Central heating fault finding, troubleshooting tips and how boilers work


If your central heating system is going to go wrong at all, you can bet it will fail as soon as the first really cold weather arrives. Central heating is rarely used during summer months so the first cold snap will put it through its paces.

Minor problems can quickly become major issues as the central heating system works hard to try and heat your home.

Hissing or Banging From Boiler or Heating Pipes

Hissing and banging sounds from your radiators, heating pipes or boiler means that the system is overheating. Overheating can be caused by a blocked chimney in solid fuel central heating systems. To remedy or prevent this, get your chimney swept to clear heavy soot build-up.

Another cause might be an accumulation of limescale in the central heating system, particularly in hard water areas. If so, shut down the boiler and pump and treat the system with descaler.

Faulty boiler thermostats may be the cause of the knocking or hissing sound from your central heating system. Shut down the boiler but leave the pump working so that it can inject cold water to quickly cool down the central heating system. When cool, turn on the boiler. If you don’t hear a click, call an engineer.

Knocks, bangs and hissing might be the result of a lack of water in the central heating system. Shut down the boiler. Check the expansion tank in your attic. If it is empty, move the ball valve up and down to restore water flow to your central heating system. Also double check that the outlet pipe has not frozen over.

Cold Radiators In Part or All of the House

If some or all of your radiators fail to warm up, the zone valve may be faulty. If so, drain the central heating system and replace the valve.

Another cause might be a faulty central heating pump. Shut down the boiler and check that the pump is switched on. If the pump is not running, turn off the power and check all wired connections. If it still fails to work, drain the central heating system, remove the pump and thoroughly clean it to remove all blockages.

Single Radiator Does Not Warm Up

If one radiator does not heat up, check that the handwheel valve is set to “open.” If it has one, also check the radiator’s individual thermostat control.

Top of Radiator Stays Cool

If the radiator heats up at the bottom but not the top, this means an airlock is preventing water from properly circulating. Fix by bleeding the radiator to release trapped air.

Boiler Not Working

If the boiler does not come on when it should, check that your thermostat is not set too low. Another cause of boiler problems is if the programmer or timer is not working. Check it and re-set the timer or programmer. If the problem continues, replace the boiler timer or programmer.

If the pilot light on a gas boiler goes out, re-light it using the instructions for the boiler. If the pilot light continues to fail, you will need the help of an engineer.

How a Gas-Fired Hot Water Boiler System Works

In some homes, the hot water is also fed to a radiant floor heating system or through baseboard heaters.

Gas-fired hot water boilers are generally preferred to fuel oil models. Currently, the price of natural gas is economically advantageous. Oil burning models also require more maintenance, especially with respect to the oil filter and the blower assembly.

Hot Water Boiler Components

These appliances can be rather complex compared to a central forced air furnace, but given proper maintenance, they are very reliable. To understand how this appliance functions, it is helpful to understand the components. Following are the major boiler components.

Supply Side Components

Water feed valve. Self-explanatory; this valve regulates the water flow to the system.
Air vent. Hydronic systems can build up unwanted air and the purpose of this vent is to expel it rather than letting it build up excessively. “Hydronic” simply means a system that uses water as a heat transfer medium.
Aquastat. This is similar in function to the thermostat that controls the temperature in a residential central air conditioning and heating system.
Expansion tank. Like other materials, water expands as the temperature rises. The expansion tank allows room for this.
Burner and gas valve assembly. This is the component that does the actual water heating. Any maintenance is similar to that on a gas hot water heater.
Tridicator. This is a gauge that monitors both water pressure and temperature. The best ones conveniently monitor pressure in both PSI (Pounds per Square Inch) and kPa (kiloPascal) as well as temperature in both Fahrenheit and Celsius.
Flow control valve. This valve controls the flow of the outgoing heated water.
Pressure release valve. This ensures that excess pressure will be relieved in a safe manner.

Return Side Components

Water drain valve. This allows the water to be drained from the boiler system, usually for periodic maintenance.
Circulating pump unit. This electric pump keeps the water circulating through the system to provide radiant heat to the home.

How the Boiler Functions

When all the components are functioning harmoniously, the system runs itself. There should be sufficient water circulating through the system. When the volume is correct, the system’s internal pressure will be around 14 PSI (96.5 kPa), give or take a few pounds. When the pressure drops too low, the automatic filling system will add water until the tridicator signals that all is well.

The expansion tank maintains a level of water and volume of air to prevent boiling. The circulation pump keeps cooled water from the radiators or baseboards flowing back into the boiler, where the water is reheated and pressured back out to the rooms.

As long as the pipes, radiant baseboard heaters, and radiators contain no trapped air, the entire system will be furnished with radiant heat. If any air is in the system, it may be removed using the bleeder valves on the radiators.


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