Most cold radiators in the first week of the heating season are fixed in ten minutes without tools you do not already own. The trick is doing the checks in the right order, because bleeding a radiator that is actually shut off at the valve achieves nothing, and topping up a system that is leaking only hides the problem for another fortnight.
First, how many radiators are cold?
Put the heating on, turn every thermostatic valve up to its highest number, wait fifteen minutes and walk round the house. Feel each radiator at the top, the middle and the bottom. The pattern you find decides everything that follows.
| What you find | Where to look |
|---|
| One radiator stone cold, the rest fine | That radiator's valves. A closed lockshield or a seized thermostatic valve pin is the usual answer. |
| Warm at the bottom, cold across the top | Trapped air. Bleeding fixes it. |
| Warm at the top, cold along the bottom | Sludge settled in the panel. A system clean, not bleeding. |
| Radiators near the boiler hot, the far ones cool | An unbalanced system. The lockshields need setting so water reaches the end of the circuit. |
| Upstairs cold, downstairs fine | Low system pressure or air collecting at the top of the house. |
| Everything lukewarm, hot water fine or not | The boiler, pump or diverter valve. A Gas Safe heating engineer, not anything on this page. |
Check 1: the gauge on the boiler
Look at the pressure gauge before you touch a radiator. On a sealed system, which is what you have if your boiler has a pressure dial and no tank in the loft, the reading with the heating off and cold should sit between 1.0 and 1.5 bar. Worcester Bosch, Vaillant and Ideal all publish the same band. Below 0.5 bar, Worcester Bosch treat the system as having lost water.
Low pressure starves the highest radiators first, so a cold top floor with a gauge near zero is not a radiator fault at all. Top the system up through the filling loop to 1.0 to 1.5 bar cold, following how to repressurise a boiler, and run the heating again. Many boilers also refuse to fire at all when pressure is too low and show a fault code instead, which is covered on boiler pressure too low.
Make a note of the reading and the date. You will want it later.
Check 2: the valves on the cold radiator
A radiator has two valves at the bottom, one at each end. One is the thermostatic valve (TRV) with the numbered head. The other is the lockshield, usually under a plain plastic cap.
The TRV pin has stuck
This is the classic first-cold-day fault. A TRV that sat on a low setting or closed all summer can seize with the pin pushed down, which holds the valve shut whatever number you turn the head to. Unscrew the head (most come off by hand or with a locking ring) and look at the small metal pin underneath. Press it gently with your thumb. It should spring back up. If it stays down, tap it lightly with the handle of a screwdriver or work it up and down a few times with grips, holding the valve body steady. Once it moves freely, refit the head and the radiator should warm from the valve end within minutes.
The lockshield is closed
Pull the cap off. If someone decorated, changed the flooring or took the radiator off the wall, the lockshield may have been shut and never reopened. Turn the spindle anti-clockwise a turn or so with a spanner or the adjuster, and do not wind it fully open unless you know the system has never been balanced. Lockshield valve covers how to set one and what it means when the spindle keeps turning.
If either valve starts to weep at the spindle once you move it, that is common on valves that have not been touched in years. A turn of the gland nut often stops it. If it does not, see radiator leaking from the valve, and our note on which radiator valves weep.
Check 3: air in the top of the panel
Warm at the bottom and cold across the top means air. Turn the heating off, let it cool, and open the bleed valve at the top corner a quarter to half a turn until the hiss turns into a steady dribble of water. Work the ground floor first and upstairs after, then top the pressure back up, because bleeding takes water out of a sealed system along with the air. The full method is on how to bleed a radiator.
Once a year, at the start of the season, is a normal amount of bleeding. Air that comes back within weeks is not normal, and it is the reason for the last section of this page.
Check 4: sludge and balancing
A radiator hot along the top and cold along the bottom has corrosion debris settled in its lower waterway. Worcester Bosch describe exactly this pattern as a sign the system needs cleaning. Bleeding will not shift it. It needs a chemical clean or a power flush by a heating engineer, followed by a fresh dose of inhibitor.
Radiators that get warm near the boiler and stay cool at the far end of the house have a balancing problem, not a fault. The lockshields near the boiler are open too far and take all the flow. Balancing means closing those down slightly and opening the far ones, then checking again with the heating up to temperature. It takes patience rather than parts.
When a cold radiator is really a leak
A sealed heating system is closed. It should not lose water, and it should not gather air, from one season to the next. So if you topped up the pressure in check 1 and it has fallen again within a week or two, water is leaving the system. Worcester Bosch put normal top-ups at once or twice a year, and say to get the system looked at if you are doing it much more often than that.
The pattern to watch for:
- The boiler gauge drops between top-ups, a little each day or each week.
- The same upstairs radiator goes cold at the top again soon after bleeding, because air is replacing the water that is getting out.
- Nothing under the boiler, around the radiator valves or at the pressure relief pipe outside is wet.
Where the water goes when you cannot see it: a pinhole in a heating pipe under a screeded floor, a joint inside a boxed-in chase, or a pipe in the ceiling void under a bathroom. A warm patch on a floor with no radiator near it is one of the strongest clues. The pressure relief valve discharging outside is another way water leaves without showing indoors; check for a dripping copper pipe on the outside wall near the boiler.
Keep topping up and the problem grows. Every refill brings in fresh oxygenated water, which dilutes the inhibitor and speeds up corrosion inside the radiators. The losses also keep soaking whatever the pipe runs through. Boiler pressure that keeps dropping goes through the causes one by one, including the expansion vessel faults that look like a leak but are not.
What you can do yourself, and when to call us
You do not need anyone to free a TRV pin, open a lockshield, bleed a radiator or top up the pressure. A cold system where the whole house is lukewarm needs a Gas Safe registered heating engineer to look at the boiler and pump, and a sludged system needs a clean. Neither is a leak job, and we will tell you so.
Call us when the pressure will not hold. We isolate and pressure-test the heating to confirm the loss and size it, find it with thermal imaging and, where the circuit can be drained, tracer gas, then repair the pipe or valve and make good the floor, wall or ceiling we opened to reach it. The work is charged at a fixed hourly rate agreed before we attend, and under our no find, no fee terms you do not pay for the time spent looking if we do not find the leak. See radiator and valve leak repair and boiler and heating leak repair for what a visit covers, pricing for the rate, or book an engineer.
