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AFUE DEFINITION, RATINGS
AGE of HEATERS, BOILERS, FURNACES
BACKDRAFTING HEATING EQUIPMENT
BACKFLOW PREVENTER VALVE, HEATING SYS
BANGING HEATING PIPES RADIATORS
BOILER CONTROLS & SWITCHES
BOILER LEAKS CORROSION STAINS
BOILER NOISE SMOKE ODORS
BOILER OPERATING PROBLEMS
BOOKSTORE - InspectAPedia
BTU USAGE MONITORS
CHEMICAL TREATMENTS for BOILERS
CHIMNEY INSPECTION DIAGNOSIS REPAIR
CIRCULATOR PUMPS & RELAYS
DEFINITION of HEATING & COOLING TERMS
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DRAFT MEASUREMENT, CHIMNEYS & FLUES
DRAFT REGULATOR, DAMPER, BOOSTER
FILTERS, OIL on HEATING EQUIPMENT
FIRE SAFETY CONTROLS
FLOODED HEATING EQUIPMENT REPAIR
FLUE VENT CONNECTORS
FREEZE-PROOF A BUILDING
FUEL OIL TYPES & CHARACTERISTICS
FUEL UNIT, HEATING OIL PUMPS
GAS BURNER FLAME & NOISE DEFECTS
GAS PIPING, VALVES, CONTROLS
GAUGES ON HEATING EQUIPMENT
HEATING COST SAVINGS METHODS
HEATING LOSS DIAGNOSIS-BOILERS
HEATING OIL PIPING TROUBLES
HEATING SYSTEM INSPECTION GUIDE
HEATING SYSTEM NOISES
HIGH EFFICIENCY BOILERS/FURNACES
GAS LP & NATURAL GAS SAFETY HAZARDS
MANUALS & PARTS GUIDES - HVAC
MOTOR OVERLOAD RESET SWITCH
NO HEAT - BOILER
NOISE, HEATING SYSTEMS
ODORS FROM HEATING SYSTEMS
OIL PUMP FUEL UNIT
PUFFBACKS, OIL BURNER
RELIEF VALVE, TP VALVE, STEAM BOILER
RESET SWITCH, HEATER PRIMARY CONTROL
RESET SWITCH, ELECTRIC MOTOR
RESET SWITCH, STACK RELAY
SAFETY, HEATING INSPECTION
SAFETY RECALLS CHIMNEYS VENTS HEATERS
SOOT on OIL FIRED HEATING EQUIPMENT
SPILL SWITCH, FLUE GAS DETECTOR S
STACK RELAY SWITCHES
STEAM HEATING SYSTEMS
THERMOSTATS, HEATING / COOLING
VIDEO GUIDES: HEATING SYSTEMS
ZONE VALVES, HEATING
Steam vents on heating systems: here we provide an inspection and repair guide to the vents found on radiators and piping used with Steam Heating Systems. We explain one pipe and two pipe steam heating vents and how they work, and the article includes diagnostic and repair advice for hissing sounds at steam radiators, steam vents that don't close when they should, steam vents that don't open when they should, noisy steam vents, and steam vent leaking or spitting.
Separately at STEAM VENT TYPES, SELECTION we explain more about the different venting requirements for 1 pipe vs. 2 pipe steam systems, we describe the extra controls and vents needed at two-pipe steam systems. And at STEAM VENTS, AUTOMATIC we describe just about every type of automatic steam vents used on residential and most commercial systems.
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Our photo (above left) shows a common model of automatic air bleed valve used on residential steam heat radiators.
[Click to enlarge any image]
This article series answers most questions about central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
At the start of a heating cycle as steam rises in the building steam pipes and begins to enter the heating radiator, air in the cool radiator is pushed out through this valve.
As the radiator gets warm, then hot due to the rising steam, a mechanical thermostatically operated valve inside of the air vent, also called a steam vent, closes the vent so that steam does not continue to escape.
When steam is first rising in a one pipe or two pipe steam heating system, the steam heating radiator will be cool as will be the steam vent. Our photo (above left) shows what looks like a Hoffman 1Afloat-type radiator steam vent. This is a residential low-pressure (1.5 psig) steam vent. Similar models include the Hoffman Model 40 (6 psig) and model 70A (11 psig).
Two Pipe Steam Heat
In a two-pipe steam heat system, steam rises through the supply side of the radiator (where the radiator valve is found) and may push air and later condensate out through the return or condensate drain side of the radiator.
One-Pipe Steam Heat
On one-pipe steam heat systems the radiator valve is normally fully open (for the radiator to operate properly) or fully shut (no heat); you may find that some one-pipe steam heating system radiators will not work properly if the valve is left "in between" these positions.
Steam rises and enters each heating radiator through a single pipe, pushing air out of the radiator through its vent. Condensate returns to the boiler through the same pipe, passing out through a special passage in the radiator control valve.
For details about steam heat piping & steam pipe radiator connections & definitions of different types of steam heat see STEAM RADIATOR PIPING CONNECTIONS
It is normal to hear a hissing sound from the steam vent as air escapes from the radiator at the start of a heating cycle.
The hissing sound should stop and the valve close when the radiator gets hot.
If steam continues to escape from the valve even after the radiator is hot, the valve is defective and should be replaced since continuous loss of steam increases the water consumption by the heating system.
Other causes of steam radiators to fail to get hot include a radiator valve that is shut or a steam radiator valve that has become clogged with rust and debris (more likely on a one-pipe steam heating system where returning condensate through the valve can become blocked), or a one pipe steam radiator that has become tipped the wrong way so that it becomes clogged with condensate.
If a steam vent stops working and fails to open, rising steam cannot enter the radiator and it will be slow to heat or may not heat at all.
If your one pipe steam radiator won't get hot, the air vent may be clogged or it may just be worn out and need replacement.
If a steam vent stops working properly and fails to close, the vent will hiss and release steam (and sometimes produce condensate or water) continually all during the heating cycle, wasting heat, increasing heating cost, and in some cases creating a moisture problem or even a burn risk at the radiator.
Steam vents on individual radiators or main steam vents may not be opening when they should. If an individual steam radiator vent fails to open, that radiator won't get hot even though its supply valve is open.
But if a main steam vent (master steam vent) is not opening, the whole heating system may continue to work, but we expect to see two problems:
Main steam vents: Our photo (left) shows an automatic air vent found on steam piping in a basement - not all steam vents are located right on the radiators. This is a line vent or main vent, and it too needs to work. (Most likely this is a 2 pipe steam system]
This vent may have been added to help speed rising of steam in a building by purging air from a section of steam piping.
If no air escapes from this valve during the heating cycle it may be that the valve is jammed, is not releasing air, and the steam radiator may not ever warm up.
The valve may need to be replaced if it is corroded, leaky, doesn't vent, or if the steam system appears not to be venting properly. We also discuss this valve at STEAM VENT TYPES, SELECTION.
It is normal for steam vents to make a hissing noise (also described as a whistling noise) at the start of a heating cycle while air is being vented out of the steam piping or steam radiator as it is pushed by rising steam.
Watch out: If the radiator continually makes noises (hissing, whistling or wheezing) at the steam vent, there is a problem that needs to be fixed: a bad steam vent, steam piping problem, steam pressure set too high, or boiler oversized for the heating distribution system.
First, make sure that the noise complaint is coming from the steam vent and not from water hammer or clanking steam piping.
Next: mineral deposits or dirt in a steam vent can prevent it from opening or closing properly. While it is often possible to disassemble and clean (soak in vinegar) a mineral-clogged steam vent, other internal vent failures can also occur, so considering the low cost of replacement steam vents, it usually makes sense to simply replace the steam vent entirely.
At RADIATOR STEAM VENTS, 1-PIPE SYSTEMS we list a variety of replacement steam vents.
A rattling sound at a steam vent may be heard at any radiator or at a main steam riser where a main line vent is installed. If your steam vent is rattling an internal part has become loose. Replace the vent.
Watch out: Steam condensate return problems can cause banging clanging steam heating pipes & radiators. And as we discuss at BANGING HEATING PIPES RADIATORS, if your heating boiler does not have an automatic water feeder and you've been putting makeup water into the boiler manually, a blocked condensate line and low water in the boiler will eventually lead to total loss of heat when the low water cutoff switch (see LOW WATER CUTOFF CONTROLS) , a key boiler safety device, simply shuts down the boiler.
Also see HEATING SYSTEM NOISES - where we list heating system noise types, sources, problem indicators, cures
If water is "spitting" out of your steam radiator's air vent it's most likely that debris, sediment, mineral deposits, or rust flakes have partly clogged the air vent. Steam experts such as Dan Holohan suggest that you may be able to clean a spitting or partly-clogged steam vent by disassembling and boiling the vent in vinegar (to dissolve minerals).
Watch out: don't remove a steam vent when the heating system is in use or hot. You can get badly burned. But some specialty steam vents (air valves) such as the Hoffman Model 508 (Part No. 401475) can be disassembled and cleaned without removing from the line.
If a lot of water is really leaking out of a steam radiator's steam vent, the radiator may also have become waterlogged (according to some writers). We don't think this is a common problem, since most steam vents are mounted at mid radiator height or higher - the radiator would have to be really full of condensate for a lot of water to be coming out of the steam vent - unlikely.
If a steam radiator contains condensate (water from condensed steam) that fills the radiator to a level above the steam vent, the steam vent cannot function, air won't be pushed out of the radiator by rising steam, and the radiator won't heat.
If your steam vent is mostly venting steam but spits a few drops of water, the vent may not be working properly and may need replacement, but it is not as likely to be a sign that the steam system has become waterlogged.
Watch out: are you sure your system is using steam heat? Hot water radiators often include a manual air-bleeder valve that must be opened to let air out of the hot water (not steam) radiator or the radiator won't heat. On a hot water (hydronic) heating system if you leave the air bleeder valve open after air has stopped coming out, indeed water will come squirting out of the valve - indefinitely. Don't do this. See AIR BLEEDER VALVES for details.
Automatic and thermostatically operated steam vents used to provide room temperature control are discussed in detail also at RADIATOR VALVES & HEAT CONTROLS.
Also see RADIATOR STEAM VENTS, 1-PIPE SYSTEMS where we explain automatic steam vents and heat regulation using air vents on steam radiators. If you don't know what kind of heat your building uses, we explain how to figure out the answer at HEATING SYSTEM TYPES. If your heating system is not working properly, see NO HEAT - BOILER or NO HEAT - FURNACE.
Because some controls are used in common on hot water heat, hot air heat, and steam boilers, readers should see these other articles: If your building uses forced hot water heat see BOILER CONTROLS & SWITCHES. Some of those controls also appear on steam heating equipment.
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Frequently Asked Questions (FAQs)
Question: steam valve replacement: mounting thread problems
July 17, 2011) RJ said:
Can i rethread a radiator in order to properly hold a valve?
Great question on radiator repair RJ.
Provided there is enough thickness in the surrounding metal (cast iron in your case?) you might have to drill out, tap to the next size, and insert a short nipple or plumbing reducer to mount the new vent or bleeder valve. When I come across a problem like this I tell myself that surely others have run up against it before me, and there is probably already a solution available. For example auto supply stores carry tools for drilling and tapping out stripped spark plug openings in automotive engines. The stripped opening is drilled to an over-size and an isert is then threaded into the opening to reduce it to the proper spark plug thread dimension.
Ask one of the older guys at your heating supply house for advice too.
Question: water is spitting out our steam vents
(Nov 9, 2011) Dean said:
We recently replaced our our old boiler with a new one. Since the heating season started we have noticed water spitting out of the two air vents on the condensate return line to the boiler after the heat has been on for a while? How can this be prevented?
Dean spitting steam vents may need to be cleaned or replaced - I agee that we don't want water spraying into the home.
But watch out: a blockage in the condensate return line could mean abnormal levels of condensate in the system and improper operation. If your steam boiler is using abnormal amounts of water I'd ask for a service call promptly.
Question: combined steam and hot water heat: does the hot water part need an air valve?
(Jan 12, 2012) Saul said:
I have a steam boiler and the house uses steam heat/radiators; however, we added a hot water baseboard system for the basement that works off the steam boiler -- hot water is pumped from the boiler through the pipes to the baseboard heating system; all of the pipes except for the pipes directly attached to the boiler are below the water level of the boiler; does the hot water system need an air bleeder valve?
Add an air vent on a hot water loop fed by a steam boiler?
Small amounts of air in the hot water heating loop may be pushed around by the circulator pump, return to the steam boiler and vent out with the steam. But larger amounts can block the loop of hot water piping, especially anywhere that it is higher than the boiler and circulator pump themselves.
In my opinion it's good practice to include a manual air bleed valve at the end of the hydronic (hot water) baseboard in any room or building level. It makes bleeding out an air-bound segment of baseboard much easier when that occurs in the future - as it can certainly occur over the life of a system.
Question: loud spitting steam vents
(Dec 30, 2011) Ashley said:
The vents on the radiators in my apartment were incredibly loud, so I had maintenance replace them today. However, one of them is now spitting (a very small amount) of water droplets. It never did that before. The radiator is getting hot, so there's no heating issue. It sounds like spitting vents are a problem and that new vents shouldn't do that! Could the vents be faulty?
(Oct 11, 2012) R said:
If I hammer a nail into my steam vents hole to stop it up, will it explode?... Our steam vent is UNBEARABLE. It is so loud, and rattles, it is next to the bed, and spits water on us. We live in a 5 story building, and we are on the top floor. There are four units below us. Do you see any problem with just corking the love thing up? YES- we could call the super... but 15 phone calls havent fixed it, so we are just taking matters into our own hands.
Ashley, very loud spitting steam vents may be damaged or improperly sized for the radiators. They're not costly to replace, but be sure to ask your installer to match the vent to the radiator and system requirements.
Don't try hammering anything into a steam vent- when you break it you may be unable to stop steam from pouring into the room. The right repair is to replace defective steam vents that don't close when they should
Question: do steam systems ever create a vacuum?
(June 26, 2012) Is a vacuum created when the fur said:
When the furnace is turned off, the steam condenses. Does this create a vacuum in the system? This could be a problem. How is the vacuum eliminated?
If you watch the pressure gauge on a residential steam boiler you will generally see that pressure rises from zero psi cold up to about 0.5 psi hot and then when the system stops heating pressures fall to zero. Residential steamnoperates at low pressure.
But yes, particularly in two pipe steam systems a vacuum can be created in the main piping system when the system shuts off and cools. Air is allowed into the system to accomodate this by vacuum breakers installed on the steam mains.
Question: steam radiators don't get hot. How do I fix this?
(Nov 7, 2012) ashley said:
some of the radiators in my steam system do not get hot what can i do to correct this
Ashley, in addition to checking for a stuck shut steam vent, see our COLD STEAM HEAT RADIATORS article for diagnostic procedures and repair tips.
Question: how to select the right steam vent
(Mar 19, 2013) John Bolan said:
We recently had a two foot section of steam baseboard installed in our bathroom. It is a small 8x5 foot room. What vent valve should be installed? We are noticing water coming from radiator.
John, first get your installer back to fix the leaky condensate return. Then take a look at
The steam vent type is first chosen based on the type of steam piping system: one pipe vs. two pipe, as in most installations different vents and controls area used.
Questions & answers or comments about steam vents on steam heating radiators and convectors
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