Friends, I love my heat pump. And as I have learned more about heat pumps, it has become ever clearer to me that heat pumps like mine are the best solution for heating a large fraction of homes in the UK.
A large fraction? Why not all homes? There are two parts to the heating required in a home:
- Space Heating: most typically provided by radiators that warm the air within a home.
- Domestic Hot Water (DHW) heating: providing hot water for taps and showers.
So-called Air-to-Water heat pumps like mine have difficulties providing the low level of space heating required in smaller properties. And additionally, in order to provide hot water, they need a hot water cylinder with a lot of associated “gubbins” that takes up a lot of space. And space is especially precious in smaller homes.
This article is about an interesting innovation which allows the use of hot water cylinders which are small enough to fit under kitchen counters, and which don’t require the “gubbins” of conventional cylinders. They could be especially relevant in medium-sized homes in which an air-to-water heat pump is suitable, but in which space for a storage cylinder is limited.
The smallest homes
For the smallest homes – typically flats with just one or two rooms – annual gas usage will be 10,000 kWh or less and peak heating requirement in winter will be 3 kW or less. For such homes, an air-to-air heat pump with one or two (or up to five) indoor units (a so-called multi-split system) will probably provide the cheapest and most efficient space-heating solution. It will also provide cooling in summer.
Such air-to-air pumps can come with a water-heating option, but often (if not too much hot water is required each day) the simplest option may be to just use a small storage cylinder heated with off-peak electricity, or possibly electric showers and water heaters.
The largest homes
For the largest homes, particularly older homes with existing water-based central heating, heat pumps offer an excellent low-carbon heating alternative. If the annual gas usage is above 25,000 kWh then peak winter heating requirement will be around 10 kWh or above.
Heat pump units with heating power up to 16 kW (example) and flow temperature up to 65 °C can heat most older properties as a direct substitute for a gas boiler. And these larger homes typically also have plenty of room for a hot water cylinder and the “gubbins” that comes with it.
Medium-sized homes
Medium-sized homes – with an annual gas usage between 10,000 kWh and 20,000 kWh – are well suited for space heating with an air-to-water heat pump and radiators, but finding room for a hot water cylinder can be a challenge.
Such homes are often currently using a combination-boiler which heats hot water for radiators, but when a tap is opened, the boiler temporarily stops heating the home and heats the hot water directly – often with a heating power above 20 kW – providing 8 litres/minute of toasty hot water.
The New Heat-Geek Heat Store Cylinder
Before I begin it’s important to explain that the Heat Geek Heat Store Cylinder is not (in my opinion) as revolutionary as its inventor purports. It is after all just an insulated metal cylinder containing hot water! But it is an interesting and innovative tweak that may help people who would like a heat pump but lack space for a conventional hot water cylinder.
In operation the Heat-Geek Heat Store allows a heat pump with a heating power of just 5 kW to deliver instant hot water like a combination boiler – but not indefinitely. The trick only works for between 10 and 20 minutes. However, that’s enough for most people to shower – and the system can recharge in around 30 minutes.
Critically the smallest store is only 110 litres and can fit under a kitchen counter top – or in the space previously occupied by a gas boiler.
The new cylinder incorporates three innovations.
- Firstly, it stores heat, not water. This removes the requirement for much of the gubbins that typically gets installed along with a cylinder. I’ll explain what I mean by this shortly.
- Secondly, as a consequence of #1, the orientation of the cylinder doesn’t matter.
- Finally, it’s pink. An appreciation of aesthetics is sadly lacking in many hydronic device, but this colour choice is inspired. It may be hidden behind a cupboard door, but you still know it’s pink 24/7.
Click on image for a larger version. The heat store can be mounted in an upright or horizontal configuration. There are four connections for fluids: see teh text below for details.
Heat Store versus Hot Water Cylinder
Click on image for a larger version. Illustration of the difference between a conventional hot water cylinder and the Heat Geek Mini Store. See text for more details.
In a conventional cylinder, the hot water which will emerge from the taps is stored in the tank. It is heated by contact with hotter water from a heat pump flowing through a coil. Depending on the temperature, the volume of the stored water will change. For example, heating 100 litres of water from 20 °C to 60 °C would increase its volume by about 0.8 litres. This means there must be an expansion vessel to allow for this, and this forms part of the “gubbins” that accompanies these sealed tanks. Because of natural stratification, the hot water is drawn off from the top of the vessel, and re-filled with cold mains water from the bottom.
In a heat store, the water which will emerge from the taps is not stored. It remains in a pipe just like the water that comes out of the cold taps. When a hot tap is opened the water flows through the coil in the tank and is rapidly-heated by the hot water stored as part of the heat pump circuit. This stored water is not discharged: the water flowing from the mains through the cylinder extracts heat from the stored water.
The heat exchange within the cylinder is good enough that the water emerging from the tap is close to the average temperature of the cylinder.
The heat pump already has an expansion vessel, and so an extra vessel is not needed. Also, by design, the coil is long and covers the whole volume of the tank so the tank is nearly isothermal. This means it can be positioned vertically or horizontally as required.
As I said above, it’s very similar to a conventional cylinder, but subtly different.
Heat Store in operation
Click on image for a larger version.
In operation the heat store is designed to hold water at typically 60 °C – this is much too hot to be dispensed from taps. The output is thus passed through a blending valve set to perhaps 40 °C or 45 °C.
As water is discharged the heat store cools, and so the temperature of water leaving the store falls too. But as long as the temperature remains above the temperature of the blending valve, the discharge temperature is not affected. All that happens is that the amount of blended cold water falls.
To prolong the time that this hot water can be discharged, the heat pump is set to re-heat the cylinder at full power (typically 5 kW) when the store temperature falls by 5 °C. The Performance is illustrated in the figure below.
Heat Store in operation
Click on image for a larger version. See text for details. The above performance graphs assume 8 litres/minute of hot water is being provided at 40 °C; a storage volume of 110 litres at 60 °C; an inlet temperature of 10 °C; and a heat pump power is assumed to be 6 kW.
The upper graph shows the way various temperatures vary with time after starting a shower. The storage temperature falls quickly at first, but once the heat pump starts to re-heat the water, the rate of cooling slows down. The shower temperature stays constant during this time. After about 9 minutes, the shower would have started to cool (dotted red line), but the heat pump re-heat extends this time to 13 minutes.
The lower graph shows flow rates of hot water flowing through the cylinder, and the blending flow of cold water used to maintain the shower temperature of 40 °C.
If the flow rate is slightly lower and inlet water temperature slightly higher, performance can be extended to 20 minutes or more (see below)
Click on image for a larger version. See text for details. The above performance graphs assume 7 litres/minute of hot water is being provided at 40 °C; a storage volume of 110 litres at 60 °C; an inlet temperature of 15 °C; and a heat pump power is assumed to be 6 kW.
The spreadsheet I used to generate these performance estimates has not been approved by Heat Geek. But my calculations make optimistic assumptions and I am pretty sure that performance will not be better than these estimates, but also probably not much worse. You can download my spreadsheet here.
Reflection
Personally I think this heat-store is an excellent idea, and there are probably a substantial number of UK homes that could benefit from this.
But thermodynamics is relentless, and for each advantage one steals in one place, the laws of the Universe will typically demand repayment in some form or other. In this case, one is squeezing more energy into a small space at high temperature. But the higher temperature means that the heat pump will operate with a lower than optimal SCOP. In most cases, hot water is not a large component of heating energy use, and so this won’t make much difference overall.
Friends, you can see a couple of videos about this heat store on You Tube: This is super-installer Syzmon Czaban installing and testing one in his house.
This is chief Heat Geek Adam Chapman hyperbolically describing the new heat store.
Protons for Breakfast 
September 21, 2024 at 4:29 pm |
We have a small house in Portugal with a 50 litre hot water tank that I set on very hot with a thermostatic shower I find it easily produces 2 showers one after the other. The immersion element is only 1200w so works well with our solar panels and will heat from cold in about 1 hour. I think as the amount of energy used for hot water in comparison to that used for heating or cooling it isn’t worth the additional complexity a simple immersion heater hot water tank is quick and easy to install. I think the hot water tank was 100 euro.
September 21, 2024 at 5:50 pm |
David, Good Evening.
Your implementation is the same idea: the mini-store just gets a little bit of extra hot water. Sounds like you have found a good design optimum!
Best wishes
Michael
September 21, 2024 at 5:42 pm |
Hi Michael,
The Heat Geek mini store or whatever they are calling it now is an interesting development. When I first saw it I twigged that the main advantage or innovation was that the coil and hot water store are reversed. The heat pump feeds the main tank volume allowing full volume flow without restriction and the coil instead of being a large diameter (28mm) bore to allow an unrestricted heat pump flow as per conventional high gain hot water cylinder design for use with heat pumps, to heat and store a bath full of hot water, is instead (having spoken to the partner manufacturers) a thin but long 15mm coil carrying mains pressure straight to a shower or hot tap. The large surface area of the long thin pipe provides a much improved thermal contact and heat exchange in a small space so that the heat pump can operate at full power with continuous full rate transfer to the consumer output side – no need for prior accumulation. Brilliant in its simplicity and beats my 300L monster tank that had to go in the loft. I did write to the Guardian green enery correspondent after reading an article explaining that the size of hot water tank was a problem for smaller properties and informed her of this innovation. She asked for more information so I provided a short summary press brief and links but I’ve not seen anything further. It certainly needs promoting.
September 21, 2024 at 5:59 pm |
John, Good Evening.
Thanks for the extra detail about pipe diameters. And yes, it’s important to address all the reasons for people to object to heat pumps.
When I started to write about the heat geek cylinder I realised that there is still a gap for the smallest properties. Air-to-Water heat pumps are too large for these properties (physically and thermally) and we should really be recommending Air-to-Air systems. I’ll try to write more about what a solution for space heating and DHW for a small flat might look like.
Best wishes
Michael
September 21, 2024 at 10:16 pm |
On the topic of air-to-air systems, this is the route we are going down for our house as – for the usual reasons of space and pipework – an A2W isn’t suitable.
I can’t help but grumble about the ineligibility of these systems for the boiler upgrade scheme subsidy. I hope this is something the government addresses because, as you rightly observe, it’s an ideal solution for smaller properties.
Matt
September 21, 2024 at 11:17 pm |
Matt, Good Evening,
Assuming you have a smaller house or flat, I think an A2A system is very smart. By putting the fan-coil units above doorways you also save radiator wall space AND you get cooling in the summer!
I agree with you concerning teh grant system. I think this anomaly arose because these systems sell very well in any case, but no one was buying A2W systems. I think the real rationale for ethical subsidy is to remove a boiler. So I think there is a pretty good case for a subsidy IF a boiler is being removed during the installation.
In any case, I wish you a warm winter!
Michael
September 23, 2024 at 2:14 pm
I had always assumed that the reason the current BUS does not subsidise A2A heat pumps is that they can be used for air conditioning and the government does not want to see growing use of electricity for cooling.
Hopefully, they will loosen the rules on this. I would think that on those few days when you really need air con in the UK, there will be such an abundance of solar power on the grid that the air con use has a low carbon cost.
Excellent article, by the way. I had not previously grasped what was different about these cylinders.
September 23, 2024 at 6:24 pm
Gareth, Good Evening. Glad you enjoyed the article.
Yes, we have an air conditioner in the house just covering our bedroom and the hall. It’s only for a few days each year but (a) it feels great to cool down and sleep well and (b) it’s all solar powered.
I don’t know the government’s exact motivation, but I think you’re roughly right. But, I think if the A2A installation was accompanied by removal of a boiler then even a small amount of subsidy would make it very popular in smaller dwellings.
Best wishes
Michael
September 21, 2024 at 10:21 pm |
Hello Michael – I’ve very much enjoyed reading and learning from your blog.
In moving from a combi setup we faced the problem of where to house a hot water cylinder, and opted for a Sunamp “heat battery”. You’ll surely appreciate the physics of these- they are units that exploit the high enthalpy of fusion of sodium acetate (+ some proprietary additives) to heat direct-flow water to ~60C.
It’s a very small – if heavy! – unit which practically fits in one of our kitchen cupboards. We have the direct immersion heating model which we run using off-peak electricity.
For the amount of DHW we use, the cost difference between that and gas is negligible and it was very satisfying to see our summer gas use go to zero!
Matt
September 21, 2024 at 11:13 pm |
Dear Matt, Good Evening,
The Sunamp route is very smart. I think the role of the Heat Geek Heat Store may be as a sort of cheaper, not quite so good, Sunamp. I meant to mention the Sunamp in the article, but it was already too long!
The physics of sodium acetate is fascinating and if you are interested, you can read about my kitchen experiments with Sodium Acetate here.
Best wishes
Michael
September 22, 2024 at 3:23 pm |
I must say, having given it a lot of thought (especially when you tweeted the first results of your model), I think the Heat Geek mini store is brilliant and really want one!
btw I don’t think (because people who think about such things tend not to live in them) people fully appreciate how efficient standard post-2000 houses are. For a 80 sq m detached house you’re looking at around 5,000 kWh gas demand a year. Even a big 180 sq m 5-bed is only about 15-20,000 kWh (depends on how many of the bedrooms are filled).
September 22, 2024 at 3:42 pm |
Dan, yes it’s curious that it’s just a tweak on an existing idea, but it does seem to shift one’s perspective.
Regarding housing, ~20,000 kWh/year for 180 square metres is ~100 kWh/m^2/year which is not great, but it’s still only a 7 kW heat pump.
But, 5,000 kWh/year for 80 m^2 is ~60 kWh/m^2/year and that is plenty good enough. It corresponds to a peak heating demand of less than 2 kW! It makes a 20 kW gas boiler seem even more insane! And for such homes every air-to-water heat pump on the market is too large by a factor of at least 2.
I think air-to-air (with cooling) is likely to be appropriate here – with a standalone hot water heater, either an electrical heater with a buffer (Heat geek mini store?) or a mini heat pump water heater. But in an 80 square metre house, living space is precious and there is no “plant room” that seems to feature in so many heat pump installation videos!
Interesting times.
Best wishes – and thanks for so many useful postings on so many interesting topics on Twitter.
Michael
September 23, 2024 at 1:33 am |
Apologies for the muddled repetitive comments! AFAIK wordpress doesn’t allow comments to be edited. So I’ll try again:
—
Hmm, when viewed from NZ, the heat-geek’s invention is a clever marketing pitch for a bog-standard electric HWC of 135L capacity whose tank has been downsized a bit (to 110L) to fit the UK use-case.
“In New Zealand, the recommended temperature for most hot water cylinders typically ranges between 60°C to 65°C (140°F to 149°F). This temperature range is deemed sufficient to prevent the growth of harmful bacteria, such as Legionella, which thrives in lukewarm water. By maintaining the water temperature within this range, you can ensure the safety of your household members by minimizing the risk of waterborne illnesses.” (https://regencyplumbing.co.nz/what-temperature-should-your-hot-water-cylinder-be-set-at-in-nz/)
“New Zealand building code requires that the maximum hot water temperature at the tap shall be 45°C or less for childcare centres, schools, homes for the elderly and similar facilities, and 50°C for other buildings.” (https://www.wattsnz.co.nz/resources/references-tools/thermostatic-mixing-valves)
From my eco-perspective, the primary advantage of having a 135L electric HWC is that it’s an inexpensive “water battery” that’ll be “charged” efficiently and opportunistically by my household’s wee 1.8kWh PV array. In the summer, there’s enough left-over power at mid-day to solar-trickle charge my EV.
September 23, 2024 at 6:39 pm |
Clark, Good Evening,
Zerothly, I think I deleted most of the copied comments. Let me know if I deleted something precious. Sorry it’s so cumbersome. M
Firstly, please excuse my slow replies. I am feeling a bit overwhelmed at the moment. Not because there is so much going but because my capacity to get things done is getting less and less!
Secondly, no! The key thing about the heat store is that the hot water which is dispensed to the taps is not stored at all. The dispensed water is (almost) instantly heated and so one has zero Legionella risk. This is one of the benefits of using the coil in the cylinder for dispensing and using the stored water as part of the heat pump circuit – that water has all the usual anti-corrosion chemicals in. This change also means that the orientation doesn’t matter which can be a big help in some circumstances.
Does that make sense? Or have I misunderstood your comment?
All the best
Michael
September 23, 2024 at 1:25 pm |
I watched the Heat Geek channel with interest when I saw their heat store idea. It seemed like a perfect solution. My colleague has just had Octopus install a standard Daikin heat pump system with a regular hot water cylinder.
there is far too much in the way of gubbins for my liking. Too much complexity, too many potential leaks and way too much space. Luckily for my colleague, his modern house previously had a system boiler with a separate hot water cylinder. This meant that he already had a sufficient space envelope in which to fit the new cylinder and gubbins.
in my situation I have a floor standing combi and although my house is large there is no ideal location for a standard cylinder and gubbins. However if I were to install a heat pump then the space left by the old gas boiler would be more than sufficient for a mini heat store.
My only concern would be the flow rate the showers. My current combi has a very high output and the flow rate is significant making my thermostatic mixer showers feel like power showers. I could happily accept some degradation in performance so I need to carry out some experiments to determine the lowest acceptable flow rate.
I think that this new development will simplify the change to heat pumps for many home owners.
September 23, 2024 at 6:28 pm |
Kevin, Good Evening.
Yes it sounds as though the heat store – mini or slightly larger – might do the trick for your. There is also the possibility of using a SunAmp Phase Change heat store which would have a larger thermal capacity in a similar volume. I’ll send you a photograph of a SunAmp under-counter installation separately by e-mail.
Regarding showers, both mini-store and SunAmp should should deliver water at close to mains pressure.
In any case, good luck with endeavours. Watch out for an e-mail!
M
September 23, 2024 at 7:57 pm
Thanks for the photo of the Sunamp that you emailed me. I first became aware of the Sunamp through Robert Llewellyn on the Fully Charged Show. I thought it was a fascinating product. I am still not clear about what happens when you start the reaction to release the heat, does it continue to release it until the reaction finishes or does it stop when you turn off the tap?
September 23, 2024 at 8:13 pm
Kevin, Good Evening,
The physics is fascinating. Here’s my “Kitchen Physics” article describing my experiments (link)
What happens is this (roughly).
First of all imagine that the container is perfectly insulating – no heat can get in or out except through the pipe that runs through the container of sodium acetate.
When you run cold water through the liquid phase, the liquid is cooled strongly and solidifies around the pipe releasing latent heat at around 58 °C (IIRC). If you keep the cold water flowing in, then more liquid condenses releasing more latent heat, but the temperature stays at 58 °C. When the water stops flowing, there is no heat loss and no more liquid condenses. Eventually, after a lot of cold water has passed through (typically about 200 litres) all the liquid condenses and all the latent heat is released, and then the water is heated to a lower temperature.
In practice, there are heat losses, so the liquid will gradually solidify over (I guess) a few days, even without water flowing through the pipes.
The technical innovation in making the product concerns longevity. If you repeat the cycle, the sodium acetate can phase separate – the water of crystallisation doesn’t mix back in with the chemical. One could combat this by stirring, but I don’t think the sunlamp has any moving parts. I think they counteract this by mixing other compounds in (one is rumoured to be wallpaper paste!) which creates a “slush” phase. So instead of changing straight from liquid (at high temperatures) to solid (at low temperatures), there is an intermediate slush phase that can exist over a range of temperatures.
I don’t know the technical details, but people who own the product tell me the product works and that they are happy with it!
Best wishes
Michael
September 23, 2024 at 6:46 pm |
Clark, good evening again.
Regarding Legionella, I think such precautions are over the top. Heat Geek has good advice on this too.
https://www.heatgeek.com/hot-water-temperature-scalding-and-legionella/
Most notable is this graphic which states that at 50 °C, 90% of legionella die after 2 hours but at 60 °C 90% die after 2 minutes. Coupled with the vanishingly small chance of infection from anything other than an AC tower, I think the risk is super low.
In case it’s of any interest I wrote about blending valves a few years ago.
Best wishes
Michael
September 28, 2024 at 7:55 pm |
Hello Michael
Adding to your as-usual informative blog, I have a 14kWh SunAmp rated equivalent to ~280l in our 4-bed terrace and a 7kWh SunAmp ~140l in its basement 1-bed rental, both charged by either their own immersion heaters or a Vaillant aroTHERM+ 5kW ASHP. I spoke with SunAmp people at an event in London over summer and they confirmed SunAmp heat batteries are generally slave units, which adds an element of difficulty in trying to optimise charging efficiency. I wish they produced more data.
I’ve added a WiFi controlled AC contractor to each SunAmp, which switches charging between ASHP and immersion, and which is able to be automated in Apple HomeKit. I’m on Octopus Go so the basement SunAmp is timed to begin ASHP charging (if necessary) at 00:30 when the 8.5p/kWh rate kicks in, and the main SunAmp begins ASHP charging at 2.30am (when the basement is timed to stop) and goes through to 5.30am, if necessary, when the 8.5p rate stops.
I’ve been charging my car and doing my clothes and dishwashing in this window as well. My electricity bills have halved. That’s why we have smart meters – The Telegraph or Sun or Daily Mail if you’re asking… you knuckleheads.
I’m achieving COPs – admittedly as calculated on the My Vaillant app which apparently once was less accurate but may have improved in recent times – of 2.9 on average, which I think is a great result given the DS18B20 temperature sensors I’ve attached to various points on the ASHP and SunAmp systems are consistently showing 65-70° during charging, with the ASHP operating in 5-12° external temperatures. For anyone interested the sensors are fed via Shelly WiFi boards into an Influx database running in Home Assistant OS installed on a Raspberry Pi 5 and graphed on Grafana. All components in the £2-16 range apart from the £50 pi – ridiculously cheap really.
I am experimenting with a circuit board called eBusd which is connected to my Vaillant bus and sends data via MQTT to HA as well. When I finally have all the data Vaillant and SunAmp should be giving me but do not for presumably many silly reasons, I hope to automate further eg doing things like using immersion charging in the cheap power window if any of the batteries need more than the allotted time I’ve set them to charge.
Fun times!
September 28, 2024 at 10:18 pm |
Guy,
Good Evening. I will just comment that I am filled with admiration for your technical prowess. I think the Sunamp technology and the Heat Geek mini-store are new pieces of technology and I think people like you will figure out clever ways to combine them.
Best wishes
Michael