Boiler working but cold radiators – why?

30 August 2023

Why is my boiler working but the radiators are cold?

It can be frustrating and concerning when you notice that your radiators remain cold despite your boiler seeming to be working perfectly. Most radiators might get hot while one or more remain stubbornly cool. In this article, we’ll explore the possible reasons and solutions behind this perplexing question.

Thermostatic radiator valves

If you’ve just got one radiator that refuses to heat up, the problem might simply be that the thermostatic radiator valve (TRV) is set too low. TRVs are bigger than regular radiator valves as they incorporate a twisting control head and some sort of readable scale (e.g. 0–10 or a red to blue grade). They’re designed to be operated by the householder. The idea is that you can use the TRV to control the temperature of just that radiator, rather than having the temperature of the whole home governed by the main thermostat. 

Sometimes, a TRV is set too low, in which case that particular radiator will never get hot as long as the room is as warm as the setting. Simply rotate the head and you should see some kind of pointer or window moving up and down the temperature scale. This is when you might find that it was set to its minimum temperature, or just very low, and that should solve the problem.

Are you sure the radiator is on?

A standard radiator valve is less obvious than a TRV. These manually control the flow of hot water entering the radiator, but there’s no scale or readout to gauge whether it’s open or closed, or somewhere in between. Sometimes they incorporate a visible tap or lever, but they are often lockshield valves, which have a cap on them to prevent accidental turning. 

Try turning the valves on both ends of the radiator anticlockwise as far as they will go. If it’s a lockshield type, you’ll need to remove the caps and gently turn the valves with a spanner. If the central heating is turned on and you hear water gushing into the radiator, it’s likely that the valves have been turned off, or were allowing just a trickle, in which case you might have located the problem.

Trapped air

Air can get trapped in a radiator system in many ways, from leaks and pumps to faulty equipment or low pressure. Because it’s lighter than water, air will always bubble to the top. And as the water inlet and outlet are always at the bottom, that means the radiators gradually get filled with air and it has nowhere to go but up. Given time, air can take up half the radiator or more. This air is not circulated and isn’t a particularly good heat conductor, so the top of the radiator might feel cool even when the bottom is hot.

The solution to this issue is to bleed the radiator using a special key, tool or spanner. You’ll find plenty of videos on how to do this online – just make sure you have something to catch the water that will squirt out once the air is gone, as it could damage wallpaper or carpets. Once it’s done, you’ll probably need to put more water in the system using the filling loop on the boiler.

Lack of water circulation

One reason your radiators may be cold even when the boiler is functioning is due to a lack of water circulation in the central heating system. A couple of factors can contribute to this issue:

  • Blockages: Over time, debris can accumulate within the central heating system, hindering the smooth flow of hot water to the radiators. This can lead to uneven heating or complete coldness in certain areas. To clear blockages, you might need a power flush or chemical flush which can dislodge sludge and grime that is restricting the flow.
  • Partial pump failure: A malfunctioning or inefficient pump can disrupt the circulation of hot water through the system, causing radiators to remain cold. Professional assistance will be required to diagnose and replace the pump if necessary.

Incorrect boiler pressure

Boiler pressure is crucial for the proper functioning of a central heating system. If the boiler pressure is too low, it can result in inadequate water flow and cold radiators. Common reasons for low boiler pressure include:

  • Water leaks: Check for any visible leaks in the system, such as dripping pipes or a faulty pressure relief valve. Addressing and repairing these leaks, then refilling the system, can help restore proper pressure levels.
  • Faulty pressure gauge: Sometimes, the pressure gauge itself may be inaccurate, showing a lower pressure reading than the actual pressure. Ask a heating engineer to calibrate or replace the gauge.

Imbalanced system

Another possible reason for cold radiators is an imbalanced central heating system. This occurs when certain radiators receive an insufficient supply of hot water while others receive an excess. Factors that can cause imbalance include:

  • Thermostat settings: Incorrect settings on individual TRVs can result in some radiators not receiving sufficient hot water. Ensure that all TRVs are correctly set to the desired temperature.
  • Blocked or misconfigured pipework: Blockages, kinks or incorrect pipework installation can disrupt the flow of hot water to some parts of the system, leading to uneven heating. A professional heating engineer can assess the system, identifying any pipework issues.

Boiler malfunction or inefficiency

Although your boiler may be working, there could still be underlying problems that affect its ability to heat the water efficiently. Potential causes for a boiler malfunction include:

  • Faulty thermostat: A faulty thermostat can inaccurately control the temperature, resulting in inadequate heating. You might need to replace or recalibrate the thermostat to ensure proper functioning.
  • Limescale build-up in the heat exchanger: Over time, mineral deposits can accumulate in the boiler’s heat exchanger, reducing its efficiency. Regular boiler servicing and descaling can help address this issue.
  • Boiler size and capacity: Sometimes, the boiler may be undersized for the heating demands of your home. In such cases, your radiators may not receive sufficient hot water to provide adequate heat. When you order a new boiler, you’ll be asked how many rooms, radiators, cylinders etc. your home has and that will determine the power of the boiler. Sometimes, people put in extra radiators or showers, or build an extension, but don’t upgrade the boiler. That can sometimes mean there’s not enough hot water to go round.

Let us assess your system

You might be able to fix certain issues yourself with valves or bleeding. However, if this fails to address the problem, you’ll need a full examination of the central heating system. At Able Plumbing, we’re experts in diagnosing and fixing all sorts of central heating and hot water problems. So whether you need a quick fix or a complete boiler replacement, why not give us a call so we can help you get all your radiators working at maximum efficiency?