"P-trap" on dry/wet return

Patrick_North

Hello all,

Yes, it's as convoluted (too me, anyway) as the post title makes it seem.

We're doing some renovation work on our church and need to reposition a one pipe steam radiator. The situation gets a bit odd from there- I'll try to be clear.

The boiler for the church is actually in the second building downhill from the church. The condensate returns in the church- though they run close to the floor- are actually dry returns within the church. As they travel downhill to the boiler, they become wet returns. Make sense so far?

This radiator in question is in the church basement, so while the main feeds most of the radiators (upstairs) upwards, this radiator is fed downward. A second pipe handles the condensate and connects to the "main" return back to the boiler.

Now here's the final wrinkle: someone piped in what amounts to a p-trap between the radiator and condensate return. In having the radiator moved we don't want to screw up whatever benefit this "trap" provides, but it also presents an opportunity to correct what might have been a bad idea in the first place.

I'm guessing that the "trap" is meant to prevent steam from short circuiting up the (dry) return. If so, is this an appropriate method to handle the situation? The radiator does heat up and produces no water hammer. In other words... it ain't broke.

Thanks,

Patrick

Dave in QCA

Devil's in the Details

I've got a few more questions for you.

Regarding the dry return running along the floor in the church building, are the steam mains trapped before they enter this line?  Or, if not, is this return line charged with steam when the boiler is running and steam has been established throughout the system? Also, what is the maximum pressure that the boiler operates at?  And, how deep is this p-trap that you describe?  3", 6", 30"?



All of these things would make a big difference so it's better to get more information before jumping in with answers.

Patrick_North

details

Hi Dave,

No, the mains are not trapped before entering this "dry" return.



You asked: "Or, if not, is this return line charged with steam when the boiler is

running and steam has been established throughout the system?"

I'm not sure I follow the question, but maybe this gets to the point. There are main vents at the end of the mains, not on the "dry return." I suppose that (minus this "p-trap") the radiator in question would act as a vent on the dry return, but beyond that there's nothing that suggests steam would be encouraged to travel very far down the dry return. Where this rad drips into the dry return is a good 50' downstream from where the mains drip into the dry return.



Boiler pressure: An earlier problem with the boiler was the pressuretrol. The boiler would get up to 5+#. This is in check now, but as I said, it didn't seem to be causing any water hammer or other problems with the radiator.



The "trap" is probably about 6"-10" high. I was viewing it from above (it's currently behind some paneling) and so I'm not of this measurement.



Thanks,

Patrick

Dave in QCA

Keep the Trap!

OK, the person who piped it up knew what he was doing and if you move it, it is important to keep all the details exactly the same.



As you describe the system, there are no traps, and so the dry return is essentially an unvented pipe connected at the end of the steam main.  As such, it is charged with air that cannot escape, and the condensate draining from the steam main flows through without any difficulty, and the pressure in this dry return is the same, or nearly the same as the pressure in the steam main and the boiler.  Since the pressures are all nearly equal, except for line losses, the pressure on the return main would be nearly the same as the pressure of the steam main feeding the radiator in question.  Therefore, the p-trap does not have to be tremendously deep.  But, if you omit the trap, there is the very real likelihood that air from the return will migrate into the radiator and would therefor allow steam to flow all the way down into that dry return, which you don't want.



A potential problem that I see with the setup comes if the supply to the radiator is shut off.  If this is done, the pressure in the radiator is going to be 0.  If the system runs a length of time that allows some pressure to be built, the pressure of the system, and the air in that dry return might be sufficient to blow the condensate out of that p-trap, and then would allow steam to flow down the return and into the outlet of the radiator.  One way to prevent this, if there is any chance that the radiator might be shut off, is to put a shutoff valve on the return connection as well, just like they did in the old two-pipe vented systems.  That is pretty much what you have, except in the two-pipe vented, the return usually connected to a wet return in the basement. 

Patrick_North

Excellent

Will do. Glad I asked!

Now that you say it, it makes sense that this radiator wasn't plumbed with a shutoff valve. We'll keep it that way.

Thanks for your help, Dave.

Best,

Patrick