Boiler Killer Strikes Again

June 12, 2021

Teddington: 10 June 2021:

Vaillant Ecotec, 10, a hardworking combination boiler, was today found dead in the garden of his family home in Teddington, West London. His body was found in the front garden, with body parts gruesomely strewn across the grass. This grizzly discovery marks fifteenth unexplained death of a boiler in Teddington this year.

We understand that he had been in good working order just recently, and his death has left other household appliances in a state of shock.

Bosch Fridge Freezer, 8, was lost for words. “He put in a fine performance over the winter, emitting almost 1.5 tonnes of carbon dioxide. You really couldn’t ask for a better boiler. And now it’s summer and he had been hoping for a bit of rest, just heating water for the odd shower. I can’t believe he’s gone.”

Early reports suggest that the slaying was pre-meditated and the killer was probably operating on a so-called ‘contract’. An Air Source Heat Pump was seen in the back garden just before the incident, and a Police spokesman confirmed that they considered that sighting was significant.

Heat Pumps#1

June 10, 2021

Installation of an air source heat pump (ASHP) in my own house is sadly on hold while the installers await delivery of a part. So I thought I would take this opportunity to update you with one or two things I have learned about how real-world heat pumps operate.

What is a heat pump?

I am preparing an article about how heat pumps work internally, but considering only their operational behaviour, they work like the device illustrated below.

Click for a larger version.

  • Powered by electricity, they extract heat from the air.
  • Cold water enters the the heat pump.
  • Warm water flows out.

The engineered ‘miracle’ of a heat pump is that 1 kWh of electrical energy can extract between 2 kWh and 4 kWh of heat energy from the air.

It might seem that nothing could be simpler or more wonderful? But the engineering reality behind the ‘miracle’ requires that the heat pump be operated carefully.

The problem

The key problem is that heat pumps require a high flow of water through them in order to enable efficient operation of the heat exchangers which extract heat from the air. Typical flows are in the range 20 to 40 litres per minute of water.

For my 5 kW heat pump, this can warm such a flow of water by only 2 or 3 °C. So how can such a device heat water to 55 °C?

Domestic Hot Water

When the heat pump is configured to heat domestic hot water – for sinks and bathrooms – then the circuit looks like the figure below.

Click for a larger version. Schematic diagram of how a heat pump heats domestic hot water. See text for further details.

In DHW-mode, the water in the heat pump circuit is passed through a steel tube wound inside an insulated water storage cylinder. This acts as a heat-exchanger between the water in the heat pump circuit, and the water in the cylinder.

But remember, the ‘hot’ water in the heat pump circuit is just a degree or two warmer than the returning ‘cold water. So how can this ever heat the domestic water to 55 °C.

The trick is having a smart heat-pump controller and low losses in the connecting pipework.

The heat pump controller first sets the heat pump operating parameters to warm the water returning from the DHW by a few degrees.

As the DHW tank warms, the returning water also warms, and the controller slowly adjusts the operation of the heat pump to increase the temperature of the water it supplies to the DHW tank. Eventually the controller detects when the water in the DHW tank has reached its set temperature.

So for example, if the outside temperature is 10 °C, and the water returning from the DHW tank is initially at 20 °C, then:

  • Initially the controller configures the heat pump to heat the flowing water to (say) 22 °C. Pumping heat from air at 10 °C to water at 22 °C can be done much more efficiently than pumping heat from 10 °C to 55 °C.
  • At first the temperature of the water returning from the DHW tank will be only slightly above 20 °C. But as heat is transferred to the DHW tank the temperature of the water returning from the DHW tank increases.
  • In response to this increase in the temperature of the returning water, the controller re-configures the heat pump to an incrementally higher temperature.

By adopting this clever strategy:

  • The first part of the heating can be done with higher efficiency – perhaps resulting in 4 units of heating for each unit of electrical work.
  • The later part of the heating is less efficient and might only results in 3 units of heating for each unit of electrical work.
  • So overall – depending on the maximum temperature required – the so-called coefficient of performance (COP) is usually somewhere between 3 and 4.

Space Heating 

When the heat pump is configured for room heating – so called ‘space heating’ in the lingo – then the circuit looks like the figure below.

Click for a larger version. Schematic diagram of how a heat pump heats radiators. See text for further details.

I was surprised to find that in this mode of operation the water from the heat pump is not passed directly through the system of radiators.

Instead, most of the water passes through a short section of tubing called a ‘low loss header’ and goes straight back to the heat pump. This allows the heat pump to operate at high flow rates.

The water used in the radiators is drawn from the top of the ‘low loss header’ and returns – cooler – to the top of the ‘low loss header’.

However there is almost no pressure difference between the top and bottom of the ‘low loss header’ – and so very little water would naturally flow through the radiators. So a hydraulic pump is used to push water through the radiators.

The cooled water from the radiators now mixes with the main flow at the bottom of the ‘low loss header’ and returns to the heat pump.

Click for a larger version. Schematic illustration of a ‘low loss header’ See text for further details.

So for example, if the heat pump is supplying 20 litres per minute of water at 55 °C to the ‘low loss header’:

  • The hydraulic pump draws perhaps 4 litres per minute of water at 55 °C leaving 16 litres per minute to flow straight through the header.
  • The return water from the radiators is cooled to (say) 45 °C.
    • From this one can calculate that the radiators have provided heating of 2.8 kW.
  • So at the bottom of the ‘low loss header’ there is a mixture of:
    • 16 litres per minute of water at 55 °C
    • 4 litres per minute of water at 45°C
    • When mixed together this makes 20 litres per minute of water at approximately 53 °C which is returned to the heat pump.

At first I was puzzled by this arrangement, but then I realised it was clever trick.

  • It allows the heat pump to operate at high flow rates and yet heat water only over small temperature differences.
  • And it allows the radiators to operate with lower flows and bigger temperature drops.

For those with experience of electronics, it is analogous to the ‘impedance matching’ effect of a transformer.

It’s complicated…  

Things are more complicated than these diagrams would suggest.

Firstly, the heat pump can only operate in one mode at a time.

So the heat pump controller changes modes by operating a valve to direct the water from the heat pump either to the DHW storage tank or the radiators.

Secondly, there are numerous features incorporated for reasons of safety or maintainability.

Some of these guard against the effects of thermal expansion of the water, some guard against the (low risk) of Legionella infection, and some are filters or energy monitoring components.

But I hope the explanations above come close to getting to the gist of heat pump operation.

I have lots more to say about heat pumps: so stay tuned!

Estimating Rates of Air Change in Homes

June 6, 2021

Air flow in modern homes

Modern homes are built with low air leakage rates and then mechanically ventilated to keep the air ‘fresh’. To prevent heat losses associated with this air exchange, the outgoing ‘stale’ air is flowed through a heat exchanger to warm the incoming ‘fresh’ air.

However this Mechanical Ventilation with Heat Recovery (MVHR) is not suitable for many older homes – such as mine – which are too leaky.

My home has gaps between floorboards on the ground floor and the air can flow in and out easily through the underfloor void.

To seal my home to modern standards would require re-building the entire ground floor – adding insulation as one worked. One would then add MVHR to the newly-sealed house. This would be very disruptive, and so I have instead chosen to remain married.

So in my old house and many like it, heat losses from air flow are highly uncertain.

Wouldn’t it be great if there were some way to measure air flow in older homes which was cheap and convenient!

Measurement 

Air flow through a building is commonly characterised by the number of air changes per hour – ACPH. But how can this be measured if one doesn’t know where the air is coming in or going out?

This building wiki suggests:

Tracer gas measurement can be used to determine the average air change rate for naturally’-‘ventilated spaces’ and to measure infiltration (air tightness)’. To do this, a detectable, non-toxic gas is released into the space and the reduction in its concentration within the internal atmosphere is monitored over a given time period.’

By ‘tracer gas measurement’ the wiki means that a gas is released into the air at a known rate, and its concentration measured versus time. If the rate of production of tracer gas is known, then the final stable concentration allows one to work out the number of air changes per hour (ACPH).

  • If the number of ACPH is small, the final concentration will be high.
  • If the number of ACPH is high, the final concentration will be low.

What this wiki frustratingly fails to point out is that carbon dioxide is an ideal tracer gas and has been used for years for this purpose.

This essential fact is pointed out in the first paragraph of an outstandingly clear and authoritative paper from Andrew Persily and Lillian de Jonge.

Carbon dioxide generation rates for building occupants Persily A, de Jonge L. Indoor Air. 2017;27:868–879. https://doi.org/10.1111/ina.12383 . It’s also available with alternate formatting here.

The first line of the abstract is:

Indoor carbon dioxide (CO2) concentrations have been used for decades to characterize building ventilation and indoor air quality.

This surprised me because in all my reading about this subject in the UK I have never seen it mentioned. But then, in the first line of the paper itself, Persily and de Jonge point out just how old the idea is:

Indoor CO2 concentrations have been prominent in discussions of building ventilation and indoor air quality (IAQ) since the 18th century when Lavoisier suggested that CO2 build-up rather than oxygen depletion was responsible for “bad air” indoors.

The gist of their paper is a thorough review and examination of the factors which affect the rates at which human beings emit carbon dioxide. I won’t deprive you of the pleasure of reading the paper but factors discussed include:

  • The ratios of fat, protein and carbohydrate in people’s diet.
  • Age, gender and ethnicity.
  • Body size and mass.
  • Levels of activity.

The paper is very readable and I recommend it in the highest terms.

A worked example: my bedroom.

At night my wife and I sleep in a room which is about 7 m long, 3.5 m wide and 2.2 m high. So it has a volume of 7 x 3.5 x 2.2 = 54 cubic metres, or 54,000 litres.

There are no obvious draughts and I had no idea how many air changes per hour there were.

But overnight, the concentration of carbon dioxide rises from about 450 parts per million (ppm) characteristic of fresh air, and stabilises around 1930 ppm.

I can work out the number of air changes per hour ACPH using the formula below.

In this formula:

  • The room volume in litres
    • In my case 54,000 litres
  • c is the measured stable CO2 concentration in ppm
    • In my case 1930 ppm
  • c0 is the concentration of CO2 in ‘fresh’ air in ppm
    • In my case around 450 ppm
  • 10-6 is the scientific way of saying “divide by a million”
    • 1/1,000,000
  • CO2 production rate is what Persily and de Jonge’s paper tells us:
    • For sleeping males over the age of 11, the answer is within 10% of 12.7 litres per hour.
    • For sleeping females over the age of 11, the answer is within 10% of 10.2 litres per hour.
    • So our joint CO2 production rate is about 23 litres per hour

Putting all those numbers in the formula……we find the rate of change of air is around 0.29 ACPH – with the answer probably being within 10% of that value.

Some other factors.

Persily and de Jonge’s paper is extraordinarily thorough and tackles some of the tricky problems about using this technique for estimating air flow in buildings.

Firstly, there is the question of the level of activity of the people in a particular space. The metabolic rate is generally measured in units of mets with 1 met being roughly the metabolic activity during sleep. Very roughly it corresponds to around 58 watts.

The paper has extensive tables showing the CO2 production rate in litres per second for different levels of activity of different sexes at different ages. (Remember to multiply these numbers by 3600 to convert them into CO2 production rate in litres per hour before using them in the formula above.)

Secondly, there is the wider question of which volume of air is relevant. My bedroom represents a small volume with well understood rates of CO2 production.

But is a CO2 meter placed in a ground floor room measuring the characteristic concentration of the room it is in, the whole ground floor, or the entire house? Resolving questions like this may take a few experiments, such as moving the meter around.

Additionally, the amount of CO2 generated in a house over a day may not be clear. For example, the number of occupants and their level of activity may be hard to determine.

Mi casa no es tu casa

The situations encountered in your home will be different from those in my home.

Nonetheless, if you are trying to assess air flow within your home, I would recommend that you consider using carbon dioxide measurements as part of your arsenal of measurement techniques.

I use two CO2 meters and can recommend them both:

COVID 19: What have we learned? Nothing.

June 4, 2021

Click for a larger image. Logarithmic graph showing positive cases, hospital admissions and deaths since the start of the pandemic. The blue arrows show the dates of ‘opening’ events. See text for further details. The red dotted line shows cases doubling every 15 days as they did in September 2020.

Friends, so here we are, 4th June 2021, and I am reluctantly concluding that – as they did last summer – the government are about to screw things up.

The graph at the head of the page shows casesadmissions and deaths throughout the pandemic.

The situation now is strikingly similar to July last year, except that the growth rate of cases is more similar to September last year.

The statistics for admissions and deaths represent ‘ground truth’ – but when the situation is changing rapidly they lag the spread of the virus by several weeks

So to assess the spread we should look at cases. And with the best part of 1 million tests a day, mostly in asymptomatic people, we should have a reasonably good track on what is happening.

In my previous blog (s), I suggested we should not care about:

  • the absolute number of cases,
  • the population prevalence of cases,
  • or even the rate of change of cases.

What mattered was:

  • Is there the potential for the pandemic to expand into the general population and kill hundreds of thousands of people?

Last summer the answer was definitely ‘Yes’.

This summer I previously thought the answer was probably ‘No’.

Now I think that in fact the virus has run away from us – spreading through schools – and has the potential to reach to the general unvaccinated population.

And although it I don’t think it can kill ‘hundreds of thousands’, it could easily kill ‘thousands‘ and cause serious illness in many more.

How?

First of all, please let me me warn you about statistics which state the fraction of the ‘adult’ population which have been vaccinated. Adulthood is not relevant.

It seems that unvaccinated and previously uninfected people can catch COVID and spread it, no matter how young, even if their symptoms are not strong.

As I write: 59% of the entire population, including practically all of the most vulnerable groups have received a first dose of the vaccine. Vaccination is reaching an additional 8% of the population per month.

Together with the 10% – 20% (roughly) of the population who have had the disease, we are close to herd immunity. This would be relevant if the virus were spreading randomly through the population. But it isn’t.

The virus appears to be spreading amongst exactly the fraction of the population who have not been vaccinated. This is an inevitable consequence of our choice to vaccinate the elderly first. And as social restrictions have eased, viral spread is barely hindered by social distancing.

There are three problems with this.

#1: More Death 

If we consider the population of people who could be infected to be the roughly 20 million people under 30: then with a fatality ratio of 0.01% this corresponds to a summer with a further 2000 dead people under 30. If we are lucky the number might be only a few hundred.

To me, these wholly preventable deaths seem like those who died in WW1 after the armistice: more tragic somehow than the previous 129,000 deaths.

This does not take account of the fact that vaccinated people are not invulnerable – merely less vulnerable.

#2: More Illness

Without further interventions, the current case rate appears to be growing at the same rate it did in schools last September. Cases are doubling roughly every 15 days. By the end of June they will exceed 10,000 per day and approach 40,000 per day at the end of the school term.

Aside from the deaths, this corresponds to a lot more illness – some of it chronic ‘Long COVID’.

#3: Rolling the variant dice

The larger the pool of infected people, the more chance the virus has to mutate and find variants which might escape the vaccine, or – heaven forbid – take a more dangerous form.

As far as I understand, nobody knows why elderly people are more vulnerable to COVID-19. But imagine a hypothetical COVID-21 which was more deadly to children? Is that an experiment we really want to conduct?

So…

The latest outbreaks have not been contained locally– yet another failure of Track, Trace, and Isolate.

The vaccination program means that unlike last summer, we are unlikely to face a wave of a further 80,000 dead people.

But I am expecting a further wave of wholly unnecessary deaths – I just don’t know how large a wave to expect.

I did write out a list of recommendations for what we should do about this situation. But having edited it, and reflected on it, I realised that the recommendations were all obvious, but that writing them down was pointless, because the Government just doesn’t care!

Stay safe.

 

 

 

COVID 19: What have we learned?

May 26, 2021

Click for a larger image. Logarithmic graph showing positive caseshospital admissions and deaths since the start of the pandemic. The blue arrows show the dates of ‘opening’ events. See text for further details. The red dotted line shows cases increasing by a factor 10 every 150 days.

Friends, so here we are, May 26th 2021, and I have spent the day listening to Dominic Cummings testify to the “Lessons to Learn” inquiry in Parliament.

  • I found his testimony compellingly plausible.

As I explained previously (link), I can forgive the government for failing to act at the start of pandemic. They should have known better, but actually very few people in this country could quite believe what was happening.

But I refuse to forgive the government’s failure to act in September last year. What was required was obvious even to an amateur like me (link).

Dominic Cumming’s testimony explained how, despite advice to the contrary, Boris Johnson refused to act, asserting he was the “The Mayor in “Jaws” and he would keep “the beaches” open.

But unlike the “Mayor in Jaws” who was responsible for a few fictional deaths, Boris Johnson was personally responsible for tens of thousands of real deaths – the majority of the 86,164 who died in the second wave.

I won’t go on about this, because this is not that sort of blog, but this is, in my opinion, a criminal failure.

So what can we learn now?

The graph at the head of page shows cases, deaths and admissions throughout the pandemic.

It is striking that deaths and hospital admissions are very similar now to what they were in between the first and second waves.

Positive cases are higher than at the end of the first wave, but this could easily be due to the current extensive testing of asymptomatic people. The actual prevalence of the virus is probably similar or less.

As I mentioned in my previous blog, we should not care about:

  • the absolute number of cases,
  • the population prevalence of cases,
  • or even the rate of change of cases.

What matters is this:

  • Is there the potential for the pandemic to expand into the general population and kill hundreds of thousands of people?

Last summer the answer was definitely ‘Yes’.

This summer the answer is still in my estimation probably ‘No’.

Why? Because 57% of the entire population, including practically all of the most vulnerable groups have received a first dose of the vaccine. Vaccination is reaching an additional 8% of the population per month.

Together with the 10% – 20% (roughly) of the population who have had the disease, we are close to herd immunity.

So what is the worst case?

The current resurgence in cases appears be localised in communities with low vaccination rates, having been seeded by people returning from India.

The public health response – local mass vaccination and surge testing – seems appropriate.

The likely worst outcome with 3000 cases/day amongst the least vulnerable groups – aged under 30 – is that deaths might amount to 0.1% of cases, or 3/day. Tragic as each death is, in the context of this pandemic, this seems to me “acceptable”.

Cases nationally are rising slowly: by a factor 10 in about 150 days – or 5 months.

However vaccinations are proceeding at a rate of about 8% per month, so in 5 months the entire population will be vaccinated with at least a first dose.

My guess – and it is just a guess – is that with continued attention to local outbreaks, and continued progress with vaccination, we will avoid any significant third wave.

So returning to the key question:

  • Is there the potential for the pandemic to expand into the general population and kill hundreds of thousands of people?

As far as I can tell, the answer is still ‘No’. Probably.

The Last Artifact – At Last!

May 20, 2021

Friends, at last a film to which I made a minor contribution – The Last Artifact – is available in full online!

It’s the story of the redefinition of the kilogram which took place on this day back in May 2019.

The director Ed Watkins and his team carried out interviews at NPL back in August 2017 (link) and then headed off on a globe-trotting tour of National Metrology Laboratories.

Excerpts from the film were released last year (link), but somehow the entire film was unavailable – until now!

So set aside 90 minutes or so, put it onto as big a screen as you can manage, and relax as film-making professionals explain what it was all about!

 

The Last Artifact

Gas Boilers versus Heat pumps

May 18, 2021

Click for a larger version. A recent quote for gas and electricity from Octopus Energy. The electricity is six times more expensive than gas.

We are receiving strong messages from the Government and the International Energy Agency telling us that we must stop installing new gas boilers in just a year or two.

And I myself will be getting rid of mine within the month, replacing it with an Air Source Heat Pump (ASHP).

But when a friend told me his gas boiler was failing, and asked for my advice, I paused.

Then after considering things carefully, I recommended he get another gas boiler rather than install an ASHP.

Why? It’s the cost, stupid!

Air Source Heat Pumps:

  • cost more to buy than a gas boiler,
  • cost more to install than a gas boiler,
  • cost more to run than a gas boiler.

I am prepared to spend my own money on this type of project because I am – slightly neurotically – intensely focused on reducing my carbon dioxide emissions.

But I could not in all conscience recommend it to someone else.

More to Buy

Using the services of Messrs. Google, Google and Google I find that:

And this does not even touch upon the costs of installing a domestic hot water tank if one is not already installed.

More to Install

Having experienced this, please accept my word that the installation costs of an ASHP exceed those of replacing an existing boiler by a large factor – probably less than 10.

More to Run

I have a particularly bad tariff from EDF,  so I got a quote from Octopus Energy, a popular supplier at the moment,

They offered me the following rates: 19.1 p/kWh for electricity and 3.2 p/kWh for gas.

Using an ASHP my friend would be likely to generate around 3 units of heat for every 1 unit of electricity he used: a so-called Coefficient of Performance (COP) of 3.

But electricity costs 19.1/3.2 = 6.0 times as much as gas. So heating his house would cost twice as much!

More to buy, install and run and they don’t work as well!

Without reducing the heating demand within a house – by insulation – it is quite possible that my friend would not be able to heat his house at all with an ASHP!

Radiator output is specified assuming that water flowing through the radiators is 50 °C warmer than the room. For rooms at 20 °C, this implies water flowing at 70 °C.

A gas boiler has no problem with this, but an ASHP can normally only heat water to 55 °C i.e. the radiators would be just 35 °C above room temperature.

As this document explains, the heating output would be reduced (de-rated to use the correct terminology) to just 63% of its output with 70 °C flow.

What would you do?

Now consider that my friend is not – as you probably imagined – a member of the global elite, a metropolitan intellectual with a comfortable income and savings. I have friends outside that circle too.

Imagine perhaps that my friend, was elderly and on a limited pension.

Or imagine that they were frail or confused?

Or imagine perhaps that they had small children and were on a tight budget.

Or imagine that they were just hard up.

Could you in all honesty have recommended anything different? 

These problems are well known (BBC story) but until this cost landscape changes the UK doesn’t stand a chance of reaching net-zero.

 

Recalesence

May 18, 2021

When I used to work at NPL, I remember being really impressed by the work of my colleague Andrew Levick.

Magnetic Resonance Imaging (MRI) machines are able to image many physical details inside human bodies.

One their little-used features is that they can also be used to image the variation of temperatures throughout bodies.

Andrew was working on a temperature standard that could be used to calibrate temperature measurements in MRI imaging machines.

This would be a device placed inside the imager that could create a volume of imageable organic material at a known temperature. But one of the difficulties was that there must be no metal parts – so it could not contain any heaters or conventional temperature sensors.

So Andrew had the idea of using a vessel containing a supercooled organic liquid. If the transition to a solid was initiated, then the released latent heat – the recalescence –  would warm the liquid back to the melting/freezing temperature, creating a region of liquid-solid mixture at a stable, known and reproducible temperature, ideal for calibrating MRI machines.

Anyway…

Early on in the research he was doing experiments on the different ways in which the liquid crystallised.

He was supercooling organic liquids and then seeding the solidification, and making videos of the solidification process using a thermal imaging camera.

I thought the results were beautiful and put them to music, and that’s what the movie is.

The music is Bach’s Jesu Joy of Man’s Desiring arranged for 12-string guitar by the inimitable Leo Kottke.

If you are interested You Tube has many versions and lessons!

That’s all. I hope you enjoy it.

COVID-19:Still Looking good, but stiiiiillll not over

May 17, 2021

Click for a larger image. Logarithmic graph showing positive caseshospital admissions and deaths since the start of the pandemic. The blue arrows show the dates of ‘opening’ events. See text for further details

Friends, so here we are, May 17th 2021, and I can finally resume the natural pastime of old men: sitting in cafes.

Obviously, matters pandemical are not ideal, but they are in my estimation looking good.

To explain my uncharacteristic positivity, allow me to remind you how this summer differs from last.

I know what happened last summer…

As we look at the figure above covering positive cases, hospital admissions and deaths across the pandemic, we see that the second wave began in July 2020 – when cases began to rise. This was before the end of the first wave as judged by the minimum in the rate of deaths.

And after cases started to rise hospital admissions and deaths both fell for a further two months!

So now we should focus our attention on cases for first signs of a problem.

However what matters is:

  • not the absolute number of cases,
  • not the population prevalence of cases
  • not even the rate of change of cases

What matters is this:

  • Is there the potential for the pandemic to expand into the general population and kill hundreds of thousands of people?

Last summer the answer was definitely ‘Yes’.

This summer the answer is probably ‘No’.

Why? Because 55% of the entire population, including practically all of the most vulnerable groups have received a first dose of the vaccine.

Together with the 10% (roughly) of the population who have had the disease, we are close to herd immunity.

Herd Immunity is not an on-off thing.

As the prevalence of immune people approaches a critical prevalence – probably around 66% for the coronavirus – the speed of viral transmission slows as the virus finds it increasingly difficult to move from an infected individual to a vulnerable individual.

Once the prevalence of immune people has passed the critical prevalence – chains of viral transmission decay with increasing rapidity as immunity prevalence approaches 100%. For any physicists reading – it’s a continuous phase transition.

Probably 

Careful readers may have noticed that I slipped in an italicised  ‘probably’ a few paragraphs back. Things could still go wrong.

Most notably, one variant of the virus or another could acquire the ability to escape the immunising effects of the vaccine.

This is possible, but it is – as far as I can tell – not a very likely outcome.

What will happen next?

Click for a larger image. Logarithmic graph showing positive cases, since the start of 2021. The blue arrows show the dates of ‘opening’ events. See text for further details

The figure above shows that the ‘openings’ (shown as blue arrows) are having an effect, because after each ‘opening’ step, the rate at which cases are falling slows down, because viral transmission chains can spread further in the more liberal environments.

And since the start of May, the daily rate of positive cases has been rising slowly.

So after today, with one has to expect that the daily rate of positive cases will rise faster, and that numbers will probably not decline for several weeks or months.

Also, the potential for tourists and returning tourists to re-seed infections of different variants around the country is a concern.

And many people will be distressed by this rise in cases and the (probably) ineffective controls at the borders.

But, as I said above, in my opinion the key question is:

  • Is there the potential for the pandemic to expand into the general population and kill hundreds of thousands of people?

And as far as I can tell, the answer is ‘No’. Probably.

Air Conditioning versus Air Source Heat Pump

May 15, 2021

Click for a larger version. Similarities and differences in how an air source heat pump (ASHP) or an air conditioning (AC) system warms a home. All the components inside the dotted green line are contained in the external units shown. A key design difference is whether or not the working fluid is completely contained in the external unit. See text for more details.

Regular readers will probably be aware that – having reduced the heating demand in my house – my plan is to switch away from gas heating and install an electrically-powered air source heat pump to heat the house and provide domestic hot water.

But next week I am also installing air conditioning, something which is traditionally not thought of as very ‘green’. What’s going on?

Why Air Conditioning?

I have two reasons.

My first reason is that, as you may have heard, the whole world is warming up! Last year it reached 38 °C in Teddington and was unbearably hot for a week. I never want to experience that again.

During the summer the air conditioning will provide cooling. But assuming the heating comes with good weather, the air conditioning will be totally solar powered, and so it will not give rise to any CO2 emissions to make matters worse!

My second reason is that in the right circumstances, air conditioning is a very efficient way to heat a house. That’s what this article is about.

Heat Pumps

Air Conditioners (AC) and Air Source Heat Pumps (ASHP) are both types of heat pumps.

In scientific parlance, a heat pump is any machine that moves heat from colder temperatures to higher temperatures at the expense of mechanical work.

Note: to distinguish between the general scientific idea of a heat pump, and the practical implementation in an air source heat pump, I will use abbreviation ASHP when talking about the practical device.

The general idea of a heat pump is illustrated in the conceptual schematic below.

As shown, the pump uses 1 unit of mechanical energy to extract two units of heat energy from air at (say) 5 °C and expel all 3 units of energy (1 mechanical and 2 thermal) as heat into hot water at (say) 55 °C.

Click for a larger version. Traditional representation of the operation of heat pump.

Heat pumps can seem miraculous, but like all good miracles, they are really just applied science and engineering.

A heat pump is characterised using two parameters: COP and ΔT.

  • A heat pump which delivers 3 units of heat for 1 unit of work is said to have a coefficient of performance (COP) of 3.
  • The temperature difference between the hot and cold ends of the heat pump is usually called ‘Delta T’ or ΔT.

Obviously engineers would like to build heat pumps with high COPs, and big ΔTs and they have used all kinds of ingenious techniques to achieve this.

But it turns out that heat pumps only operate with high COPs when the ΔT is small and when the heating power is low. There are two reasons.

  • Firstly, the laws of thermodynamic set some absolute limits on the COP achievable for a given ΔT.
    • Most practical heat pumps don’t come close to this thermodynamic limit for a variety of mundane reasons.
    • The maximum COP for moving heat from 5 °C to 55 °C is 6.6.
    • The maximum COP for moving heat from 5 °C to 20 °C is 19.5.
  • Secondly, in order to heat a room to (say) 20 °C, the hot end of the heat pump needs to be hotter than 20 °C.
    • Typically the hot end of the heat pump must be 5 °C to 10 °C warmer than the room in order that heat will flow out of the heat pump.
    • Additionally the cold end of the heat pump must be 5 °C to 10 °C colder than the external air in order that heat will flow into the heat pump.
    • The interfaces between the ends of the pump and the environment are called heat exchangers and designing ‘good’ heat exchangers is tricky.
    • A ‘good’ heat exchanger is one that allows high heat flows for small temperature differences.

So now we have seen how heat pumps are characterised, let’s see how heat pumps are used domestically.

Air Source Heat Pump (ASHP) versus Air Conditioner (AC)

The schematic diagrams  below show how a house is heated by an ASHP and an AC system. Both systems operate using a working fluid such as butane, which is ingeniously compressed and expanded. The details of this process are not the topic of this article so here I am glossing over the fascinating details of the device’s operation. Sorry.

Click for a larger version. How an air source heat pump (ASHP) warms a home. All the components inside the dotted green line are contained in the external unit shown. A key design feature is that the working fluid is completely contained in the external unit and heat is transferred to the central heating water by a heat exchanger.

Click for a larger version. How an air conditioner (AC) warms a home. All the components inside the dotted green lines are contained in either the external unit or the fan coil unit shown. A key feature is that the working fluid itself flows into the fan coil unit and heats the air directly.

We can compare the operation of the two systems in the table below.

Air Conditioner Air Source Heat Pump
Air at (say) 5 °C is blown over a heat exchanger and evaporates the working fluid.

 

The same.
The working fluid is then compressed – that’s the bit where the work is done – and liquefies, releasing the captured heat.

 

The same.
The hot working fluid – now at ~30 °C then flows through a pipe to an indoor heat exchanger (fan coil unit) where air is blown over the pipe and heated to 20 °C. The hot working fluid – now at ~60 °C then flows through a heat exchanger and transfers the heat to water in my central heating system at ~55 °C
No corresponding step  

The 55 °C water then flows through a radiator in my room, heating the room by radiation and by convective heat transfer to air at ~20 °C.

Looking closely at the figures and table above, one can see that the operation of the ASHP and the AC system are broadly similar.

However the ASHP has to operate with a bigger ΔT (~55 °C versus ~25 °C) than the AC system, and also has to transfer heat through an extra heat exchanger.

Both these factors degrade the achievable COP and so for my application, the specified COP for an ASHP is just over 3, but for the AC system, it is just over 5.

In my well-insulated house, when the external temperature is 5 °C, I require typically 36 kWh per day of heating, equivalent to 1.7 kW continuous heating. I can achieve this in several ways:

  • Using gas I must burn ~40 kWh of gas at 90% efficiency costing 40 x 3p (£1.20) and emitting 40 x 200 g = 8 kgCO2
  • Using an ASHP with a COP of 3, I must use ~36 kWh/3 = 12 kWh of electricity costing 12 x 25p (£3.00) and emitting 12 x 200 g = 2.4 kgCO2
  • Using an AC system with a COP of 5, I must use ~36 kWh/5 = 7.2 kWh of electricity costing 7.2 x 25p (£1.80) and emitting 7.2 x 200 g = 1.4 kgCO2
  • Using a domestic battery and buying the electricity at night for 8p/kWh, I can reduce the cost of using an ASHP or AC system by a factor of 3 to £1.00/day or £0.60/day respectively.

[Note: In these calculations I have assumed that the carbon dioxide emissions per kWh are same for both gas and UK electricity (200 gCO2/kWh) which is roughly correct for 2021]

So using an AC system I should be able to achieve domestic heating with lower carbon dioxide emissions than an ASHP.

My plan

In my case I need to heat water for my home to 55 °C for use in showers and basins. So I need an ASHP for that. And since I already have radiators in every room, hooking up the ASHP to the radiator circuits is smart double use.

The AC system I am having installed will have 1 external unit and 2 internal ‘fan coil units’. One unit will be in my bedroom (a sheer indulgence) and the other will be high up on the stairs, allowing air to be either blown down to the ground floor where I hope it will circulate, or blown towards the bedrooms.

My hope is that, when used together, the AC system (COP~5) will reduce the heating output required from the radiators so that I can reduce the flow temperature of the water from 55 °C to perhaps 40 °C. This reduces their heat output, but increase the COP of the ASHP from 3 to perhaps 4.

The main difficulty that I foresee is the extent to which the AC heating will actually permeate through the house and so reduce the amount of heating required by the ASHP.

So I am not sure how much heating will be required by the ASHP acting through the radiators, and whether the radiators will work at low flow temperatures. It is possible I might need to replace a few radiators with ones which work better at low temperatures.

It is not at all obvious that this plan will actually work at all – but I think it is worth a try.

Kit

The air conditioning I am having installed is a Daikin 2MXM40 multi-split outdoor unit with two FTXM25 indoor air units. (Brochure)

The model of heat pump I will have installed is a Vaillant Arotherm plus 5 kW. It can supply up 5 kW of heating at 55 °C with a COP of 3  – i.e. it will use just 1.6 kW of electrical power to do that – and heat water to 55 °C. Water storage will be a 200 litre Unistor cylinder. A brochure with technical details can be found here, and a dramatic video showing the kit is linked at the end of this article.

When I have come to terms with how much money I am spending on this, I will share that information. But at the moment it hurts to think about it!

Anyway: the adventure begins next week!

 


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