Spitting Wet Legs

BRader

Working on a one-pipe, pumped-return system. A previous contractor had run the returns out of the F&T traps at the ends of the mains, up about 18" to the basement ceiling, then almost dead level across the floor joist from one end of the basement to the other where the condensate pump and boiler are. They then wondered why it wouldn't return condensate and hammered like crazy. Apparently water doesn't like to flow uphill? We replaced the F&Ts, which were ancient Trane traps with new Watts traps. We had to create a wet leg to get the condensate back to the condensate pump due to the layout of the building. Basically the mains end on one side of the basement and the boiler is on the other. Having slope without dropping down to a wet leg would put the return right about waist level in the main path through the basement of this commercial facility. They use this space for storage, so that's a no-go. While this is less than ideal, it should work. Three 3/4 F&T traps tie into this wet leg, which is 1 1/4. We installed street 90s on the spare outlet of the traps, turning them up, adding about a 14" nipple and placing a Barnes and Jones Bigmouth vent on each to vent air and act as a vacuum break for the wet leg. This all seemingly worked well when we first fired up the system after the changes. Almost all the hammer was gone, condensate was coming back, and everything seemed good. That is until the boiler cycled off on the pressuretrol. When it cut back in, the vents all started to spit and sometimes gush water/condensate. We tried increasing the height of the vents, but that didn't help the situation. The boiler is cutting in at 1/2 psi and cutting out at 1.5 psi. From what we can tell the traps appear to be functioning properly. They are passing condensate. We're not getting steam in the returns. We just have this issue with the condensate backing out of the vents. Anyone run into a similar situation or have any ideas?

Steamhead

Vacuum in the returns?

BRader

Steamhead said:

Vacuum in the returns?

That was one of the things we considered. I think we are going to try adding an additional vacuum break where the three lines come together in the wet leg.

EBEBRATT-Ed

Does the wet leg drop to the floor and run along the floor to the condensate pump? Does it have to rise up to go in the pump or is the pump in a pit? Condensate tank vented? (I know its a stupid question). If the return rises up to the pump how much?


I am not getting what you are doing. I am assuming the steam mains are not counterflow. Are these 3 traps carrying all the condensate for the entire system? If so are they not dripping the steam main as well as all the condensate from the radiation seeing it is one pipe.

If the mains are dripped into a wet leg why do you need traps? To keep the steam out of the condensate pump. If the new condensate pipe is low enough so it has to rise to get in the pump and each drip is individual you won't need traps

pedmec

Can you sketch a drawing of a piping layout. I'm confused of what you have.

BRader

This is a very rudimentary drawing. I figured out that the F&T traps might be the cause of my problem. When the hot condensate moves through them, it seems to flash to steam because of the difference in pressure. This steam was backing up the return lines because of the water seal in the wet legs. I added a vent where the wet legs come together and it seems to have solved the problem. It’s returning condensate well and no longer spitting from the vents. I ran it for a couple of hours today and didn’t have issues after the changes, so fingers crossed that it will work. 

EBEBRATT-Ed

@BRader

Hope u have the fix.

I had a similar job in a building that dated back to the civil war. When they added the steam some rooms they left unheated. So after some renovations they added steam to those rooms it was a mish mash of one pipe steam and two pipe steam with traps.

I had one spot where the one pipe rads the vents on the rads would spit even though I was running the pressure very low.

They had a 3/4 F &T at the end of the main on the one pipe rads. One of our other guys changed the trap....still no good.

Turnes out the 3/4 trap was way too small.

On two pipe you only need to drip the mains and they don't condense that much. Most of the condensate goes out the radiator traps.

But on one pipe (as long as it's not counterflow) and you drip the end of the main your getting what condenses in the main PLUS all the condensate from the rads on that main.

If that is your situation your 3/4" traps at the end of the mains could be too small.

The job I was on I looked at it twice before the light bulb lit up and, in my case, it was only one trap. Since the rads were vented, I just replaced the trap with a 5' loop seal problem fixed.

BRader

@EBEBRATT-Ed Thanks for that insight. It's definitely a weird system and it's been chopped up and changed many times over the years by people that didn't really understand it. When we arrived the hammer was so bad the building would shake. We got rid of that but are still having this issue. I appreciate the suggestion about the traps being too small. That makes some sense since they have to handle the condensate from the entire system. That's what was there before, but that doesn't make it right and it's dangerous to assume the guy that put them in knew what he was doing. I think we're going to look at doing loop seals if this continues to be a problem. Thanks again for your help.

EBEBRATT-Ed

@BRader

Seems like you're on the right track.

In my case I should have spotted the problem sooner. It is so common to see a 3/4" trap dripping a main and they are always large enough for that purpose that I just stared at it and thought it was ok. Then I took a step back and thought about the one pipe rads and it finally dawned on me.

Because the pipe is in the basement and the rads are above it doesn't make it obvious.

BRader

@EBEBRATT-Ed That's true and a lot of times you don't have access to what's above. In this case, we couldn't get into the second floor apartments. Sometimes the hardest thing to do is look beyond what you think you know. Thankfully this isn't a huge system and there are 6 traps, so hopefully they'll handle it.

pedmec

You added a wet leg between the condensate pump and the f&T trap? the condensate pump is the vent for the whole system. By creating a water seal you are preventing the condensate pump from doing its job. Which is not to just pump condensate to the boiler but also vent the system. You could have added a crossover vent that would have allowed the air to go up and over and then tie back into the return after the wet leg. The set-up you did will only work until the vent fails.

BRader

@pedmec Thanks for responding to my questions. I appreciate the suggestion. There weren’t a lot of other good choices to get the condensate back across the basement due to the layout and the way the customer uses the space. Originally there was a buried wet leg below the basement floor, but that’s long been abandoned due to leaks. A previous contractor had installed an overhead crossover pipe from the F&Ts, which allowed air to vent but would not return condensate because water doesn’t like to flow uphill. We would’ve liked to have run the condensate return at a gentle slop back to the pump, but that would’ve cut right through their warehouse/storage area for their business. The wet leg at floor level, while not ideal, seemed like the best solution. It did solve the hammering issues that they were having. The added vent on the wet leg is just a riser that’s open to atmosphere just like the condensate pump vent. It’s a piece of 3/4 pipe before the water seal that terminates between the floor joists with a 90. We could easily add a crossover to the condensate pump if it becomes a problem. That’s not a bad idea. This system has taken some time and work to make it operate decently because it’s been so chopped up and modified by many people over the years. 

pedmec

the condensate can go under and back but you add a tee. at the branch you go horizontal at a level just below the water level before you go down, basically a trap. it does not stop the flow, just the air. if the condensate is not returning its because the condensate is plugged at the low point