My (new) baseboard heating system is causing a racket, and I've been trying to figure out a way to properly purge the air from it. Following the flow of water through one zone of my four-zone system, it goes:

Furnace output → Circulating pump → Expansion tank → Hand valve (A) → Thermostat-controlled valve → Spigot (A) → Radiators throughout house → Hand valve (B) → Water main input → Spigot (B) → Furnace input.

I've been trying to figure out a way that I can force the air in the pipes out with new water from the main. If there were a spigot right before hand valve (B), for example, I could close hand valve (B), open the spigot right before it, and let the water pump through the system and out that spigot to purge the air. However, the spigot (A) is instead on the other side of the piping through the radiators.

I have read about using the pressure from the main line rather than the pump to purge the air, and I've also heard mention of "reverse purging" (which I suspect my system might be set up for, given that the spigots immediately follow the zone control valves). I'm still not sure what to do, though—any suggestions?

  • 3
    You'd typically remove air via a bleeder valve located at a high point in the system (often each radiator will have one). You may need a screwdriver or small wrench to operate it. Does your system have these? And are you sure the noise is from air and not expanding pipes?
    – BMitch
    Commented Nov 23, 2012 at 12:19
  • @BMitch, that should be an answer, not a comment. I totally agree, he shouldn't need to add anything in order to bleed the system.
    – GdD
    Commented Nov 23, 2012 at 17:33
  • @GdD, I may convert it later. But if the answers to my questions turn out to be "no" then someone else should chime in with more knowledge of this situation.
    – BMitch
    Commented Nov 23, 2012 at 18:27

2 Answers 2


The hand valves and spigots near the furnace are for isolating and draining the zones to servicing the radiators. To bleed the system, you would find a bleed valve at a high point, often at each radiator. You may have to remove a cover to find it, and it may require a screwdriver or small wrench to operate the valve. Make sure to have some rags handy to cleanup any water that comes out. When you're finished, check the pressure in the system and add more water from the water main input if necessary.

  • I would also question the location of the expansion tank within the system. See separate answer for explanation.
    – pdd
    Commented Dec 3, 2012 at 20:14

In addition to what BMitch said, I would also question the location of your expansion tank.

Your pump should be located after the expansion tank as you always want to pump away from the expansion tank, like this:

Furnace output → Expansion tankCirculating pump → Hand valve (A) → Thermostat-controlled valve → Spigot (A) → Radiators throughout house → Hand valve (B) → Water main input → Spigot (B) → Furnace input.

Here's the explanation:

The expansion tank is the point of no pressure change. Anything between the tank and the inlet of the pump will be at a negative dynamic pressure (below the static pressure of the system, which is the pressure of the system when nothing is running). Anything from the outlet of the pump to the expansion tank will be at a positive dynamic pressure.

Heat Source → PDP → Expansion Tank → NDP → Pump → PDP → Heat Source

PDP = positive dynamic pressure | NDP = negative dynamic pressure

If your system is setup so that the majority of the run is at a negative dynamic pressure, water within the system will be allowed to dissolve into the water making air elimination difficult. keeping the system in a positive dynamic pressure will help prevent the air from dissolve into the water which will make air elimination more effective.

Here's a short PDF that explains in more detail.

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