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For better or worse, I'm one of those folks who not only want to know what we usually do — or what we may be required to do — but why. When I was younger I drove folks nuts that way — parents and teachers! — and I still do it, many decades on. Sorry…

@clammy can't say I agree 100%. I certainly agree that everyone should be installing proper equalizers. But it is equally important to understand what things do and how and why they work. This conversation is purely hypothetical, but it certainly enhances our understanding. Open minds are awesome.

I'm still staying with it's a drain for the header and a vent for the loop to keep it from siphoning out of the boiler. Without the vent the loop wouldn't matter it would just siphon over the hump.

Not understanding the applying pressure thing. You can't push down into a tee or wye without pushing in both directions. It's gonna push equally outward from the boiler as it is into it.

@ChrisJ

Just my opinion.

When the boiler is not firing there will be more pressure at the bottom of the boiler than above the water line. Atmospheric pressure will be pushing down on the water and the top of the boiler is vented to the atmosphere.

As the boiler builds pressure the pressure on the water at the water level pushes down on the water which will tend to force water out the return. Since the same pressure is pushing down on the water from the header through the equalizer the water level is equalized and the water level in the equalizer should roughly match the sight glass. Without the equalizer water could be pushed into the return.

This may change depending on the system. In a one pipe system with the end of the steam main connected to the return the equalizer may not be needed as @ethicalpaul has shown. In a two-pipe system with no pressure in the returns due to steam traps that could be a different issue as could a large one pipe system in an apartment building with 100s of feet of pipe the pressure at the far end of the steam main may not equalize as well as it would at the boiler.

Weather you need an equalizer to drain the header is another issue. Properly piped boilers with low velocity in the boiler risers may not require an equalizer for a header drain.

This site abounds with picture on various posts of boiler with crappy piping that work fine without an equalizer or an improperly piped equalizer.

But if you install a boiler how do you know if it will work or not?

Most would install the equalizer because the MFG recommends it even if it is not "needed". It would cost a lot more to go back and install it if the job does not work than the cost of a few fittings and some pipe and some labor.