Radiators not heating up? What to check before calling

If your radiators are not heating up, it can be worrying, especially in colder weather. Before you assume the worst about your boiler, there are several safe checks you can do yourself that often solve the problem or at least narrow down the cause.

Safety first when radiators are not heating

Before you start, safety should always come first. You must never remove your boiler casing or attempt to work on any gas components, wiring, or the flue. This work is strictly for Gas Safe registered engineers only.

If you ever smell gas, see signs of scorching around the boiler, or your carbon monoxide alarm sounds, switch the system off at the controls, ventilate the area, and contact the gas emergency number immediately. Do not attempt any further checks.

Simple checks to do before calling an engineer

Many heating issues are caused by settings that have been changed accidentally or by the system protecting itself. These are straightforward things you can safely look at without tools or technical knowledge.

Check the thermostat and timer controls

Start at your main controls. If your room thermostat is set too low, the heating may not come on even if the radiators feel cold to you. Try turning it up a few degrees and listen for a click and the boiler firing.

Next, check your programmer or timer. Make sure the heating is actually selected to "on" or "auto" and that the current time and day are correct. Someone may have changed the schedule without realising.

• Room thermostat turned up above current room temperature

• Heating set to "on" or "auto" on the programmer

• Boiler switched on at the spur and not in "off" or "summer" mode

Look at the boiler pressure gauge

On most modern combi and system boilers you will see a pressure gauge on the front or underneath. When the system is cold, many manufacturers recommend a pressure in the region of 1.0 to 1.5 bar. Check your boiler manual for the exact range.

If the needle has dropped close to zero, the boiler may lock out to protect itself, leaving radiators cold. Some homeowners feel comfortable topping up using the filling loop, but if you are unsure, it is safer to contact an engineer rather than risk over-pressurising the system.

Try a safe boiler reset

If you can see an error code or flashing light on the boiler, your manual will usually explain what it means and whether a reset is allowed. Follow the manufacturer instructions carefully, usually involving holding a reset button for a few seconds.

Never keep resetting a boiler that repeatedly locks out, as you may be overriding a safety feature. If the fault comes back quickly or the boiler cuts out again, it is time to book a professional inspection.

Check TRVs and radiator valves

Thermostatic radiator valves (TRVs) on individual radiators can easily get knocked into the off position. Ensure each TRV is turned up to a higher number so water is allowed to flow through the radiator.

On the opposite side of the radiator is usually a lockshield valve, often with a plastic cap. This is normally set by an engineer, but if the cap has been removed or turned fully clockwise, it may be restricting flow. Do not force it if it seems stuck; note the radiator and mention it during your engineer visit.

Whole house cold or a single radiator?

Knowing whether the issue affects the whole system or just one area can tell you a lot. Walk around your home and feel each radiator carefully from bottom to top, taking care with any that may suddenly get hot.

If every radiator is cold, or only lukewarm, the problem is likely at boiler or system level. If just one or two radiators stay cold while others are hot, you may be dealing with a localised problem such as air, a sticky valve, or sludge in that part of the pipework.

Common causes of radiators not heating properly

Once you have checked settings and valves, it helps to understand the issues that commonly sit behind cold or patchy radiators. Some are simple, but others need specialist tools to diagnose properly.

Airlocks and the need for bleeding

If a radiator is hot at the bottom but cold or cool at the top, there may be trapped air stopping hot water from circulating fully. Bleeding with a radiator key can release this air and restore heat.

Before you bleed any radiators, make sure the heating is off and the system has cooled. Keep a cloth and small container handy to catch any drips, and check the boiler pressure afterwards, as bleeding can reduce it slightly.

Low pressure and circulation problems

Consistently low pressure can prevent the pump from circulating water around the system effectively. If pressure keeps dropping after you top it up, there could be a leak or expansion vessel issue that needs professional attention.

A failing or incorrectly set pump can also leave radiators at the far end of the system cold while those closest to the boiler are hot. You might notice humming, grinding noises, or a pump that is very hot to touch, which should be reported to an engineer.

Diverter valve faults and hot water only issues

On combi boilers, a diverter valve controls whether heat from the boiler goes to your radiators, hot water taps, or both. If this valve sticks, you can end up with good hot water but little or no heating.

Valve faults are not a DIY job and usually require an engineer to test, repair, or replace the component safely.

Sludge build-up in pipes and radiators

Over time, internal corrosion can create a thick sludge that settles in radiators and pipework. Signs include cold spots at the bottom of radiators, gurgling noises, and some radiators never quite getting hot.

This can be more common in older properties, including many in and around Aberdeen, where original pipework and steel radiators are still in place. Regular servicing and maintaining inhibitor levels significantly reduce the risk of sludge building up to the point where it affects performance.

• Cold patches on radiators that never clear

• Radiators slow to heat compared with the boiler firing

• Dirty, dark water if you bleed a radiator

Troubleshooting: Narrow Down Your Radiator Issue

Working through a simple decision process can help you understand what is likely going on before you speak to an engineer.

If your hot water works but heating does not: the issue may relate to your programmer, room thermostat, or diverter valve.

If both heating and hot water are affected: this points more towards a boiler-wide fault, low pressure, or an electrical or gas supply problem. Check your pressure, controls, and any visible error codes, then contact an engineer.

If only one or two radiators are cold: suspect trapped air, local sludge build-up, or a stuck valve feeding those radiators. Bleeding, gently checking valves, and noting which radiators are affected will help your engineer diagnose quickly.

What a heating engineer will typically check

When you do need professional help, knowing what an engineer is likely to do during a visit can be reassuring. A proper diagnosis goes beyond quick visual checks and relies on measurements and specialist equipment.

System pressure and leak checks

The engineer will usually start by confirming system pressure and looking for signs that it is dropping over time. They may check safety valves, expansion vessels, and visible pipework for leaks that could be affecting radiator performance.

Pump performance and circulation

Next, the circulation pump will be inspected and tested to ensure it is moving water around the system efficiently. This might involve checking speeds, electrical readings, and listening for internal wear or blockage.

Poor circulation can explain cold radiators on upper floors or at the far end of the system, even when the boiler itself seems to be working.

Water quality, inhibitor levels and filters

A good engineer will also consider the health of the system water. They may draw a small sample to look for sludge, debris, or corrosion and test whether there is enough inhibitor chemical present to protect the system.

If you have a magnetic or inline filter fitted, it will typically be inspected and cleaned. A heavily blocked filter is a strong sign that further cleaning, such as a powerflush or chemical flush, could be beneficial.

When to call Westhill Boiler Services

If you have tried the simple checks above and several radiators are still cold, or your boiler keeps cutting out, it is time to bring in an expert. Leaving circulation or pressure problems unresolved can make faults worse and reduce the life of your boiler.

For professional diagnostics, boiler repairs, and help getting your heating working reliably again, contact Westhill Boiler Services on 01224548803 to book a repair visit and get your home warm and comfortable again.