When a NRG control displays “HEAT ON” and a heater is not coming on, this would indicate that the heater has overheated and tripped out on high limit.

The most common cause for a warm air heater to overheat are:

• Loss of power during operation. This could be the result of a power cut or simply the heater being turned off from its local isolator (always use the control to shut the heater off when not required).
• Lack of ventilation around the heater (on new installations the ventilation requirements for heating equipment that is installed should always follow regulations and manufactures instructions).
• Lack of ventilation around the heater (on existing installations the area around heating equipment must not be obstructed e.g. placing pallets, boxes, installation of racking can result in heating equipment overheating).
• Lack of ventilation due to poor maintenance, e.g. blocked vents.
• Lack of ventilation due to ductwork (some heaters have ductwork applied - on new installation all ductwork should be of a suitable size and meet the requirements of the installed heater, fire dampers if fitted within the ductwork may have activated and should be checked along with inlet air filters that may be blocked, as these type of obstructions will cause nuisance over heat trips).
• “Over fired” incorrect burner pressure. If a heater is commissioned serviced correctly, this should not occur (remember any commissioning process, service or maintenance should only be carried out by a gas safe engineer).
• Stratification (this can occur when heating equipment is installed normally at higher levels, warm air emitted from the heater rises and remains trapped in the roof void. The tapped warm air gets re-circulated through the heater to a point where the heater cannot maintain a maximum temp in the heat exchanger).
• Distribution fan failure (the heat exchanger requires air flow to keep cool and give required warm air for heating).

All warm air heaters have an overheat reset function, located inside or outside of the heater according to its age. This is a simple procedure to rest a warm air heater - attention to Health and safety must be taken into account e.g. the heater may need access equipment to carry out a reset procedure and electrical circuits may be live. TURN OFF POWER SUPPLY BEFORE ATTEMPTING ANY INTERNAL RESET.

On the latest models for CTU and CTCU type heaters, the overheat reset button is located internally - please see the excerpt from the current CTCU ISO manual.

On the latest models for MGB and MOB type heaters, the overheat reset button is located externally - please see the excerpt from the current ISO manual.

Limit Thermostat

This protects the heater from overheating caused by a reduction of air flow over the heat exchanger.

It is a manual reset device and once operated prevents the heater restarting until reset.

To reset, wait until the heater has cooled sufficiently and then press in and release the red reset button located on the front of the heater cabinet below the burner position. After the operation of the limit thermostat, efforts must be made to locate the problem causing its operation.