new appliance in place of the two old devices. That's significant. It's almost as much electricity as we used for all our heating last winter. It's also about the same as the output from two of our solar panels. I think it's worth noting that much of the saving will occur during night-time, on the shorter days of winter, or when there's not much sun. i.e. at times when it's especially valuable for us to save electricity because we don't have so much of it available from our own panels.

In the Netherlands the total retail cost of electricity including all taxes is currently around 25 c / kWh so a saving of 600 kWh of electricity is worth about €150 annually. Having paid €799 for the appliance the appliance should pay for itself in about five years. It will also save 134 kg of CO2 each year (at 223 g / kWh average emissions for NL). The actual cost for us is much more difficult to work out because we generate more solar electricity than we use.

Disposing of the old devices

The people who delivered the new fridge/freezer took the old fridge away for recycling, but they would only take one appliance, so I had to take the freezer on a 7 km trip to the local recycling centre by bike. Both of the old appliances were the same size as this.

At the recycling centre they picked it this way, putting considerable pressure on the radiator at the back and risking causing a leak of the refrigerant. That's why I didn't buy a CFC containing freezer in the first place. I suspect that even when they're supposed to be safely removed, only a small percentage actually are safely removed.

An earlier trip to the recycling centre. Judy and I transporting parts of the kitchen cabinets, mostly rather nasty damaged doors but also the parts which contained the old built-in fridge.

Update: Energy meter problem
It turns out that our plug-in energy meter has a problem. It's reporting 270 V as the voltage of our mains supply instead of the ~225 V which the multimeter reports. 270 V is above the legal maximum supply voltage. I believe the multimeter. Anyway, this 20% high voltage reading probably translates into a 20% over reading when measuring Watts as well, so the first week of the new refridgerator was probably a lot closer to the manufacturer's specification than what I measured.

Twelve years of rooftop solar power

Our rooftop solar power system was installed 12 years ago today. In total they've generated 40900 kWh since installation. While last year (2022-2023) was a record year for output from these panels, this year (2023-2024) was not. The end of 2023 and beginning of 2024 were marked by particularly grey and dull weather so this  output is not surprising. But the output of 3362 kWh over the whole year is still only slightly below the 12 year average of 3378 kWh per year.

Total output of our solar power installation per year. The blue columns are the contribution of the 12 year old rooftop system. The red shows the additional power generated by the extra panels we've installed on our garage roof.

The garage roof panels added an additional 1416 kWh to the total for the year. These were installed as we wanted to to compensate for the consumption of the heat pump and electric water heater that we installed when we got rid of our gas connection.

This winter was the dullest that we've recorded, resulting in the blue bar for March 2024 being easily the lowest in the graph. Even the substantial contribution of the new panels, shown in red, didn't result in higher total output than we have seen in brighter March months in past years from the rooftop panels alone. But the extra panels still helped us to generate a higher proportion of our consumption this winter than we have done without them.

Our rooftop panels having been operating for 12 years also of course means that the inverter has been operating for 12 years. The inverter actually only lasted for six years and three months before it failed due to poor soldering and the manufacturer refused to fix it. I fixed it myself and the repair that I made has very nearly doubled the life of the inverter thus far. I am still very irritated that ABB, the inverter's manufacturer, preferred to tell us that the whole inverter had reached the end of its life and needed to be thrown away and replaced with a new one when they could have made the same simple repair in order to keep it operating.

Anyway, the system as a whole is still working very well. It's difficult to work out exactly by which date this system paid for itself because the electricity price has changed over time. But the cost of electricity to consumers, including all taxes etc., has always been higher than 19.5 c, so I think we can now reliably state that the original purchase price of €8000 has been repaid by the solar panels and inverter together. Since 2012 the cost of solar panels has dropped precipitously and I expect the extra panels on our garage will cover their cost in under 4 years.

If you've read this far you'll probably also be interested in my blog post from four days ago about how the heat pump, electric water heater and solar panels together have reduced our energy bill to less than zero.

Relative cost of heating with a heat pump vs. gas (How to reduce your heating bill by 100%)

We've now had our heat pump for one whole winter. For the purposes of this blog post, "winter" refers to each period of November through to and including March, as those are the months when we use heating. Because we've gone through our first winter period with the heat pump we can now make a comparison with other winters when we used gas for heating and work out both the financial cost and (more importantly) the emissions due to both heating options.

This house was cold
We've lived in our home since 2007 and I've recorded our energy consumption consistently each month. Our home was built in 1972, just after the discovery of gas under the Netherlands. At this time it was assumed that gas would always be an inexpensive fuel source and no-one was making much effort to make housing efficient. We had very little insulation and single glazing in most of the windows. Only the living room, dining room and kitchen had old and basic double glazing.

When we worked out how much the first winter's energy bills were going to be this really shocked us and we ended up setting the thermostat as low as 14 C and in order to reduce the gas consumption we even turned the heating completely off on some of the coldest days of winter until our children were about to come home from school. This was quite unpleasant so we started some steps to insulate our home even in the first winter in order to reduce the energy requirement.

Each insulation job contributed to reduced heating requirements and a warmer house. At first there isn't much of a reduction in consumption to see, but that's because we were compensating by living with a bit more warmth each winter. The difference between the blue and red line was the consumption of the pilot light in an old gas water heater that we replaced with an electric water heater last year.

By the winter of 2022/2023, the last in which we still had a gas connection, our heating consumed just 287 m3 of gas rather than the average of just over 1110 m3 of gas per winter that we had burnt over the first four years that we lived here. That's about a 70% reduction in energy input, without changing the heating system.

This winter we used an air-air heat pump in our living room as almost the only heat source between November and March. There's a heated towel rail / IR panel in the bathroom which comes on with a timer every morning but otherwise no permanent heating upstairs to replace the no longer used radiators. We did make very occasional use of portable electric heaters. We've been quite warm. It's certainly far more comfortable upstairs now without any heating than it was in the first years that we lived here when hot radiators struggled to keep uninsulated bedrooms with large single glazed windows up to a comfortable temperature.

Energy consumed
Our heat pump consumed 682 kWh of electricity over the five month winter period. That's roughly equivalent to the energy content of 68 m3 of gas. i.e. this winter we used only about 6% as much energy to heat our home as was the case when we first moved here.

I have to admit that this was not a particularly cold winter, but it was especially grey and wet so the solar panels that were supposed to run the heating didn't do as well as was expected. Despite that, even this winter, they still generated more than 30% of the electricity that we consumed over those five months so we had to buy just 470 kWh of electricity from the grid to run our heating.

CO2 output
We're signed up to a green tariff which promises us 100% green electricity ("100% groene stroom uit Nederland") but I never know if you can really trust such a claim so I will instead take the an average carbon intensity of 223 g per kWh for Dutch electricity in 2023 as a worst case scenario for our CO2 output.

That worst case scenario suggests that our heating may have produced 105 kg of CO2 this winter (470 * 0.223), which is a huge reduction compared with the emissions of our gas boiler when we first moved in. Our gas heating CO2 output averaged around 2000 kg of CO2 per winter over the first four years that we lived here. i.e. the worst case scenario gives us a 95% reduction in CO2 output for heating due to a combination of insulation and electric heat pump replacing the gas boiler. The first 75% or so of that reduction in emissions is due to the insulation and the heat pump is responsible for the rest.

What would it have cost if we were still using gas ? What has it cost this year all-electric ?
We could never afford that level of gas usage so we shivered more than most when we first came to live in the Netherlands. If we'd continued burning gas at the rate that we did in the first four years that we lived here then it would have cost over €1600 for heating this winter. Instead of this, we insulated our home in order to consume far less heat. If we had stayed as we were last winter, with the same insulation but with gas heating instead of the heat pump, then we'd have consumed about €400 worth of gas over the winter period. That's already quite a saving over the average.

According to our energy supplier, the gas consumption of an average home like ours (semi-detached / 2 onder 1 kap) over the winter period is around 1360 m3, which is actually higher than what we found unacceptable when we moved here.  At the current price of gas quoted by the energy that gas costs an average family in an average house like ours about €1930 each winter and each of those homes will produce over 2400 kg of CO2 for heating over that period.

But our gas supply was removed last year and we're all electric now, so what did it actually cost us to heat with the heat pump ?

For the last year we've been paying our energy company €5 per month for the electricity connection only (there is no gas connection). We've just come to the end of the yearly billing period and they now owe us more than €200 for the excess electricity that we produced from our solar panels and contributed to the grid. i.e. our energy bill is negative.

End of year summary from our energy company. They owe us €252, and we can continue to pay them €5 per month next year.

Insulate ! It makes a huge difference to your comfort and your bills. Then you can install a small and inexpensive heatpump.
Every step that we've taken in the past to better insulate our home has led to lower bills, lower CO2 output, and more comfort. It took us a while to do everything because our income is small so we couldn't do it all at once. But do everything you can, as soon as you can. It's really worthwhile to do all of this before you even think of replacing the heating system.

If you're in a rented home that makes things more complicated, but encourage your landlord to everything they can. This is a no-brainer for any sensible landlord as any work done adds to the value of their asset. If you're in an apartment and shared ownership of walls and roofs is a problem, then do whatever you can to get the organisation (probably a VVE in the Netherlands) to make changes. That's a difficult situation because you have to get a lot of people to agree. But it's all worthwhile. Every penny spent on heating is wasted, every penny helps to pollute the planet, so let's stop spending so much on it.

Reducing energy input
Something that seems quite crazy to me is that people replace absurdly oversized gas-powered boilers (ours was rated at 28 kW!) with equally absurdly oversized heat pumps. Yes, they'll cost less to run and have lower emissions than gas heaters, but they still consume a lot of electricity. Those high powered heat pumps require a three phase connection (single phase is limited to 16 A / 3.5 kW in the Netherlands) in order to suck in enough current to produce their huge outputs. What we installed has a 3.5 kW maximum output and it consumes a maximum of about 1 kW when it's in use. It it really warms the room up quickly when it starts up, but it soon settles down to a lower power mode to maintain temperature.

I think it's important that we try not only to switch away from fossil fuels but also to reduce our total energy consumption. The energy transition certainly won't be made easier if we try to achieve that transition by installing lots of electrical devices which consume enormous amounts of energy.

Once a house is well insulated it just doesn't need much heat input. At that point, an inexpensive air-air heat-pump like ours can keep your home at a comfortable temperature. Our heat pump and everything I needed to install it myself together cost significantly less than one year's average winter heating bill. The result of installing this device is that we no longer have a heating bill. Why would anyone not try to do this if they can ? We may now install a second one upstairs in the room where I work, but that's really a luxury: I've been fine this winter. If I didn't work from home I'd probably not be considering it at all.

Effect of heat pump and electric water heating on our electricity bill in December 2023

Since we had our gas supply removed last year we've used electricity for both our water heating and our home heating. Unsurprisingly, this means we're using more electricity, especially in winter months as we no longer burn gas for heating.

We consumed 222 kWh more electricity from the grid in December 2023 than we did in December 2022.

Our heat pump consumed 168 kWh of electricity in December and the water heater used about 70 kWh. It's been a bit chilly upstairs sometimes so we've also used some small electric heaters occasionally, but clearly we also managed to reduce our consumption of electricity elsewhere as otherwise the numbers don't quite add up.

We had hoped to compensate at least some of the increased electrical consumption by expanding our solar power system. Unfortunately, due to the last quarter of 2023 being incredibly grey and rainy (a new record for rainfall was set, largely due to rainfall in the last three months of the year), the expanded system produced just 42 kWh in December, vs 60 kWh from the smaller system a year before.

Part way into January, waiting for ice to melt off the extra panels so that they could have full performance, if only the sun came out properly...

The gas we didn't burn, and the resulting CO2 emissions
In December 2022 we burnt 125 m3 of gas. That's less than an average apartment and well under half the average for a house like ours. This year we of course burnt no gas at all. 125 m3 of gas contains the equivalent of about 1250 kWh of energy, so the 222 kWh extra electrical energy that we drew from the grid was considerably less than that contained in the gas that we used to burn.

The 125 m3 of gas which we burnt in December 2022 produced 223 kg of CO2 (factor of 1.78). The average CO2 intensity of Dutch electricity for 2022 was 321 g / kWh meaning that our extra 222 kWh of electricity consumption in December 2023 will have led to 71 kg of CO2 emissions if our electricity was of average CO2 intensity for the Netherlands. That's a worst case scenario as even in the exceptionally grey month which just passed, 8% of our electricity still came from our solar panels. We are of course also signed up to a tariff which claims to supply us with zero CO2 green electricity (despite this not always being possible to do).

Therefore in the worst case our emissions in December as a result of replacing the gas supply with electricity were less than a third of what they would have been if we'd continued to burn gas. In the best case we did a lot better than that, but we're then in the realm of guesswork based on where our electricity might really have come from. When a large proportion of Dutch electricity still comes from burning fossil fuels it's nonsense to ever claim that electricity has zero emissions.

An average Dutch household in a home like ours will have consumed around 300 m3 of gas in December, resulting in around 530 kg of CO2 being emitted so in the worst case we had around 1/7th of the emissions of an average household.

Update: Dutch emissions per kWh electricity may actually be much lower.
It's possible that emissions in 2023 per kWh were actually much lower than 321 g. A smart guy on Mastodon calculated that the true figure was actually around 223 g / kWh for the Netherlands in 2023. This would have the effect of reducing our worst case emissions for heating in December to just 50 kg, meaning that we emitted about a fifth so much CO2 this year compared to last, or around a tenth of the amount emitted by an average similar size household using gas for heating.

Costs
It's difficult to work out exactly what the cost of gas would have been, but based on pretending to take a new contract out with our electricity supplier it appears that they would have charged us about €200 for the 125 m3 of gas had we used it in December. The cost of the extra electricity that we used is about €100.

But actually we deliver more electricity to the grid each year than we consume, so we only pay €5 a month for energy. At the moment our supplier says they still owe us about €260. This amount becomes due in mid February so we won't get quite that much returned to us because we expect to use more electricity than we produce for heating in January and February as well.

How well did the heat pump work in the cold ?
The lowest temperature in the morning that we've seen so far was about -7 C. There was plenty of heat from the heat pump. It does need to pause and defrost itself occasionally when it's cold outside.

Onward and hopefully downward
December is the worst month of the year due to the short daylight hours. Let's hope we can take proper advantage of the sun in January, February and March as more sun means lower emissions.

This may look like a grey rectangle but it's an actual photo of the sky today. The sun is roughly in the centre (that's a guess as I couldn't see it). Not exactly ideal weather for solar power.