First, check what the radiators are doing
Put the heating on for 30 to 45 minutes and walk round every radiator. What you find narrows it down quickly.
| What you find | Likely cause | Where to start |
|---|---|---|
| All radiators only lukewarm | Flow temperature set low | Boiler heating temperature setting |
| Near ones hot, far ones lukewarm | System out of balance | Lockshield valves |
| Radiators cold at the bottom | Sludge | System clean |
| Radiators cold at the top | Air | Bleed the radiators |
| Radiators hot, rooms still cold | Heat loss, small radiators or short heating times | Controls, draughts, insulation |
| Radiators cold, boiler not firing | Controls or boiler fault | Thermostat, timer, fault code |
If the radiators are completely cold, see our guide to heating not working when the hot water is instead.
Flow temperature too low
The flow temperature is how hot the boiler makes the water it sends to the radiators. On a combi or system boiler it is set with the radiator symbol dial or in the heating menu. Lowering it is a popular way to save gas, because condensing boilers run more efficiently when the water coming back is cooler. It works well in mild weather, but if it is set too low for a cold spell, the radiators may not give out enough heat to warm the house.
As a rough guide, many homes with standard radiators are comfortable with a flow temperature of around 55 to 60°C for most of the winter. On very cold days, or in homes with small radiators or poor insulation, it may need to go up to around 65 to 70°C. If you have turned it down and the house is now cold, turn it up in small steps, a few degrees at a time, and give the house a day to respond.
Some boilers have weather compensation, which raises the flow temperature automatically as it gets colder outside. If yours has an outdoor sensor fitted, check the settings with the manual or your installer rather than overriding it.
Balancing, air and sludge
Water takes the easiest route round the system, so radiators near the boiler can take most of the heat while those upstairs or at the far end stay lukewarm. Balancing the lockshield valves fixes this. Our guide to one radiator not heating explains how.
Radiators cold at the top need bleeding, and radiators cold at the bottom usually have sludge that needs flushing out. See our guides to a radiator cold at the top and radiators cold at the bottom. After bleeding, top the pressure back up to 1 to 1.5 bar if it has dropped, using our guide to repressurising a boiler.
Low pressure matters here too. A boiler running with pressure on the low side may still fire but circulate poorly, and some models reduce their output or cut out. Check the gauge with the system cold before assuming the boiler is too small or faulty.
Controls, timings and boost
- Thermostat position. A thermostat in a warm hallway, near a radiator or in the sun switches the heating off while other rooms are still cold. A cooler, central living space usually works better.
- TRV settings. A TRV on setting 1 or 2 closes early. For a main living room, 3 to 4 is typical. Check none are stuck closed.
- Heating times. A cold house takes time to warm up. If the heating comes on only 30 minutes before you get up, it may never catch up on a freezing morning. Starting it earlier often works better than turning the thermostat higher.
- Boost. Most programmers and smart thermostats have a boost button that runs the heating for an extra hour or so. Use it on very cold days rather than leaving the heating on constantly at a high setting.
- Eco or summer modes. Check the boiler is not in an eco mode that limits heating output, or in summer mode.
Heat loss and radiator size
If every radiator is hot and the settings are right but rooms are still cold, heat is escaping faster than the radiators can replace it, or the radiators are too small for the room. Draughts around windows, doors and floorboards, uninsulated lofts and single glazing all add up. Cheap fixes include draught strips, a chimney balloon in an unused fireplace, thick curtains and topping up loft insulation to around 270mm, which is the depth commonly recommended in the UK. If a single room is always cold, a heating engineer can check whether its radiator is big enough for the room.
Furniture matters more than people expect. A sofa pushed against a radiator, long curtains tucked behind it or a shelf directly above it can trap much of the heat. Keeping the front of each radiator clear and closing curtains at dusk lets the heat you are paying for reach the room.
When to call an engineer, and typical costs
Call a Gas Safe registered engineer if the boiler will not reach its set temperature, shows fault codes, makes kettling or banging noises or cuts out. These can point to a weak pump, sludge in the heat exchanger, or a gas pressure or burner problem, all of which are inside the casing. A circulation code such as Ideal L1 is a common sign that water is not moving through the boiler fast enough.
It is also worth asking whether the boiler is set up correctly for your home. The engineer can check the maximum heating output, pump speed and flow temperature, and confirm the boiler is big enough for the heat loss of the house. A combi sized mainly for hot water is usually more than big enough for the heating, so an undersized boiler is less common than a settings or circulation problem, but it does happen after extensions or loft conversions add radiators.
Do not open the boiler casing or try to adjust its gas settings. If you smell gas, call the National Gas Emergency Service on 0800 111 999.
| Job | Typical UK cost |
|---|---|
| System balancing | Typically £60 to £150 |
| Powerflush, 6 to 10 radiators | Typically £400 to £700 |
| Pump replacement | Typically £200 to £400 |
| Larger replacement radiator, fitted | Typically £200 to £450 |
Costs rise in London and the South East, for designer radiators and where pipework needs moving. An annual service is a good time to ask about flow temperature and balancing; see our guide to boiler service costs.