Heat Battery vs Hot Water Cylinder: Which Should You Choose?

What a heat battery is

A heat battery stores heat in a solid material instead of in a tank of hot water. Inside the casing is a phase change material, a substance chosen because it melts at a set temperature. When the battery charges, the material absorbs heat and melts. When you open a hot tap, cold mains water passes through a heat exchanger running through the material, picks up that stored heat and leaves as hot water, and the material slowly sets solid again as it gives the heat up.

Melting and setting a material holds a great deal of energy for its size, far more than simply warming the same volume of water. That is the whole point of the technology: similar usable hot water from a noticeably smaller box. Sunamp is the best known maker in the UK, and its domestic range is sold as Thermino, so you will often see the words heat battery, Sunamp and Thermino used for the same thing.

There are versions charged by their own electric element, versions charged by a boiler or heat pump in the same way an indirect cylinder is, and versions set up to soak up surplus power from solar panels. Which one suits depends on where your heat comes from today.

How it differs from a hot water cylinder

A cylinder heats and holds a body of water, anything from around a hundred litres upward, and hands it over when you open a tap. An unvented cylinder does this at mains pressure, which is why it carries an expansion vessel, a pressure reducing valve and a temperature and pressure relief valve, and why it has to be fitted and serviced by someone qualified under Part G3 of the Building Regulations. If you are not sure what you have now, our guide to unvented vs vented cylinder systems shows how to tell them apart.

A heat battery holds no stored hot water. The water in it at any moment is only what sits inside the heat exchanger, and it is heated on its way through. That changes several things at once: there is no large tank to insulate, much less stored water sitting at a lukewarm temperature between uses, and the unit can go in spaces a cylinder never could. It still needs the valves and safety devices the maker specifies on the incoming supply, so it is not a plug in appliance, but the installation is laid out differently from an unvented cylinder.

In a sense a heat battery is a cousin of the thermal store, which also holds heat rather than drinking water and heats the mains as it passes. If you have met one of those, the comparison in our thermal store vs unvented cylinder guide will feel familiar.

Where a heat battery beats a cylinder

For a one or two person flat that currently runs an old vented cylinder in a cramped cupboard, those advantages can be decisive.

• Space. A flat with no airing cupboard, or a kitchen where the only free space is a wall cupboard, can take a heat battery where a cylinder of useful size simply will not fit, although a horizontal hot water cylinder or a slimline model is worth checking first.
• All electric homes on a time of use tariff. The battery charges in the cheap hours and holds the heat well, which suits the same households that would otherwise have an electric cylinder on an off peak rate.
• Solar panels. A battery version designed to take surplus solar power does a similar job to a solar diverter on an immersion heater, using power you would otherwise export.
• Low standing loss. There is no large tank of water losing heat through its jacket all day, although any store loses some heat.
• Less stored water. With almost nothing stored between uses, there is less stagnant warm water in the system than in an oversized cylinder that is rarely emptied.

Where a cylinder is still the better answer

A heat battery is not the right answer for every home, and it helps to be honest about the limits.

• Big demand at once. Two showers running together plus a bath filling needs a lot of heat very quickly. Each battery model has a maximum flow and capacity, and a large family house is usually better served by a properly sized unvented cylinder.
• Weak mains. Like an unvented cylinder, a heat battery is fed from the mains. If the incoming supply is poor, the hot water will be poor too, and a battery does not fix that.
• Water temperature. The material melts at a fixed point, so the water comes out at a fairly fixed temperature, usually a little below what a cylinder stores at. Most people never notice, but anyone used to very hot water at the kitchen tap should know.
• Repairs and parts. Almost any competent plumber can find parts for a Megaflo, a Joule or a Gledhill cylinder within a day in London. A heat battery is a more specialised product, and anything inside the casing is a matter for the maker and its approved engineers.
• Up front cost. A heat battery usually costs more to buy than a cylinder of similar output. The case for it rests on the space it frees and how it charges, not on a cheaper purchase.

Hard water and heat batteries in London

London water is hard, and scale forms wherever water is heated. In a cylinder that means scale on the coil and the immersion element, and it builds slowly because the water sits there at temperature. In a heat battery the scale forms inside the heat exchanger that the mains water passes through, which is narrow by design.

That makes the water treatment the maker specifies worth taking seriously. If the installer recommends a scale reducer on the incoming supply, it is there to protect the part of the battery you cannot easily get at. The same principle applies to plate heat exchangers in combis and thermal stores, which is why hard water shortens the life of all of them in the capital.

Running cost: is a heat battery cheaper?

Heating the same amount of water by the same number of degrees takes the same energy, whatever box does it. The differences in what you pay come from three places: which fuel or tariff you charge from, how much heat the store loses while it waits, and how much hot water is wasted in pipework and oversized storage.

A heat battery charged on a cheap overnight rate or from surplus solar can cost very little to run, but so can an electric hot water cylinder on the same tariff with a good jacket and a timer set properly. Where a battery can pull ahead is standing loss, because there is no large tank cooling all day, and in homes where an old cylinder is far larger than the household needs. If you are comparing against an immersion heated cylinder, our guide to immersion heater running cost works through what an element uses and how tariffs change it.

Be wary of any running cost figure that does not tell you the tariff, the household size and the cylinder it is being compared with. Those three things decide the answer.

Questions to settle before you choose

If the answers point to plenty of space, a gas boiler and a family house, a well sized unvented cylinder is hard to beat. If they point to a small all electric flat with no cupboard to spare, a heat battery deserves a serious look. Our guide on how to choose a hot water cylinder covers the cylinder side of that decision in more detail, and our guide to hot water cylinder sizes is the place to start before any quote.

• What does your incoming mains actually deliver in litres per minute? That rules options in or out for both a battery and an unvented cylinder.
• How many people shower, and do they shower at the same time?
• Where will the heat come from: gas boiler, heat pump, off peak electricity or solar?
• How much space do you really have, including access for the pipework and the discharge from any safety valve?
• Who will service and repair it where you live, and how quickly can they get parts?

Talk it through with an engineer

We fit, service and repair hot water cylinders across London, and we will tell you plainly if your property would be better off with something else. If you already have a heat battery, the plumbing around it, the mains supply, valves, leaks and the pipework to the taps, is ordinary plumbing work we handle every day, while anything inside the casing goes back to the maker.

If your current cylinder has failed and you are weighing up the options, our hot water cylinder replacement service covers the whole job, and an unvented cylinder installation is the work we do most. Call 020 4652 5111 to book a visit. Our rates are on the pricing page and are agreed before we attend.