Craft Quantities
Plan & Takeoffs

What to check, and in what order

A radiator is cold at the top, cold at the bottom, or cold all over

Where it is cold identifies what is in it. Cold at the TOP is air, which rises. Cold at the BOTTOM is sludge, which settles. Cold ALL OVER, with the others hot, is a flow or valve problem. Three different faults with three different fixes, separated by one observation.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

A radiator heats by circulating hot water through itself, so a cold area is an area the hot water is not reaching — and what is displacing it depends on where the cold area is.

AIR collects at the high point, because it is lighter than water. A radiator cold across its top with a hot bottom has air trapped in it, and that is what bleeding a radiator removes.

SLUDGE — iron oxide and debris from the system — settles at the low point, because it is heavier. A radiator cold across its bottom with a hot top has a layer of sediment the water is passing over rather than through, and bleeding does nothing for it.

A radiator cold ALL OVER while the others are hot has a flow problem rather than a contents problem: a closed or seized valve, a failed thermostatic head, or a system so out of balance that this one is being starved.

The pattern across the WHOLE system matters too. One cold radiator is a local fault; the ones furthest from the boiler being cold is a balancing or pump problem; all of them cold at the top is air throughout, which usually means air is being drawn in somewhere rather than simply left over from filling.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. Air trapped at the top

    Most common

    Cold across the top with a distinctly hot bottom, often with a gurgling sound, and frequently in the highest radiators or the ones furthest along the circuit.

    How to check: Open the bleed valve with a key and a cloth. Air hissing out before water arrives confirms it.

  2. Sludge settled at the bottom

    Most common

    Cold across the BOTTOM with a hot top — the opposite pattern to air — usually in an older system, often worst in the radiators at the low points.

    How to check: Feel the radiator across its width at the bottom and at the top. Bleeding a radiator with this pattern releases nothing and changes nothing.

  3. A thermostatic valve stuck closed

    Common

    The whole radiator cold while others are hot. Common after a summer with the valve left closed, and often on a radiator nobody uses.

    How to check: Take the head off the valve and see whether the pin beneath moves freely. A pin stuck down holds the valve shut regardless of the setting.

  4. The system is out of balance

    Common

    A pattern rather than one radiator: those nearest the boiler hot and those furthest cool, consistently, with everything working but unevenly.

    How to check: Note which radiators are affected in relation to the boiler. A distance-related pattern is balancing, not a fault.

  5. A lockshield valve closed or nearly closed

    Common

    One radiator cold or cool while its thermostatic valve is fully open and its neighbours are fine.

    How to check: The valve at the opposite end from the adjustable one, under its cap. It sets the flow through that radiator and can be closed.

  6. Pump or circulation fault

    Less common

    Many radiators poor at once, slow warm-up throughout, and often boiler behaviour that has changed — cycling, or reaching temperature quickly and stopping.

    How to check: Whether the problem is system-wide rather than local. That moves it away from the radiators entirely.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    It is the whole diagnosis for the two commonest causes, and it takes seconds. Top cold means air; bottom cold means sludge; and the two remedies have nothing in common.

  2. 2

    One cold radiator is a local fault; a pattern related to distance from the boiler is balancing; everything cold is the pump or the boiler. The scope changes who needs to do what.

  3. 3

    Bleeding is the right first action for air and a waste of time for sludge. Doing it on a bottom-cold radiator and concluding nothing is wrong is a common dead end.

  4. 4

    A radiator can be cold simply because it is turned off at the end nobody looks at, and a stuck thermostatic pin looks identical to a failed radiator from the outside.

  5. 5

    Air that keeps coming back is being drawn in somewhere, which is a different problem from air left over after filling — and repeated bleeding will not fix it.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

Hydronic Circulator Pump Head Loss Calculator →Hydronic Closed-Loop Expansion Tank Sizing Calculator →BTU/hr to kW Calculator →

When to stop and get somebody in

  • Where air keeps returning after bleeding, since it is being drawn into a sealed system somewhere and repeated bleeding only loses more water.
  • Where the system pressure falls repeatedly and has to be topped up, which indicates a leak or a failed expansion vessel rather than a radiator fault.
  • Where sludge is the answer across several radiators — a power flush or a chemical clean is a system operation, and the corrosion behind it will continue otherwise.
  • Anything involving the boiler itself, its controls or its flue, which in most jurisdictions is restricted work for gas and is not a DIY diagnosis in any case.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.