Mixergy Cylinders Explained

What is a Mixergy cylinder

It is a stainless steel hot water cylinder with active control over where the heat sits inside it. Mixergy is a British manufacturer and the range covers most of the arrangements found in a house or a flat. We are an independent firm that works on these units alongside every other make, not an approved or authorised agent for the manufacturer, and nothing here is a manufacturer statement.

The difference is not the steel. Any modern unvented cylinder is a stainless vessel with an inlet control group, expansion provision, a temperature and pressure relief valve and a discharge route to a tundish, and a Mixergy has all of that too. What it adds is an internal circuit that can place heated water at the top of the tank deliberately, a column of temperature sensors so the controls know where the hot and cold water meet, and a connected controller that decides when and how much to heat. That is why owners call it a smart cylinder rather than an efficient one. The efficiency follows from the measurement.

• Indirect unvented models heated by a boiler through a coil, usually with an immersion element as well.
• Direct electric models heated by immersion elements only, which suits a flat with no gas supply.
• Heat pump models built around the lower flow temperatures a heat pump delivers.
• A retrofit kit that brings the same charge control to a suitable existing cylinder.

How does heating from the top down actually work

A small internal pump draws water from the cooler lower part of the tank, sends it past the heat source, and returns it to the very top through a diffuser that lays it in gently enough not to stir the contents. Hot water is less dense than cold, so once it is placed at the top it stays there. The result is a sharp boundary between the hot layer above and the cold water below, and that boundary moves downward as charging continues.

A conventional cylinder has little say in the matter. Its coil sits low in the vessel and heat rises by convection, so stratification happens as a by product rather than as something anyone measures, and the heat source generally runs until the whole cylinder reaches the set temperature. If you only needed enough for a shower, you paid to heat a full tank anyway.

Sensors up the height of a Mixergy tell the controller where the boundary is, so it can be asked to charge to a proportion of the volume rather than to a temperature alone, stop there, and report what is left. Ask for a third of a tank and you get a fully hot third rather than a lukewarm whole. That is the important distinction: a partly heated conventional cylinder gives you a mixed result, while a partly charged Mixergy gives you a smaller quantity of properly hot water.

Top down charging is also why a Mixergy produces usable hot water quickly from cold. The first useful volume arrives at the outlet early in the cycle rather than at the end of it, which matters when a household has just run out.

What the Mixergy app actually controls

The app sets how much hot water you want, when you want it ready, and where the energy should come from. It is a scheduling and charge level interface rather than a remote thermostat, and that difference is the whole point. You are telling the cylinder to have half a tank hot by seven in the morning, not asking it to hold one temperature all day and hoping that works out.

What it does not do is worth saying plainly. It does not change the mains cold supply, so it cannot improve poor flow or pressure at a shower, and it cannot make a boiler heat faster than the boiler can. It is also no substitute for the mechanical safety devices on a pressurised cylinder, which work on physics and do not consult the controls. A connected cylinder with a healthy app screen can still have an inlet control group that needs attention.

• Set a target charge level and a ready by time rather than one fixed temperature.
• Schedule charging into an off peak or time of use tariff window.
• Accept surplus solar photovoltaic generation as a heat source.

Does a Mixergy cylinder actually save money

It saves by not heating water nobody will use, and by moving the heating it does into the cheapest hours available. How much that is worth depends almost entirely on what heats the water and what you pay for that energy, so a single headline number would be meaningless. The mechanism is easy to follow without one.

Three effects are in play. The first is avoided volume: heating a third of a tank instead of all of it uses roughly a third of the energy for that cycle, and a household that heated a full cylinder to take one shower was wasting the rest. The second is standing loss, because a partly charged tank has less hot water sitting there cooling into the cupboard overnight. The third, usually the largest on an electric system, is tariff shifting, where the same energy simply costs less because it was bought at night.

The saving is largest on a direct electric cylinder on a time of use tariff, or on a property with solar generation that would otherwise be exported. It is smaller on a gas fired indirect cylinder in a household that already empties a modest tank every day, because there is little waste to remove. Anyone weighing it up should ask honestly how much hot water the household wastes today. Where the answer is not much, a plain cylinder well controlled is a rational choice, and how to choose a hot water cylinder is the wider decision this sits inside.

What a Mixergy adds in complexity

It adds moving and electronic parts to an appliance that historically had almost none, and that trade should be understood before buying rather than discovered later. A conventional indirect unvented cylinder is a vessel, a coil, a thermostat, a motorised valve and the safety ancillaries, which is why faults on one are usually in the ancillaries. A smart cylinder has all of that plus an internal pump, temperature sensors and their harness, a control board, firmware and a network connection.

None of that makes it a worse appliance. It makes it a different kind of appliance, closer to a boiler than to a tank in terms of what needs attention. It also makes commissioning matter more. A conventional cylinder that is plumbed correctly works. A smart cylinder plumbed correctly but commissioned carelessly, with sensors sited wrongly or the heat source configured wrongly, will run and will disappoint, and the owner tends to blame the concept rather than the setup.

The other practical point is parts. Boards, sensors and pumps on a smart cylinder are manufacturer specific rather than generic merchant stock, so a control side fault usually means sourcing through the maker rather than picking something up locally the same day. That is a serviceability difference worth knowing about, not a criticism of how anything is built.

Nothing here suggests a manufacturer warranty is affected by who attends. Warranty terms are between the owner and the manufacturer, and they are worth reading before any work is arranged.

What goes wrong with a Mixergy cylinder

The faults divide cleanly into two groups: the ones any unvented cylinder can have, and the ones that exist only because this one is smart. Knowing which group a symptom belongs to saves a great deal of time.

On the conventional side a Mixergy is subject to the same things as any pressurised cylinder. Water at the tundish, a weeping relief valve, a leak at a connection, or no hot water because the heat source is not delivering are ordinary unvented problems, diagnosed in the ordinary way. On the smart side the list is different. A cylinder that has dropped off the network keeps working on its last schedule but stops answering the app. A sensor giving a false reading makes the reported charge wrong, so the owner sees a full tank and gets a cold shower. A pump that is not circulating cannot place heat where it belongs, so the cylinder behaves like a poor conventional one.

A useful first question with any smart cylinder complaint is whether there is no hot water at all, or hot water at the wrong time or in the wrong quantity. The first points at the heat source or the ancillaries and is a conventional diagnosis. The second points at sensing, control or configuration. Establishing which of the two it is gets an engineer there considerably faster than opening the front cover and starting with the electronics.

• Lost network connection: the app goes quiet while the cylinder runs its last schedule.
• Sensor reading wrong: reported charge does not match what the taps deliver.
• Internal pump not circulating: charging becomes slow, uneven or ineffective.
• Controller or power supply fault: no scheduling and no charge management.
• Configuration set for the wrong heat source or tariff window, which looks like a fault.

Mixergy cylinders and London hard water

Hard water is the defining condition across almost the whole London supply area, and it treats a smart cylinder much as it treats any other. Scale forms on the hottest surfaces it can reach, which means immersion elements first and heat exchange surfaces second. A stainless vessel resists corrosion well, but stainless does nothing about scale, and any cylinder here with a direct electric element will collect it.

Where this design differs is in how visible the decline is. Because the controller records how long a charge takes and what it costs in energy, a scaled element or a fouled exchange surface shows up as charge cycles that are quietly getting longer, while the same decline on a conventional cylinder stays invisible until somebody notices the hot water is not what it was. That is a real diagnostic advantage and it is worth using. The rest of the London logic is unchanged, and hard water and hot water cylinders in London covers what this water does to a cylinder in more detail.

Does a Mixergy suit a London property

It depends far more on the building than on the household, and in London the building usually decides. In a Victorian conversion the cylinder normally lives in a cupboard on the first floor, up a staircase never designed with appliance delivery in mind. The questions are whether the unit will go up the stairs and turn into the cupboard, whether there is height for it, and whether the safety valve discharge can reach a lawful termination. A smart cylinder is no harder to carry up than a conventional one, but it needs a permanent electrical supply and a usable wireless signal in that cupboard, which is not a given in the middle of a lath and plaster house.

In a purpose built block the common arrangement is a thermal store or an electric cylinder with no gas supply to the flat. This is where a direct electric Mixergy makes the most sense, because the tariff shifting and the avoided volume matter most when electricity does all the heating. The constraints there are the incoming electrical supply, whether the meter arrangement supports a time of use tariff, and what the lease and the building management allow.

In a new build the cylinder is usually already unvented and already in a dedicated cupboard, sometimes with liquid screed and underfloor heating downstairs and increasingly with a heat pump rather than a boiler. A smart cylinder fits that well, because a heat pump wants to run long and steady into a well stratified tank and the sizing was already done for a pressurised unit. A like for like footprint also keeps the job sensible, which is not always true when a hot water cylinder replacement changes the type of system rather than just the tank.

What servicing a Mixergy involves

It needs the same annual attention as any other pressurised cylinder, plus a look at the parts only a smart cylinder has. The first half is standard work on a pressurised system: the safety ancillaries, the discharge arrangement, the expansion provision, the controls and the condition of the connections. That part is not optional and it is not different because the cylinder is clever.

The second half is checking that what the controller believes is true. Do the reported temperatures match what is measured at the tank. Does a charge cycle behave correctly, with the boundary moving down the tank as expected. Is the pump running when it should. Is the configuration still matched to the heat source and the tariff, which drift when somebody changes supplier. One thing catches people out and has nothing to do with plumbing: the app account is tied to whoever set the cylinder up, so when a property is sold or a tenancy changes the account needs transferring, or a new owner ends up with a cylinder that works but cannot be managed. An unvented cylinder service is the natural point to check all of this, because the covers are already off.

Only somebody properly qualified for unvented hot water work should be opening a pressurised cylinder or its safety devices. That applies to a smart cylinder exactly as it does to a plain one.

Who a Mixergy suits, and who it does not

It suits a household where the energy going into hot water is expensive, variable in price, or partly free. An all electric flat on a time of use tariff, a house exporting surplus solar, a heat pump installation, or a household whose demand swings wildly will all get real value from controlling the charge. It suits far less well a house on gas with a steady routine that empties a modest cylinder daily, because there is little waste to remove and the extra hardware still has to be bought and maintained.

The fair verdict is that this is a real innovation rather than a marketing exercise. Controlling stratification deliberately and measuring the state of charge solve a genuine waste problem, and the top down charging gives a benefit anybody can feel on the day the tank runs out. The cost is a cylinder with more inside it, manufacturer specific parts on the control side, and a dependence on commissioning being done properly. Bought for the tariff, the solar or the heat pump, it earns its place. Bought because it is smart, it is an expensive way to be told how much hot water is left.