Introduction to Hot Water Cylinders

There are two types of hot water cylinders found in homes today. The newer pressurised unvented hot water tanks and the older style vented hot water tanks.

Unvented hot water cylinders

Unvented hot water cylinders were only made legal in the UK in 1986, but have since grown rapidly in popularity. In an unvented system there is no cold water tank – instead, the sealed hot water cylinder is fed directly by the cold water mains. Since they are operating at mains pressure, they offer much better flow rates, meaning your shower and bath performance should be higher.

The other major benefit is that you don’t need to maintain a cold water tank in the loft (which vented systems require). This is good news since not only does it free up space, it also removes the potential freezing issue during our long cold winter periods.

In addition, since you aren’t relying on gravity to move the hot water around the home, the unvented cylinder can be located pretty much anywhere in your property.

Other advantages of installing an unvented system include reduced noise in the system (since there is no cold water filling of the water storage cistern), and since there is no water storage cistern and the system is essentially sealed, the cold water is not at risk from contamination.

Unvented water cylinders and the water expansion issue

Since water increases in volume as it gets warm, unvented cylinders need to include a mechanism that allows the expansion to take place, thereby keeping the cylinders operating at a safe pressure.

There are two methods of allowing this expansion to take place safely. The first is the bubble top unit, which uses an internal air bubble that is produced and trapped at the top of the cylinder when it is installed. The other type is the external expansion unit that utilises an expansion vessel to contain the expanded hot water.

The major issue with unvented hot water cylinders is that since hot water flow depends on the cold water main pressure; if for any reason the mains water is turned off, your home will be without access to any hot water.

Since unvented hot water tanks operate at higher pressure than vented systems and have additional safety features installed, these cylinders need to be installed by boiler specialists who hold a qualification that complies with G3 of building regulations. This means they tend to be far more expensive to install than traditional vented hot water systems.

Vented hot water systems

Vented hot water tanks are still the most common type of hot water system found in the UK. Unlike newer unvented tanks, these copper tanks are fed by cold water from a header tank (normally located in the loft) and they use gravity to drive the hot water around the home. A vent pipe links the vented hot water cylinder and the cold water in the header tank.

As with the unvented system, expansion of warm water is still an issue, but in this case the expansion simply takes place via the vent pipe and in the header tank.

The hot water pressure tends to be governed by the height of the water tank above the tap or shower feed. This means that although on the ground floor of the home the pressure might be excellent, in rooms on upper floors the pressure will be lower. As a result, many showers in homes with vented hot water tanks use electric pumps to drive the hot water to the shower at increased pressure.

Vented hot water cylinders are far less complicated than the pressurised vented systems and for this reason they are much simpler to maintain and install. This makes them a far cheaper option when compared to the unvented system.

Indirect systems

Most hot water cylinders are heated via an external heat source such as a gas boiler or solar thermal. In this case the hot water is heated and then travels through a copper coil in the hot water tank. The heat is then transferred from the from the external heat source to the water inside the hot water tank.

Indirect cylinders tend to be fitted with a direct backup (such as a immersion heater). Even if the boiler is broken, you can still produce hot water as and when you need it.

You can get both vented and unvented indirect systems.

Direct systems

In a direct cylinder system, the hot water is heated directly by an internal element such as an immersion heater. The hot water tends to be more expensive to produce in direct systems. Some homes have no access to gas, for example a mid-level flat. In this case they are forced to go with a direct system for their hot water, so they may choose to take advantage of Economy 7, which will give them a cheaper electricity at night to heat the hot water with.

Normally this type of cylinder would be fitted with two different immersion heaters, one for the peak electricity and one for the off-peak electricity. If this is the case, you really need to make sure that the immersion heaters are set up on the timers correctly to ensure you are paying the least possible for the hot water. There is no point heating water via your peak immersion heater during the middle of the night.

You can get both vented and unvented direct systems.

Combi boilers

We have covered combi boilers in more detail here, however it is worth mentioning that they can produce hot water for your home without the need for a hot water cylinder. While you can use electric point of use hot water heaters, gas currently is about ¼ of the price of electricity. Therefore it makes sense to use gas to heat the water as well as to carry out your heating.

The issue with combi boilers is that since they operate at mains water pressure, as soon as there is a pressure drop (i.e. more than one tap is opened in the home) then the hot water pressure is split between the two outlets. They are however ideal for smaller properties since there is no need to store hot water which is not very energy efficient.

Installing an unvented hot water cylinder

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