See all of the symptoms guides.
Short answer
Three patterns, three causes. Cold at the top and warm at the bottom means trapped air, and bleeding the radiator fixes it. Warm at the top and cold at the bottom means sludge or debris settled inside. Cold all over, while other radiators are hot, points at a closed valve, a failed zone valve, or a circulation problem.
Those three patterns are genuinely diagnostic, which makes hydronic heating easier to describe over the phone than most systems. Before calling anyone, note which of the three you have and whether it is one radiator or several. One is usually local to that radiator or its valve, while several on the same loop points back toward the zone or the circulator serving them.
Read the radiator with your hand
Your hand is the first tool. Run the heat until the system is up to temperature, then feel across the whole emitter: top, middle, bottom, and both ends. The pattern is the diagnosis, and it takes less than a minute.
| Where it is cold | Cause | Who fixes it |
|---|---|---|
| Top cold, bottom hot | Air trapped at the high point | Homeowner, bleed it |
| Bottom cold, top hot | Sludge or debris settled inside | Technician, flush or replace |
| Cold at one end only | Poor flow through that emitter | Balancing, or a partly closed valve |
| Entirely cold, others hot | Closed valve, failed zone valve or actuator | Check the valve, then a technician |
| Whole zone cold | Zone valve, thermostat, or circulator | Technician |
| Everything lukewarm | Boiler temperature or system-wide flow | Technician |
Bleeding a radiator, which is genuinely a homeowner job
- Run the heat, then turn it off. Bring the system up so you can identify which emitters are affected, then switch the heating off and let it settle for a few minutes so the water is still and air can gather at the top.
- Find the bleed valve. It is a small square or slotted fitting at the top of one end of the radiator. A radiator bleed key costs very little. Some take a flathead screwdriver.
- Hold a cloth and a cup underneath. Water will come out, and it will be dirty. Protect the floor.
- Open it slowly. Turn a quarter to half turn counter-clockwise. You should hear air hissing out. Do not open it fully: it can come out.
- Close it as soon as water runs steadily. When the hiss stops and a steady dribble of water appears, the air is out. Close the valve firmly but without forcing it.
- Check the boiler pressure afterwards. Bleeding removes water as well as air, so system pressure falls. Read the gauge and top up to the normal cold pressure if needed. Bleeding several radiators without checking pressure is a common way to end up with a system that will not circulate.
Bleed with the system off and cooled slightly, not while it is hot and running. The water in a hot system is above boiling point at system pressure and can scald when released.
When bleeding does not help
If air keeps returning to the same radiator within weeks, something is introducing it, and repeating the bleed treats the symptom. The usual sources are a leak drawing air in on the suction side, an automatic air vent that has failed, or a system that is being topped up repeatedly, which brings dissolved air in with the water.
That last one connects two symptoms that people usually treat as unrelated: a boiler that keeps losing pressure and a radiator that keeps needing bleeding are frequently the same underlying leak, seen from two ends.
Why one room is cold in a Vermont house, when nothing is wrong with the heating
Before assuming a heating fault, it is worth ruling out the building, because in this region the room is often the problem, not the emitter. A room over an unheated garage, in an addition built to a different standard, on the north-west corner taking the prevailing winter wind, or with an uninsulated band joist beneath it will run cold with a perfectly functioning radiator.
The tell is consistency. A room that has always been the cold one, every winter, in the same conditions, has a heat-loss problem, not a heating problem, and the fix is insulation and air sealing rather than a bigger emitter. A room that used to be fine and now is not has a heating fault. That distinction decides whether you should be calling a heating contractor or an energy auditor, and Efficiency Vermont's programmes exist for the second case.
Common questions
What does it mean when a radiator is cold at the top?
Air has collected at the highest point of the emitter and is displacing the hot water that should be there. Air rises within the radiator, water cannot get past it, and the top section stops transferring heat while the bottom continues to work. It is the most common and most easily fixed heating complaint there is, and it accumulates naturally in any system where water has been added, so it is not a sign that anything is broken unless it keeps recurring.
Can I bleed radiators myself?
Yes: it is one of the genuinely simple homeowner heating jobs, needing only a bleed key and a cloth. The two details people skip are doing it with the system off and slightly cooled instead of hot, and checking boiler pressure afterwards, because bleeding lowers it. If you bleed several radiators in one go and do not top the system back up, you can leave it unable to circulate, which turns a small job into a no-heat call.
What happens if I leave a radiator that is cold at the top?
The room stays under-heated and the rest of the system works harder to compensate, so you pay for heat you are not receiving. There is no acute damage from trapped air alone. The reason to deal with it anyway is diagnostic instead of mechanical: air that keeps returning indicates a leak or a failed vent somewhere, and treating the recurrence as normal means the underlying fault goes unexamined for years.
How often should radiators need bleeding?
In a healthy sealed system, rarely: perhaps once after any work that let air in, and otherwise not at all for years. Needing to bleed the same radiator every heating season is a signal, not a chore: something is admitting air, most often a slow leak, a failed automatic air vent, or repeated topping up of a system that is losing pressure. If that is your pattern, the more useful question is where the water is going.
How do I know if it is the radiator or the zone?
Feel the others. One cold emitter with the rest of the house warm is a problem at that emitter or its valve. A whole floor or wing cold points at a zone valve, its actuator, the thermostat for that zone, or a circulator. You can often hear a zone valve click and feel its body warm when it opens, which is a useful thing to check before calling. Knowing whether the zone is being called at all narrows the visit considerably.
Is a cold room the landlord's responsibility in Vermont?
If the heating system is not delivering adequate heat, yes: Vermont's warranty of habitability covers the heating facilities, and a rental has to be capable of being heated adequately. Where it gets less clear is a room that is cold because of the building's insulation rather than a broken system, which is a harder argument but not automatically a losing one if the unit cannot be kept reasonably warm. Document indoor temperatures with dates and report in writing. A temperature log is the most useful evidence a tenant can produce.