A guide to selecting the right hot water cylinder for the job

Designed for professionally qualified and trained installers, this three-step guide walks you through how to select the right hot water cylinder for the job. It factors in all you need to consider to ensure your customers end up with a cylinder that’s right for their home and their needs.

Step 1: Does the system need an unvented or a vented hot water cylinder?

There are three common systems for storing hot water. It’s important to identify which one your customer needs.

Unvented hot water cylinder

Unvented hot water cylinders use the cold water supply from the mains to deliver high flow rates for filling the bath up quickly, and mains pressure hot water (regulated to 3 bar) to make for a powerful shower. Unlike vented cylinders, they don’t use a cold tank in the loft.

Unvented cylinders can support a home where there’s a need to support several hot water outlets at the same time. Examples include houses with more than one bathroom, and those with a utility room as well as a kitchen.

Unvented hot water cylinders are well insulated and have an integrated casing to protect them. This means that heat loss is minimal, and they have a smooth and slick appearance.

Vented hot water cylinder

Vented cylinders have their cold water delivered by gravity from a header tank, with the tank often situated in the loft. They were extremely common in Britain from the 1940s right through to the 1990s. Unlike unvented cylinders, they don’t use cold water directly from the mains.

Vented hot water cylinders deliver good water pressure and are ideal for use as simple replacements – where you need to replace a worn out vented cylinder with a new one. This straightforward switch will get the hot water back online as cheaply and simply as possible, with the minimum amount of disruption for your customer.

Thermal store

Thermal stores were developed before the days of unvented hot water cylinders. As their name implies, they store hot water at high temperatures. This hot stored water is then used to heat the mains pressure cold water by passing it through the cylinder’s in-built heat exchanger. The heat exchanger takes the heat out of the cylinder, meaning the thermal store works in reverse to a traditional vented or unvented hot water cylinder.

Thermal stores are ideal for use either when the heat source is unregulated, or where there are multiple heat sources. One such example is a solid fuel wood burner.

Step 2: Choosing the cylinder you need according to the source of heat

The heat source has implications for the type of cylinder required, as we detail here.

Oil or gas boiler

Choose an “indirect” hot water cylinder when a conventional boiler is the home’s heat source.

Renewable energy – heat pump or solar thermal

Select a “heat pump” or “solar thermal” hot water cylinder in situations where the property has rentable heat sources. Solar indirect hot water cylinders are all set-up for occasions when there’s not enough solar energy available – their two heat exchanger coils connect to the solar heat source and the conventional boiler respectively.

Electric immersion heater

You should select a “direct” hot water cylinder when the only option for your customers’ home is to heat the cylinder electrically via an immersion heater.

Aga, solid fuel or other unregulated heat source

Choose a “thermal store” as your preferred option if you’re working with either multiple heat sources or an unregulated heat source.

Step 3: Selecting the right sized cylinder

Once you’ve selected the cylinder type, the next thing to consider is its capacity and size. This will depend on your customer’s current and future hot water needs. For example, if you know of future likely changes in their home, such as building an extension or intend for extended family to live with them, these should be considered when planning the hot water capacity they’ll need.

The number of litres of hot water needed per person per day is less than many people initially think, as hot water is typically stored at around 60°C, but is then mixed with cold water to a safe and usable temperature of 40°C on delivery. This means that a shower uses only 11 litres of 60°C water to deliver a 40°C shower of 18 litres. Similarly, it takes only 60 litres of 60°C water to fill a 100 litre bath at 40°C.

Levels of low, average and mixed hot water consumption (40°C) per person, per day are as follows:

20 to 30 litres = low consumption

30 to 50 litres = medium or average consumption

50 to 70 litres = high consumption

This table of recommendations is based on old British standards, although it does not completely take into account how many people will be using using the hot water system:

Hot water demand

Bedrooms

Indirect Cylinders

Direct Cylinders

1 standard bath or shower

Bedsit/1 bed

120

150

2-3 bed

120

180

3-4 bed

150

210

1 standard bath

2-3 bed

120

180

3-4 bed

150

210

1 bath and en-suite

2-3 bed

150

210

3-4 bed

150

210

4-5 bed

180

250

2 standard baths

2-3 bed

180

210

3-4 bed

180

210

4-5 bed

210

250

3 bathrooms

3-4 bed

250

300

4-5 bed

250

300

5-6 bed

250

300