What is the difference between a direct and an indirect cylinder?

A direct cylinder is heated only by electric immersion elements inside it. An indirect cylinder is heated by a coil carrying hot water from a boiler or heat pump, and normally has an immersion as a backup. The water in the coil never mixes with the water you draw from the taps.

Is a direct or indirect cylinder cheaper to run?

An indirect cylinder heated by a gas boiler is usually cheaper, because gas costs much less per kWh than electricity. A direct cylinder heated overnight on an off peak tariff narrows the gap and is the right choice where there is no gas. Running a direct cylinder at the full day rate is the expensive option.

How do I know if my cylinder is direct or indirect?

Count the pipes. An indirect cylinder has two extra connections on the side for the boiler flow and return, usually with a cylinder thermostat strapped to the tank and a motorised valve nearby. A direct cylinder has only a cold feed, a hot draw off and one or two immersions. The data plate on the casing will also say which it is.

Does an indirect cylinder have an immersion heater?

Almost always, yes. The immersion is there as a backup so you still have hot water if the boiler breaks down. On a normal day it should stay off, because heating the tank through the boiler is cheaper.

Can I change my direct cylinder to an indirect one?

Only by replacing the cylinder. A coil cannot be added to an existing tank. You would also need a boiler or heat pump to heat it, flow and return pipework, a motorised valve, a cylinder thermostat and wiring, and in a flat without gas, a gas supply and flue the building may not allow.

Which type is better for a heat pump?

Neither a standard direct nor a standard indirect cylinder. A heat pump needs a cylinder with a much larger coil, because it heats at a lower temperature than a boiler. Fitting a standard indirect cylinder to a heat pump gives slow reheats and lukewarm water.

Why does my direct cylinder run out of hot water so quickly?

Usually because only the top element is working. The top element heats roughly the upper third of the tank, which is about one shower. If the bottom element or its thermostat has failed, the overnight charge does nothing and you live on the boost. A timer that has drifted off the off peak hours produces a similar complaint.

Direct vs Indirect Hot Water Cylinder: Which Do You Need?

A direct cylinder heats its water with electric immersion elements and nothing else. An indirect cylinder has a coil inside it that carries hot water from a boiler or heat pump, with an immersion fitted only as a backup. If your home has a working gas boiler, indirect is the right answer almost every time, because it heats faster and costs far less to run. Direct cylinders make sense where there is no gas and no boiler, typically flats in purpose built blocks, and they work best on a tariff with cheap overnight electricity. Both types come in vented and unvented versions, and neither can be converted into the other: changing type means a new cylinder.

The difference in one sentence each

Direct: the cylinder is a tank with one or two electric immersion elements in it, and those elements are the only thing that ever heats the water. Indirect: the cylinder has a coil of pipe running through it, the boiler pumps hot water round that coil, and the heat passes through the pipe wall into the stored water. The water in the coil never mixes with the water you use.

Everything else follows from that. An indirect cylinder is part of the heating system, with a cylinder thermostat, a motorised valve and a programmer deciding when the boiler heats it. A direct cylinder is an electrical appliance, controlled by the thermostats on its elements and usually a timer that matches the cheap hours of your tariff.

There is a third arrangement worth knowing about. A twin coil cylinder is an indirect cylinder with a second coil, so a solar thermal panel or a heat pump can feed the same tank as the boiler. And a thermal store, such as a Gledhill Pulsacoil, is different again: the stored water heats the mains as it passes through a heat exchanger, rather than being the water you use. Our explainer on how a Pulsacoil works covers that arrangement.

How to tell which one you have

Look at the pipework on the cylinder. Every cylinder has a cold feed going in low down and a hot draw off coming out at the top. An indirect cylinder has two more pipe connections, usually on the side, lower half: the flow and return from the boiler to the coil. You will normally also find a cylinder thermostat strapped to the tank or pushed into a pocket, and a motorised valve with a small grey or white actuator head on one of the nearby pipes.

A direct cylinder has only the cold in and the hot out, plus the electric immersions. Many have two: one entering near the top and one lower down, or a single long element from the top with a dual switch on the wall marked Bath and Sink, or Day and Night. If there is no boiler anywhere in the property, the cylinder is direct.

The data plate settles it. It is usually on the front of the casing or near an immersion boss, and it will say direct or indirect along with the capacity and, on an unvented unit, the pressure settings of the safety valves.

Four pipes plus an immersion, cylinder thermostat on the side: indirect
Two pipes plus one or two immersions, no boiler in the property: direct
Six pipes, two coils: twin coil, usually for solar or a heat pump
Mains pipe passing through a heat exchanger on a store of hot water: a thermal store, not a cylinder

Running costs: why indirect usually wins

An indirect cylinder is heated by gas through the boiler, and gas costs much less per kWh than electricity. Heating the same tank electrically in a direct cylinder costs several times more at a standard day rate. That gap narrows a great deal if the direct cylinder is heated overnight on an off peak tariff, which is exactly how direct cylinders are designed to be used.

So the fair comparison is not direct against indirect in the abstract, but a direct cylinder heated on a cheap night rate against an indirect cylinder heated by your boiler. In a flat with no gas supply, the direct cylinder on a night rate is the sensible, economical choice. In a house with a gas boiler, fitting a direct cylinder would mean paying for electricity to do a job the boiler does more cheaply. The detail of what an element costs per hour and per tank is in our guide to immersion heater running cost.

Speed: how quickly each one recovers

Recovery is where you feel the difference day to day. A 3kW immersion takes around two and a half to three hours to heat a 150 litre tank from cold. A boiler coil transfers heat several times faster: an older copper cylinder with a modest coil comes up in 45 to 90 minutes, and modern unvented indirect cylinders with large coils are built to reheat in well under an hour.

That is why direct cylinders are sized larger than the indirect equivalent for the same household. If you cannot reheat quickly, you need to store more. A couple in a one bedroom flat can live comfortably with a direct cylinder charged overnight. A family with two bathrooms drawing hot water through the evening will run a direct cylinder dry and then wait hours for it to recover. Our guide to what size hot water cylinder do i need works through the numbers.

Which suits which home

Choose indirect if there is a gas boiler, or will be, and the cylinder can sit within reach of the heating pipework. This covers most London houses and conversions with a system or regular boiler.

Choose direct if there is no gas, if the building does not allow a boiler or a flue, or if you are on an electric heating system with an off peak tariff. Many purpose built blocks in London were built this way, often with an unvented direct cylinder in a hall cupboard.

Choose twin coil, or a cylinder specifically designed for a heat pump, if a heat pump or solar thermal is a real prospect during the life of the cylinder. A heat pump delivers heat at a lower temperature than a boiler and needs a much larger coil surface to heat a tank in a reasonable time. Adding a coil later is not an option.

Gas boiler in the property: indirect, with an immersion as backup
No gas, electric heating, night rate tariff: direct, sized generously
Heat pump planned or fitted: heat pump cylinder or twin coil
Solar thermal panels: twin coil

Can a direct cylinder be converted to indirect?

No. The coil is built into the vessel at the factory, and there is no way to add one to an existing cylinder. Going from direct to indirect means a new cylinder, a boiler to heat it, flow and return pipework between them, a motorised valve, a cylinder thermostat and wiring into the heating controls. In a flat that has never had gas, it also means a gas supply and a flue, which the building may not permit.

Going the other way is simpler but rarely sensible. An indirect cylinder already has an immersion, so if the boiler is removed it will still heat, just slowly and at electric prices. It is a stopgap, not a plan.

If you are replacing a cylinder anyway, this is the one moment you can change type. A hot water cylinder replacement is the time to ask whether the household has changed, whether gas is now available, and whether a heat pump is likely in the next ten years.

What goes wrong with each type

Direct cylinders fail through their elements and element thermostats, because those are working every day. In London hard water the elements scale up, run longer and eventually burn out. The classic sign is hot water that runs out after one shower because the bottom element has died and only the top third of the tank is being heated. A timer that has drifted off the cheap hours is the other common complaint, and it shows up on the bill rather than at the tap.

Indirect cylinders fail in the control chain more often than in the cylinder itself. A motorised valve that no longer opens, a cylinder thermostat that has stuck, or a programmer fault leaves the radiators working and the cylinder cold. That is the pattern behind no hot water but heating works, and it is usually a part and an hour rather than a new cylinder. Over many years the coil furs with scale and reheat times stretch, which is not repairable in place.

Unvented direct and indirect cylinders

Direct and indirect describe how the water is heated. Vented and unvented describe how it is fed and pressurised. The two choices are independent, so you can have a vented direct cylinder fed from a loft tank, or an unvented direct cylinder running at mains pressure in a flat. If you are unsure of that second distinction, see unvented vs vented cylinder.

Any unvented cylinder, direct or indirect, must be installed and worked on by an engineer holding the unvented hot water qualification, and it should be serviced every year. The safety components are the same on both types: a pressure reducing valve and expansion relief on the cold side, an expansion vessel, a temperature and pressure relief valve, and a discharge route through a tundish. On a direct unit the thermostats and cut-outs sit on the elements; on an indirect unit there is also a high limit cut-out wired to the motorised valve, so the boiler cannot keep heating a tank that is already too hot.

Getting the right one fitted

If you are choosing a new cylinder, start with what heats it and then size it, rather than the other way round. Our guide on how to choose a hot water cylinder covers the incoming main, capacity and fit in the cupboard. For an unvented cylinder installation or replacement in London, call 020 4652 5111. We will ask what heats the water now, what the property has for gas, and how many people and bathrooms the cylinder has to serve. Our rates are on the pricing page.

Questions we get asked

What is the difference between a direct and an indirect cylinder?: A direct cylinder is heated only by electric immersion elements inside it. An indirect cylinder is heated by a coil carrying hot water from a boiler or heat pump, and normally has an immersion as a backup. The water in the coil never mixes with the water you draw from the taps.
Is a direct or indirect cylinder cheaper to run?: An indirect cylinder heated by a gas boiler is usually cheaper, because gas costs much less per kWh than electricity. A direct cylinder heated overnight on an off peak tariff narrows the gap and is the right choice where there is no gas. Running a direct cylinder at the full day rate is the expensive option.
How do I know if my cylinder is direct or indirect?: Count the pipes. An indirect cylinder has two extra connections on the side for the boiler flow and return, usually with a cylinder thermostat strapped to the tank and a motorised valve nearby. A direct cylinder has only a cold feed, a hot draw off and one or two immersions. The data plate on the casing will also say which it is.
Does an indirect cylinder have an immersion heater?: Almost always, yes. The immersion is there as a backup so you still have hot water if the boiler breaks down. On a normal day it should stay off, because heating the tank through the boiler is cheaper.
Can I change my direct cylinder to an indirect one?: Only by replacing the cylinder. A coil cannot be added to an existing tank. You would also need a boiler or heat pump to heat it, flow and return pipework, a motorised valve, a cylinder thermostat and wiring, and in a flat without gas, a gas supply and flue the building may not allow.
Which type is better for a heat pump?: Neither a standard direct nor a standard indirect cylinder. A heat pump needs a cylinder with a much larger coil, because it heats at a lower temperature than a boiler. Fitting a standard indirect cylinder to a heat pump gives slow reheats and lukewarm water.
Why does my direct cylinder run out of hot water so quickly?: Usually because only the top element is working. The top element heats roughly the upper third of the tank, which is about one shower. If the bottom element or its thermostat has failed, the overnight charge does nothing and you live on the boost. A timer that has drifted off the off peak hours produces a similar complaint.