I heard some gurgling in our baseboard heat recently, so I decided I should bleed the air out of our system. Never having done this before, I watched a few youtube videos to familiarize myself with the procedure. The basic idea seemed as follows:

  1. Turn off the boiler
  2. connect a garden hose to the zone's output spigot
  3. turn off the valve below the output spigot (to prevent water from cycling back through)
  4. turn the switch from auto to manual so that the water will flow out the spigot.
  5. in some cases, videos recommended turning the refill valve from normal to fast-fill to speed it up, or to have enough pressure to push air through an upstairs zone.

I did this, and nothing happened. And in the process, I looked around for the refill valve switch to put it in to fast-fill...and I couldn't find anything that looked like the ones from the videos. Well, I found one thing that looked a little like a fast-fill thing from one of the videos, but it didn't seem to do anything. And there was no flow at all - not just slow flow.

After poking around, I found a valve that, when I turned it, I heard the flow of water, and so feeling brave, I left it open and a little while later, lo-and-behold, water (and air) was coming out the other end of the garden hose.

So I went through each zone, opening and closing this valve for each one. I didn't want to leave the valve open once I closed the zone valves, since in the videos, people made it clear that leaving systems in 'fast-fill' mode with the spigots closed could cause pressure to build up and blow out the high-pressure release valve. I didn't know what the pressure of this input was, so being cautious, left it how I found it (closed).

But my question is this - is it a problem if the refill valve is closed normally? My impression is that this exists to replace water that leaves the system over time. I don't know exactly where this water would be going (absent a leak), but the fact that each of the systems I saw on the instructional videos had them made me think it is standard for the refill valve to be applying a constant input source (of around 12 psi, I think). But my system seems to have just a manual on/off, and it seems to live in the off position.

Here are some photos of the system to give you a sense of what I'm working with:

Overview of the heating system Overview of the heating system

Stuff on the side of the unit Stuff on the side of the unit

Various Zone pipes with valves and spigots Various Zone pipes with valves and spigots

Pipe above the unit which had the valve I ended up using on it Pipe above the unit

Thing that looked like a fast-fill switch but did not seem to do anything Thing that looked like a fast-fill switch but did not seem to do anything

Valve above the previous thing which did seem to work when I opened it Valve above the previous thing which did seem to work when I opened it

  • 12 psi doesn't sound right. the rating is on the pressure reducing valve. 25 is more typical with the relief valve at 30psi. Your relief valve and tube look heavily corroded - any idea why? Dec 1, 2023 at 2:46

2 Answers 2


It's actually normal, arguably good, fairly standard practice to leave the valve to the pressure regulator closed, because the only reasons you should need to add water are to make up for all the dissolved air coming out of solution you'll need to bleed out having just dumped a bunch of new water in there, and leaks. And leaks should be fixed, not allowed to keep leaking and automatically get refilled.

From the shutoff valve downwards, you have a backflow preventer and a pressure regulator and an additional valve to permit changing the regulator and backflow preventer without draining the loop. The toggle on the top of the pressure regulator is what you're calling a "fast fill device" unless you're calling the whole pressure regulator that (which would be incorrect.) With the shutoff valve open that will make water go through the regulator faster. If the lever is not actuated, the regulator will regulate the pressure, unless it's broken.

A closed heating loop should not leak. If it does leak, your cast iron boiler and pump keep rusting (because you keep adding oxygen with each addition of new water) which is something that the standard practice of using cast iron boilers and pumps assumes won't be the case - in a leak-free system the oxygen gets used up making some rust, you don't add more water, with no more oxygen, no more rust forms.

Also, if it's leaking you have water going somewhere out of the heating loop, which can rot parts of your house.

So, if the pressure in your system drops, and you have to open the valve to refill it, you would then NOTICE (one hopes) and take corrective action. Or, you throw the valve open, ignore the root problem, and you have bad things happen after some time passes due to corrosion and leakage.

You should check for the presence and proper operation of automatic air bleeders on your system. Normally there's one on an air scoop over your expansion tank, but it's just out of the picture so I can't be sure it's there. That's the normal and "a lot less fuss" way that air bubbles should come out of your heating loop before they grow so large that you need to do what you just did.

  • Ah - so you are suggesting that, had I just left that valve in the on position, the thing below it would work like that fast-fill toggle valve I saw in various videos?
    – Doug
    Oct 11, 2023 at 2:53
  • 1
    I like your answer though most/all of the plumbers I've had typically tell me to keep the fill valve partially open. It does mask the expansion tank having failed though. The boilers generally have safety features ( pressure troll ? ) to prevent turning on with too low water so not sure why they'd suggest keeping it partially open. I seemed to think I asked and the plumbers had the this is just the way it is always done answer for me. Dec 1, 2023 at 2:39

Fairly common to keep the filler valve closed -- you should never need to use it except to refill the system, and if a pipe upstairs breaks, you'd rather not have water being pumped upstairs and dumped on the floor of your house.

There's another reason to keep your automatic feeder valve closed: if your expansion /pressure tank fails, the water will cool off, and the feeder valve will add water to make up for the lost pressure. When it all heats up again, some of this will be dumped on the floor.

In addition to @ecnerwal s advice above, look for hi-hat / bleeder valve at the top of the circuit in question. They can get stuck open and let air in or let water out through evaporation.

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