How to bleed air out of a heating system (C) Daniel FriedmanAir-bound Hot Water Heating System Diagnosis & Repair

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Air in heating system pipes, baseboards, radiators makes a gurgling noise and ultimately leads to loss of heat.

How to diagnose & fix trapped air in heating systems causing cold baseboards, convectors or radiators: beginning right here we describe six different causes & cures for cold radiators and baseboards due to air trapped in the heating system. These diagnostic routines are described in the order they should be tried.

We start by confirming that we think the problem is that air trapped in the hydronic (hot water) heating system piping, boiler, radiators, baseboards or convectors is preventing heat from being delivered to the building.

This article series provides a detailed guide fixing no-heat calls in which the heating boiler and circulator are running but heat is not being delivered to one or more building areas.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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How to Recognize & Fix Problems Caused by Air In Hot Water Heating System Pipes, Radiators, Baseboards

Radiator air bleeder valve (C) D Friedman

After cofirming that the building is calling for heat but some areas are not getting it, here we explain how to find where the trapped air is actually located and we give a series of steps to fix the trouble, starting with a simple simple procedure for bleeding trapped air at registers or baseboards.

[Click to enlarge any image]

If necessary there are additional methods used to remove air from air-bound hot water heating systems using two different service procedures to force air out of airbound pipes in a hot water heating system.

What to check first if a hot water heating radiator or baseboard is not getting hot

Check hot water baseboards, radiators, or heating convectors: if some of these heating devices are hot and others cold, are they all on the same heating zone?

Feel the hot water piping leaving the heating boiler - it should be hot when the boiler is running and the thermostat is calling for heat.

If the building has multiple heating zones each zone will be controlled by its own thermostat and each heating zone will either have its own hot water circulator pump (controlled by a thermostat and pump relay switch), or each hot water heating zone will have its own thermostat and a zone valve that opens to let a common circulator pump send hot water through that individual heating zone

Are all of the thermostats turned up high enough to call for heat in each heating zone?

If some heating baseboards or radiators are hot and others cold and we're sure that they're on the same heating zone, then the system is probably air-blocked.

Watch out: it is not obvious to people not expert in hot water heating systems, but the circulator pump on a typical hot water (hydronic) heating system does not have enough oomph to push a bolus of trapped air around through the heating system pipes. If more than small bubbles of air remain trapped inside the heating piping the circulator may spin without actually pushing hot water through the system. By purging the trapped air we get the hot water circulating again.

5 Approaches to Removing Hydronic Air / Fixing an Air-Bound Hydronic Heating System

Flamco NexusValve Pressure Step Deaerator (PSD) removes dissolved air from sealed chilled & hot water systems (C) adapated from Flamco cited in this article In sister articles we discuss these approaches to removing air that is preventing proper water circulation in a hydronic (hot water) heating system.

You are likely to find at least two of these automatic and manual air removing devices on both residential and commercial hot water heating systems: air bleeder valves, both automatic and manual, and air scoops/separators.

  1. AIR BLEEDER VALVES - installed at the boiler and at other strategic locations automatic or manual air bleeders remove air from the system at high points.
  2. AIR SCOOPS SEPARATORS PURGERS are specialty valves that in essence are a more-sophisticated "air bleeder valve" - the first de-aeration approach in this list. Example air separator products include

    Nexus Valve Flamco Flamcovent Smart air separator. Retrieved 2017/12/14, original source:
    Excerpt from the company's web page:

    The Flamcovent Smart air separator removes even the smallest microbubbles from the system water. It is virtually maintenance-free with a negligibly low flow resistance. The Flamcovent Smart can be incorporated in a variety of heating and cooling systems and in every desired direction.
  3. AIR BOUND HEAT SYSTEM REPAIR by PUMP - using a pony pump to force air out of an air-bound heating system
  4. AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE - using a pressure over-ride on the boiler water feeder to try to force air out of an airbound heating system
  5. PRESSURE-STEP DEAERATORS [PDF] - Nexus Valve Flamco Pressure Step Deaerator, Aalberts Industries, 9982 E. 121st St., Fishers IN 46037, USA Tel: 888-900-0947, Website:

    Pressure-Step Deaerators or PSDs are a more-recent innovation used to remove entrained air in hot (or chilled) water hydronic systems.

    "Entrained" air can be taken to mean air that is dissolved in the hydronic system's water. As water is warmed that entrained air changes to gas bubbles that in quantity can result in an air-bound piping system.

    A pressure-step deaerator uses vacuum to "deaerate" or remove air entrained in the hydronic system's water. The PSD from FLAMCO uses a combination of duplexed pumps, isolation valves, and a vacuum deaerator cylinder along with additional controls and valves to remove air dissolved in the system water supply. Retrieved 2017/12/14, original source:

Technical note: building plumbing systems may include other water system air removal devices not associated with heating and cooling, such as AIR ELIMINATORS for POTABLE WATER SUPPLIES

Air in Heating Systems: Articles on Diagnosis & Cure

Pony pump used to force water through a boiler (C) Daniel FriedmanOur discussion of radiators or baseboards that do not get hot when they should includes these articles on how to get rid of air in hot water or hydronic heating systems in order to prevent or cure air-bound heating systems (sections of baseboard or radiators that don't get hot):

  1. AIR BLEEDER VALVES - Guide to manual and automatic air bleeder valves & vents for hot water type heating systems: how to find and use manual and automatic air bleeders to fix an air-boundheating system causing cold radiators convectors, or baseboards or noisy bubbling, gurgling heating pipes.

    The air bleeder valve article series also explains how to troubleshoot, fix, install, or repair manual air purging or air bleeding valves on heating equipment. Some of these valves also work on cold water used in chilling and cooling equipment.

    Our photo at above left (red arrow) shows a conventional air bleeder valve on a cast iron hot water heating radiator.
    The blue arrow shows the little nipple out of which air will hiss when the valve is opened (using a rectangular "skate key" type wrench or key available from your plumbing supplier). Close the valve as soon as water starts squirting out from this opening.

    Watch out: if an air vent is not installed properly it may make matters worse, actually admitting air into the hydronic heating system and increasing its tendency to become airbound. Details are
  2. AIR SCOOPS SEPARATORS PURGERS: guide to automatic air purging devices: since on many hot water heating systems a key air bleeding or air vent point is at the air scoop or air separator closer to the boiler.

    A quick visual inspection can tell us if there is obvious leakage, corrosion, or clogging of some automatic air separator devices, and fdor some automatic air purge valves, a simple step of opening a valve cap may be all that's needed to restore the valve to operation - or perhaps we close the cap to stop dripping there.

    Air vents necessary for steam radiators to get hot for steam heating systems are discussed separately at STEAM VENTS.
  3. AIRBOUND HEAT SYSTEM REPAIR by WATER FEED VALVE - describes how to use built-in controls and valves to force air out an air-bound heating system whose radiators, convectors, or baseboards are not warming up. We try this approach if trapped air in the heating system piping or baseboards is not being successfully vented by the manual or automatic vents already installed.
  4. AIR BOUND RADIANT HEAT - a no heat problem in one zone of radiant heating may show up as a "vacuum" or as "heat is running backwards" or may be mistaken for a thermostat or control problem, depending on the controls installed and where you look
  5. AIR BOUND HEAT SYSTEM REPAIR by PUMP - (photo above) how to use a portable pump, short sections of garden hose, and heating system valves to force air out an air-bound heating system whose radiators, convectors, or baseboards are not warming up.
  6. STEAM VENTS - are installed around the middle of one end of steam heat radiators to vent air as steam rises into the radiator. These vents are automatic. If the steam radiator does not heat and its feed valve is open try replacing the steam vent.
  7. WATER FEEDER Valves, Hydronic - a defective automatic water feeder valve on a hot water heating boiler can result in too little starting water pressure in the system - radiators on upper floors may fail to receive heat.

    Gurgling noises in the heating system pipes or low heating boiler water pressure could be due to a clogged or damaged pressure-reducer water feed valve. Here is an example:

    After I was unable to figure why the furnace had a low water level that led to gurgling in the radiators, I decided to change the auto-fill valve above the expansion tank. I'd already changed the tank the week before but decided to cheap out and reuse the old valve.

    When I disconnected and upended the tank/valve it spilled a bunch of black sulfur laden water out. The sulfur flakes may have clogged the screen or valve for as much as several years.
    - M.F. 12/24/2014

    On a steam heating boiler a defective automatic water feed valve can cause the heating system to shut down completely or can lead to boiler damage or even unsafe conditions. In this article we discuss problems with water feeder valves that also control water pres sure in the boiler and we explain how to adjust the water feeder valve and thus boiler water pressure.

    WATER FEEDER VALVE, STEAM is the article you want if your heating system is by steam rather than hot water.
  8. At BOILER PRESSURE & TEMPERATURE SETTINGS we discuss the pressures needed for hydronic or hot water heating systems - a taller building needs higher starting (cold) pressure in the heating boiler.
  9. At OIL HEAT NOISE DIAGNOSIS we discuss the diagnosis and repair of other heating system noises on both oil and gas fired heating equipment.

This article series answers nearly all questions about diagnosis & repair of residential & light commercial central heating systems.


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