Domestic Thermal Storage: Part 1: Hot Water

Friends, writing about the ‘Sand Battery’ fiasco the other day brought to mind smaller thermal stores that are used domestically. And so I thought it would be interesting to write about the physics of thermal storage.

But it has all got out of hand and now this this is the first of three articles over which I will compare three different types of thermal storage, one most people are familiar with, and two that are less familiar:

  • A domestic hot water tank.
    • This stores thermal energy in water which is then used directly within a household.
    • A typical Domestic Hot Water (DHW) cylinder stores between 7 kWh and 10 kWh of thermal energy.
  • A phase-change thermal storage device.
    • This stores thermal energy in the so-called ‘latent heat’ of a material which absorbs thermal energy when it is melted, and releases it at a constant temperature as the material freezes.
    • A typical Phase Change Thermal Store stores between 4 kWh and 8 kWh of thermal energy, comparable with a DHW cylinder, but requiring only approximately half the volume.
  • A Zero Emission ‘Boiler’.
    • This stores thermal energy in the heat capacity of a ‘thermal core’ – a cylinder of concrete weighing ~300 kg – which is heated to an astonishing 800 °C.
    • This can store up to 40 kWh of thermal energy.
  • A ‘big thermal store’.
    • Like a Zero Emission Boiler, but heavier – and ‘only’ heated to 500 °C.
    • This can store up to 100 kWh of thermal energy.

Click on the image for a larger version. Schematic illustration of four different types of thermal storage devices and a human being for scale.

The key role of all these devices is to separate two events:

  • The time when energy is consumed from a central resource – such as the electricity grid,
  • The time when energy is used domestically – such as when you take a shower.

Separating these events has two benefits:

  • It allows users to store thermal energy slowly but to release large amounts of thermal energy quickly – such as when you need a flow of hot water ‘instantly’.
  • It allows users to store thermal energy when it is cheap or convenient .

For each device we need to consider how it is heated (‘charged’) and how it passes on its stored heat (‘discharged’).

In this article we will look at how a domestic hot water (DHW) cylinder works and in the following articles we will look at how Phase Change Material Stores works and how Zero Emission Boilers and big thermal stores work.

Domestic Hot Water (DHW) Cylinder

When I first heard a DHW cylinder described a ‘thermal store’, I was initially confused. I had always considered them as storing water!

In the other thermal stores, heat is first stored in a material, and subsequently transferred to circulating water or DHW only when it is required. In a DHW cylinder the storage material is the water which will itself later emerge from a tap.

The amount of stored thermal energy can be estimated as the product of:

  • The volume of water in the tank
  • The heat capacity of water in the tank (4,200 J/°C/litre)
  • The difference between the storage temperature and the charging temperature.

For a 200 litre tank storing water at 55 °C which has been heated from 20 °C this amounts to ~29 MJ or 8.2 kWh.

One can store more energy in a cylinder of a given size by storing water at a higher temperature: at 75 °C the stored energy in the cylinder above would be 12.8 kWh. To prevent discharge of scaldingly hot water, a blending valve would be used on the top of the cylinder and set to (say) 50 °C.

Click on the image for a larger version. Schematic illustration of the structure of a DHW cylinder showing the internal coil for heating the stored water. On the right is a manufacturer’s illustration of the coils within their cylinder.

A DHW cylinder can be charged in one of several ways.

  • In the simplest way, an electrical heater immersed in the water heats the water directly. A 3 kW heater can charge a 200 litre cylinder to 55 °C in just under 3 hours. The heater could be powered by either grid or from excess solar PV.
  • Alternatively, hot water heated by a gas boiler or a heat pump can be flowed through a coil inside the cylinder, passing on its heat to the stored water. The rate of heating in this method will generally be slower than using an immersion heater.

Discharging the cylinder is simple: one opens a tap and the mains water pressure forces water out of the top of the cylinder replacing it with cold water at the bottom.

The rate of discharge of thermal energy is given by the product of:

  • The discharge flow rate (litres/second)
  • The heat capacity of water in the tank (4,200 J/°C/litre)
  • The difference between the storage temperature and the charging temperature.

So if 10 litres of water at 50 °C is discharged per minute, thermal energy is being released at a rate of 21 kW. This is a very high rate of energy use.

‘Combination boilers’ can provide this very high heating rate, but only at the cost of releasing (at the specified flow rate) around 0.1 kg of CO2 for each minute of operation.


One of the difficulties with a DHW cylinder is that natural convection within the cylinder causes the hot water to rise to the top. And the stratification within the cylinder can be very dramatic.

Since most cylinders have only a single thermometer somewhere in the middle of the cylinder, even after reading the thermometer it is difficult to know how much heat is currently stored in the cylinder.

Additionally since the heating coil or immersion heater is typically in the lower third of the cylinder, practically the whole cylinder must be re-heated before any sufficiently hot water is available at the top of the cylinder – which typically takes several hours.

Some modern cylinders made by the Mixergy company exploit the stratification by heating the water from the top and then carefully mixing it with the colder water below.

These computer-controlled cylinders can give a reasonable estimate of the state of charge of the cylinder, and also allow rapid heating of small volumes of water at the top of the cylinder. However, I don’t understand precisely how the technology works.

Update: This video gives a clear – if rather glossy – explanation of how the system works. It turns out that Robert Llewelyn was given one as part of a research study!

Click on the image for a larger version. The water in a conventional DHW cylinder is hotter at the top but the temperature gradient from top to bottom is not well-defined. More modern computer-controlled cylinders from the Mixergy company can precisely control the location of the temperature gradient.

Heat Losses 

A DHW cylinder holding 200 litres is typically 1 metre high with a diameter of 50 cm and insulated with 50 mm thick layer of polyurethane foam with a typical thermal conductivity of 0.025 W/°C/m.

Click on the image for a larger version. Heat losses from a DHW cylinder are typically 10% of the stored energy per day.

For a cylinder at 55 °C, this leads to a heat loss of roughly 35 watts, or 0.85 kWh/day. i.e. the cylinder loses about 10% of its stored energy per day.

This loss rate increases if the water is heated to a higher temperature. For a cylinder at 75 °C the loss rate is ~ 55 watts or 1.32 kWh/day – again, about 10% of its stored energy per day.

The only way to reduce the heat loss is to apply either better insulation (which is expensive) or to apply a thicker layer, which makes the cylinder larger.


A DHW cylinder holding 200 litres is a simple way to store hot water for use around the house.

In the context of renewable energy, it allows a heat pump with a COP of 2.5 to use perhaps 1.5 kW of electricity for 2 hours (3 kWh) to fully charge a cylinder with ~8.5 kWh of thermal energy. This can then be discharged at 10 litres per minute i.e. releasing stored energy at a rate of 21 kW.

The downsides of a DHW cylinder, (large size, 10% leakage per day, unknown temperature gradient within the tank) are generally considered acceptable.

But there are alternatives and we will look at one of these in the next article.

24 Responses to “Domestic Thermal Storage: Part 1: Hot Water”

  1. James (ex of NPL) Says:

    Hi Michael. Thought you might be interested in the system we are getting put in. ~400 l tank. inputs from thermal solar panel (to lower in tank), wood burner (to lower in tank), pellet boiler (switchable from only very top to top half of tank). Outputs to hot water via heat exchanger and mixer and oversized radiator circuit via mixer. Room for more connections too…

    • protonsforbreakfast Says:


      Good Morning. I would ask if you are well, but the fact that you are engaged in such an intriguing project tells me that life must be pretty good!

      So the philosophy of this storage device is to use water – keeping simple – and then to have multiple inputs to capture heat from a variety of sources. Those sources can achieve very high temperatures so my suggestion is to consider using a blending valve on the output to control discharge temperatures.

      Best wishes


  2. Ben Farris Says:

    We have need of help creating a heat store. We are willing to pay you to consult.
    We intend to store summer heat for winter use.
    I have read your articles, and believe that this is feasible in our application based on your numbers.
    We are converting an old mill building to apartments. We have some very large “rooms” with thick concrete walls that could be used for storage. We have a 3/4 acre roof to collect heat in the summer (in addition to what we may collect from the residential space via heat exchanger cooling)
    We were interested in using sand, as in the Finnish sand battery, but your articles have disabused is of that idea.
    We now think a 40’x60’x10′ pool of water heated to 90C would be sufficient if well insulated (2′ thick wool).
    The apartments will be app. 500 sq ft. with 1 exterior wall.
    Are you interested enough to talk to us?
    If so, my email address is

  3. Ross Mason Says:

    I remember seeing a nice demo of the stratification in my first year as a lowly technician at DSIR. Roy Benseman got a beaker, rolled a circular piece of nichrome, placed it half way down the beaker of cold water, hooked up the ubiquitous Variac to it and heated it for a few minutes. Then dropped some food colouring into it. I was amazed at the delineation. It has stuck with me for ever and been very useful in analysing liquid heating problems over the years.

    • protonsforbreakfast Says:

      Good Morning Ross,

      Yes, the stratification can be incredibly dramatic. My insight came when I was trying to grow a large crystal by inducing a temperature gradient in a vertical stainless steel tube. I tested out the heater by putting water in it and measuring the temperature at the top and the bottom. To my surprise, I could boil the water at the top but 5 cm below, there was absolutely no heating detectable!

      Warm Best wishes


  4. Malcolm Reeves Says:

    Gas boilers heat water faster than 3kW immersion heater. With a fast reheat coil this could be 30min. Gas boiler has 20-40kW available. Limitation is usually the DHW cylinder coil.

    • protonsforbreakfast Says:


      Thank you for that. I know that the boiler can heat water at 20 kW or so, but the heat transfer within the cylinder will – as you say – depend on the design of the heat exchanger and the chosen flow temperature. I didn’t mention an exact rate for heating using a Combi boiler and an indirect heating coil because I didn’t know the answer.

      Best wishes


  5. john ridge Says:

    Very interesting, will keep for future reference.
    Not sure you captured the domestic thermal store case fully though.
    I used a system where there was
    – a reservoir of water R that was heated by either tip or bottom electric coils on cheap electric as required
    – another coil for mains water that ran through the reservoir R which then came out hot etc

    Believe it was 70C in R, water came out nice and hot.
    Lousy pipework that ended up corroding it that’s another story

    • protonsforbreakfast Says:

      Good Morning,

      That sounds like a really well-designed system (aside from the corrosion). The feature I would love but which is probably too complex to engineer would be to capture the waste heat from runoff hot water (baths showers and dishwashers etc.). So much is possible with a little thought ahead of time.

      Best wishes


  6. Eric Hawkins Says:

    Hi James, having been 1 0f just 3 in 1990, who began the development of an alternative to unvented pressurized HW only cylinders, now known as Thermal Stores, can I sugest we have a phone call some time, as my latest thermal stores which provide both heat and hot water, but 100% vented, no heating coils, or for solar thermal, as heat stores direct. Only coils in my store, is to delivery of HW at mains pressure indirect at high flow rates, as low at store at top half of 48c, last week the ground temp was 19c, that was my cold water out.
    I have overseen 2 x400L thermal store installs over last month, both heated by solar thermal direct, one has a back up of 2 x 3kw Immersion heaters to heat 14 rads, as well as for HW use. This is a test case to compare a £100 spend on 2 Immersion heaters compared to a 8kw heat pump installed at £8,000
    Some good figures I have copied down, thanks

  7. Nic Houslip Says:

    I have been using a Thermal store since 1990. My AGA cooker stores heat in a 600lb stack of high temp ceramic blocks, fitted with electric heating elements to to raise it to 750 deg C overnight on E7 Electricity. It draws around 6 KW per hour. Heat is distributed to ovens and heating rings by an internal fan that circulates hot air around them. Cooking is possible at any time, with no warmup time. Heat loss, which is gradual during the day, goes into the fabric of the house which offsets offsets and keeps my LPG Consumption [no mains gas Here] to minimum.

    • protonsforbreakfast Says:

      Nic, thanks for your comment. I did initially include the Aga in my article because the ZEB sounded in many ways similar, But I couldn’t find enough information to write anything useful.

      As far as I can tell, the storage system is not very efficient i.e. it leaks quite a bit.

      Have you made any measurements on your Aga?

      Best wishes, Michael

  8. Chris Malcolm Says:

    We have a hot water system that you may find interesting.
    We have a stainless steel 300 litter tank with heating elements and thermostats at the bottom and halfway. We have an evacuated glass tube solar hot water system drawing cool water from the base and returning it a bit higher up for summer this is more than adequate most of the time and water temperatures of 80 C are achieved so a tempering valve is required.
    We also have solar electric power. Surplus solar electricity is directed to the heating element halfway up the tank . It is set to heat to about 55 C. Mostly this will compensate for any shortfall for throughout the year. Backing this up we have two conventional mains power heating periods after diner and before we rise in the morning.
    This arrangement ensures that whatever direct solar water heating in the cooler times is maximised. If the sun can only directly heat the water to 35 C, we maximise that opportunity and lessen the load on the solar electricity system. (The 2nd law of thermodynamics, entropy and all that)

    • protonsforbreakfast Says:

      Chris, Good Morning.

      That sounds like a thoroughly well thought out system. With a 300 litre tank storing water at 80 °C, you are able to store ~21 kWh – roughly twice the storage of a typical cylinder. I am guessing that you use a lot of hot water!

      I have two thoughts. Firstly I am happy that you are using a tempering valve – they prevent scalding and save energy: I wrote about them here.

      Secondly, my strategy for the use of solar water heating is to use solar PV (sometimes via a battery) to operate a heat pump that heats the water which a COP of 3.0. This 300% efficiency gain more or less compensates for the fact that solar PV is only 20% efficient while solar thermal panels are typically 60% efficient.

      Anyway: Good luck with low carbon adventures.


  9. Nonstop Says:

    I have an air source heat pump system to provide hot water and heating for my house.

    During the first year I have had the hot water set to run continuously and the 250l tank maintains a 50°C temperature. Previously we had a much smaller tank at a significantly higher temperature, but with the capacity to reheat quickly. 50°C is plenty hot enough for domestic use and the lower the temperature, the more efficient the heat pump.

    However, I’m now on an electricity tariff where I can buy at $0.1/kWh at night and $0.5/kWh during the day. How would I work out whether it would be advantageous to heat to a higher temperature at night and if so, how hot would the water need to be to meet my needs during the day?

    • protonsforbreakfast Says:

      John, Hi.Good Afternoon.

      Heating to high temperatures is one area where heat pumps have improved a lot. My own heat pump (A Vaillant Arotherm Plus) installed one year ago can heat to 70 °C.

      However like you, we heat to lower temperatures. We have a 200 litres tank set to nominally 55 °C so – compared with 20 °C – it stores roughly 200 x (55-20) x 4200/ 3,600,000 = 8.2 kWh. We also fit a ‘blending valve’ to limit the output temperature to 49 °C. I wrote about these ‘blending’ or ‘tempering’ valves here:

      If we wanted to store more heat we could heat the cylinder to (say) 70 °C then we would store ~11.7 kWh of heat, but the blending valve would keep the water in the taps at the same temperature.

      Regarding the time-of-day tariff, during the winter we top up the hot water cylinder at night and that normally lasts through the day. But if it didn’t, I would increase the temperature of the stored water as described above.

      So coming to your question. You say that the hot water runs continuously. I suggest you set it to run just during the cheap period and see if the water lasts through the day. If it does: great. If it doesn’t, then try heating to a higher temperature. As I explained above, heating to 70 °C would give you 40% more stored thermal energy, but the blending vale would keep the tap temperatures safe.

      Does that make any sense to you?

      In any case: best wishes


  10. Tim Says:

    Good morning,
    What purpose is this energy being used for?
    Obviously I can use them as heating in the Winter, And as hot water.
    Can this be used for electricity At a home owner level?
    Is there an indirect way to use this for cooling?

    • protonsforbreakfast Says:

      Tim, Good Afternoon.

      I can’t quite understand your question. The energy is being used for hot water for showers and handbasin etc.

      I couldn’t quite understand the other parts of your query.

      All the best


      • Tim Miller Says:

        Depending our your configuration, a water tank can be used
        1. purely for hot water

        2, as a heat sink- to expand-
        a. Using water to cool your solar panels- the “”waste heat” can be stored in your tank.
        b. is the tank is large enough, there are water based heat pumps where you can cool your house on one side of a pump, and use that heat to heat the water.

        3. While this is more than often a higher storage level, the energy that you are storing as heat can be used to produce electricity.
        a. Thermo-Electric Generator. a device that uses a temperature delta to create electricity. (really low tech example:
        that is a 9 dollar 12 volt TEG.

        b. To use heat for Electricity in scale you are looking at creating steam, likely beyond a residential scale operation.

        A rather dry document on this is at
        title: A Comprehensive Review of Thermal Energy Storage

        I like your descriptions.

        to Digress- did you look at insulation inside your walls, or was that not practical. I am interested in Air infiltration and efficiency, but your solution, while very cool, is not practical on our house…

      • protonsforbreakfast Says:


        Hi. There’s quite a bit to respond to there!

        1. Yes.
        2. Using the tank as a store for low-grade heat (e.g. from cooling solar panels or scavenged from hot water run-off) is a great idea. But as you say, it generally requires a large tank.
        3. I would generally advise against generating electricity from a temperature gradient because the efficiency is always low. For example, even with a 30 °C temperature difference between cold and hot stores, the maximum conceivable efficiency is about 9%, but it would almost impossible to get anywhere near that.

        Regarding insulation. I didn’t look closely at Internal Wall Insulation because it steals precious centimetres from the inside of the house and would be quite disruptive – which would not be popular with my wife. Also, it cools the out fabric of the house which opens up the possibility of condensation somewhere in the brickwork. In contrast, external wall insulation increases the temperature of the brickwork and generally makes condensation less likely.

        Regarding Air Infiltration – I made measurements using CO2 as a tracer gas. I described them here.

        Best wishes


  11. Rodney Holland Says:

    What a great subject and article. Lots of interesting questions and technical issues.

    Firstly I feel sure that in the coming months we will see greater flexibility across the national power grid for their sources of power and we should be awakened and ready for the fact that utility companies are going to run smart very soon on pricing plans for different times of the day. I know for sure the Octopus CEO is very interested in this area and wants to have pricing plans.

    We have these DC power line interconnects between scotland and england and norway and france and soon morocco all providing green intermittent energy at lowish cost. It means that any fossil fuel at today’s inflated pricing wlll be inefficient as a supply heat source and all that surplus wind energy at night will need to be utilised. What I’m interested to learn is how much insulation to use when we are talking about 500 °C or 800 C temps being stored and how can we thermally or electrically get the heat into our chosen store…whether concrete or water. And what sort of insulation can withstand 500 °C.

    Getting heat out of the store and monitoring it I wouldn’t have thought is much problem as software and sensors connected to valves can do the job and i know a clever software/hardware engineer who can make it easy. I do think the addition of solar power panels is a good option as the price of 350 watt panels was recently around £135 ea. plus the cost of the inverter. It all starts to become reasonable pricing in the face of demand heating by gas-oil-electricity at peak times.
    Keep thinking guys.

  12. Stuart Gibbons Says:

    Does battery storage of energy become part of the process?

    • protonsforbreakfast Says:


      Good Evening. I am not sure what you are thinking of.

      This article is about storage of thermal energy whereas batteries store electrical energy.

      The domestic battery that we use stores 13.5 kWh of electrical energy – more energy than our 200 litre storage cylinder at 70 °C. But a battery is dramatically more useful than a storage cylinder.

      Using the battery we could produce hot water if we wanted with 100% efficiency. But we can also run TVs and computers etc. In fact we can run a heat pump and make hot water with 300% efficiency! But this flexibility comes at a cost – the battery cost around 10 times as much as the cylinder.

      So battery storage of energy is part of running a home efficiently, but it still helps to have a DHW storage tank.

      Best wishes


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