Radiator Cold at the Top? What It Means and the Fix
One radiator cold along the top is air, and a radiator key fixes it. Several, or all the upstairs ones, usually means the system pressure is low, and bleeding them first makes it worse.
A radiator cold at the top and warm at the bottom has air trapped in it. Air rises, so it collects above the hot water and keeps it from filling the panel. If it is one radiator, bleeding it with a radiator key fixes it in a few minutes. If it is several radiators, or every radiator upstairs, look at the boiler's pressure gauge before you bleed anything. Low system pressure is the more likely cause, and bleeding lowers it further.
Key takeaways
- One radiator cold at the top is air in that radiator. Bleed it.
- Several radiators cold at the top, especially upstairs, usually means low system pressure. Check the boiler gauge first.
- Most sealed systems run at about 1 to 1.5 bar when cold. Below 1 bar, top it up before bleeding.
- Every bleed lets water out of a sealed system, so the pressure drops each time.
- Topping up the pressure every few weeks means a leak or a faulty part. That is an engineer's job.
One radiator cold at the top, or several?
Where the cold patch sits is the clue. Feel the radiator across its whole face after the heating has run for half an hour.
| Pattern | Likely cause | First step |
|---|---|---|
| One radiator cold along the top | Air trapped in that radiator | Bleed it |
| Several radiators cold at the top, mostly upstairs | Low pressure in a sealed system | Check the gauge, top up, then bleed |
| Cold at the top again within days of bleeding | Air getting into the system | Engineer |
| Warm at the top, cold at the bottom | Sludge, not air | Flush the radiator |
| Cold all over, others hot | Valve stuck or shut | Free the valve pin |
A cold top and a gurgle often arrive together, because both come from the same pocket of air. If yours is making noises too, what each heating noise means covers the rest.
Check the pressure gauge before you bleed
Most boilers installed in the last few decades run a sealed system. The water is held at a set pressure, shown on a dial or a display on the front or underneath the boiler. That pressure is what lifts water to the top of the highest radiator.
When it falls too low, the upstairs radiators lose water at the top first, and air takes its place. Bleeding them lets out the air, but it also lets out water, so the pressure falls further. Bleed every radiator in the house on a low-pressure system and the boiler can lock out entirely.
So start at the boiler. With the heating cold:
- 1 to 1.5 bar: pressure is fine. The cold top is local air, so bleed the radiator.
- Below 1 bar: top the pressure up first, then bleed, then check the gauge again.
- Above about 2.5 bar: too high. Do not add water. Bleeding a radiator lets a little out.
Vaillant's guide to repressurising gives 1 to 1.5 bar for most of its boilers and warns against going above 2.5 bar. Other makes are similar, but the figure in your boiler's manual is the one that counts. An older system fed from a cold water tank in the loft is not sealed, and usually has no pressure gauge. If yours works that way, this section does not apply.
How to top up a sealed system
Vaillant itself recommends having an installer do this. It is a common household job, though, and its own guide sets out the steps for a boiler with an external filling loop, a braided hose with a valve at each end, usually under the boiler.
- Turn the boiler off and let it cool. The pipework near the boiler runs hot.
- Find the filling loop. Some boilers keep it behind a panel or use a built-in filling key instead. If you cannot see one, check the manual before going further.
- Open the valves slowly. Turn them anticlockwise and watch the gauge. You will hear water running in.
- Close them at 1 to 1.5 bar. Turn both valves clockwise until they are fully shut. Go slowly near the end, because the needle moves faster than you expect.
- Disconnect the loop if it is a detachable one.
- Turn the boiler back on and let it heat fully. Check the pressure holds and the valves are not dripping.
How to bleed a radiator that is cold at the top
With the pressure right, bleed the cold radiators with the heating off and cooled for half an hour.
- Put a cloth under the bleed valve at the top corner of the radiator.
- Turn the radiator key anticlockwise, no more than a quarter turn. Air hisses out.
- Close the valve when water appears. A steady trickle with no spitting means the air is out.
- Check the gauge again. If it has dropped below 1 bar, top it up.
- Run the heating for an hour, then feel the radiator. The top should now heat with the rest of the panel.
Mistakes that bring the cold top back
- Bleeding every radiator before checking the pressure. On a low system it makes the problem worse and can lock the boiler out.
- Overfilling when topping up. Above about 2.5 bar the relief valve may open and dump water outside through a pipe, and you lose the pressure you added.
- Leaving the filling loop open. The system keeps filling and the pressure climbs. Check both valves are fully closed.
- Bleeding with the heating on. The pump keeps mixing air into the water instead of letting it settle at the top.
- Topping up every few weeks without asking why. A system that keeps losing pressure has a leak, a weeping valve or a fault in the expansion vessel, and each top-up brings in fresh water and fresh air.
When it keeps coming back
If the same radiator is cold at the top again within days, or the pressure needs topping up more than a couple of times a season, call a Gas Safe registered engineer. Air is getting in or water is getting out, and finding where usually means testing parts inside the boiler casing, which is not a household job.
While you wait for them, a radiator that is cold at the top is still working for the rest of its surface, and closing curtains and sealing the drafts around the windows keeps more of that heat in the room.
Frequently asked questions
Why is my radiator cold at the top and hot at the bottom?
Air has collected at the top of the panel. Air rises, so it sits above the hot water and stops it filling the radiator. Bleed it with a radiator key while the heating is off, close the valve when water appears, then check the boiler pressure gauge, because letting air out of a sealed system lowers the pressure.
Why are all my upstairs radiators cold at the top?
Usually because the system pressure is low. On a sealed system the water may not reach the top of the highest radiators, and air fills the space. Check the gauge on the boiler first. If it reads below 1 bar, top it up to the level in your manual before bleeding anything.
What should my boiler pressure be?
For most sealed systems, about 1 to 1.5 bar with the heating cold, and a little higher when it is hot. Vaillant gives 1 to 1.5 bar for most of its boilers and warns against going above 2.5 bar. Always check the figure in your own boiler's manual.
My radiator is still cold at the top after bleeding. What now?
Check the pressure gauge and top up if it has dropped, then run the heating for an hour and bleed it again. If air comes back within days, or you have to top up the pressure regularly, something is letting air in or water out, and a heating engineer needs to find it.
