Radiators Not Heating

Cold radiators in a house that's otherwise warm normally trace to one loop or one valve, so call and name the rooms that have gone cold.

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When this becomes an emergency

A cast iron radiator stays cool to the touch through a full burn.
The top half of a radiator is cold while the bottom is hot.
There's knocking in the pipes when the system comes up.
An upstairs bedroom has been cold since the piping was last worked on.

What a pro does

A house full of working radiators and one cold one is a distribution problem, not a boiler problem. On hot water systems the classic cause is air riding to the high point of the piping and stalling flow, and a radiator that's hot low and cold high is the signature of it. On steam, the culprit is usually the air vent: it has to let air out for steam to get in, and a vent that has crusted shut keeps that radiator cold no matter how hard the boiler works. Valves are the other half of the list, whether a supply valve partly closed years ago or a steam valve that must be all the way open or all the way shut. Knocking pipes point at condensate sitting in a line that has lost its pitch, which is common in houses that have settled for a hundred and fifty years.

Frequently asked questions

What does a steam radiator's air vent actually do?

It lets the air already inside escape so steam can take its place. Once that vent plugs up, air stays trapped, steam never enters, and the radiator stays cold.

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

On a hot water system that's trapped air sitting above the water line inside the radiator, occupying space the hot water would otherwise fill.

Should a steam radiator valve ever be partly open?

No. Those valves are built to be fully open or fully closed, and leaving one halfway is a reliable way to produce banging and uneven heat in that room.

Can settling in an old house cause heating problems?

It can. Steam piping depends on pitch to drain condensate back, and floors that have moved over a century leave low spots where water collects and gets slammed around.

Radiators Not Heating?

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