Quick answer
One radiator cold while the others are hot is a local fault, not a boiler fault. Cold at the top and warm at the bottom means trapped air, which bleeding fixes. Cold at the bottom means sludge. Cold all over points to a stuck valve or a seized thermostatic valve pin, or, on steam, a blocked air vent. The radiator farthest from the boiler may simply be starved of flow.
When one radiator is cold and the others are hot, the boiler, the pump and the thermostat are all proven good, because they are heating the rest of the house. The fault is in that radiator, its valve, or the share of flow it receives. Where the radiator is cold tells you which: run a hand over the top, the bottom and the pipe at each end.
Cold at the top is trapped air; cold at the bottom is sludge
A hot-water radiator that is warm along the bottom and cold across the top is holding a pocket of air. Air rises to the highest point of the radiator and water cannot fill the space, so the top sections never heat. Releasing it takes a couple of minutes, and the steps are in how to bleed a radiator.
The reverse pattern, hot across the top and cold along the bottom, often worst in the middle, is sludge. Black iron oxide from decades of slow corrosion settles in the lowest part of the radiator and blocks the waterway. Bleeding does nothing for it. The cure is to flush that radiator or the whole system and dose it with inhibitor, which is what a hydronic system flush does.
Cold all over: a stuck radiator valve, a seized TRV pin or a blocked steam vent
A radiator that is cold from end to end while the pipe feeding it is hot right up to the valve has a valve that is not passing water. On old cast-iron radiators the disc can drop off the stem of a hand-wheel valve, so the handle turns and nothing opens. Check as well that nobody closed the valve during painting or flooring work and forgot it.
A thermostatic radiator valve (TRV) fails in its own way. Under the numbered head is a small spring-loaded pin; when the head calls for heat the pin rises and lets water in. After a summer pressed shut, the pin can seize in the down position, and the radiator stays cold at any setting. With the head unscrewed, a free pin springs back when pressed a couple of millimetres; one that stays down needs freeing or a new valve body, and it should never be pulled out or struck.
On a steam system, look at the air vent: a one-pipe steam radiator heats only if the air inside it can get out. The small bullet- or disc-shaped vent on the side, opposite the supply valve, lets air escape and closes when steam reaches it. A vent that is stuck shut, clogged with scale or sealed under layers of paint traps the air, and steam cannot enter a radiator that is already full. The supply valve on a one-pipe radiator must also be fully open, never throttled.
To tell which system you have, count the pipes: a steam radiator of this kind has one pipe and an air vent partway up the far side, while a hot-water radiator has two pipes and a small bleed valve at a top corner. Steam survives mostly in pre-war houses and older walk-up apartment buildings in Hamilton and Toronto.
Balancing: why the radiator farthest from the boiler is starved
Water takes the easiest path. Radiators close to the boiler, with their valves wide open, can take most of the flow and leave the last one on the run with a trickle that has already given up its heat. That radiator is lukewarm on mild days and cold on the coldest ones, and it is usually in a back bedroom, an addition or the room over the garage.
The fix is balancing, not a bigger boiler. The valves on the radiators nearest the boiler, or the balancing valves on their returns where those are fitted, are throttled until every radiator heats at a similar rate. It is slow, methodical work with a thermometer on the supply and return pipe of each radiator, booked as radiator bleeding and balancing.
Check this yourself first, and when to book a visit
- Radiator valve fully open: turn the hand wheel counter-clockwise until it stops.
- TRV head set to its highest number, then feel whether the pipe after the valve warms up.
- Top versus bottom: a cold top means bleed it; a cold bottom means sludge.
- Furniture, long curtains or a radiator cover blocking the air around it.
- Boiler pressure gauge: an upstairs radiator is the first to go cold when the cold reading slips under 12 psi.
Book a service visit when one of the following is true.
- The radiator needs bleeding again every few weeks, which means air is entering through a leak or a failing expansion tank.
- The bleed valve releases neither air nor water.
- Water is weeping from the valve stem, the union or between the radiator's sections.
- Several radiators on the same floor are cold; that is a zone or pressure fault, covered under no heat upstairs with a boiler.
A single cold radiator is usually a modest call: the $90–$150 diagnostic visit, applied to the work if you proceed, plus a valve, a vent or a flush priced in writing before anything is taken apart. A seized TRV body or a snapped valve stem means draining part of the system, and that labour, more than the part, sets the price.
ZK Mechanical works on cast-iron radiators, panel radiators and fin-tube baseboard throughout Burlington, Hamilton, Oakville and the west side of Toronto. To get one cold room sorted, start with boiler repair and hydronic service or tell us which radiator and which end is cold.