Difference Between A Direct And Indirect Hot Water Cylinder

Last updated: June 8, 2021

Many of us turn on our hot tap with little thought of what it takes to supply that hot water. The route that the water takes and the way in which it is heated are dependent on several factors but they are broadly categorised as direct and indirect systems. There are advantages and disadvantages to each. Both will supply hot water, but in different ways. You may also have heard the terms ‘vented and unvented systems which we shall look at as well.

What is a Direct Hot Water Cylinder?

A direct hot water cylinder is one in which cold water is fed in, then heated directly by an electric element (an immersion heater) . This hot water is then fed directly from the tank to the hot tap. The simplest of these systems is a vented system. As the water is heated, the water pressure and the pressure of the adjacent air will rise, and that pressure must be allowed an escape route for safety reasons. In a vented system, that escape rout is via a pipe which leads back up to the cold water tank in the loft.

In an unvented system, there is no open pipe to take the excess pressure. Instead, the system is kept constantly full with an expansion vessel dealing with the rise in pressure.

Direct Hot Water Cylinder Advantages And Disadvantages

The main advantage of a direct system is its relative simplicity, particularly if it is of the vented variety. It is also more straightforward to install than some other types of heater. If it is an immersion heater, then it can be a source of hot water even if the boiler is not working and can draw power from clean energy sources such as solar panels.

The main disadvantage is that the water tank may need to be heated from cold which can be time consuming and use quite a lot of electricity. In addition, water storage itself can give rise to bacterial growth if the thermostat is not set to a high enough temperature, which is obviously undesirable in water used for washing. It can also be a potential problem if the same water is used for washing and the domestic heating system, as radiators are prone corrosion, which will end up in the water tank.

What is an Indirect Hot Water Cylinder?

In an indirect water heater the water in the cylinder is heated by a coil which in turn is heated directly from the boiler. While these cylinders can also be fitted with electric elements as back up, the prime source of heat energy is the gas boiler. What this means is that the hot water which circulates in the central heating system never comes into direct contact with the water in the cylinder, hence the term ‘indirect’.

Indirect Hot Water Cylinder Advantages and Disadvantages

In the case of indirect water heaters, the hot water coil which heats the water in the cylinder , is fed with hot water from the boiler.Because this water is separate from the water in the hot water cylinder, it follows that any contaminants such as rust particles, which will inevitably be shed by the radiators, can never end up in water which is used for washing. The hygiene advantage is obvious. There is also the advantage that even if the electrical power fails, an indirect cylinder will continue to supply hot water, since the boiler is gas powered.

The bonus with indirect cylinders is that should the boiler need to be switched off for maintenance or servicing, the electric heating element can still be used to heat the water in the tank. In the case of both types of water heater, good insulation around the water tank and associated pipework will maximise the efficiency of the process.

Of course, there is never a perfect solution, and this is particularly true in the plumbing world! The main disadvantage of an indirect hot water cylinder is its relative complexity. This means that installation can be more costly than the direct type of heater due to the necessity of allowing for both electrical and gas fired heat sources. And it also follows that the more complex something is, the greater that the range of potential problems that will go with it is.

Which is Better?

As usual when this question is asked of two different types of heating equipment, the answer is it depends. Both will supply you with hot water, but how they do so is the real issue here.

With the direct system, the advantage of relative simplicity is certainly worth having. It does mean however, that there is only one method of heating the water in the tank. Should the power supply be interrupted, for whatever reason, you could be left with no means of supplying hot water until it has been restored.

It may be that the property has no domestic gas supply, in which case a direct hot water cylinder is going to be fitted anyway. This is particularly true of rural properties where it would be uneconomical to lay a gas main. Even if this is not the case, it is fair to say that the direct water heating system takes up relatively little space and is easier to install.

In the case of indirect hot water cylinders, the issue of its relative complexity is certainly there, but it is largely negated by the reliability of modern heating components. Regular boiler servicing will greatly reduce the chance of a problem further.

Where indirect hot water cylinders score points, is the fact that they rely on two separate sources of energy to heat the water, so if one is out of use, the other can do the job. While the boiler and pipework will take up more space, they provide the security of knowing that hot water will be there when it is required. In addition, with the hot water in the heat exchanger coil never coming into contact with the water in the tank, no cross contamination between heating water and washing water can take place.

Assuming access to a gas main, the indirect hot water cylinder is the more versatile choice.