Diagnosing and fixing a radiator
Where it is cold tells you what is wrong. That one observation picks the tool and saves taking anything apart.
Where it is cold tells you what is wrong
Cold at the top and warm at the bottom is trapped air, and a bleed key fixes it in a minute. Cold at the bottom and warm at the top is sludge settled in the base, which needs the radiator flushed or the system power flushed.
Cold all over while the pipe feeding it is hot means the valve is not opening, usually a seized pin under a thermostatic head. Cold all over with a cold pipe means the flow is not reaching it, which is a balancing or a circulation problem rather than a radiator fault.
- Cold top, warm bottom
- Air. Bleed it. If it fills with air again within days, there is a leak drawing air in somewhere.
- Cold bottom, warm top
- Sludge. Flushing the radiator or the system, not a valve or a bleed.
- Cold all over, hot pipe
- The valve is not opening. Usually a stuck pin under the thermostatic head.
- Cold all over, cold pipe
- Not a radiator fault. Balancing, circulation or a closed valve further back.
Bleeding, and why it keeps needing doing
Bleeding releases air trapped at the top of a radiator. Open the valve a quarter turn with a key, wait for the hiss to become water, then close it. A cloth and a cup underneath, because what comes out is black and it stains.
A radiator that needs bleeding once a year is normal. One that needs it every few weeks is drawing air in, either through a weeping joint, an open vent on a feed and expansion tank, or corrosion producing hydrogen. That is a system problem and bleeding only treats the symptom.
- Quarter turn only
- Enough to release air. Fully out and the spindle comes with it, followed by the system contents.
- Cloth and cup
- The water is black with iron oxide and it marks carpet and paint permanently.
- Heating off, then on
- Bleed with the system off and cooled so air has collected at the top rather than circulating.
- Repeat air means a leak
- Weekly bleeding is a diagnosis, not a chore. Something is letting air in or generating it.
Stuck valves and seized pins
A thermostatic valve works by a head pressing a pin down against a spring. Over a summer of being fully open, the pin corrodes in place, and in autumn the radiator stays cold or stays hot depending which way it stuck.
Take the head off and the pin is visible. Free it with pliers, working it up and down and adding penetrating oil, and it usually recovers. If not, the valve body can be replaced without draining the whole system by fitting the new one with the radiator isolated at both ends.
- Take the head off first
- The fault is nearly always the pin under it rather than the head itself.
- Work it, do not force it
- Up and down with pliers and a little oil. Levering hard snaps the pin off inside the body.
- Exercise valves in summer
- Turning every thermostatic head fully closed and open once in the summer prevents most of these.
- Lockshield is not a control
- The plain valve at the other end sets balance. Opening it fully to make a radiator hotter unbalances the system.
Leaks and what each one means
A weep at the union nut is usually the olive or the nut, and both are repairable with the radiator isolated. A weep from the valve spindle is the gland, which some valves allow to be repacked. A drip from the bleed valve means the spindle seat is damaged, and a whole new bleed plug is pennies.
A wet patch on the panel itself is different. That is a corrosion pinhole from the inside, there is nothing to tighten, and sealant products are a temporary measure at best. That radiator is due for replacement.
- At the nut
- Olive or nut. Isolate both valves, undo, replace the olive, refit. A small job.
- At the spindle
- The gland. Some valves repack with PTFE, others need a new valve body.
- At the bleed plug
- A damaged seat. Replace the whole plug, which costs almost nothing.
- On the panel
- Corrosion from inside. Nothing to tighten, and the radiator needs replacing rather than patching.
Draining and refilling properly
Most radiator work does not need the system drained, because both valves can be closed to isolate that radiator alone. Count and note the turns on the lockshield before closing it, so balance can be restored exactly afterwards.
Where the system does have to be drained, refill it slowly with the highest bleed valve open, then bleed every radiator from the lowest floor up. Add inhibitor before you finish, because a system refilled with plain water starts corroding immediately.
- Isolate rather than drain
- Both valves closed and the radiator alone comes off. Note the lockshield turns first.
- Refill slowly
- Air out through the highest vent. A fast fill traps air in every horizontal run.
- Bleed lowest floor first
- Working upward pushes air ahead of the water rather than trapping it behind.
- Inhibitor every time
- Fresh water plus steel plus air makes sludge. Inhibitor is the difference between ten years and three.
Frequently asked questions
- What tools do I need to fix a radiator?
- A bleed key, a radiator spanner, two adjustable spanners, water pump pliers, PTFE tape, a cloth and a cup, and a hose for draining. That covers bleeding, freeing a valve, and taking a radiator off to deal with a leaking union or to flush it.
- Why is my radiator cold at the top but hot at the bottom?
- Trapped air, which collects at the highest point. Turn the heating off, let it cool, then open the bleed valve a quarter turn with a key until the hiss becomes water and close it. If it needs doing again within weeks, air is getting in and that needs finding.
- Why is my radiator cold at the bottom but hot at the top?
- Sludge, which is iron oxide that has settled in the base and blocked the flow through the lower channels. Bleeding will not help. The radiator needs taking off and flushing out, or the whole system needs power flushing and fresh inhibitor.
- How do I fix a radiator valve that is stuck?
- Take the thermostatic head off and look at the pin underneath. It corrodes in place over the summer. Work it up and down with pliers with a little penetrating oil and it usually frees. Do not lever hard, because the pin snaps off inside the valve body.
- Can I take a radiator off without draining the system?
- Yes, in most cases. Close the thermostatic valve and the lockshield at the other end, counting and noting the lockshield turns first so balance can be restored. Then undo the unions with a bowl underneath. Only a small amount of water comes out.
- What is a lockshield valve for?
- It balances the system by restricting flow, so radiators near the pump do not take all the heat and leave distant ones cold. It is set once and left. Opening it fully to make one radiator hotter takes flow away from the rest of the house.
- My radiator is leaking from the panel, can it be repaired?
- Not properly. A wet patch on the panel itself is a corrosion pinhole from the inside, so there is nothing to tighten and no gasket to replace. Sealant additives may slow it briefly. The radiator needs replacing, and its presence suggests the system water needs attention.
- Do I need to add inhibitor after draining a radiator?
- Yes, if you have drained any real volume. Fresh water with steel and oxygen produces the iron oxide sludge that blocks radiators and wears pumps. Inhibitor is inexpensive and it is the single biggest factor in how long a system lasts.
