Maintenance of the central heating system

Although all central heating systems should be serviced at least once a year by qualified engineers, you can keep the system running reliably by correct use and regular maintenance.

It is not advisable to switch off heating at night during cold weather. A small amount of heat—to ensure that the temperature throughout the

When a central heating system ceases to function properly, it becomes an expensive liability. But it does not always take an expert to repair it. Many common faults can be cured just as easily by the householder.

The most complicated part of the average domestic central heating system is the boiler. This may require specialist knowledge and tools to put it right should it fail—but failures in boilers which are regularly serviced are rare. More prone to trouble are circulation pumps, thermostats and radiators which only have a limited life.

Faults here are usually easy to identify and even if a specialist has to be called in, being able to pinpoint a problem will help keep repair bills to a minimum.

Wet central heating systems

In order to identify a particular fault in a central heating system, it is worth having some idea of how the system works.

In most modern systems, hot water flows from the boiler to the radiators and hot water cylinder, releases its heat and returns to be reheated. The flow is created artificially by an electric circulation pump which is normally mounted adjacent to the boiler. The pump is controlled by a time clock and, in most cases, by a room thermostat as well.

At pre-selected times, the mechanism in the clock switches on the pump. The pump then sends hot water to the radiators, heating the house. house never falls below 10°C — cuts the time needed to reach full operating temperature and may, in the long run, save fuel. It will also help to reduce condensation and prevent frost damage to the system.

The boiler should never be run at too low a thermostat setting. There is no economic advantage to be gained and it can shorten the life of the boiler. The boiler thermostat in a conventional small bore system should be set at up to 82°C in winter. In summer, when the system is required for hot water only, it should be kept at not less than 54°C.

If the system is oil-fired, the oil tank should be examined annually. Any external rust should be removed with a wire brush and glasspaper and then painted over with black bitumen paint. Keep the vent pipe on top of the tank clear, removing any obstruction with a stiff piece of wire. A piece of fine wire mesh can be fitted over the end of the vent pipe to ensure that leaves do not enter the tank and restrict the flow of fuel to the boiler.

To clean the oil filter on an oil tank, turn off the stop cock and remove the filter bowl. Clean the element with paraffin, dry it and refit. At the same time check the oil line from the tank to the boiler for leaks, tightening joints where necessary.

When a solid fuel boiler is not in use it should be left clean. Remove sooty deposits from the combustion chamber and flue and leave the damper and boiler doors open to allow a current of air to pass through. Have the flue cleaned at least once a year.

If a central heating system has to be left drained for any length of time and stop valves are fitted on either side of the circulation pump, you can close the valves, remove the pump and dry it thoroughly to prevent rusting.

Once or twice a year, the circulation pump valves and all the radiator valves should be turned as far as they will go in both directions and then back to their original setting. This will prevent them becoming fixed.


Overheating is one of the most common faults found in wet central heating systems. In all cases of overheating, if the fault cannot be rectified at once, the supply of gas or oil to the boiler should be cut off as a precaution. If you can run the circulation pump with the boiler off, keep it circulating water so that the heat is dissipated through the radiators. With a solid fuel boiler, rake the fire into the ashpan and remove it.

If the house feels abnormally hot, check the time clock and, if there is one, the room thermostat. These may be failing to turn the pump off when they should or have had their settings accidentally advanced. Start by turning the time clock down to the present setting. If the radiators do not cool down at the time they are supposed to, the mechanism of the clock has probably jammed and will have to be replaced with a new one.

To check a room thermostat, turn it down to its lowest setting and then back up again. A click should be heard as the switch inside turns the pump on. If there is no click, the unit will have to be replaced.

If the whole system is overheating seriously, the radiator pipes may make prolonged knocking or hissing noises and there will be excessive temperature in the boiler delivery pipe. One possible reason for this is failure of the circulation pump.

To find out whether the pump is working, hold one end of a screwdriver against the casing with the other end to your ear and listen for the hum of the rotor inside : if there is no noise, this is probably stuck. On pumps with a screw-on glass inspection cover, the rotor can be freed quite easily. Turn the pump off, unscrew the cover and insert a screwdriver into one of the slots in the rotor. If the rotor does not spin freely, it should be possible to free it by levering gently with the screwdriver.

On pumps which have all metal casings, the water supply must be cut off before opening the cover. In most cases, there are stop valves on each side for this purpose but where no such valves are fitted, the system will have to be drained before carrying out any work on the pump.

If the pump is heard to be working but water is evidently not circulating, there is probably an air lock. At the top of the pump you will find a vent valve—operated either by a key or a screwdriver—from which the pump can be bled.

To do this, turn the pump off and leave it for a few hours to allow the water in the system to settle. Then open the valve to bleed the air off. A hiss of air will be followed by a trickle of water: when the trickle becomes constant, close the valve.

If the fault is not in the pump, the boiler thermostat may have failed. The thermostat, a small box with a dial on the top, is located behind the boiler casing. Remove the casing and check that the electrical connections on the thermostat are sound. Check also that the sender bulb on the end of the copper capillary from the thermostat to the boiler has not fallen out of its socket. If so reposition it and replace the securing clip.

Note the setting on the boiler thermostat dial and turn it down low. After a few minutes turn it back towards its maximum setting and listen for a click. If there is no click, it may mean that the thermostat has jammed and you should call in a qualified engineer to check it.

If the boiler thermostat appears to be working, check to see whether the boiler flue outlet outside has become blocked in some way. Depending on the nature of the blockage, expert help may be needed in order to clear it.

If the flue is free of any obstruction, the next thing to check is the expansion tank. The ball valve supplying it may have become jammed or seized, in which case there may not be enough water in the system to absorb the heating action of the boiler. If the level in the tank is more than 150mm from the valve outlet, free the valve and introduce more water into the system. Where the valve is completely seized, replace it with a new ballcock, arm and piston unit.

Central heating too cool

If all the radiators are cool and the boiler is working correctly, the fault probably lies with one of the thermostats, the time clock or the circulation pump. Carry out checks outlined above under ‘Overheating’, paying special attention to the position of a room thermostat if fitted. This reacts to the temperature around it, and a nearby heat source can cause it to give a false reading even though the mechanism may be perfectly sound.

To work efficiently, the thermostat should be mounted on an internal wall at least 1.5m above the floor and away from draughts, radiators and direct sunlight. It should not be placed in rooms which are in constant use— such as lounges—because people generate extra heat, nor in kitchens, because of the heat from cooking and appliances. However, it should be accessible so that changes in setting can be made conveniently.

Draining the system

Before doing any major repairs or modifications to your central heating, you will have to drain, or partially drain the system. Start by turning the boiler off and leaving the system for a few hours to cool down. Turn off the electricity supply to the time clock and the immersion heater—if the system includes one.

Shut off the water supply to the boiler by closing the stop valve on the pipe into the expansion tank. If no stop valve is fitted, lash the ball valve in the expansion tank to a piece of wood laid across the tank.

When the system has cooled, return to the boiler and identify the main drain cock. This is usually at the front end of the boiler near the pump where it is always built into the lowest pipe. Alternatively, it may be found on a ground floor radiator. Attach one end of a garden hose to the nozzle and run the other to an outside drain. Open the drain cock by turning the nut beneath with a spanner or adjustable wrench and allow as much water as you require to drain away.

Refilling the system

Before refilling, close the main drain cock securely. Open the valve on the pipe leading to the expansion tank, or untie the ball valve, to admit fresh water into the system. Regulate the position of the valve so that the tank fills slowly—keeping the risk of air locks to a minimum. Also check the drain cock for leaks.


Noise is another common problem with wet central heating systems. Creaking under the floorboards and around radiators is caused by pipes— which expand and contract according to the temperature of the water— rubbing against the floor joists on which they rest. Creaking can also occur where a pipe rises through the floorboards to feed a radiator.

The creaking can often be reduced by turning the boiler thermostat down so that the radiators remain switched on for longer periods instead of constantly heating up and cooling down.

If the noise persists, take up the floorboards around the suspect area. Eventually you will find a point where one or two pipes cross a joist and are notched into the woodwork. If the notch is so small that it causes the pipes to rub against each other, enlarge it to give a better clearance. Make sure, though, that the notch does not exceed one sixth of the depth of the joist or it will seriously weaken the timber. Use a piece of rubber pipe lagging, felt or carpet, trimmed to the approximate size of the notch, to cushion the pipes.

Where a pipe rises through a gap in a floorboard, either enlarge the gap by filing it away or pack the space around the pipe with padding. Metal pipe brackets—another common source of noise—can be bent back slightly, and stuffed with felt to prevent them making direct contact with the pipes.

Creaking behind radiators is usually caused by the hooks on the back of the panels rubbing against their corresponding wall brackets. For serious cases, on smaller radiators, special nylon brackets can be fitted in place of the normal pressed steel type. A simpler solution is to place pieces of felt or butyl between each hook and bracket. This can be done, with the help of an assistant, by gently lifting the radiator away from its brackets, slipping the pieces of felt into the hooks and then replacing it.

Immersion heaters

In many systems, hot water for sinks and baths is heated by a thermostatically controlled immersion heater in addition to the boiler-fed heat exchanger. The thermostat is pre-set to turn the heating element off when the water reaches the selected temperature. If the water is unbearably hot, the thermostat may simply need adjusting.

The thermostat control is found at the top or on the side of the hot water cylinder. To adjust it, turn off the electricity supply to the heater then unscrew the element cover where you will find a small dial marked centigrade, fahrenheit, or both. By hand, or with a screwdriver, turn the regulator screw to the desired temperature—normally 60’C in hard water areas or 80 °C in those with especially soft water.

If the water heats up slowly, or the hot tap cools too quickly, check that the cylinder is sufficiently lagged and that the lagging is in good condition. If it is, try adjusting the thermostat. When water fails to heat up at all, either the thermostat control or the heating element are defective and will have to be replaced.

Radiator controls

Most radiators are fitted with two valves—a handwheel and a lockshield valve. The handwheel allows radiators to be shut down individually or the temperature of a radiator to be reduced by restricting the flow of water. The lockshield valve is set when the system is installed, to give a balanced flow of water through the radiator.

There is no basic difference between the two valves except that the lock- shield valve is locked into position to prevent casual adjustment. A lock-shield valve should normally need adjusting only when a radiator has to be removed for decoration or repair. When this is necessary, both the lock-shield valve and the handwheel should be closed. To close a lockshield valve, unscrew the cover and turn the valve with a spanner or a wrench.

In some cases, thermostatic radiator valves are fitted in place of hand-wheels. A radiator thermostat can be pre-set to maintain any desired temperature and is controlled by temperature sensitive bellows. As the water temperature falls, the bellows contract to allow more hot water into the radiator. Radiator thermostats are usually only suitable for use in a two-pipe system.

Bleeding a radiator

When air accidentally enters a wet central heating system, it can find its way to a radiator and prevent this from functioning efficiently. All radiators should be bled of air once or twice a year to clear the small amounts that inevitably get into the system. But if a radiator becomes cold whilst others are functioning normally, the cause is probably a substantial air lock and the radiator should be bled immediately. The top of a radiator remaining cold while the bottom is scalding also suggests an air lock.

On most radiators a square ended hollow key—obtainable from ironmongers—is needed to open the air vent valve at the top. To prevent air being sucked into the system, turn down the room thermostat and switch off the time clock so that the pump stops working.

Place a towel underneath the radiator to catch any drips, then open the valve by turning the key anticlockwise until a hiss of escaping air is heard. As soon as water begins to flow, re-tighten the valve.

If air locks occur frequently in a certain radiator, you can fit a screw bleed valve or an automatic air eliminator. These save you from constantly having to bleed it by hand.

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