7: Water heating – part 1 of 2

Hot water storage and supply

There are two ways to supply hot water to the point of need…

  1. Heating in bulk and intermediate storage
  2. On demand instant heating

With instant heating hot water the delivered water is heated only when the tap is opened. This heat will normally come from an electric element, such as is used in an electrically powered shower, or by using a combi boiler or separate gas powered geyser.

The key advantages are that no hot water storage is needed so saving space for the hot water tank and eliminating heat loss from the water stored in the tank. The disadvantage is that the rate of heating of the water is limited by the heating element capacity (or boiler capacity) which normally limits the supply to one consumption point at a time. So it is inadvisable to draw off kitchen hot water whilst filling the bath.

When hot water is stored it will lose heat constantly. Most modern hot water tanks come provided with foam spray insulation and a minimum of 50mm should be sought. If you currently have such a tank with less than this level of insulation then the single most cost effective energy improvement you can make is to boost this insulation.

Looking at the overall picture for hot water supply there are several facets that need to be considered…

  • Temperature of supply at point of use
  • Temperature of storage
  • Pipe lengths
  • Method of heating & Storage
  • Pressure of supply

Let’s look at each in turn.

Temperature of supply at point of use

This is the temperature at the actual tap outlet. The minimum temperature should be 40oC (130oF) or it will not be hot enough to wash greasy dishes. On the other hand the temperature should not exceed 60oC (180oF) or there is a serious danger of scalding.

Where the hot water is stored this can be achieved either by storing at the appropriate temperature or by the use of a thermostatic mixer valve on the outlet from the hot water tank if the storage temperature is higher.

Temperature of storage

Should the domestic hot water be stored at the hottest temperature or the cooler one?

This is not a simple question. Storing at the cooler temperature means no complications (thermostatic valves etc.) in delivery and an inherently safer delivery. But the overall tank’s heat capacity is smaller so that that an unusually heavy drawdown will empty its contents followed by a wait for more supply.

Storing at the cooler temperature also carries a risk of legionella infection as the water should be at 60oC or more to kill of such bacteria.

Storage at the higher temperature will avoid both of these problems but will incur higher heat loss (unless insulation is improved). Also, using solar water heating alone makes it difficult to get to such high temperatures.

We will return to this later.

Pipe lengths

If at all possible the dwelling should be laid out so that hot water use is in rooms that are close to the source of the hot water supply. In larger dwellings this means that multiple supply points will be needed.

The acknowledged target is to have the maximum hot water pipe run contain no more than 0.5 litres (a bit under a pint) of water. That’s a bit over 2m (7ft) of pipe run. This is to minimise delay in supplying the hot water and also to minimise the heat loss from those pipes. Where a pipe feeds a little used point, such as a vanity unit in a guest bedroom, then the runs may be longer as the losses will still be small.

It is also important to ensure that all hot supply pipes are well insulated, especially any frequently used ones such as the kitchen area, to minimise losses between uses. Wherever possible they should be run within the heated building envelop so as to minimise the temperature differential.

Link to Part 2 (from 12 September 2010)

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