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Electric Water Heater Selection By Installation Type:
Single Basin Electric Water Heaters
It's worth noting that electric water heater suppliers differentiate between basins (only big enough to wash your hands in) and sinks (big enough to wash up plates and crockery).
If you fit a water heater designed for a basin to a kitchen sink, they may try to get out of the warranty if you have a problem. It makes sense up to a certain point, as a heater used for occasional hand-washing is going to work less than half as hard over 3 years as one that's feeding a kitchen sink where someone washes up 2-3 times a day or more.
Hand wash unit
These neat little boxes with spouts sit directly over your sink and instantaneously heat the water as it passes through the unit. They're usually 3kw and you can get auto-sensing ones for increased hygiene and economy. Great for hand-washing, not so for washing-up.
Over-sink vented water storage unit
Looks like a big brother to the hand wash, but works by storing the water in the tank (5, 7 or 10 litre versions are most common) and keeping it hot.
A thermostat on the front of the unit allows you to control the stored water temperature. Water is delivered to the basin via a spout similar to that of the hand wash unit.
Plenty of hot water to do a bowl of washing up, a bit of an over-kill if it only gets used for hand-washing.
Instant water heaters
These very compact units can be discreetly tucked away under sinks or inside cupboards. They work in a similar way to an instant electric shower, taking a cold water feed and heating the water as it passes through with a high power element (usually between 9.5 and 12kw single phase). The element only draws power when the internal flow switch senses water passing through, so they can be a surprisingly efficient on-demand hot water supply system.
Usually suitable for one or two outlets (one basin and one shower or two basins). Check with your electrician that you have a big enough supply cable and capacity back at your fuse-board when considering this option.
For commercial installations, there are larger 3 phase instant water heaters available.
Small unvented water storage heaters
Typically 10 or 15 litres of water held in a compact casing that can be fitted under the sink or cupboard.
The water is kept heated when switched on with (typically) a 3kw element. Suitable for up to 4 outlets, which can include larger sinks. Existing and standard taps/mixers can be used.
Requires a minimum amount of pipe length between the cold water inlet and the first draw-off point to allow for water expansion when the unit is in use. In rare cases where this pipe length isn't available, you can purchase an additional cold water expansion vessel to solve the issue.
A thermostat on the front of the unit allows you control the stored water temperature.
Large Capacity, Multiple Outlet Electric Water Heaters
Wall-mounted electric water heater options:
Flatback vented wall-mounted water heaters - For larger installations, such as bathrooms, commercial kitchens, cloakrooms, and canteens, a large rectangular heater can be wall-mounted to provide heated wall volumes between 25 and 100 litres. Handy for applications where larger amounts of water can be required but there isn't room for a floor-standing cylinder in the building.
Medium unvented wall-mounted water heaters - These storage heaters are typically available in sizes between 30 and 75 litres. They are best suited for medium-sized applications such as commercial sinks
Whole-house electric water heater options:
Unvented cylinder water heaters
If you have the room, these cylinder heaters can be used for direct water heating applications up to 300 litres (enough capacity to supply all the hot water for an average 6 bedroom house).
These 'direct' models are used with either one or two immersion heaters and can be used on 'economy 7' peak/off-peak tariffs if required.
'In-direct' models are also available for use in conjunction with a gas-fired or electric boiler up to 200 litres.
Electric central heating boilers
Can replace a gas boiler to provide hot water for 'wet' central heating or underfloor systems.
For central heating and hot water supply, use in conjunction with an unvented electric cylinder water heater.
Suitable for any dwelling between small flat up to a 4 bedroom house.
Much lower maintenance costs than a gas combi-boiler. Some units have multiple output settings to suit a range of dwellings whereas others require you to purchase the correct size boiler to suit your application. Always get direct advice from the boiler manufacturer as to the suitability of the equipment for your particular installation.