Video by BOXT. BOXT is a UK heating and boiler platform, and this guide to balancing radiators is a clear, practical walk-through of the lockshield valve method — the approach most plumbers use. Good on the temperature-differential target and on how to tell which radiator is nearest the boiler.
1. Bleed all the radiators first
Before balancing, bleed every radiator to remove any trapped air. Air in the system creates cold spots at the tops of radiators and throws off your temperature readings. Use a bleed key and hold a cloth under the valve while you work — a small amount of water will follow the air out.
Once all the radiators are bled, top up the system pressure at the boiler filling loop if it's dropped below 1 to 1.5 bar. Check the boiler gauge. Most boilers show a warning light or drop pressure indication if it's too low.
2. Turn all lockshield valves fully open
Every radiator has two valves: the thermostatic radiator valve (TRV) on one side, which the occupant adjusts, and the lockshield valve on the other, which controls the flow rate and normally has a plastic cap over it.
Remove each lockshield cap and, using a small adjustable spanner, turn the valve fully anti-clockwise to open it completely. Do this on every radiator in the house. Write down how many turns it takes to close each one — you'll use that as a reference when adjusting.
3. Run the system and note which radiators heat first
Turn the heating on with all thermostats and TRVs fully open. Walk round the house every few minutes and note the order in which radiators become hot. The radiator nearest the boiler will heat up first; the furthest away will heat last.
Make a simple list. This order is your balancing sequence — you'll start with the first to heat and work through to the last.
4. Start balancing the nearest radiator
Once all radiators have reached their maximum temperature, start with the radiator that heated first. This one is getting too much flow at the expense of those further away, so you'll restrict it.
Use a clip-on thermometer or a digital thermometer to measure the temperature of the flow pipe (the pipe bringing hot water in) and the return pipe (the pipe leaving). The target difference is 12°C. If the difference is less than 12°C, the radiator is getting too much flow — close the lockshield valve slightly and check again.
5. Measure the temperature differential
The 12°C flow-to-return differential is the standard target for a well-balanced system. It means the radiator is extracting roughly the right amount of heat from the water as it passes through.
Restrict the lockshield valve a quarter-turn at a time, wait a few minutes for the reading to stabilise, and check again. Patience here is well rewarded. Rushing the adjustments means you overshoot and have to come back.
6. Work through the rest of the radiators
Move to the second radiator in your sequence and repeat. As you restrict earlier radiators, more flow will reach those further away, so later radiators will need only small adjustments or none at all.
The last radiator in the circuit — the one furthest from the boiler — should usually be left fully open. If it's still not reaching temperature after everything else is adjusted, the system may have other issues worth investigating.
7. Do a final walk-round check
Once you've worked through the whole house, wait for the system to reach a steady state and walk round again. Every radiator should feel hot across its full surface within a few minutes of each other. Cold bottoms suggest an air lock (bleed again); cold tops are almost always air too. Persistent cold patches in the middle of a radiator can indicate sludge build-up — a different problem altogether.
Write down your final lockshield settings so you can restore them if you ever need to drain the system for maintenance.
When to call a handyman
Call Richard if the system pressure keeps dropping after you've balanced it (suggests a leak or faulty expansion vessel), if a radiator stays cold despite valve checks and bleeding, or if the boiler is making unusual noises. Balancing sorts heat distribution; it won't fix a failing pump, a blocked heat exchanger, or a combi boiler that's losing pressure. Those need a Gas Safe engineer.
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The Sandwich Handyman can help with radiator balancing, bleeding, and general heating maintenance around the home.
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