Boiler Working but Radiators Stay Cold

Are all the radiators cold, or only some?

This one question splits the problem in two. If every radiator in the house is cold while the boiler is on, the boiler is not sending heat into the heating circuit at all, and the cause is in the controls, the pump or a valve. That is what this page covers.

If some radiators are hot and others cold, the boiler is doing its job and the fault is in the pipework or the radiators: air, sludge, a stuck radiator valve or an unbalanced system. Our guide to fixing a radiator that will not heat up walks through those in order, and one radiator not working covers a single cold radiator.

Also note, carefully because they can be very hot, whether the pipes coming out of the bottom of the boiler warm up when the heating should be on. Hot pipes at the boiler with cold radiators points to the pump or a valve. Cold pipes at the boiler points to the controls or the boiler not firing for heating.

Safe checks before you call anyone

None of these needs tools or the boiler casing off. Do them in order, because the first two are the ones most often at fault after a power cut or the first cold night of the year.

• Programmer: make sure the heating channel is on, not just the hot water. Set it to constant or a one-hour boost to test.
• Room thermostat: turn it up well above the room temperature, to 25°C or more, and listen for a click. On wireless thermostats, check the batteries and that the receiver near the boiler shows a heating-on light.
• Boiler heating temperature: check the radiator temperature dial or setting on the boiler has not been turned right down. Some digital boilers also have a summer or hot-water-only mode, which switches the heating off.
• Pressure: a sealed system should read about 1 to 1.5 bar cold. Well below 1 bar, many boilers lock out and show a pressure fault, which stops the heating.
• Radiator valves: check the thermostatic heads are turned up. After summer, a TRV pin can stick shut. See a stuck thermostatic radiator valve.

Leave the pump and valves alone beyond looking at them. Tapping a seized pump with a hammer is an old trick that can crack the casing and flood the cupboard.

Why the boiler runs but the radiators stay cold

If the controls are calling for heat and the pressure is right, work through the causes in this order.

The pump has failed or seized. The circulating pump pushes hot water round the radiators. On a combi or system boiler it sits inside the casing; on older regular boilers it is usually a separate Grundfos or Wilo pump in the airing cupboard. A seized pump often hums or feels hot with nothing moving, and some boilers show an overheat fault shortly after the burner lights, because the heat has nowhere to go. After a summer standing idle, pumps are especially prone to sticking.

On a combi, the diverter valve is stuck on hot water. This is the reverse of the fault where the radiators get hot instead of the taps. The valve that switches between heating and hot water has jammed in the hot water position, so the taps are fine and the radiators never warm. If your hot water is good but the heating has not worked since the autumn switch-on, this is a common reason.

On a system or regular boiler, the heating zone valve is not opening. On an S-plan system, a two-port valve controls the heating circuit. If its actuator head has burnt out, the boiler may heat the cylinder normally but never open the heating valve. Some systems will not fire the boiler for heating at all until that valve proves it is open.

Heavy sludge or an airlock in the main pipes. Years of corrosion produce a black magnetite sludge that can block the pump inlet, the heat exchanger or a magnetic filter. A heavily blocked filter can stop the whole circuit. Air trapped at a high point after a repair or a top-up can do the same.

Worcester boiler not heating radiators

Worcester Bosch combis follow the same pattern, but a few model features catch people out. Check the central heating temperature knob or setting first: on some Greenstar models the reset is done by turning the central heating knob to a reset position, so it is easy to leave that knob turned too low afterwards. On digital models, check the display for a heating-off or summer setting.

If the display shows a fault code, look that up before anything else. For example an EA 227 on a Worcester means the boiler has failed to light, which is a different problem from cold radiators with a working burner. Our Worcester boiler repair visit covers the pump and diverter faults these models are known for.

What the engineer does to find the fault

The engineer starts by confirming the boiler is being asked for heat, measuring for a live signal at the boiler heating terminal when the thermostat calls. If there is no call, the fault is in the thermostat, programmer or wiring centre, not the boiler.

With a call for heat confirmed, they watch the flow and return temperatures at the boiler. A flow pipe that climbs fast while the return stays cold means water is not circulating, so the pump is checked for power, then for a stuck rotor. Some pumps have a screw on the front that gives access to the shaft so it can be freed, and an engineer will try this before condemning the pump.

If the pump is running, they check the diverter valve or zone valve by watching whether the heating side actually opens. Where sludge is suspected, they check the magnetic filter and the colour of water from a radiator bleed point. Black water means the system needs cleaning, not just the failed part replacing. Gas work is carried out by Gas Safe registered engineers.

What it usually costs

The £60 call-out covers getting an engineer to you, and the repair price is agreed with you on site before any work starts. See our pricing page for the rate card, or our central heating repair page for what the visit covers. These are typical UK market ranges, parts and labour together.

• Seized pump freed on the visit: usually just the call-out and a short labour charge
• Circulating pump replaced: typically about £150 to £350
• Diverter valve motor or cartridge on a combi: typically about £150 to £400
• Two-port or three-port valve actuator head: typically about £100 to £250
• Powerflush for a heavily sludged system: typically about £400 to £800 depending on the number of radiators