Loop seals vs. F&T traps on 2 pipe mains

What "system" is it? (Webster, Dunham, etc)

You're not missing something. The combination of loop seal and crossover trap is much cheaper and simpler — and considerably more reliable — and offers higher capacity than an F&T.

However.

It this condensate path is a loop seal to the dry return, as it sometimes is, it is limited to relatively low pressures. Even if the loop seal is a generous 60 inches, any steam main pressure over 2 psig will allow steam to blow through the loop seal and pressurize the dry return. This is not good.

Now if the loop seal isn't really a loop seal, but consists of two drips (one from the main, one from the dry return) going to the wet return, this limitation doesn't apply — but… now if the boiler pressure acting on the wet return will raise the condensate in the wet return up to the dry return if the pressure is too great — using that 60 inches, 2 psig again.. This isn't good, either.

So. Bottom line: if you boiler pressure is much over a couple of pounds, you are going to need an F&T. Otherwise, you're better off with a crossover trap and a loop seal or double drips.

Note that an F&T makes a truly miserable main vent — so you'll need a main vent at the end of the steam main.

For the 1874 AA Griffith's two pipe radiator in our parlor, I made a loop seal below in the basement because I wanted to keep my New (2001) Vapor system extremely simple , no traps to confuse some guy many years from now. I will post a picture. Mad Dog

My loop seal has its very own closet!

Nah Ed…a loop seal is a basic plumbing P Trap..very obvious what it does even to the dopiest plumba...I've seen countless very good plumbers having no clue what a steam trap actually does. Mad Dog

shouldn't have to… but it does no harm to plumb it so you can.

I need someone like @Mad Dog_2 if I ever replace the orangeburg under the street because I want someone to dig the joint out of the clay main branch and caulk PVC in to the clay and put a fernco over the hub and pipe as a second seal and I don't know who will do that in Michigan.

I flush every few seasons..mad Dog

@JohnNY , here is a discussion of the Tudor system (which became popular in Europe), using these loop seals. On page 5 of the PDF there is a diagram showing one:

https://www.heatinghelp.com/systems-help-center/european-heating-systems-circa-1907/

Why would you need a loop seal to a wet return? Just askin?