Buying a water heater can seem very confusing! In truth, when you get down to the basics, water heaters are pretty simple things. There are a few different sorts of water heater available, but for any given application there is usually one best option and you really don't need to worry too much about the rest.

To make a start in the product selection process, we would suggest that you first ask yourself a few basic questions to clarify what you are trying to achieve. In particular, you should consider:

  • How much hot water do I need, where and how often ?
  • How much space is available for the heater and where do I want to locate it?
  • What heat sources are available? (do you just have electricity, or is gas available too?)

This guide then takes you through a range of typical situations from small scale to large, and proposes a sensible water heating solution for each case. All you then need to do is to identify which case most closely matches your situation.

The suggestions are not brand specific, instead they are designed to point you towards the right type of heater. Within each section we have created a hyperlink that will take you straight to some competitively priced, quality options of the suggested type. Simply click an underlined hyperlink to go straight to the relevant page on our website to view some suitable products.

Remember that if you find this guide hard to follow, or just want to just confirm your understanding, you can contact us at any time for further selection advice either by email at or telephone by on 01751 411965 (email is preferred as we do tend to get a lot of calls !) and we will do out best to answer your questions on a personal level.

1. Occasional handwashing at a single sink -- Aquahot prices from about £40 (ex VAT)

If you just need enough hot water for occasional handwashing at a single sink, an instant water heater is a good place to start.

These are small electrically powered units that heat water 'instantly' as it flows through the product. Instant water heaters have the advantages of being compact, relatively cheap to buy and easy to install above a sink but they can only serve one sink and they can normally only heat water relatively slowly.

With these products the temperature of the outlet water depends on the water flow rate; the faster the flow rate the less the temperature rise will be and vice versa. This means that you might have to accept quite a slow flow rate when the incoming water is very cold, for example in winter. However an instant water heater will never run out of hot water and will not use energy when it is not being used.

2. Dish washing at a single sink -- Aquahot prices from about £75

If your typical application is dish washing at a single sink, you will need larger quantities of hot water at a time than an instant water heater can easily supply. In this case a storage water heater is likely to be best. These pre-heat a quantity of water in a tank for later draw-off. Over-sink storage water heaters are the most economical and easiest to install. These are mounted over a sink, and are supplied complete with a valve and a spout. However you are usually limited to locating them above a sink and you can also only serve one sink at a time.

3. Dish washing at several sinks -- Aquahot prices from about £85

If you want to serve multiple sinks or wish to locate the heater out-of-sight, e.g. beneath a sink, you will need a mains pressure storage water heater (sometimes called an unvented water heater). These heaters let you install the heater more or less anywhere but because the tank pressurised at the incoming cold mains pressure you will need to fit a safety valve and possibly other accessories too if there is a cold water drinking supply nearby or if the incoming water pressure is unusually high. This makes installation a little more complex and the heaters are more expensive but overall we believe these heaters offer a much better solution than either instant or oversink water heaters and they are our best selling water heating products.

4. Hot water for 4 or more sinks or for a shower or a bath -- Aquahot prices from about £350

The best solution is a large mains pressure unit. These can be located flexibly and give high pressure for showers without needing a pump or a header tank in the loft. A small flat would require a unit of a least 90 Litres capacity and a 3-4 bedroom house at least 150L.

Capacities of approximately 100L or more can usually be heated by a central heating boiler as well as electrically. Use of a central heating boiler is termed 'indirect' in contrast to electric heating which is termed 'direct'. Indirect heating is generally recommended in the long term as running costs will be lower, although the initiual capital cost would be a higher.

Mains pressure units do need various safety valves to be fitted. Everything required is included with the heaters, but plumbing experience is very valuable for installation.

You should note that traditional low pressure copper cylinders are an option too. We do not sell these as they do not offer good water pressure, but they are relatively cheap and simple to fit.

5. Hot water for 4 or more sinks, or for a shower or both and a low pressure heater is required - Aquahot prices from about £350

If for any reason mains pressure heaters are not suitable combination units are an option. These have an integral cold header tank and do not need any special valves. All that is required is to connect to the water supply and to power. They are thus ideal when speed and simplicity of installation is the key but they provide poor pressure for showers and have to be wall mounted as high as possible.

6. Boiling water for hot drinks -- Aquahot prices from around £55

Water for drinks needs to be heated to nearly 100oC and special products are designed for this purpose. Your choice comes down to traditional manual catering urns and automatic fill wall mounted or counter-top units.

Traditional manual units are relatively inexpensive but have to be re-filled by hand and boiling water is unavailable as they re-heat. Gas heated versions are also available.

Automatic units are connected directly to the water mains and provide near boiling water for hot drinks on demand without requiring any intervention but are significantly more expensive.

Please note that the information in this guide is intended to provide high level information of a generic nature and should not be taken to imply the suitability of a specific product in a specific situation. Please also note that all products must be installed by a competent person in accordance with the manufacturers' instructions and appropriate regulations.


If you need further information please do not hesitate to contact us. We will do our very best to help you in your exact situation. We have many years of experience with water heating products and have access to a wider range of products than we can show on our website so if you have an unusual requirement please contact us with the details and we will to our best to help.

© Aquahot 2005-2015

Further Information

Water Usage

The following data give an indication of water consumption for a range of situations and activities. Actual requirements may deviate from these typical figures and these data should therefore be taken as being indicative only. It should also be borne in mind that the water heating requirements are frequently determined by the peak rather than average demand, and that a water heater may need to be specified accordingly.

Hot Water Cylinder Selection

1 Bed 1 Bath & Shower     125L 
2 Bed 1 Bath & Shower     125L
3 Bed 1 Bath & Shower     145L
4/5 Bed 2 Bath & Shower  170L
4/5 Bed 3 Bath & Shower  250L
Light Commercial Usage   300L
Other Comercial Usage     300L

Please note that correct hot water cylinder selection is often determined by the requirement to fill more than one bath within a short period of time. If the capacity of the cylinder is exhausted by the initial bath, then second and subsequent baths may run cold unless sufficient time has been allowed for water in the cylinder to re-heat. Cylinder re-heat times depend on the available heating power and cylinder size. As general rule re-heat times are longer for the 'direct' type of hot water cylinder (where only a 3kW electric immersion is available) than for indirect cylinders that may be heated by a much more powerful external boiler. Selection of a suitably sized hot water storage cylinder that allows a large volume of hot water to be drawn off in a short period of time avoids the requirement to wait for a cylinder to re-heat.

Hot Water Consumption

 Hand Washing              1 to 2 Litres at 35 to 40oC  
 Kitchen Sink                 2 to 7 Litres per meal at 60o
 Cleaning (General)        2 to 7 Litres at 60oC
 Bath                            60 Litre per bath at 60oC (Plus mixing cold water)
 Hair Dressing                10 Litres per shampoo at 35 to 40o
 Dishwasher                  2 Litres per wash at 60o
 Washing Machine         20 Litres per wash at 60oC
 Shower                      15 Litres per shower at 40oC