Off The Wall: Steam boilers and wet returns

September 1, 2012
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I am in the process of getting bids to replace my old (circa 1930s) steam boiler. 



I am in the process of getting bids to replace my old (circa 1930s) steam boiler. The system is one-pipe steam. The wet returns are very long and have a slight pitch to them as they travel across the front wall of the basement. As they turn the corner and head toward the boiler, they pitch more steeply and then finally enter the bottom of the boiler. One of the wet returns is positioned about 1 ft. below and runs parallel to the other one.

This system is very old, so no Hartford Loop, equalizer or such. The system has run fine, no banging, etc.

I have had two different opinions on the existing wet returns. One contractor wants to repipe the wet returns and bring them down to floor level. He is not happy with the current configuration. The other contractor said there will be no problem and the location works in our favor.

I re-read a portion of one of Dan Holohan’s books and this seems to be similar to an example he gave that produced negative results for the installer. With the existing boiler, if you measure up 28 in. from gauge glass center, those wet returns are indeed wet; 95% of that piping is below that line.

If we go to a shorter boiler, over half of that wet return piping will be above the 28-in. measurement. Will this cause banging upon startup? Am I missing something here? Steve Nichols



I think you are confusing something in reference to the 28-in. measurements. Dimension A (28 in. above water line) states that all steam-carrying pipes must be at least 28 in. above the water line in the boiler. That has nothing to do with the level of wet returns.

A wet return MUST be below the water line of the boiler. This means that all portions of the wet return must be below the water line. Since the water line can fluctuate, having it several inches below the water line would be advisable.

The big question is what is the water level of the new boiler compared to the old boiler? If it is lower, and it probably is, then you need to measure the level of the wet returns to make sure they are all below the water line. If they are not, you might consider a different model boiler, setting the new boiler up on blocks, lowering the wet returns or installing a false water line setup. Dave in QCA



Thanks, Dave. I was under the impression that if the wet returns are above the boiler water line that they, too, become steam-carrying pipes. As it stands now, the wet returns are mostly above the existing boiler water line. They are pitched ever so slightly, as noted in my first post. These returns average at a good 4 ft. above the ground for a good portion of their travel back to the boiler. I don’t know how high the boiler would need to be placed to achieve the same result. Maybe if I can get some dimensions on the new units I can figure this out. Steve Nichols



What kind of shape are the wet returns in? If they are about to rust through, you’re looking at re-piping in a few years anyway. They tend to rust through first at the threaded ends, where the pipe is thinnest. Hap_Hazzard



If your pressure is high enough, the return water level is probably rising due to boiler pressure and making the needed water seal to keep steam out. If the pipes are in good condition, you could find an upstream joint to make a vertical drop to the floor and reuse the dry sloping return as a new wet return. Dry returns are frequently pretty clean inside and may be good for another 20 years. Put in some flushing valves at the boiler end to keep them free from rust particles.

Make sure you get a good low-pressure gauge (0-3 psi) on your new boiler, and enough main vents to keep the venting back-pressure below 2 ounces. Nicholas bonham-carter



Hap: The wet returns are in good shape from what I can see.

NBC: Boiler pressure when running never goes above 1.5 psi. I think I understand about dropping the lines down to ground level and reusing the piping, but I guess my question still focuses on how do the new boilers function (sensitive to water level, much less water within the boiler, etc.) should these returns get dropped below the new boiler water level to avoid problems?

And if a wet return is defined as return piping below the water line, how much piping is necessary for these new boilers or is that not an issue? I have about 15 ft. of return piping that is below the current water line and about 25 ft. above. 

Would a false water line work in this case as was suggested and do most steam professionals know how to do this?

I’m not against re-piping to accommodate the new boiler but I want to make sure I understand what is needed since I am getting conflicting views from contractors currently placing estimates on the job. Steve Nichols



If you look at “The Lost Art of Steam Heating,” you will find tables that indicate what size dry returns and wet returns should be based on the EDR they are serving. Your lines may be one size too small. However, the installer appears to have attempted to compensate for that by adding unusual amounts of slope.

So, do you have wet returns or dry returns? They are dry returns, except that the last portions of the lines are too low. They should travel at the recommended rate of fall until getting to the boiler and then drop vertically down to well below the water level. The horizontal portions of the piping should be no lower than 28 in. above the boiler water line. The other option would be to drop straight down at the end of your steam mains and run them back as wet returns.

The question remains as to why your returns are not banging up a storm. It appears to me that your main vents are probably located at the end of the main near the point that the drip line drops into the dry return. If that is the case, it leaves the return line unvented, which is OK. We all know that steam cannot travel into a pipe, radiator or whatever unless the air can get out. If your dry returns are not vented, the steam can’t get in, and thus there is no banging.

What works well with your old boiler may suddenly change when you install a new boiler with higher exit velocities and a different water level. If it were my system, I would re-pipe the returns. Wet returns are notorious for filling up with sludge, I would opt for dry returns. However, I would increase the pipe size and keep the main vents at the end of the steam main, assuming vents are already at this location. If not, I would install them.

Leave the dry return unvented and then there is not much need to insulate it. When the lines come around to the boiler, they should drop to well below the water line before they tie together, then they should rise up and connect to your new equalizer line to form a proper Hartford Loop. Dave in QCA



You are correct, sir! The two steam mains are vented as you describe, just before they turn into the returns. It would make sense now that with no venting after this, the steam would have a difficult time getting into them, thus no water hammer. I’ll go back and revisit the wet return pipe dimensions based on the EDR for each pipe.

If, as you suggest, I opt for dry returns, are you saying that they should be at least 28 in. above the water line, then before entering the boiler, drop down? Steve Nichols



Yes, that is what I meant to say. Even though you may not be getting steam into those returns, it could be possible. Also, check the pipe sizes and make sure they are adequate. If they are smaller than they should be, your system may be working because of the steep pitch of the pipes. If you raise the low end and decrease the pitch, it may cause a problem if the pipes are undersized. Dave in QCA



I would favor a dry return over a wet return whenever possible. I’ve noticed that a return like yours, with so much pitch toward the boiler, does not cause as many issues as you would think, as long as the return drops fully below the water line. It’s the truly horizontal pipes that may be a concern.

You could always fix the piping after the new boiler is installed. See how it reacts and gauge its priority. JStar



Thanks JStar. I just got a quote for re-piping the returns down to the floor and it’s a budget-buster for me. If the new system works fine with the existing return piping, that would be great. That is what I am hoping for and somewhat what I am paying for (good results).

The option for re-piping later is something that I didn’t consider. I guess at some point I need to trust that the individual doing the work will do it right and deliver the expected results. I’m not a specialist, just a curious and concerned homeowner who likes steam heat. Steve Nichols



I would at least drop the end of the return down to the floor before rising up into the Hartford Loop/equalizer. That should be easy because you’ll need to cut those pipes anyway to remove the old boiler. JStar


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