The choice between using an instantaneous water heater or some form of tank to provide hot water for your 1 (or 2) bedroomed apartment or flat is something we get asked at least once a day by our customers.

There are advantages and disadvantages of both types of heater, but there are also some straightforward questions which will easily make the decision for you. So to start with, work through these simple questions. This explains the advantages of each, and hopefully makes your decision straightforward.

Firstly, an instantaneous heater is what most people have in mind as the ideal solution, because it offers a number of advantages:

  • You only heat the water you use, so it is as efficient to run as it possibly can be.
  • No need for expansion vessels or pressure relief valves
  • Small, easily to position units – perfect where space is limited


However, there are certain situations where an instantaneous water heater will not work in practice:

  • If you have a bath in your bathroom rather than just a shower, even the most powerful instantaneous water heater will NOT be powerful enough. The Zip DEX 12 will not produce more than 5 litres/ minute at bath temperature, and you will need getting on for 100 litres for a bath. Zip themselves do claim this model will cope with a bath, but we do not recommend it.
  • An instantaneous heater for anything more than light hand-washing, will require a thicker cable (for example 10mm) from your fusebox. Sometimes installing this is impractical.
  • Each outlet you are hoping to serve must be no more than 2 metres away from the heater, otherwise it will lose too much heat in the connecting pipework.
  • Using a hot tap when the shower is in use will reduce the flow, so in practice, although an instantaneous water heater can be plumbed to multiple outlets, they cannot be used at the same time.


If none of these are a problem for you, then an instantaneous water heater is ideal for you, and although it is not the cheapest model, we recommend the Zip DEX12 – we sell a high number of these and they have an extremely high level of reliability.

If any of the above issues apply to you, however, then you need to consider a stored water heater. Apart from not having the issues listed above, this type of heater does have some significant advantages which may apply to your situation:

They typically use a maximum of 3kW and so can be wired into your existing ring main.

The pressure you get from your hot taps will be very close to what you currently get from your cold tap, which usually means you will have a good, powerful shower.

They are not as ‘ugly’ as a traditional hot water cylinder, and so do not need to be hidden away in a cupboard, and they can be wall hung (as long as you have a sound and sturdy wall!)

A 90 or 100 litre model will be adequate for a good bath or 3 showers in a row before it needs to reheat.

Reheat time is approximately 45 minutes.

For many people, these advantages are significant, and they end up choosing something like the Hyco Powerflow 90L, which can be wall hung and is just 930mm high. That model, again, extremely popular and good quality, has a ‘Standing Heat Loss’ of just 1.25kWh per day, in other words the running cost of keeping it hot all day (if you did not use it) is approximately 15p.

If space is a real problem, then you should also consider that you will need to install a small expansion vessel (supplied) with units such as this.

If you have any questions about your specific requirements or need any more information please do not hesitate to contact us via the online chat on our site or call one of our experts on 01934 253600.