What is your steam system actually doing?

Air has to leave a radiator before steam can enter it. If the air vent on the side has failed closed, or is blocked with paint or scale, the steam never arrives and the radiator stays cold while everything around it heats.

• Check the valve first. A steam radiator valve is either fully open or fully closed — never part way. A half-open valve traps condensate and is the second commonest cause of this.
• Feel the vent early in a cycle. A working vent puffs air for a while, then closes as steam reaches it.
• Vents are inexpensive consumables and they are sized. Replacing one with whatever was on the van is how a system gets unbalanced.
• If the radiator is cold at the far end only, that is usually pitch rather than venting.

Water hammer at a single radiator is almost always pitch. The radiator, or the pipe feeding it, has settled so that condensate collects where steam is trying to travel, and the two meet violently.

• A one-pipe radiator must slope back towards its valve so condensate drains out the way the steam came in. Shimming the far legs is often the whole repair.
• Check the floor as well as the radiator. Old buildings settle, and a radiator that was pitched correctly in 1930 may not be now.
• If the banging is in the pipe rather than at the radiator, the run below has lost its pitch and that is a different job.
• Nothing about this requires a new boiler. Be wary of anybody who says otherwise on the basis of a bang.

Some hiss is the vent doing its job — that is air leaving. Continuous hissing after the radiator is hot, or water spitting from the vent, is not.

• A vent that has failed open will hiss throughout the cycle and let steam escape into the room.
• Spitting water usually means either a failed vent or wet steam arriving from the boiler, which is a system problem rather than a radiator one.
• An oversized vent on one radiator makes it fill fast and hiss loudly while starving everything downstream.
• Vents are cheap. Replacing one with the correct size is a small job with a disproportionate effect.

A steam radiator has no throttle. It is either receiving steam or it is not, and the only real control is how fast it is allowed to fill — which is set by the vent.

• Do not close the valve part way. It will not moderate the heat and it will cause condensate to trap and bang.
• A smaller vent slows the rate at which that radiator fills, which is the correct way to turn it down.
• Thermostatic valves designed specifically for steam exist and work, and they are fitted on the vent side on a one-pipe system.
• If one radiator roasts while others never warm, the system is unbalanced and the fix is vent sizing across the whole system rather than at this one.

This is the classic one-pipe complaint and it is nearly always main venting. The air in the steam mains has to be pushed out before steam can reach the branches, and if the main vents are missing, undersized or failed, the far end of the system never gets its turn.

• Main venting is the most neglected item in one-pipe steam. Radiator vents get replaced; main vents frequently have not been touched since installation.
• Symptom to look for: the near end of the building heats quickly and the far end only warms at the very end of a long cycle, or not at all.
• A waterlogged return will do the same thing and needs looking at separately.
• Adding boiler capacity does not fix this and is an expensive way to find that out.

Banging along a run rather than at one radiator points at the piping: a main or a return that has lost its pitch, sagged between hangers, or is holding water where steam passes.

• Sagging mains are common in buildings where hangers have corroded or a ceiling has been rebuilt beneath them.
• A wet return that no longer drains will hammer every time the boiler fires.
• Excessive boiler pressure makes all of this worse by pushing water up into the mains — worth checking before any pipework is touched.
• This is real work, but it is pipework and hangers rather than equipment.

Hissing across a whole area usually means main vents that have failed open, or steam pressure high enough to drive water along with it.

• Check the pressure control before anything else. Steam heating runs at ounces, not pounds, and a control set high is the commonest expensive fault in this trade.
• Wet steam — water carried along with the steam — makes every vent on that run spit and every joint hammer.
• Carryover is often a boiler water quality or near-boiler piping problem rather than anything in the run itself.
• If vents on one run have all failed at once, look upstream rather than replacing them one at a time.

One area overheating while the rest is fine is a balance problem, and on one-pipe steam balance means venting. That area is taking its steam first and holding it longest.

• The fix is to slow that section down and speed the cold section up, by vent sizing at the mains and at the radiators.
• Tenants opening windows in January is the visible cost of an unbalanced system, and it is the fuel bill that pays for it.
• Balancing is a survey and a set of vents rather than a capital project, which is why so few people offer it.
• Thermostat location matters too: if the thermostat is in the cold end, the whole building runs to satisfy the worst room.

When nothing heats properly, the question moves to the boiler and its controls rather than to the distribution.

• Water level and the low-water cutoff — a boiler that cannot see its water will not fire, and that device is a safety rather than a nuisance.
• The burner and its safeties, tested with instruments rather than watched.
• Pressure control set correctly, and actually working.
• Main venting across the whole system, if it has never been assessed. A system that cannot get its air out cannot get its steam in anywhere.
• Fuel and combustion on oil or gas, checked as a matter of course before the season.

System-wide hammer that starts every time the boiler fires points at the boiler room rather than at the building.

• Near-boiler piping that does not match what the boiler needs is the classic cause, and it produces wet steam that hammers everywhere at once.
• Dirty boiler water carries over into the mains. Skimming is a real maintenance task and it is skipped constantly.
• Pressure set too high pushes water up into the system. Check it first; it costs nothing.
• A boiler oversized for the connected radiation makes steam faster than the system can take it, which produces the same symptom.

This is the one that is usually a single setting. Steam heating is designed to run at ounces of pressure. A pressure control left at several pounds drives water into the mains, makes every vent spit, and burns fuel to do it.

• Check the pressure control and the gauge. Many systems still carry a gauge that cannot even read the pressures the system should be running at.
• Wet steam from carryover produces the same symptoms and needs skimming and a look at the near-boiler piping.
• Vents failing across the building at the same time is a symptom of this, not a coincidence.
• Correcting it is frequently an afternoon and no capital spend at all, which is why it does not get proposed.

Overheating everywhere is a control question, and in a multi-family building it is usually the most expensive habit in the place.

• Thermostat location decides everything. One sensor in a lobby or in the coldest apartment runs the whole building to satisfy that one spot.
• Cycle length matters more than temperature setting on steam, because a long cycle keeps making heat after the building is satisfied.
• Outdoor reset and proper cycle control exist for steam and are worth discussing where the building runs long and hot.
• Windows open in January is the measurable version of this, and it is fuel you have already paid for leaving the building.