Combi boiler breakdown

Finding the cause of any combi boiler breakdown efficiently is an easy step providing the gas safe engineer is a) experienced with the generic causes of combi boiler breakdown b) understand the concepts and principles of combi boiler components and how they actually work.

Understanding Combi boiler breakdown

In order for a Gas Safe Engineer to be able to deal with any combi boiler breakdown efficiently, he must be able to point where the fault is, to do this he must understand the combi boiler sequence of operation.

Typical sequence of operation for combi boiler breakdown

Demand >boiler Thermostat> Over heat thermostat > APS Co-NC >Fan >APS-NO > Ignition/Pilot>FRD>Main Burner

Although there are more than 100’s of combi boiler models and many variations, luckily their operating sequences are almost the same as above. 

Any Safe safe engineer attending combi boiler breakdown should follow a logical approach including using safe working practices, using specialist instruments to test components in order to spot the faulty part that causes the combi boiler to breakdown.

Generic causes for combi boiler breakdowns

(Gas Safe Egineers only)

• No power
• Gas supply
• Low water pressure
• Water leaks from the boiler causing low pressure sensor to activate
• faulty component pump- flow switch, gas valve, diverter valve, Air pressure switch and sensors
• Blockage causing no primaries flow

What make us special

Once the faulty part is proved to to be the cause of the combi boiler breakdown, then the gas safe engineer must go the extra mile and investigate the cause of the breakdown. There is no point of trying to replace a faulty component in poorly installed/maintained central heating without curing the cause. A typical example such as replacing a combi boiler faulty primary pump with a new one without flushing the system if the sludge proved to be the cause of the combi boiler breakdown, or replacing scaled up plate to plate heat exchanger with out installing a water treatment solution.

It is paramount that any gas safe engineer carrying out repairs on combi boilers to replace like for like parts only bought from a reputable wholesaler.

The most common cause of combi boiler breakdowns is a faulty thermistor often thought the PCB

Understanding Thermistors

Thermistors do a similar job to thermostats but they work on a different principle. Thermistors work by electrical resistance, a law current around (5 volts dc in most cases) is sent up one wire to the thermistor and down a second wire back to the PCB. The PCB measures the current it is sending to the thermistor and compares it to the current coming back from the thermistor. depending on the current the PCB decides whether to reduce or increase the gas output to the burner (modulating).

The Most common thermoistors used are those NTC, very often the PCB indicates that there is a fault, a common mistake service engineer do that they rush to change the PCB.

It is the law that only A COMPETENT person in accordance with the Gas Safety Installation and Use Regulations 1998 (also MUST be (listed) registered gas safe engineer) allowed to repair install or maintain gas installations/appliances in the UK.