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Home & garden.

Updated: 9 Nov 2021

How to balance and bleed your radiators

Find out how to tell if your radiators are balanced, how to balance them and also how to bleed your radiators
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Sam Morris
Father and son checking radiator_main 386084

A poorly balanced heating system can add a significant amount of cash to your annual bills.

Many people aren't aware of the impact that a poorly balanced system can have, chiefly that it can drive up your bills during the colder months.

The same goes for radiators that need bleeding. They don't heat up fully so you have to leave your radiators on for longer, driving your heating bills up.

Read on to find out the best why to diagnose unbalanced heating systems and how to balance and bleed your radiators. 


If you're planning on giving your heating system an overhaul, make sure you read our five tips to reduce your heating bills before you do


How to balance your radiator

Why do I need to balance my heating system?

 In an unbalanced system the temperature your home reaches may not match the one you set.  For example, when you set the thermostat to 20°C you would expect the whole home to heat to, and stay at, 20ºC.

However, with an unbalanced system, the upstairs might actually reach 23°C, while the downstairs remains relatively cool at 17°C. You may then be tempted to turn up the thermostat (in this example to 23°C), which will create a comfortable temperature of 20°C downstairs, but superheat the upstairs to 26°C. 

The Energy Saving Trust estimates that turning up the thermostat by just one degree costs an additional £75 a year in heating costs. So turning up the thermostat will just increase your bills, whereas getting your radiator rebalanced will save you money in the long run.

How do I know if my heating system is unbalanced?

thermal house reading
The telltale signs of a poorly balanced system include some radiators heating up much faster, or getting much hotter, than others in your home.

Another clear sign is if certain parts of your house (such as upstairs) get uncomfortably hot, while other parts stay cool. 

Why is my system unbalanced?

radiator knob
Your system can become unbalanced for a wide variety of reasons:
  • Sludge and debris Old systems may be clogged up, causing blockages in some radiators, meaning they take longer to heat up. 
  • Redecorating When radiators are removed and replaced, perhaps during decorating, this can impact the balance of the heating system.
  • Misuse of Thermostatic Radiator Valves (TRVs) If TRVs have been fitted to radiators around your home, and not adjusted correctly, this can lead to system-balancing issues. 

It's easy to tell if you have inadvertently unbalanced your own heating system with TRVs: 

  1. Turn all radiator thermostats to 0.
  2. Turn the central heating off and wait for at least two hours for the system to completely cool off.
  3. Starting from the radiator furthest from the heating source if you have a combi boiler, or furthest from the water pump (usually found in the airing cupboard next to the hot water cylinder) if you have a heat-only boiler, turn all thermostats to 2.
  4. Starting from the radiator furthest from the heating source, check if all radiators get hot evenly across the radiators at the same rate.

If they don’t, the system is not balanced properly and we would recommend that you call a Which? Trusted trader to assist with what will be a much more technical method of balancing your heating system. 


How a heating engineer will balance your system

radiator knob
  1. Turn off the boiler for at least two hours for the system to cool off.
  2. Turn on the boiler from cold.
  3. Measure whether there is a flow differential (DeltaT) of at least 20°C at the boiler.
  4. Measure whether the return temperature is below 55°C (this is essential for condensing to occur, and for your boiler to be operating at its advertised efficiency).

If the flow differential is less than 20°C, or the return temperature is higher than 55°C, then the system is either unbalanced or the radiators are too small. Either way, you will need a heating engineer to fix it.

A heating engineer will first completely drain the system and remove any air pockets by bleeding the radiators. They will then refill the system with all of the radiator valves fully open. The next step will be to adjust the valves on the radiators that get hottest the fastest. This it to restrict the flow of water to the radiator and even out the spread of heat to the rest of your home.  

Costs for this will vary between £100 and £200, but expect the heating engineer to take at least half a day to complete the work. 

If you're ready to call in a heating engineer to balance your radiator, you can use our Which? Trusted Traders search tool below to find an accredited trader near you. All of our traders have been vetted so you can trust them to do a great job.


If you decide it's time for a new boiler, make sure you buy from the best gas and oil boiler brands


How to bleed your radiators

How do I know if my radiators need bleeding?

Radiators need bleeding every few years to get rid of trapped air bubbles. The air bubbles rise to the top of a radiator so place your hand on the top each radiator while the heating is on. If the top of the radiator is cool, or much colder than the bottom of the radiator, it's a sign your radiators need bleeding. 

Instructions on how to bleed your radiator

Bleeding a radiator is a task you can carry out yourself. All you need is your radiator bleed key, and a spare cloth or towel and container to catch any water that could potentially leave your radiator when you start bleeding it.

  1. Turn the heating off Let the radiator cool down for 20-30 minutes so you don't risk burning yourself with hot water. 
  2. Locate the radiators bleed valve Usually located on the side of the radiator, at its top. If you're bleeding multiple radiators, start with the one on the ground floor furthest from your boiler.
  3. Place a container under the valve This is to catch any water that may drip out of the valve. Also hold the spare cloth or towel under the valve.
  4. Insert the radiator bleed key and open the valve Open the valve slowly, turning anticlockwise, until you hear a hissing sound. This is the trapped air escaping. Only open the valve a maximum of half-a-turn, so any water doesn't escape too quickly.
  5. Close the valve when water starts leaking out At this point all the trapped air has been removed so turn the key clockwise to close it. Repeat for all the radiators you need to bleed.
  6. Check your boiler's pressure After bleeding several radiators, it is normal for the heating systems pressure to drop. Check the boilers pressure indicator, it should be between 1.0 and 2.0 bar (often highlighted green). You can increase the boiler pressure if needed, follow the manufacturer's instructions to do this. If you’re unsure contact your boiler manufacturer’s technical department or a qualified heating engineer.
  7. Turn the heating on Give your radiators time to heat up and check the cold areas at the top of the bled radiators have gone. 

Read our guide on getting the best boiler service to get your boiler running as efficiently as possible