Who makes Joule cylinders

Joule is a hot water cylinder manufacturer based in Ireland, selling across the UK through merchants and distributors. The domestic range most people have is called Cyclone and covers unvented cylinders for boilers, cylinders designed for heat pumps, slimline and horizontal versions, and twin coil models for solar. We work on them as an independent firm and have no formal relationship with the manufacturer.

Are Joule cylinders stainless steel

Yes. The Cyclone range uses a welded stainless steel vessel rather than enamel lined steel, so there is no sacrificial anode that somebody has to inspect and replace on schedule. Stainless is the right material for a sealed hot water store and it is what nearly every serious manufacturer now uses. The trade off is that a stainless vessel cannot be patched, so if the steel itself weeps the cylinder is finished.

How long does a Joule cylinder last

A stainless cylinder that is serviced annually, fed by a correctly set inlet control group and not run hotter than the household needs should give many years of service, and the vessel normally outlives several sets of valves and immersion elements. What shortens that life in London is hard water scaling the coil and the element, an unnecessarily high store temperature, and years of skipped services letting a worn relief valve pass unnoticed.

What usually fails on a Joule cylinder

The fittings rather than the steel. In rough order: the immersion element scales up and burns out, the expansion vessel diaphragm perishes and the tundish drips, the pressure reducing valve drifts or sticks, a relief valve starts to pass, the motorised valve on the heating side sticks, and the control thermostat fails. Every one of those is a replaceable part on a cylinder with years left in it.

Are Joule cylinder parts easy to get

The wearing parts are, because they are standard industry components rather than proprietary ones. The immersion screws into a normal boss, the expansion vessel is a commodity item and the inlet valves are the same families used across the trade. Anything genuinely model specific can take longer than for the longest established British brands, because Joule reaches the UK through a narrower distribution network.

Is a Joule cylinder good for a heat pump

The heat pump models are among the better options, because the coil surface area is properly sized for a low flow temperature heat source rather than carried over from a boiler product. That is the specification that decides whether a heat pump heats your water in a sensible time. A standard indirect cylinder connected to a heat pump reheats slowly, and no amount of controls adjustment will fix an undersized coil.

Can a Joule cylinder be repaired or does it need replacing

It depends which part failed. A dead immersion, a perished expansion vessel, a drifted pressure reducing valve, a passing relief valve or a stuck motorised valve are all ordinary repairs. The exception is the stainless vessel itself, which cannot be patched, so a weeping vessel is a genuine replacement. Always get a diagnosis before accepting a replacement quote, because replacing a repairable cylinder is the costliest mistake here.

Why does my Joule cylinder take so long to heat up

Slow recovery is usually one of three things. Scale on the coil after years in hard London water reduces heat transfer. The coil may be undersized for the heat source, which is common where a boiler specification cylinder has been paired with a heat pump. Or the heating side is not delivering, typically a motorised valve or control fault. An engineer can separate those on one visit.

Which immersion heater should I fit in a Joule cylinder in London

Specify an Incoloy or titanium sheathed element rather than the cheapest copper one. London water is hard across almost the whole supply area, scale builds on the element surface, the scale insulates it, the element then runs hotter than intended and eventually burns out. A better sheath tolerates that for much longer. Also check the element has physical room to withdraw before anybody sets the cylinder.

How do I find out which Joule model I have

Look for the data plate, normally on the front of the insulation near the top of the cylinder. It gives the model name, capacity in litres, the coil arrangement and a serial number. Photograph it before you ring anybody, because the difference between a standard indirect and a heat pump rated model changes the whole performance conversation and makes any parts enquiry far quicker.

Joule Cylinders Explained

A Joule cylinder is a stainless steel hot water cylinder built by the Irish manufacturer Joule, sold across the UK mainly as the Cyclone range of unvented and heat pump cylinders. They are conventionally engineered in the ways that decide whether a cylinder is easy to live with: a stainless vessel, an external expansion vessel on the cold feed, a standard screw in immersion boss and an inlet control group made of individual parts rather than one sealed assembly. That means almost everything that fails on a Joule is a commodity component an engineer can carry and change on the visit. The brand is best known here for its heat pump cylinders, where the coil surface area is genuinely sized for a low flow temperature heat source rather than carried over from a boiler product.

What is a Joule cylinder

Joule is a hot water cylinder manufacturer based in Ireland whose products are sold throughout the UK through merchants and distributors. The domestic range you are most likely to have in a London property is called Cyclone, and it covers unvented cylinders for gas and oil boilers, cylinders designed specifically for heat pumps, slimline and horizontal versions for awkward spaces, and twin coil versions for solar thermal. They are stainless steel throughout rather than enamel lined steel, so there is no sacrificial anode to inspect or replace.

We are an independent plumbing and hot water firm. We are not an approved, authorised or official installer, service agent or partner of Joule or of any other manufacturer, and nothing here is endorsed by them. We simply work on these cylinders across London every week and this is our honest engineering view of them.

If you have inherited one and do not know which model it is, the data plate is the answer. It is normally on the front of the insulation near the top, and it gives the model name, the capacity in litres, the coil arrangement and a serial number. Photograph it before anybody quotes you, because the difference between a standard indirect Cyclone and a heat pump rated one changes the whole conversation about performance.

Cyclone unvented: the mainstream indirect cylinder for a system boiler.
Cyclone heat pump models: much larger coil surface area for low flow temperatures.
Slimline and horizontal versions: for cupboards and lofts that will not take a standard footprint.
Twin coil versions: a second coil for solar thermal or a second heat source.
Direct models: immersion heated only, no coil, for properties with no boiler.

How is a Joule cylinder built

The construction is stainless steel, with the vessel welded rather than lined, and the insulation foamed directly onto the shell. That is the right material choice for a sealed hot water store and it is what nearly every serious manufacturer now uses. A stainless vessel does not depend on somebody remembering to check an anode, and it does not rust from the inside out the way an unprotected steel cylinder will.

The part of the build that matters most for you is how everything else attaches to that vessel. Joule uses an external expansion vessel on the cold feed, which is a separate component with its own part number, so when its diaphragm eventually perishes an engineer replaces the vessel rather than doing anything model specific to the cylinder. The immersion boss is a standard screw in fitting, so a replacement element does not have to come from the manufacturer. The inlet controls are discrete parts on the cold feed rather than one moulded group, so a passing relief valve is a valve change and not a whole assembly purchase.

None of that is glamorous, but it is the difference between a cylinder that gets repaired in year twelve and one that gets condemned because a small part has a lead time. Joule sits firmly on the repairable side of that line.

A stainless vessel that is weeping cannot be patched or welded back into service. If the steel itself has failed, that is a replacement. If a valve, an immersion, a thermostat or the expansion vessel has failed, it is not.

Which Joule cylinder do I have

Read the data plate and look at what is connected. A standard indirect Cyclone has one coil, a boiler flow and return pair on the side, an immersion boss and a cold feed carrying the inlet control group. A heat pump model looks similar but the coil connections are usually larger, the data plate states a heat pump coil rating and the cylinder is often taller for the same litres because the coil takes up more of the height.

A twin coil version has two pairs of coil connections, the lower one intended for solar thermal or a second heat source. Owners often assume a twin coil means twice the performance. It does not. If nothing is connected to the second coil it is doing nothing, and we regularly find twin coil cylinders in London houses where the solar panels went years ago.

The other thing worth establishing early is whether you actually have a cylinder at all. Plenty of purpose built London blocks run a thermal store fed by a communal system rather than an unvented cylinder with a boiler, and the two are confused constantly by householders and occasionally by quoting salesmen. A thermal store fails differently, is serviced differently and is not comparable to a Joule on any of the points on this page.

One pair of coil connections plus an immersion boss: standard indirect.
Larger coil connections and a heat pump rating on the plate: heat pump model.
Two pairs of coil connections: twin coil, usually for solar thermal.
No coil connections at all: direct, heated by immersion only.
No boiler in the property and a communal supply: probably a thermal store, not this.

Are Joule cylinders any good for a heat pump

Yes, and this is where the brand is strongest. Joule designed its heat pump cylinders around the problem rather than adapting a boiler cylinder to it, and the coil surface area on those models is substantially larger than on a standard indirect of the same capacity. That is the whole game with a heat pump. The heat source delivers water at a much lower flow temperature than a gas boiler, so the only way to move the same amount of energy into the store in a sensible time is a much bigger heat exchange surface.

The failure we are called to on heat pump installations is almost never the cylinder leaking. It is a household complaining that hot water takes forever and that the heat pump runs constantly to produce it, and when we look, the cylinder is a standard indirect that somebody specified from a boiler era price list. No amount of controls tuning fixes an undersized coil. If a heat pump is in the property now, or is a realistic plan within the life of the cylinder, the heat pump rated model is the one to buy at the outset.

The corollary matters too. With a gas system boiler and no heat pump in your future, you do not need to pay for a heat pump rated cylinder. A standard indirect with a coil matched to your boiler output recovers perfectly well.

What goes wrong with a Joule cylinder

The same things that go wrong with every unvented cylinder, because the failures live in the fittings rather than the steel. In rough order of how often we see them: the immersion heater element scales up and burns out, the expansion vessel diaphragm perishes and the tundish starts to drip, the pressure reducing valve on the inlet drifts or sticks, the expansion relief valve starts to pass, the motorised valve on the heating side sticks or its microswitch fails, and the control thermostat fails. All of those are ordinary parts and all of them are replaceable without touching the cylinder itself.

Scale is the London specific one and it deserves its own mention. Our water is hard across almost the whole supply area, so anything that runs hot deposits calcium on its surface. On the immersion element that scale insulates the heating surface, the element then runs hotter than it was designed to, and eventually it fails. On the coil, scale reduces heat transfer and shows up as a cylinder that takes longer and longer to reheat, which householders usually blame on the boiler.

What we do not see much of on Joule cylinders is early vessel failure. The stainless work is sound. When a Joule vessel does fail it is normally either a very old cylinder or one that has been run at an unnecessarily high store temperature for years, which is hard on every cylinder ever made and buys the household nothing that correct sizing would not have given them.

Immersion element scaled and failed: the most common single fault in London water.
Expansion vessel diaphragm perished: shows up as a dripping tundish.
Pressure reducing valve drifted or stuck: poor flow or erratic pressure at the taps.
Expansion relief or temperature and pressure relief valve passing: discharge at the tundish.
Motorised valve stuck or its switch failed: heating works, hot water does not.

A tundish that is discharging is a safety device doing its job. It is telling you a relief valve is passing or the cylinder has lost control of its temperature. Nobody should ever cap, plug or pipe away a discharging valve instead of finding out why it is discharging.

Can you get parts for a Joule cylinder

For the wearing parts, yes, easily, because they are not really Joule parts. The immersion element screws into a standard boss and comes from any merchant. The expansion vessel is a commodity potable water vessel. The inlet control components are the same families of valves used across the industry. An engineer with a reasonably stocked van fixes most Joule faults on the first visit without phoning anybody.

Where Joule is a step behind the biggest names here is in the distribution depth for anything model specific. Telford and Gledhill have been sold through British merchants for so long that even a small branch will have something on a shelf, and a Megaflo part is available almost anywhere. Joule comes into the UK through a narrower distribution network, so if a job genuinely needs a model specific component or a documented service kit rather than a generic valve, the wait can be longer than it would be for the domestic incumbents. That is a supply chain observation, not a criticism of the engineering, and it affects a minority of jobs. But it is worth knowing before you choose.

Keep the paperwork and photograph the data plate. Model and serial turn a parts conversation from guesswork into a five minute phone call, and on a cylinder with a manufacturer warranty the service history is normally a condition of that warranty rather than an optional extra. That is a condition between you and the manufacturer, and it is one to honour rather than work around.

How does London water treat a Joule cylinder

Hard, the same way it treats every cylinder here. Almost the whole London supply area is hard water, and the consequences for a stored hot water system are the same regardless of badge: scale on the element, scale on the coil, slower recovery over time and shorter element life than in a soft water region.

Two specification decisions genuinely help. The first is the element. Fit an Incoloy or titanium sheathed immersion rather than the cheapest copper one, because a better sheath tolerates the scale and the heat far longer. The second is store temperature. Running a cylinder hotter than the household needs, permanently, accelerates scale on every heated surface. Get the sizing right and you do not need the extra temperature in the first place.

The third thing is access, and it is nothing to do with the manufacturer. An immersion element needs real physical room to withdraw, and its length can be considerable. In Victorian conversions with the cylinder in a first floor cupboard, we regularly find a cylinder set so close to a wall, a boxed in stack or a cupboard frame that the element cannot come out without dismantling joinery. That turns a straightforward element change into a much bigger job. Before anybody installs a cylinder, ask them to show you where the element will withdraw to.

Is a Joule cylinder worth fitting

On the criteria that matter for the next twenty years, a Joule Cyclone is a sound choice. Stainless vessel, external expansion vessel, standard immersion boss, discrete inlet controls: that is the specification we ask for regardless of brand, and Joule delivers it. For heat pump work specifically it is one of the first ranges we would look at.

The honest caveat is the one above about distribution depth for model specific items in the UK. If you want the cylinder with the absolute deepest parts presence on a British merchant shelf, Telford and Gledhill have a longer head start here. If you want a well engineered stainless cylinder that services easily on commodity parts, and particularly if there is a heat pump in the picture, Joule earns its place on the shortlist.

What we would not do is choose any cylinder on litres alone, on a warranty headline with no annual service behind it, or on whichever brand a quote happens to name. The decisions that actually change your experience are the coil specification against your real heat source, the measured incoming mains flow and pressure at the property, and whether the installation leaves room to service the thing.

Gas system boiler, one bathroom conversion flat: standard indirect, coil matched to boiler output.
Two bathrooms and simultaneous showers: more volume and a higher output coil, not just more litres.
Heat pump now or within the cylinder life: the heat pump rated model, specified at purchase.
Awkward London cupboard or loft: slimline or horizontal, but check element withdrawal first.

What replacing a Joule cylinder involves

Diagnosis first. The most expensive mistake in this whole subject is replacing a repairable cylinder, and lukewarm water, slow recovery, a dripping tundish or noise during reheat are ordinary faults on ordinary parts far more often than they are a dead vessel. Get somebody qualified for unvented work to establish which part has actually failed before you accept a replacement quote from anybody.

Where the steel has genuinely gone, the replacement job is a day of work in most London properties: isolate and drain, disconnect the coil, the cold feed, the immersion and the discharge pipework, get the old cylinder out of the cupboard, set the new one, remake all connections, fit and set the inlet control group, run the discharge properly, refill, vent, commission and test. What drives the cost is the cylinder specification you choose, how much pipework has to be remade because the new connection positions differ from the old ones, whether the discharge arrangement needs redoing, and how bad the access is. Our rate is on our pricing page, not buried in an article.

Access is the variable that surprises people most. A cylinder in a ground floor utility is a very different day from one in a first floor cupboard at the top of a Victorian staircase with a turn in it, and a loft is different again. Our twenty engineers see all three across London every week, which is why the first question on a replacement is normally where the cylinder sits rather than which brand is going in.

Questions we get asked

Who makes Joule cylinders: Joule is a hot water cylinder manufacturer based in Ireland, selling across the UK through merchants and distributors. The domestic range most people have is called Cyclone and covers unvented cylinders for boilers, cylinders designed for heat pumps, slimline and horizontal versions, and twin coil models for solar. We work on them as an independent firm and have no formal relationship with the manufacturer.
Are Joule cylinders stainless steel: Yes. The Cyclone range uses a welded stainless steel vessel rather than enamel lined steel, so there is no sacrificial anode that somebody has to inspect and replace on schedule. Stainless is the right material for a sealed hot water store and it is what nearly every serious manufacturer now uses. The trade off is that a stainless vessel cannot be patched, so if the steel itself weeps the cylinder is finished.
How long does a Joule cylinder last: A stainless cylinder that is serviced annually, fed by a correctly set inlet control group and not run hotter than the household needs should give many years of service, and the vessel normally outlives several sets of valves and immersion elements. What shortens that life in London is hard water scaling the coil and the element, an unnecessarily high store temperature, and years of skipped services letting a worn relief valve pass unnoticed.
What usually fails on a Joule cylinder: The fittings rather than the steel. In rough order: the immersion element scales up and burns out, the expansion vessel diaphragm perishes and the tundish drips, the pressure reducing valve drifts or sticks, a relief valve starts to pass, the motorised valve on the heating side sticks, and the control thermostat fails. Every one of those is a replaceable part on a cylinder with years left in it.
Are Joule cylinder parts easy to get: The wearing parts are, because they are standard industry components rather than proprietary ones. The immersion screws into a normal boss, the expansion vessel is a commodity item and the inlet valves are the same families used across the trade. Anything genuinely model specific can take longer than for the longest established British brands, because Joule reaches the UK through a narrower distribution network.
Is a Joule cylinder good for a heat pump: The heat pump models are among the better options, because the coil surface area is properly sized for a low flow temperature heat source rather than carried over from a boiler product. That is the specification that decides whether a heat pump heats your water in a sensible time. A standard indirect cylinder connected to a heat pump reheats slowly, and no amount of controls adjustment will fix an undersized coil.
Can a Joule cylinder be repaired or does it need replacing: It depends which part failed. A dead immersion, a perished expansion vessel, a drifted pressure reducing valve, a passing relief valve or a stuck motorised valve are all ordinary repairs. The exception is the stainless vessel itself, which cannot be patched, so a weeping vessel is a genuine replacement. Always get a diagnosis before accepting a replacement quote, because replacing a repairable cylinder is the costliest mistake here.
Why does my Joule cylinder take so long to heat up: Slow recovery is usually one of three things. Scale on the coil after years in hard London water reduces heat transfer. The coil may be undersized for the heat source, which is common where a boiler specification cylinder has been paired with a heat pump. Or the heating side is not delivering, typically a motorised valve or control fault. An engineer can separate those on one visit.
Which immersion heater should I fit in a Joule cylinder in London: Specify an Incoloy or titanium sheathed element rather than the cheapest copper one. London water is hard across almost the whole supply area, scale builds on the element surface, the scale insulates it, the element then runs hotter than intended and eventually burns out. A better sheath tolerates that for much longer. Also check the element has physical room to withdraw before anybody sets the cylinder.
How do I find out which Joule model I have: Look for the data plate, normally on the front of the insulation near the top of the cylinder. It gives the model name, capacity in litres, the coil arrangement and a serial number. Photograph it before you ring anybody, because the difference between a standard indirect and a heat pump rated model changes the whole performance conversation and makes any parts enquiry far quicker.