Archive for October, 2021

Heat Pump Operation: What it costs. And what it costs me.

October 26, 2021

Friends, yesterday I wrote about the way the new Vaillant 5 kW Arotherm plus Heat Pump performed during 24 hours of a mildly-cold autumn day.

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 24th October 2021.

This was really important data for me. I have spent the best part of £60,000 insulating the house; installing solar PV and a battery; and plumbing in the air source heat pump (ASHP).

But this was the first real-world indication that the system worked more-or-less as I had anticipated.

Understanding how the heat pump works is complex. And surprisingly, explaining what it costs to operate is more complicated than I expected, and slightly personally embarrassing.

Let me explain


Recently I switched to Octopus Energy and they charge me a daily standing charge plus a rate dependent on how much gas or electricity I use.

Standing Charges 

  • Gas: 23.85p per day.
  • Electricity: 25p per day.


  • 3.83p per kWh of gas.
  • 5.0p per kWh of electricity between 00:30 and 04:30.
  • 16.26p per kWh of electricity between 04:30 and 00:30.

All these rates are likely to change – increase – in the future.

Domestic Hot Water

As the graph shows, on 24th October, the heat pump used 1.6 kWh of electricity to create 4.9 kWh of heat in the hot water cylinder.

If I had used gas to do this using a boiler which was 90% efficient I would have needed to consume 4.9/0.9 = 5.4 kWh of gas which would have cost 5.4 kWh x 3.83p/kWh = 20.7p.

Using the heat pump I consumed 1.6 kWh of electricity at 5 p/kWh (because it was heated between 00:30 and 04:30) which cost 1.6 kWh x 5.0p/kWh = 8p.

So for heating domestic hot water, the heat pump is 62% cheaper than using gas.

If we had considered heating hot water during the summer, then the electricity would have been drawn from the battery which would had been charged by solar electricity, and so would be free.

Space Heating

As the graph shows, on 24th October, I used 2.3 kWh of electricity to create 10.0 kWh of heat in the house.

If I had used gas to do this using a boiler which was 90% efficient I would have needed to consume 10/0.9 = 11.1 kWh of gas which would have cost 11.1 kWh x 3.83p/kWh = 42.5 p.

Using the heat pump I consumed 2.3 kWh of electricity at 16.26 p/kWh which cost 2.3 kWh x 16.26p/kWh = 37.4p.

So for space-heating in this mild climate, the heat pump is 12% cheaper than using gas.

Actually its a bit better than this. Because we have a battery, we fill up with cheap rate electricity at 5 p/kWh so the actual cost of using the heat pump during the day is closer to 2.3 kWh x 5p/kWh = 11.5p i.e. 73% cheaper than using gas.

And actually, its even a little better still. Even at the end of October typically 50% of our electricity is derived from the solar PV system and stored in the battery. So the actual cost per unit is closer to 2.5p/kWh, so the actual cost of using the heat pump during the day at this time of year is closer to 2.3 kWh x 2.5p/kWh = 6p i.e. 85% cheaper than using gas.

The solar contribution will go down as we head into winter, but even in midwinter we still get ~ 2 kWh/day of generation, all of which is now captured in the battery. See the graphs below for more details.

Click image for a larger version. This complicated graph shows solar generation since the solar panels were installed in November 2020. The thin green lines with circles are daily generation. The thick pink line is 7 day running average. The big green circles are monthly averages, and the yellow circles are monthly averages for this location from 2005 to 2016.

Click image for a larger version. This graph shows household electricity consumption (averaged over ±1 week) during 2021. After the battery was installed, we used very little grid electricity for about 3 months. The blue line shows actual household consumption As we head into winter, our use of grid electricity is increasing. The dotted green lines show the range into which I expect grid electricity to fall depending on how cold the winter is, and how well the heat pump works.

Preliminary summary

For heating hot water, using a heat pump costs less than half the cost of using gas when using cheap-rate electricity.

For space-heating, using a heat pump costs 12% less than using gas even when using full-price electricity. And accounting for the solar generation and use of cheap-rate electricity stored in the battery, space-heating will cost a small fraction of what it would have cost using gas.

When I take account of all the variables, including the standing charges, the heat pump, the solar PV and the battery, I estimate that my annual bills from electricity and gas will be reduced from roughly £1600 to about £500 including £178/year of standing charges.

So financially, I am benefiting from my £60,000 of investment to the tune of around £1,100 per year.

But there are some complications…


I feel obliged to disclose that when I installed the heat pump, in addition to a £5000 government grant, the kind staff at Enhabit pointed out that I could I apply for funding under the Renewable Heat Initiative (RHI).

This scheme funds people who switch to using renewable technologies to heat their homes, compensating for the fact that electrical heating is generally more expensive than gas.

It didn’t seem to matter that – as I showed above – in my circumstances the heat pump would be cheaper than using gas.

So in addition to saving around £1,100 per year, the RHI scheme will additionally pay me £129.83/quarter or £519.32 per year for the next seven (7) years. Yes, I really did write that.

So broadly speaking, I don’t expect to pay anything at all for electricity or heating for the next 7 years.


I also feel obliged to disclose that when I installed the heat pump, the kind staff at Enhabit also pointed out that I could I apply for funding to monitor the heat pump performance using the so-called Metering and Monitoring Service Package (MMSP).

Under this arrangement, I needed to pay £1,156 to have a system installed which monitored the performance of the heat pump, and reported the data back live to OFGEM every 2 minutes.

This helps OFGEM establish the real world performance of heat pumps rather than relying on manufacturer’s specifications.

In return, I get:

  • Access to the data – that’s the data I used for the previous article.
  • A one off payment of £805
  • Annual payments of £115 (paid quarterly) for the next 7 years.

Overall this amounts to £1,610. This easily covers my outlay; provides important data to OFGEM; allows me to write endless blog articles; and reduces even further the cost of heating my home for the next seven (7) years.

Final Summary 

Friends, even without any Government deals, the heat pump would already be saving me money over a gas boiler.

And the use of solar PV and a battery makes the advantage even larger.

But taking advantage of the Government’s subsidies I find myself in receipt of a financial windfall – and it seems unlikely I will need to pay for electricity or heating for the next seven years.

Frankly, I am a little embarrassed.

However, even without these deals, switching to a heat pump would already be saving me money.

Keep warm!



Heat Pump: First Space-Heating Results

October 25, 2021

Friends, the unseasonably warm autumn has meant that I have had to wait until 23rd October for our 5 kW Vaillant Arotherm plus system to switch itself on.

I only have a couple of day’s data, but the results are interesting and promising. First I will show the data for each day and then discuss what it means at the end.

23rd October 2021

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 23rd October 2021.

From midnight until 6 a.m., there is no space heating, but the heat pump operates for 1 hour to heat the domestic hot water (DHW) tank. Performance for hot water heating is described in these articles (1, 2)

From 6 a.m. until midnight the system is responding to the thermostat set at 19.5 °C and heated water is circulated around the radiators.

The heat pump operates 12 times up until 6 p.m. after which no additional heating was required.

As shown on the graph, 12.1 kWh of heat was delivered using only 2.9 kWh of electrical power. which over 12 hours amounts to an average heating power of around 1 kW using only 250 W of electrical power.

The ratio of thermal to electrical power is known as the coefficient of performance (COP) and this is summarised in the graph below

Click image for a larger version. The graph shows Coefficient of Performance (COP) of the heat pump during the 24 hours of 23rd October 2021.

The COP seems to increase slowly through the day peaking for a few minutes in each cycle at values as high as 6, but the space heating average is 4.2.

Click image for a larger version. The graph shows various temperatures relevant to heat pump operation during the 24 hours of 23rd October 2021. The external temperature; the internal temperature; the flow temperature in the radiators; the temperature of water in the DHW cylinder.

The initial flow temperature is ~34 °C which falls through the day to ~28 °C. This reduction in flow temperature is in response to the increase in external temperature from ~12 °C to ~15 °C.

This so-called ‘weather compensation’ allows the use of lower flow temperatures, which enables the system  to operate with the highest possible COP.

I was surprised that even with my unaltered radiators, flow temperatures of 35 °C were sufficient to warm the house.

24th October 2021

Here is the equivalent data to that for the 23rd – the graphs are similar and I show them only to show that the system seems to be behaving reproducibly.

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 24th October 2021.

Click image for a larger version. The graph shows Coefficient of Performance (COP) of the heat pump during the 24 hours of 24th October 2021.

Click image for a larger version. The graph shows various temperatures relevant to heat pump operation during the 24 hours of 24th October 2021. The external temperature; the internal temperature; the flow temperature in the radiators; the temperature of water in the DHW cylinder.


My first conclusion is that the system works. This is quite a relief!

My second conclusion is that the system works slightly better than I had been hoping for.

Being able to heat the house with these low radiator temperatures means that over winter the average COP could be higher than my prior estimate of 3. This means I will use less energy, emit less CO2, and spend less than I had estimated.

My third conclusion is that average heating power was about 1 kW during both days. But the system has plenty of margin to deliver more power either by changing the duty cycle – the pump could stay on for longer – or by increasing the flow temperature.

As soon as the weather settles down to being reliably cold I will begin to carry out experiments to optimise the weather compensation.

My fourth conclusion will be the basis of another article – but I can see already that as far as heating is concerned – I am going to have a very cheap winter.

Keep warm.

New Things

October 19, 2021

Friends. My name is Michael, and I am a Technological Utopian. It’s been three weeks since I last bought a new thing.

There is an undeniable pleasure to be obtained from ‘New Things’. From opening packages, to discovering how each ‘New Thing’ works and how it will fill an imagined gap in one’s life.

But as I wrote previously, and as many others have pointed out, every single thing we buy will end up as rubbish.

In a change of format, here is a song I wrote about my difficulty with ‘New Things’.

I like your new thing, it’s got that feature.
I always wished that my old thing would do.
It’s so whizzy and so snappy,
And you seem so proud and so happy,
I think I’ll throw out my old thing and get one like you.

Do you like my new thing? it’s future rubbish.
Do you like my new thing? it’s future scrap.
This feast we are consuming is delicious,
But in a little while, it all turns to crap.

The new one is much faster than the old one.
It’s half the size and it can recognize my face!
But the old one was a classic,
So I’ve put it in the attic.
With a pile of other stuff I keep there ‘just in case’.

Why can’t I tame my acquisitory habits?
No matter what I have it never seems to be enough!
There always seems to be just one more thing,
And the little buzz of happiness it brings.
But in the end there’s just a bigger pile of stuff.

Do you like my new thing? it’s future rubbish.
Do you like my new thing? it’s future scrap.
This feast we are consuming is delicious,
But in a little while, it all turns to crap.

Well I know that I cannot take it with me.
Just like me, it’ll end up in a hole.
But this pile of earthly treasures,
Brings momentary pleasures,
And distracts me from the void within my soul.

So I called up my old friend the Dali Lama.
I said ‘Dali, babe’ – I’m suffering I don’t understand?
He said “This world is e-labor-ate illusion
Filled with sadness and confusion,
So be kind to yourself, and do the best you can”.

And then he looked me in the eyes…
The Dalai Lama looked me in the eyes.
And he took my hand in his.
And he said “Michael…

Do you like my robes? They’re future rubbish.
Do you like my robes?? They’re future scrap.
Yes, this feast we are consuming is delicious,
But in a little while, it all turns to crap.


COVID-19: Wave#3: The Effect of Vaccines

October 16, 2021

Click on the figure for a larger version. Charts showing the rates of cases, hospitalisations and deaths across different age groups – with categorisation according to vaccination status: Double-Vaccinated (blue) or Not-Vaccinated  (orange). Individual charts are also shown below.

Friends, as I mentioned in my previous blog, I have been puzzled about “what is going on” in the pandemic right now.

The data I have seen seem complex and difficult to interpret. The statistics involve different age groups, geographical locations, and vaccination status.

But I came across some graphs on-line that seemed significant. So I tracked the data to a regularly-updated Public Health England (PHE) surveillance report for the 4 weeks up to 10th October 2021 (Link). I then reproduced the graphs from the tables therein.

The charts show rates of…

  • positive COVID-19 cases,
  • hospitalisations, and
  • deaths,

…versus age group, categorised according to vaccination status. Note that these are rates per 100,000 people, not absolute numbers.

CLARIFICATION added on 17 October.

In each age group, the two rates shown are:

  • the number of cases, admissions or deaths per 100,000 vaccinated people and
  • the number of cases, admissions or deaths per 100,000 unvaccinated people.

They are NOT the number of cases, admissions or deaths per 100,000 members of the population in that age group.

I think they tell a story.

Click on the figure for a larger version.


As has been the case throughout the pandemic, age is the primary risk factor.

This data tells us that in any age group, being un-vaccinated is typically 4 times more dangerous.

The risk ratio is worst in the 50-59 year-old age group where the rates of death amongst the un-vaccinated are more than 8 times higher.

Note: this death rate is affected by both the number of people infected in each age group and their risk of death. The high risk ratio in this age group is likely to be because these people are more likely to be still working, unlike people aged over 60. 

It’s also important to note that amongst the older age groups most people are double-vaccinated – more than 80% of the over 50’s are double-vaccinated.

Actual deaths in the under 18 age group are sadly not zero. Over the four weeks of coverage, the tables record the deaths of 3 unvaccinated youths, and 1 who had only recently had a single shot.


Click on the figure for a larger version.

Hospitalisation data show that age is again the primary risk factor – but now even younger people are being affected.

Actual admissions in the under 18 age group were 408, of whom all but 12 were un-vaccinated.

Taking the over-50’s all together, there were 3360 hospitalisations – 660 of whom had not received a double dose. So around 2700 had been double-vaccinated. This ratio (2700/660) is around 4.1, not far off the ratio of the number of double-vaccinated people to unvaccinated people.

This seems to indicate that the vaccines have only a partial ability to prevent illness serious enough to warrant hospitalisation.


Click on the figure for a larger version.

The ‘case data’ show a striking contrast with the hospitalisation and death data. This data shows that the epidemic is spreading predominantly amongst young people.

It is also striking that amongst those older than 30, the case rates-per-100,000, are higher amongst the doubly-vaccinated than amongst the un-vaccinated.

This effect arises because (a) even double-vaccination does not fully protect against infection and (b) there are many more doubly-vaccinated people than un-vaccinated people.

This seems to indicate that the vaccines do not offer strong protection against catching COVID-19.

And so the story is…

I think the COVID-19 epidemic is being kept going by high infection rates amongst younger people – presumably in schools.

Schools – presumably – form infection hotspots through which pupils and support staff infect people in the wider community.

Double-vaccination does not protect against infection, but it does seem to reduce the rates at which infections warrant hospitalisation, and is highly – but not perfectly – effective at preventing death.

So what will happen next?

The future is amongst the most difficult things to predict. But given the Government’s laissez-faire policy, it is hard to imagine that they will not allow the epidemic to continue to infect everyone it can possibly infect.

And some fraction of those people – between 0.1% and 1% – will die. This probably amounts to tens of thousands more deaths over the coming winter.

The only thing which I can see that might cause the Government to think otherwise would be if the Health Service became overwhelmed.


Personally, I find this data deeply depressing.

This data speaks of an ongoing crisis, killing more than 100 people each day. But a crisis which the Government seems to refuse to acknowledge.

Having reflected on this data, I (double-vaccinated and aged 61) will be taking even more care than I have been up to this point.

And if you have not yet been vaccinated…

COVID 19: Wave#3. 10,000 deaths

October 14, 2021

Friends, I last wrote about the pandemic six and half weeks ago on August 29th. At that point the COVID third wave had killed around 4,300 people.

Since then, the disease has been killing just over 100 people per day. And as the death toll ticks over the 10,000 mark, it’s probably a good time to look and ask: what is happening?

Click the image for a larger versions. Logarithmic graph showing positive caseshospital admissions and deaths since the start of the pandemic. The bold horizontal dotted lines are to help one reference the situation 1 year ago. The blue arrows show the dates of recent ‘opening’ events. The green dotted line shows an extrapolation from the first week of June. The blue dotted line shows an extrapolation of trends at the start of September, doubling every 42 days. Also highlighted in purple are the Euro finals, and the dates of returns to school and university in 2020 and 2021.

Compared with a year ago…

There are currently:

  • Almost 40,000 cases per day (x 3 compared with ~ 15,000 per day at this time last year).
  • Around 800 admissions per day (roughly the same as this time last year).
  • Just over 100 deaths per day (roughly the same as this time last year)

So nominally everything is the same or worse than last year!

But last year the epidemic was in a phase of exponential growth doubling every 11 days or so.

This year, things are more-or-less stable.

What does ‘stable’ mean?

  • The epidemic is still with us– more than 30,000 people per day are being infected.
  • The prevalence of infected people has been roughly constant for roughly 10 weeks.

By ‘stable’, I mean that the epidemic overall, is not in a phase of exponential growth or exponential decline. But these ‘stable’ statistics reflect a dynamic balance between different factors.

By this I mean that the factors which reduce transmission (masks, social distancing, vaccination, acquired immunity) are collectively sufficient to prevent increasing numbers of cases. But not sufficient to reduce prevalence.

The disease prevalence is high – more than 1% among many sub-populations – so anything which affects this balance could cause the epidemic to rapidly shift into a phase with exponential growth.

This could be an increase in indoor gatherings, a decline in the percentage of people wearing masks, or other small changes in behaviour.

I was happily surprised that the return to school in September did not have a large effect – despite many infections in schools.

And similarly I have been surprised that we have not seen (or at least not yet seen) a signal from the return to Universities in October.

However, the autumn has been mild so far, and it could be that the onset of winter coupled with ever more ‘normal’ activities could tip the dynamic balance in favour of exponential growth.

Factors against this would be the slowly-growing vaccination rate among young people, and the large number of previously-infected people with acquired immunity.

My expectation – for what it is worth – is that the prevalence (as evidenced by the number of cases per day) will grow as we go into winter. And there is the potential for exponential growth.

What to do?

Back in August I said “I don’t know!“. And I still don’t know. And indeed, what is ‘advisable’ doesn’t seem to matter to this government.

My guess is that if the death rate and hospitalisation rates remain similar to current rates, then ‘people’ will accept almost any level of infection rates. – no matter what the eventual harm from Long COVID, or the risk of generating further variants.

But if death and hospitalisation rates rise to the point where the health service is even more critically stressed than it is now. Or if the death rate rises much above 170 per day – 10% of normal death rates. Then further restrictions will become inevitable, even if the doubling time of the epidemic is very slow.

However my recent experiences – some of them traumatic – have led me to believe that large groups of people are extraordinarily and aggressively unsympathetic to other people’s caution, and would disregard any restrictions.

As we stare into the coming winter, I find it very hard to see how the epidemic will evolve; how the government will respond or how people will respond.

Let’s hope that the winter is kind to us all.

1000 days of data

October 9, 2021

Friends, those of you who work with spreadsheets may know that Excelworks out dates in terms of the number of days since 1st January 1900. Or in other versions of Excel, the number of days since 1st January 1904!

Inspired by this arbitrary choice, sometime ago I chose 1st January 2018 as ‘day 1’ for my measurements of energy use around the house.

And from this arbitrary starting date, I have been measuring for just over 1000 days!

So I thought it would be nice to summarise what has happened in the last three years, and to speculate on what the coming winter might hold.

Heating Demand

Our internal thermostats have been set to 19 °C since ‘Day 1’.

So I calculate the demand for heating as the difference between 19 °C and the average weekly outside temperature.

The heating demand (averaged over ±2 weeks) for the last 3 years is shown below.

Click image for a larger version. Graph showing smoothed temperature demand versus day for since the summer of 2018. Also shown is a projection of what the coming winter has in store if it is the same as last year.

I obtained this data from the weather station in my back garden, but you can use the wonderful Meteostat website if you don’t have a nearby station. I used Meteostat to fill in occasional gaps in the data.

So far, this autumn seems to be several degrees warmer than the equivalent period in 2020 i.e. demand is lower.

Gas Use 

Since just before day 1, I have been reading the gas meter once a week. From this I can work out the average rate at which I am using energy (i.e. average power). In this blog I express this as kWh/day.

[To convert kWh/day to watts, multiply the number by 1000/24 = 41.7 e.g. 50 kWh/day = 2083 W i.e. ~2.1 kW.]

Click image for a larger version. Graph showing smoothed temperature demand versus day for since the summer of 2018 as in the previous graph. Also shown are the dates of various interventions, and smoothed gas consumption (kWh/day) plotted against the right-hand axis.

It is clear that gas consumption roughly follows temperature demand. However the Triple-Glazing and External Wall Insulation (EWI) have reduced the gas used to meet a given temperature demand by about half.

In the summers of 2019 and 2020, gas consumption fell to roughly 5 kWh/day, most of which (around 3.5 kWh) seems to have been for hot water, with the balance being used for cooking.

Since the heat pump installation, in July 2021, the gas is only used for cooking (~1.5 kWh/day) , and this will continue until my wife and I can get our heads around installing an electric oven & hob.

Electricity from the grid 

Since just before day 1, I have been reading the electricity meter once a week. From this I can work out the average rate at which I am using electricity from the grid.

Click image for a larger version. Graph showing smoothed temperature demand versus day for since the summer of 2018 as in the first graph. Also shown are the dates of various interventions, and smoothed electricity consumption (kWh/day) plotted against the right-hand axis. Also shown is the small amount of electricity which was exported to the grid.

It is clear that electricity consumption was – through 2019 and 2020 – roughly independent of temperature demand.

Solar panels were installed at around the same time as the EWI (~day 660) but this did not substantially affect the amount of electricity we drew from the grid until we installed a battery (~day 810). After that, we drew very little electricity from the grid during the period March to September.

After the heat pump installation (~day 930), we began heating the hot water with electricity rather than gas. But heat pumps require only about 30% of the energy which a boiler would use to heat water.

Looking to the winter ahead, we expect solar generation to fall to around 2 kWh/day on average, but electricity use to rise above our normal ~ 10 kWh/day because the heat pump will be used for space heating (varying with the temperature demand) as well as hot water (~1 kWh/day).

Last winter gas consumption peaked at 50 kWh/day. If the heat pump operates with a coefficient of performance of 3 – which seems a safe guess – then this should require around 50/3 ≈17 kWh/day of electricity.

Carbon Dioxide emissions. 

From the data above it is possible to roughly estimate the corresponding carbon dioxide emissions.

I have assumed that:

  • Each kWh of gas consumption results in 0.2 kg of CO2 emissions.
    • This is fixed by the chemistry of methane combustion.
  • Each kWh of electricity imported from the grid results in 0.24 kg of CO2 emissions
    • This is an average of the last three years carbon intensity (Link).

There is some uncertainty in the figures above, but the assumptions are pretty uncontroversial. These represent estimates of actual amounts of CO2 which entered the atmosphere due to ‘my’ actions.

How one should deal with exported solar electricity is more controversial. Some people point out that because of the way electricity is ‘dispatched’, solar generation directly displaces gas-fired generation. Thus each kWh of my solar generation avoids the emission of  0.45 kg of CO2 emissions from a gas-fired station.

One might argue that such exports are therefore equivalent to negative emissions even though no CO2 is actually removed from the atmosphere.

With this assumption the daily carbon emissions are summarised in the graph below.

Click image for a larger version. Graph showing estimated household carbon dioxide emissions per day since the summer of 2018. Also shown are the dates of various interventions, and the expected emissions for the coming year. As discussed in the text, avoided emissions due to exports of electricity are counted as negative emissions.

The graph shows that – subject to the uncertainty of the projection – since 2018:

  • Winter emissions will have fallen from 25 kg/day to 5 kg/day – a 5-fold reduction
  • Summer emissions will have fallen from emitting ~3 kg/day to avoiding ~1 kg/day of someone else’s emissions.

Click image for a larger version. Table shows estimated carbon dioxide emissions in tonnes for the last three years (period July to June) along with a forecast of the emissions in the coming year.

The net effect of all these changes is gas emissions are now negligible. Electricity emissions were roughly halved by installing solar panels and a battery, but in the coming year they will probably return to roughly their previous value because of the electricity used to operate the heat pump.

There are two interesting things to note about the forecast aside from the fact that it’s a forecast and we don’t know what the winter will be like.

Firstly, this assumes the heat pump COP will be 3. My hope is that it will be better than this because the well-insulated house should require such a small amount of heating that I should be able to lower the flow temperature in the radiators to 40 °C. At this temperature the heat pump has a specified performance closer to a COP of 4 even with an external temperature of -5 °C.

[Aside. As of 9th October 2021, I am still waiting excitedly for the weather to get cold enough that the heat pump will switch itself on so I can test this! The insulation appears to be good enough that the internal temperature is still greater than 19 °C (~ 20 °C) without any heating!]

Secondly, 90% of the CO2 emissions now arise from electricity. So as the grid gets greener (we hope) in coming years, these CO2 emissions should naturally reduce. If the target 100 gCO2 emissions per kWh is reached in 2030, the overall household emissions will fall to under 0.5 tonnes.

What else?

There are still one or two things I could do to reduce household CO2 emissions. But at this point my intention is just to measure how these existing interventions perform for a year or two.

In 2018/19, household CO2 emissions comprised the largest category of ‘my’ emissions, and now they are more similar to emissions from other activities: consumption, transport and investments (i.e. my pension)

In the coming year I hope to turn my attention to these much trickier categories.

I’ll let you know how it goes…

Controlling Tap Temperatures with a Blending Valve

October 6, 2021

Friends, I wrote the other week about how controlling for Legionella in a domestic hot water system could lead to overly hot temperatures at hot water taps.

This presented the risk of scalding people using hot water for a day or two after the anti-legionella heating cycle had run.

The solution was to install a thermostatic blending valve on the output of the hot water cylinder.

This article is a short follow-on, showing how the blending valve behaves.

Blending Valve

Click image for a larger version. A blending valve mixes cold water with hot water from a domestic hot water tank to achieve a blended flow with a thermostatically-controlled temperature.

The Caleffi 5218 series valve I installed (data sheet as pdf) was specified to be settable for output flows between 45 °C and 65 °C, with each unit on the thermostatic control corresponding to a 2 °C change in flow temperature.

Obviously, this had to be checked!

Click image for a larger version. Testing the temperature of the tap water.

I tested the flow temperature using a thermocouple inserted in the water flow and waited for the temperature to stabilise.

As the data sheet makes clear, on first operation, there can be a short-period where the water temperature at the taps exceeds the set temperature. This seemed to be limited to about 10 to 15 seconds after which the water temperature was stable to within ±0.1 °C.

Click image for a larger version. The graph shows the measured flow temperature versus the thermostatic setting of the blending valve. The cylinder temperature was – evidently – about 56 °C, and the sensitivity of the setting was very close to its specified 2 °C per setting unit.

As the graph above shows, the valve performed exactly as specified. And although there is still a small risk of scalding due to the transient response of the valve, in practice, I think this risk is low.

Why? Because if the pipes through which the blended water is delivered are initially cold, then the over-temperature water will lose heat to the cold pipework.

I have now set the valve to a nominal 49 °C, and I propose to stop thinking about this problem. It’s a lovely day and I really want to get outside!


October 5, 2021

Friends, I gave a talk last week to the Richmond U3A – The University of the Third Age.

Disappointingly it was still a Zoom affair, but it appeared to be pass adequately.

After the talk I rashly thought I would run through it again and create a video presentation that I could share.

Having stared at my own face for several hours while trying every possible permutation of sound and video options in Windows™, I am no longer sure it was such a good idea.

But I’ve done it now. And as the saying goes, “I’ve suffered for my art, now it’s your turn…

You can download the Powerpoint file here.

The video of the presentation is in three parts, respectively 12, 16 and 24 minutes long.

Carbonaut: Part 1: 12 minutes

The first part is about the Climate Crisis and features a nice version of the Keeling curve – the curve that shows the increase in atmospheric carbon dioxide since 1959 – but expressed in tonnes of carbon dioxide in the atmosphere rather than concentration.

This makes it easier to understand how tonnes of emissions per person can lead to gigatons of emissions globally.

Carbonaut: Part 2: 16 minutes

The second part is about my ‘personal’ carbon dioxide emissions and explains how I have assessed the carbon dioxide emissions from house.

The key recommendation is to start reading your gas and electricity meter once a week.

Carbonaut: Part 3: 24 minutes

The final part is about what I have done to reduce carbon dioxide emissions from my house by 80%. It covers the installation of:

  • External Wall Insulation
  • Triple Glazing
  • Solar Panels
  • Battery
  • Air Source Heat Pump

It also covers what all this cost, the embodied carbon cost, and the likely carbon savings before the estimated date of my death. It’s quite a bit!

Cheap Electric Cars Are Coming!

October 3, 2021

Friends, back in March 2021 I wrote about the Wuling HongGuang – the most popular electric car in China – a car with a base model starting at $4,200.

Looking through the Fully-Charged channel’s coverage of last year’s Cheng Du motor show, many similar models are being manufactured in China – models which fill in all the gaps between the ultra-cheap and the normal price of an EV in the UK  – close to £30,000.

I’ve posted the video above, but I have also compiled a table of the cars showing a basic cost, battery size, and range.

Click for a larger version: Table showing basic statistics of cars discussed in the video above. All the numbers are uncertain and subject to change, but range of numbers is sufficient to show that EV’s can be made much cheaper than models currently on offer in the UK. Most prices are rounded up.

Summary: the future

From a Chinese perspective, I guess the UK looks like a small island in the Far West. But eventually their manufacturing capability will turn to exporting some of these models to us.

As I wrote previously, the mere existence of these models confirms that EV’s are actually simpler and cheaper to manufacture than ICE vehicles. And many of these models will be well-suited to retired people living in Teddington. And within their restricted budgets.

Of course, just replacing the overwhelming number of ICE vehicles with an equivalently overwhelming number of EV’s does not solve the UK’s problems with carbon emissions.

But the possibility that in a few years normal people will be able to buy EV’s with a range of properties and prices to match their needs does make clear that a realistic pathway is emerging for finally transitioning away from fossil-fueled vehicles.

And once that transition begins it will be hard to stop.

EV’s are cheaper to run and petrol stations run on low margins. As the amount of fuel purchased declines, petrol stations will go out of business, and in an ironic twist, ICE vehicle drivers will find themselves with “range anxiety”. They will have to carefully plan their journeys so as to be sure of finding an old-fashioned ‘filling station’.

Change is coming!

Legionella protection with heat pumps

October 1, 2021

Friends, I am loving our new Vaillant Arotherm+ heat pump.

Click image for larger version: Vaillant Arotherm+ Air Source Heat Pump has replaced our gas boiler. It provides up to 5 kW of heating but uses only (about) 1.5 kW electricity! It can either supply hot water – at up to 70 °C – to a cylinder, or circulate hot water through our radiators.

At the moment it is just heating our domestic hot water (DHW) but in the next couple of weeks I expect that it will begin to circulate water through our radiators in order to warm the house.

One aspect of using a heat pump for DHW at first sounds very alarming: it is a requirement to run an ‘Anti-Legionella’ cycle once a week or so.

‘Legionella’ is the name of an amoebic bacterium which lives naturally in water. It can breed in ‘warm’ water but is killed in ‘hot’ water. Very roughly it thrives in temperatures between 20 °C and 50 °C.

The bacteria are harmless to people when in the water – it’s fine to wash in the water. But if ultra-fine droplets are breathed directly into the lungs, they can cause a potentially lethal pneumonia called Legionnaire’s disease.

Even the droplets from a shower are unlikely to be capable of causing infection – they are too large.

But the disease presents an interesting risk profile.

  • IF the disease is caught (which is unlikely) it is potentially fatal to the elderly,
  • BUT prevention is trivial – just heat stored DHW to 60 °C or above.

What’s this got to do with heat pumps?

Most gas boilers are ‘Combi’ boilers which instantly heat the water as required, and don’t store hot water at all – so they offer no risk of harbouring Legionella bacteria.

So-called ‘system’ gas boilers which store water in a DHW tank typically heat the water to 70 °C – and so kill any legionella bacteria present.

But heat pumps typically struggle to heat water above 50 °C, and typically store water at 50 °C – which is easily hot enough for normal domestic purposes – but just opens the window to potentially allow Legionella to thrive in a DHW tank.

For that reason, modern heat pumps typically run an ‘Anti-Legionella’ cycle once a week which heats the water to above 60 °C. For older heat pumps this can involve the use of a direct electric immersion heater, but more modern heat pumps (such as mine ;-)) can heat water to 60 °C or even 70 °C with no problem.

No problem? So why am I writing this?

When we first started using the heat pump, I noticed that occasionally the water from the taps was extremely hot. It felt dangerously hot.

So I began to measure the temperature of the tap water at three outlets in the house: the kitchen, and two of the bathrooms. The temperatures were typically within ±0.5 °C of each other.

I found that on the morning after the anti-legionella cycle, the tap temperatures were just a bit less than 70 °C. It felt to me like it would be just a matter of time before someone was hurt by this.

Looking on-line (1, 2) I saw that the time to ‘scald’ was less than one second at these temperatures.

Click image for larger version: Graph showing the immersion time before ‘scalding’ for water at various temperatures. For flowing water, these times would be reduced. Notice that the vertical scale is logarithmic.

Safety Action Step#1

After about 1 month of use, I realised why the water was getting so hot. The anti-legionella cycle had been timed to run exactly after the daily heating cycle. It was this ‘double-heating’ which was producing the high water temperatures.

So cancelling the daily DHW heating cycle for that day (Wednesday) meant that the water coming out of the taps was now only about 60 °C – still ‘too hot’ in my opinion.

Safety Action Step#2

A little research on line showed that thermostatic devices existed which could prevent excessive temperatures reaching the taps. They were called ‘blending valves’ or ‘anti-scald’ valves.

These devices act like the valves in thermostatic showers and blend cold water with the hot water to maintain a set temperature – set-able between 45 °C and 65 °C.

The wonderful Twickenham Green Plumbers installed such a device on the top of the DHW cylinder and now my concern about scalding  is a thing of the past. The hot water temperature at the taps is now a consistent 47 ± 1 °C independent of the day of the week.

Click image for larger version: Graph the temperatures of the water emerging from 3 outlets in the house versus time. The anti-legionella cycle takes place early on Wednesday mornings. The installation of a blending valve on the cylinder means that now the tap temperatures does not vary from day-to-day, and does not reach potentially harmful temperatures.

Coefficient of Performance

The practical miracle of heat pumps is that they extract heat from the environment in order to warm our houses, and so provide more heating energy than the electrical energy used to operate them.

The ratio of the heating effect of a heat pump to its electrical energy consumption is called the coefficient of performance or COP.

The graph below shows the COP for the DHW heating cycles (in which the water was warmed to 56 °C) and the anti-legionella cycles in which the water was warmed to either a nominal 70 °C or 60 °C (in both cases these temperatures were exceeded by about 5 °C).

Click image for larger version: The graph shows the COP for the DHW heating cycles in which the water was warmed to a nominal 50 °C and the anti-legionella cycles in which the water was warmed to either a nominal 70 °C or 60 °C. In all cases actual temperatures were exceeded nominal temperatures by about 5 °C.

For the normal DHW cycle, the average COP is 3.3; for the very high temperature combined DHW and anti-legionella cycle, the COP fell to 2.5; but for a normal anti-legionella cycle the average COP is 2.9.


The idea that preparing domestic hot water could potentially create a life-threating hazard is at first alarming.

But in fact the anti-legionella heating cycle – when programmed correctly! – is very simple and reduces COP by only a small amount.

Adding a blending valve to the DHW cylinder output maintains a safe temperature at the taps and has one additional benefit: it allows the DHW cylinder to store extra thermal energy.

Assuming the water is heated from 15 °C, a tank of water at 60 °C contains 28% more thermal energy than a similar tank at 50 °C. If hot water demand were high – e.g. visitors! – the tank could supply 30% more water at a safe discharge temperature of 47 °C.

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