How to Balance Radiators: A Step-by-Step Guide
Some radiators too hot, others barely warm, even after bleeding? How to balance a central heating system with the lockshield valves, how long it takes, and when unbalanced radiators point to a different fault.
To balance radiators, turn the heating off, open every TRV and lockshield valve fully, then turn the heating on and note the order in which the radiators warm up. Starting with the radiator that heats first (usually the one nearest the boiler), close its lockshield valve and open it again by about a quarter turn. Open each later radiator a little more, until the last one to heat up is left fully open. Let the system settle for an hour or so and fine-tune until every radiator warms at roughly the same rate.
Key Takeaways
- Balancing only helps when radiators heat unevenly. Bleed first and rule out cold spots caused by air or sludge.
- It's done on the lockshield valve, the one with a plain plastic cap, not on the TRV head you turn day to day.
- The radiator nearest the boiler gets the least open lockshield. The furthest one is left fully open.
- Expect to spend an afternoon on it, because each change needs time for the system to settle.
- If radiators stay uneven after balancing, the cause is more likely sludge, a weak pump or a stuck valve.
To balance radiators, you adjust the lockshield valve on each one so that hot water is shared evenly around the system, instead of rushing through the radiators nearest the boiler and leaving the far ones lukewarm. In short: open all the valves, note the order the radiators heat up, then partly close the lockshields on the first radiators to heat and leave the last one fully open. It takes an afternoon, needs only a lockshield key or small spanner, and is one of the most useful jobs to do as the heating season starts.
What balancing radiators means
Hot water from the boiler takes the easiest route round the pipework. Radiators close to the boiler, or on short pipe runs, get a strong flow and heat up quickly. Radiators at the end of long runs, often upstairs or in an extension, get what's left. The result is a kitchen that's too hot and a back bedroom that never warms up.
Each radiator has two valves. One is the control valve you use every day, usually a thermostatic radiator valve (TRV) with numbers on it. The other, on the opposite end, is the lockshield valve, normally hidden under a plain plastic cap. The lockshield sets how much water can pass through that radiator. Balancing means setting every lockshield so that each radiator gets its fair share. Our guide to what a lockshield valve does shows how to tell the two apart.
Signs your radiators need balancing
- Radiators near the boiler get hot quickly while those furthest away take much longer or stay lukewarm.
- Upstairs rooms are noticeably colder than downstairs, even with the TRVs turned up.
- A radiator is warm all over but never gets properly hot, and bleeding it releases no air.
- The problem started after new radiators, a new boiler or pipework changes.
Uneven heat has other causes too. A radiator cold at the top and warm at the bottom has air in it. One that's warm at the top and cold at the bottom usually has sludge. One that's cold all over may have a stuck TRV pin. Sort those out first, because balancing won't fix them. Our guide to why a radiator won't heat up covers each pattern.
Before you start
You'll need:
- A lockshield key or an adjustable spanner. Many lockshields turn with a small spanner once the cap is off, and plastic lockshield keys are cheap from DIY shops.
- A radiator bleed key and an old cloth.
- Paper and pen to note the order the radiators heat up.
- Ideally, a clip-on pipe thermometer or an infrared thermometer. Not essential, but it makes the result more accurate.
Bleed every radiator first, with the heating off and the system cool, then check the boiler pressure. On most sealed systems the gauge should read about 1 to 1.5 bar when cold. Bleeding lowers it, so you may need to top it up. See our guide on how to repressurise a boiler.
How to balance radiators step by step
Turn the heating off and let it cool
You need the whole system cold so you can see which radiators warm first when it comes back on.
Open every valve fully
Turn every TRV to its highest setting and open every lockshield fully, anticlockwise. Remove the plastic caps so you can reach them later.
Switch on and note the order
Turn the heating on with the room thermostat set high. Go round the house and write down the order in which each radiator starts to get warm at the pipe where the water comes in. This order is your balancing list.
Let it cool again
Turn the heating off and let the radiators go cold, so the next round starts from the same point.
Set the first radiator
Turn the heating back on. On the first radiator on your list, close the lockshield fully, then open it by about a quarter turn. Wait for the radiator to warm and check it heats across its whole surface.
Work down the list
Open each following radiator's lockshield a little more than the one before: perhaps half a turn for the next ones, then more. The last radiator on the list stays fully open.
Let it settle and fine-tune
Leave the heating running for an hour, then go round again. If a radiator is still slow, open its lockshield a little more. If one near the boiler is still scorching, close it slightly. Small changes work best.
Refit the caps and reset the TRVs
Once you're happy, put the lockshield caps back on so no one adjusts them by mistake, then set the TRVs back to the levels you want in each room.
Using a thermometer for a better result
Balancing by feel works in most homes. For a more precise result, measure the temperature of the pipe going into each radiator (the flow) and the pipe coming out (the return). On a traditional system, engineers commonly aim for a difference of roughly 10 to 12°C between the two on every radiator. If the difference is much smaller, water is rushing through too fast, so close the lockshield slightly. If it's much bigger, the radiator is starved, so open it a little.
Some modern condensing boiler setups are designed to run with a larger difference between flow and return, so treat these figures as a guide rather than a rule. The aim is that every radiator shows a similar difference, not that they all hit one number.
When balancing doesn't fix it
If you've balanced carefully and some radiators are still cold, the problem is probably something balancing can't reach:
- Sludge. Magnetite sludge settles in the bottom of radiators and narrows pipes, so water can't get through however you set the valves. Radiators cold at the bottom are the classic sign. See signs your radiators need a power flush.
- A stuck TRV or lockshield. If turning a valve makes no difference at all, the pin or valve may be seized. Our guide to a stuck TRV explains how to free one.
- A weak or wrongly set pump. If the whole house heats slowly, the pump may be failing or set too low for the size of the system.
- Undersized radiators. A radiator that is too small for its room will never keep it warm, however well balanced the system is.
- Pressure problems. If the boiler pressure keeps dropping, there's a leak somewhere that needs finding, not balancing.
When to call a heating engineer
Balancing is a reasonable DIY job, but it can be slow and fiddly, especially in a large house or a system with a lot of radiators. Call an engineer if valves won't turn, if a lockshield starts weeping when you move it, if radiators stay cold after balancing and bleeding, or if you'd rather have it done once and done properly.
Our radiator balancing service covers the full system, and if a radiator or valve needs replacing we can do that on the same visit. For wider problems see radiator repair and central heating repair. Our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before any work starts.
Some rooms never warming up?
Tell us which radiators stay cool and what you've tried. We'll explain the likely cause and confirm the price before starting.
📞 Call 020 7870 3200Or see our radiator balancing and pricing.
Frequently Asked Questions
Which radiator do you start with when balancing?
Start with the radiator that gets hot first after you switch the heating on. In most homes that's the one closest to the boiler. It gets the least open lockshield valve, and each radiator that heats up later is opened a little more, with the last one left fully open.
Should the heating be on or off when balancing radiators?
Start with it off and the system cold, so you can open all the valves and see clearly which radiators warm first once you switch it on. The adjustments themselves are made with the heating running, because you need to see the effect each change has.
How many turns should a lockshield valve be open?
There's no fixed number. As a starting point, the radiator nearest the boiler is often opened about a quarter turn from closed, the next ones half a turn, and so on, with the furthest radiator fully open. You then fine-tune by feel or with a thermometer.
How long does it take to balance radiators?
Allow a few hours for a typical house. The work on each valve is quick, but after each change the system needs time to settle before you can judge the result. Larger homes or systems with many radiators take longer.
Do I need to balance radiators after a power flush or a new boiler?
It's worth checking. A power flush, a new boiler, a new pump or new radiators all change how water moves round the system, so settings that worked before may no longer suit. A good engineer will usually check the balance as part of that work.