How to Balance Radiators: A Step-by-Step Guide

To balance radiators, you adjust the lockshield valve on each one so hot water is shared evenly around the house instead of rushing through the radiators nearest the boiler. In practice that means bleeding the system, opening every valve fully, noting the order in which the radiators warm up, then turning the lockshields on the fastest radiators down and leaving the slowest ones more open. A pipe thermometer tells you when each radiator is right. It takes an afternoon, needs no gas work and no draining, and on a system that is otherwise healthy it is one of the few heating fixes a careful homeowner can do. For why an unbalanced system leaves some rooms cold in the first place, see our radiator balancing guide.

If you want to know why systems go out of balance in the first place, our explainer on what radiator balancing is and why it matters covers the theory. This guide is the method.

What You Need Before You Start

You need a radiator key, a lockshield adjuster or small adjustable spanner, and ideally a clip-on pipe thermometer or an infrared thermometer. A notepad and an hour or two with the heating running are the rest.

• Radiator key for bleeding.
• Lockshield key or spanner. Most lockshields have a plastic cap that pulls or unscrews off, with a small square spindle underneath.
• Thermometer. You can balance by touch, but it is guesswork. Two clip-on pipe thermometers, or one infrared thermometer used on both pipes, make it accurate.
• Old towel under each valve, in case a gland weeps when the spindle turns.

If you are not sure which valve is which, the lockshield is the one with a plain cap and no numbers. A TRV has a numbered head; the lockshield is set once and then left alone.

Step 1: Bleed the System and Open Everything

Balancing only works on radiators that are full of water, so bleed first and check the boiler pressure afterwards. Then open every TRV to its highest setting and every lockshield fully.

Bleed with the heating off and the radiators cool; our guide on which radiator to bleed first gives the order. On a sealed system, check the pressure gauge on the boiler once you have finished. If it has dropped well below where it normally sits, repressurise it or ask an engineer before carrying on.

To open a lockshield fully, turn the spindle anticlockwise until it stops, counting the turns as you go and writing the number down for each radiator. That record lets you put everything back if you change your mind. Opening every TRV fully stops them closing mid-test and confusing the readings.

Step 2: Find the Order the Radiators Heat Up

Let the whole system cool down completely, then switch the heating on and go round the house noting which radiators get warm first. The fastest ones are taking more than their share of the flow.

Feel the pipe going into each radiator, or read it with the thermometer, every few minutes. Write down the order. The radiators that heat first are usually closest to the boiler on the pipe run, which is not always the one physically nearest to it. The slowest are usually at the far end of the house or on the top floor. This list is the order you will balance in.

Step 3: Set the Fastest Radiator First

Start with the radiator that heated first. Close its lockshield completely, then open it about a quarter of a turn and let the system settle for several minutes.

Now measure the temperature of the pipe going into the radiator (the flow) and the pipe coming out of it (the return), a few centimetres from each valve. On a typical gas boiler system, the aim is a difference of around 11 to 12°C between the two. If the difference is much smaller, water is rushing through too fast: close the lockshield a little more. If it is much larger, open it a little. Small movements make a big difference, so adjust in fractions of a turn and wait a few minutes after each change for the reading to settle.

Systems designed for lower temperatures, including those running from a heat pump, are set up for a smaller difference. If you have a heat pump, follow the installer's figures rather than the gas boiler rule of thumb.

Step 4: Work Down the List

Move to the second radiator on your list and repeat, then the third, and so on. Each radiator further along will usually need its lockshield open a little more than the one before.

By the time you reach the last few, some may need the lockshield fully open to reach the target difference. That is normal. The whole point is that the radiators near the boiler are held back so the water has a reason to travel to the far ones. Note the final number of turns open for every radiator, and put the caps back on.

When you have finished, set the TRVs back to the settings you normally use. Then run the heating from cold once more and check that the far radiators now warm up in a sensible time.

When Balancing Will Not Fix It

Balancing shares out the heat the system already has. It cannot fix a radiator full of sludge, a radiator too small for its room, a weak pump or a boiler that cannot keep up. If you have balanced carefully and the same rooms are still cold, one of those is usually why.

• Cold at the bottom, warm at the top: that is usually sludge, not balancing. A flush is the answer, not the lockshield.
• Hot all over but the room is still cold: the radiator may be undersized for the room's heat loss.
• Nothing reaches the top floor however you set the valves: the pump speed, a stuck zone valve or a blocked pipe is more likely.
• Readings will not settle: air still in the system, or a TRV stuck partly closed.

Do not change pump speed settings or anything on the boiler itself. Those belong to an engineer, and any work on a gas appliance must be done by a Gas Safe registered engineer.

Doing It Yourself or Calling an Engineer

A careful homeowner with a thermometer can balance a straightforward house. It is worth calling in an engineer when the system is large, has zones or a mix of radiators and underfloor heating, or when balancing has not solved the problem.

An engineer brings the same method plus the ability to check the pump, bypass, zone valves and whether the radiators suit the rooms. If the system is dirty, balancing it before a flush is wasted effort, because the flush will change the flow again. Our radiator balancing service covers the full rebalance and tells you honestly if the real problem is somewhere else. If you are unsure where to start, get in touch and describe which rooms stay cold.