Heater and Thermostat Positioning

Heater Positioning in Water Heaters

Heat rises and this forms a convection loop in the vessel.  The water within the convection loop will heat relatively quickly.  The water underneath the immersion heater can only heat by conductions and this takes a while.  Unless there is agitation or pumped circulation, the general rule is that an immersion heater will only heat the water above it.

If you want to heat the full vessel, position the immersion heater as far down the vessel as possible.  If you only want to heat the top of the vessel, for example providing a top up or boost, position the immersion heater around a third of the way down the vessel from the top.

Thermostat Positioning in Water Heaters

All vessels should have a means of controlling the temperature.  This is normally by thermostat but temperature sensors can also be used.  Where thermostats are used, there should be a control thermostat and  an over-temperature, safety cut-out thermostat in the vessel.  The role of the over-temperature safety cut-out thermostat is to switch the immersion heater Off in the event that the control thermostat fails in the On position and the vessel starts to over-heat.  The over-temperature safety cut-0ut thermostat should switch the immersion heater Off permanently until the fault is investigated and corrected and only then should the over-temperature thermostat be reset.

Often heaters are fitted with thermostats, however, this is normally one of the worst positiosn to locate the thermostat as the thermostats will experience warm water coming from the heating elements.  This can cause cycling and can wear out your thermostat and any control equipment connected to it.  We do not recommend fitting the control thermostat to immersion  heaters over 3kW except for removable core heaters.  So why do we supply heaters with control thermostats fitted to the heater?  The answer is because customers ask for it.

So where should you position the control thermostat?  Referring to the discussion above on heater positioning, the control thermostat needs to be above the heater but not by much.  So it is not affected by the warm water coming from the immersion heater we recommend, the control thermostat is positioned approximately 45 degrees around the vessel from the immersion heater.

The position of the over-temperature safety cut-out thermostat should be above the hottest water in the vessel and this comes from the immersion heater.  We recommend the over-temperature safety cut-out thermostat is positioned vertically above the immersion heater.  It can even be in the immersion heater but it must be above the heating elements.  To avoid nuisance tripping we recommend that the over-temperature thermostat is set a minimum of 15 degrees centigrade above the setting of the control thermostat, but below 80 degrees centigrade.