How does this sytem work? 3 pipes and a vent to a radiator.

2x_Tom · December 2014

The other day I was repairing some water lines in a building I regularly work in. The heating system there has always intrigued me. I don't service it but would like to know exactly how it works.

The building has two floors. The steam boiler is on the first floor and heats both floors. The main runs straight up to and through an inaccessible attic area. All the radiators have a 1" or 3/4" drop down from the main feeding the top of one side of the radiators. All of the radiators return through the bottom of the opposite side. This is where it's a little strange.

The return comes out and immediately tees off. One side of the tee goes up to a radiator trap that is installed sideways and located about half way up the radiator. The pipe coming out of the trap is reduced to 3/8" and I'm not really sure where it goes once it goes into the wall. I'm assuming it goes back up the the main because it doesn't come back down to the first floor. The other side of the tee drops down to the level below, goes through a switch check, into a dry return, through an F&T trap and into the wet return. Some of the radiators have coin vents and some have steam vents installed at the top of the radiators which seems pretty odd, like someone was trying to toy with the system and fix something.

I've seen a ton of residential and some commercial steam systems but nothing piped like this and I'm trying to figure out exactly how it works. Steam feeds up the main, once the main is full it works it's way down the the radiators. Condensate of course goes down through the check, into the dry return, through the F&T trap and into the wet return.

How does the system deal with air though? Are those traps piped back up to the main bringing the displaced air into the main? I don't see how that could work. Wouldn't you have a higher pressure in the main then what you have after the trap? Is all the air going down and being passed through the condensate pump? If that's the case why wouldn't the traps be installed in the normal manor?

Jamie Hall · December 2014

I wonder if that system originally had a vacuum pump connected to those small lines? There were a few vapour system designs that did that.

Of course, that doesn't say where they go now...

Steamhead · December 2014

If the boiler is on the same floor as the lowest radiation, this arrangement makes sense.

As Jamie said, this is a Vapor system. Originally the condensate tank wasn't there, and the lowest pipe was a wet return. It may or may not be lower than the present boiler's waterline, but it would have been lower than the waterline of the original boiler.

The radiators are above the original boiler's waterline plus an "A" dimension, otherwise they would fill with water. That really is an "A" dimension since the traps do not block the path between the rads and the wet returns. If the traps were installed at the radiator outlet connections, it would be a "B" dimension.

The flow of air and water out of the radiators splits at the radiator outlet. The air goes up, thru the trap and into the dry return. The water drains down into the wet return.

The condensate tank and F&T trap were added later. I'd bet neither of these are needed. A False Water Line is all you need, to keep the wet return wet if the current boiler's waterline is too low to do this by itself.

Are they having any problems with this system?

2x_Tom · January 2015

Thanks for the response and explanation.

The floor the boiler is on has a radiator off the zone in the drawing. It is piped the same as the radiators shown with the radiator hung on the wall such that it is only a couple feet lower then the 2nd floor radiators. It also has a piece 15' or so piece of fin tube down near the floor that of course isn't original.
One of the other zones has about 100' of fin tube down near the floor. I guess they added the condensate tank and F&T traps when the did the renovation and put in all the fin tube.

The system actually works decently. The only issue I know of is one of the radiators piped like the one in the diagram is always hot. It's right in the middle of the system with other radiators on either side of it but it's always hot even when the others are cold. The one that's always hot has a new valve and trap on it. All the other radiators have the original valves which I don't have a picture of but they're not standard valves, they go about a quarter turn.

Actually they have another issue where every once in a while the boiler room will fill with steam blowing out from the condensate tank. That one is probably just an F&T trap failing intermittently.

So this system should really be running on very little pressure if it is a vapor system right? They're running about 5psi right now as that's what the oil company that put it in set it at.

Jamie Hall · January 2015

Yes indeed. Turn the pressure down! You don't need that much. Try to get it to .5/1.5 if you can.

Oil companies... grr...

Steamhead · January 2015

You can't fix stupid.............

How does this sytem work? 3 pipes and a vent to a radiator.

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