Central Heating Operation

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This article is about operating Central heating systems using hot water fed radiators. It assumes the system is functional.

There are also articles about:

  • Central Heating Design which may help understanding what sort of system you have.
  • Central Heating Repair for help diagnosing and fixing a faulty system
  • Other heating topics are found by clicking the heating category link at the bottom.


Fanned warm-air ducted systems are occasionally found in the UK. These operate quite differently and their use is not covered here. (There is a discussion on updating existing warm-air systems here)


Time & temperature controls

  • Programmer and room thermostat
    • check there is a room stat
    • check no other sources of heat or valves turned down in stat area
  • Programmable thermostat
    • fully programmable or set-back
    • optimum start (Honeywell issue)
  • combis with pre-heat

Setting boiler temperature control

As well as a room thermostat, there is also a thermostat on the boiler itself. This thermostat adjusts the temperature of the circulating heating water.

The lower the temp of this water, the more efficient the boiler operates, but the lower is the system heat output. If a heating system is failing to keep the house warm enough, turning this thermostat up often resolves the problem. In milder weather it can be turned down to improve efficiency.

Some systems experience kettling when this thermostat is adjusted too high. Kettling is water boiling in the boiler exchanger when the pump stops. It sounds like banging and thumping, and can prematurely damage some boilers.


Run 24/7 or turn up and down

Total energy use is always less when the system is turned down when not needed. At any moment, rate of heat loss is proportional to the temperature difference between the house interior and outdoors, so total heat loss is reduced when house temp is reduced when heat ins't needed.

Frost protection when house empty

Set the room thermostat to minimum or 5 degrees C.

Switching the system fully off instead can sometimes result in water or heating pipes freezing and bursting, potentially causing thousands of pounds of damage.


Radiator controls

Setting Thermostatic Radiator Valves (TRVs)

If there is a TRV on the radiators in the room with the room thermostat, these TRVs should be set to max temp setting.

Radiators in other rooms that have TRVs should have their lockshield (manual) valves set full on, and the TRV adjusted to give the desired room temp.

Setting manual radiator valves

Check the valves on the radiators in the room with the room thermostat are fully open. To turn these valves full, screwing the caps anticlockwise. The temperature in this room is now controlled by the room stat rather than the rad valve.

Adjust rad valves in other rooms to achieve even temperature through the house. With some houses the room temps take a day to stabilise. Little used rooms will generally be adjusted to provide a lower room temperature.

With a new or very maladjusted system, all valves fully open is a good place to start from.


User maintenance

Balancing radiators

Radiators are balanced by adjusting the manual lockshield valves or TRVs. See the Radiator controls section above.


Bleeding a CH system

Bleeding radiators is needed when some of the radiator is hot, but the top is not. This problem is caused by air buildup in the radiators, which is caused by

  • corrosion
  • sucking in air from the header tank
  • or pumping over in the header tank, which oxygenates the water

To bleed a radiator, loosen the little bleed screw at one end of the top of the radiator using a radiator key or pliers. Air will escape with a hiss, when the hissing stops and just water comes out, retighten the screw. Dribbles can be anything from clean water to black, so have a towel ready to catch a little spillage.

The need to bleed any more than very occasionally indicates some sort of problem. Topping up the system corrosion inhibitor solves this in a lot of cases.

Topping up a sealed system

  • why? check for how pressure being lost

Setting a system bypass

See Also