7.11.3 - Instantaneous water heaters

Water heaters of this type are in general use to provide hot water for showers, making drinks, and so on. They transfer heat from the element directly to the water flowing over it, and therefore will be arranged to switch an only when water is flowing. In most cases, the element of the heater has a fixed rating, and so transfers energy in the form of heat to the water at a constant rate. The temperature rise of the water passing over the element therefore depends on the inlet water temperature and the rate of water flow. If the discharge rate is high, the energy provided by the element may be insufficient to raise the water temperature to the desired level.

The slower the rate of flow, the hotter will become the water at the outlet. This is the reason for the common complaint of being scalded whilst under the shower if someone turns on a tap elsewhere, reducing the water pressure and the rate of flow over the element. Some heaters are provided with an automatic cut-out to switch off the element if a preset outlet temperature is exceeded. {Figure 7.17} shows, in graphical form, the expected outlet water temperature for various water flow rates from 3 kW, 6 kW and 8 kW heaters assuming an inlet water temperature of 10°C.

Some heaters are provided with thyristors or triacs to continuously vary the heater rating to maintain a desired water outlet temperature so long as the variation in the rate of flow is not too great. These devices adjust the effective current flow by delaying the instant in each half-cycle of the supply at which current begins to flow.

Fig 7.17 - Outlet temperatures from instantaneous water heater

The Regulations point out that a heater with an uninsulated element is unsuitable where a water softener of the salt regenerative type is used because the increased conductivity of the water is likely to lead to excessive earth leakage currents from the element. The agreement of the Water Supply Authority is usually needed before installation.

It is essential that all parts of this type of heater are solidly connected to the metal water supply pipe, which in turn is solidly earthed independently of the circuit protective conductor. The heater must be controlled by a double pole linked switch. In the case of a shower heater, if this switch is not built into the heater itself, a separate pull switch must be provided adjacent to the shower, with the switch itself being out of reach of a person using the shower. The arrangement of an instantaneous heater is shown in {Fig 7.18}. If the neutral supply to a heater

Fig 7.18 - Instantaneous water heater

with an uninsulated element is lost, current from the phase will return via the water and the earthed metal. Therefore, a careful check is necessary to ensure that there is no fuse, circuit breaker or non-linked switch in the neutral conductor. A problem frequently arises due to the increasing use of 3 kW instantaneous water heaters in caravans because supply systems have not usually been designed to allow for such heavy loading.