Central Heating Repairs: System Controls


Gas and oil-fired ‘wet’ (ie. water-circulating) central heating systems rely on a series of electrical controls to turn the boiler on and off and monitor the output of heat or hot water. If one or more of these controls fails, or is incorrectly set, the system will run inefficiently — often reflected in higher fuel bills — and may break down altogether.

Not all systems are controlled in the same way, and some have more controls than others, so the first step towards tracking down faults is to examine your system and identify the key components.


Which system?

Some systems work on the gravity flow principle, in which the radiator part is pumped but the hot water part flows by convection (ie the water rises to the cylinder as it is heated and falls back to the boiler as it cools). In this case the controls are likely to be a simple combination of programmer/boiler thermostat/room thermostat. How they work is described below.

If the system is fully pumped, in which all the water flows under pump pressure, a cylinder thermostat and control valves will be added to the list. The most common control set-ups for fully, pumped systems are described below.

Both types of system may also be equipped with thermostatic radiator valves, but these are mechanical devices which operate independently and don’t affect the workings of the other controls.



When the programmer is switched to an ‘on’ cycle, current is passed to the boiler thermostat. This activates the boiler, which heats the system water.

controls in a simple gravity flow system

If ‘hot water only’ is selected, the current goes no further.

Water flows by convection around the hot water circuit, but is prevented from flowing to the radiators by a spring-loaded valve.

If ‘hot water on, heating on’ is selected, the programmer passes on current to the room thermostat. This in turn passes the current to the pump, which overcomes the pressure of the spring valve and pumps water around the radiator circuit.

When the room thermostat senses that the house is hot enough, it shuts off the electrical supply to the pump. Water then reverts to flowing around the hot water circuit only. Meanwhile, the boiler thermostat will shut off the current to the boiler (which stops firing) as soon as the system water starts to exceed its set temperature.

When the programmer is switched to an ‘off’ cycle, neither thermostat receives current and the system shuts down completely.



In all pumped systems, the path taken by the current after it leaves the programmer depends on which programme is selected.

controls in a pumped system with zone valves

When the programmer is set to ‘hot water on, heating on’, current passes to the cylinder thermostat and to the room thermostat. If either sense they are below their set temperatures, they ‘call’ for heat by passing on the current to one-way (‘two-port’) zone valves. The valves open, and switches inside them pass on the current again, this time to the boiler thermostat and pump. The system water is then heated and pumped around both circuits.

When one of the thermostats is ‘satisfied’ (ie. reaches its set temperature), it shuts off the current to its zone valve, which springs closed and shuts off the current to the boiler and pump. Water then ceases to flow around this part of the system. If both thermostats are satisfied, no current passes to the boiler and pump. The system then shuts down until a further call is made.

If ‘hot water only’ is selected, only the cylinder thermostat receives current; the room thermostat remains inactive and its zone valve stays closed.

Likewise, some programmers have the facility to select ‘heating only’, in which case the room thermostat receives current and the cylinder thermostat remains inactive during the programme.

At any of the above stages, the boiler thermostat will switch off the boiler if the system water starts to exceed its set temperature.

However, when the programmer switches to an ‘off’ cycle, none of the thermostats receive current and the system shuts down regardless of whether they are ‘calling’ or ‘satisfied’.



This arrangement works in the same way as the one above, except that the one-way zone valves controlling the hot water and radiator circuits are replaced by a single two-way (‘three-port’) diverter valve with outlets to both circuits.

pumped system with diverter valve

Both the cylinder and room thermostats are connected to the diverter valve. When one of them ‘calls’ for heat, the motor inside the valve opens the outlet to that part of the system. Likewise, when the same thermostat is ‘satisfied,’ the outlet is closed off.

26. May 2011 by admin
Categories: Heating, Plumbing | Tags: , | Comments Off on Central Heating Repairs: System Controls


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