Why is my radiator cold at the top and hot at the bottom?

Trapped air. Air collects at the radiator's high point and displaces water, and bleeding it out through the valve restores full heat. Cold at the bottom instead suggests sludge, a different maintenance conversation.

How often do radiators need bleeding?

Once at the start of each heating season is the classic ritual, plus whenever a radiator gurgles or runs cold on top. Needing to bleed constantly means air is entering somewhere, which has a cause worth finding.

What pressure should a residential boiler run?

Hot-water systems typically run 12 to 15 psi cold, rising a few psi when hot. Pressure at zero means no water where it belongs; pressure near the 30 psi relief setting means the expansion tank or fill valve needs attention.

Do steam radiators get bled like hot-water ones?

No. One-pipe steam radiators vent air automatically through their hissing air vents; there is no bleed key ritual. Steam maintenance centers on the boiler's water level, vents, and radiator pitch instead.

Boiler Systems and Radiators: Basics and Bleeding

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Hydronic heat, a boiler warming water that warms radiators, is the quiet aristocrat of heating systems: even, silent, and durable, with maintenance rituals all its own. The most useful of them is knowing how to bleed radiators, the two-minute cure for the gurgling unit that runs cold on top. This reference covers how the systems work, the hot-water versus steam distinction that changes everything, and the bleeding procedure.

The system in brief

A boiler heats water (it rarely boils it, despite the name), a circulator pump pushes it through supply piping to radiators or baseboard convectors, each surrenders heat to its room, and the cooled water returns for another lap. Around that loop sit the supporting cast: an expansion tank absorbing the water’s swelling as it heats, a pressure-reducing fill valve keeping the loop at roughly 12 to 15 psi, a relief valve (30 psi on residential hot-water systems, the boiler cousin of the water heater’s T&P), and air-management devices, because air is hydronic heating’s one persistent pest. Zone valves or multiple circulators split larger houses into thermostat zones, and modern condensing boilers add the efficiency-and-condensate story familiar from furnaces.

Hot water versus steam: know which you own

The distinction matters because the maintenance differs completely. Hot-water (hydronic) systems run full of water under modest pressure, use circulator pumps, and their radiators have small bleed valves at the top: this article’s bleeding ritual is theirs. Steam systems, common in pre-war housing, boil water and send steam up through large pipes to radiators that hiss through automatic air vents; they run at ounces of pressure, have a sight glass showing boiler water level, and are never bled with a key. Steam’s rituals instead: keep the water level in the sight glass’s happy zone (low-water cutoffs guard the boiler’s life), flush the low-water cutoff on its schedule, keep radiator air vents working (a permanently silent or permanently hissing vent is failed), and pitch one-pipe radiators slightly back toward the supply valve so condensate drains, the cure for the famous steam-hammer banging, a percussion section unrelated to supply-side water hammer but equally curable.

Bleeding a hot-water radiator

Air rises to each radiator’s high point, displaces water, and produces the signature complaint: gurgling, and heat at the bottom with cold on top. The cure:

Run the heat until the system is warm and the circulator has pushed air to the high points, then, preference, switch the circulator or thermostat off so air is not being churned while you work.
At the cold radiator’s top-side bleed valve, fit the radiator key (or flathead, on some), hold a cup and rag beneath.
Open a quarter to half turn: air hisses out. Wait through the hiss.
Close the moment water spits steadily. That radiator is bled.
Repeat house-wide, starting at the lowest floor’s farthest radiator and working up, since air migrates upward.
Check the boiler’s pressure gauge afterward: bleeding removes volume, and a system now below about 12 psi needs water added via the fill valve to spec.

Baseboard systems hide the same ritual at bleed fittings on the loop’s high points or at purge valves near the boiler, where a hose-and-bucket purge does the whole loop at once.

Reading chronic complaints

Bleeding required constantly means air is entering: a waterlogged expansion tank breathing through the fill valve, a weeping connection, or an air separator due for service, a diagnosis worth a technician since repeated fresh water also feeds corrosion and the scale economy inside the boiler. Radiators cold at the bottom point at sludge (magnetite), the argument for system cleaners and magnetic filters at the next service. One zone cold with others fine indicts that zone’s valve or circulator. Pressure at zero, or relief-valve discharge, moves the conversation to the fill valve, expansion tank, and a same-week professional visit. And any boiler’s multi-year professional service, combustion, controls, relief valve, expansion tank charge, is the same longevity math as every other combustion appliance, venting checks included.

The bottom line

Water carries the heat, air is the pest, and the bleed key is the cure: warm the system, hiss the air out top-floor-last, and top the pressure back to spec. Know whether you own hot water or steam before reaching for the key, and let chronic air, sludge-cold bottoms, and wandering pressure buy the professional visit they are asking for.