Hoffman Differential Loop

WayneMech

I am working on a 1926 2-pipe system, with a Hoffman Differential Loop system, in a 2 story house. The boiler was replaced about 5 years ago. The first floor heats okay, but the second floor is not doing well. Since the vast majority of them were bad, I decided to rebuild them all. The customer says the problem is persisting. The vent seems to work. I looked at a cross-section of the Loop, and I am wondering if it could need cleaning. There is a (drain?) plug in the cap at the bottom of the loop. Any other suggestions?

New England SteamWorks

Some pictures of the boiler and Differential Loop and their associated piping would be a big help.

Steamhead

What vent is installed on that system?

Jamie Hall

And where is the vent? There should be one, and only one, vent location for a system with a differential loop (you can have as many vents there as you like, but only there): right where the dry returns drop into the loop. Nowhere else. Nowhere. The ends of the steam mains must vent into the dry returns using crossover traps and those must be functional -- have you checked them?

Once that is straightened out...

Pressure is critical. The Hoffman Differential Loop, by design, keeps the pressure difference between the steam mains and the dry returns to 7 ounces per square inch or less. That's ounces. Anything over that, and it will allow live steam into the dry returns at boiler pressure. That's what it's meant to do. This will, of course, stop the heating in its tracks. Therefore, you have to have a vapourstat set to cutout at 7 ounces or less. Typically it can cut back in at around 2 ounces.

There should be no need to clean a differential loop. True, there is a plug at the bottom of the loop, installed by the factory, but the only thing that gets into the loop is condensate from the dry returns. If and when the loop trips, the action is quite sufficient to eject any sludgy material which might get in there.

WayneMech

Okay, guys, I will get the pictures ASAP. While rebuilding the radiator traps, I found 1 crossover trap, and rebuilt it. I suspect there is another one, inside a soffit in the basement ceiling. Is there a way I can check for this, without opening the soffit? The pressure was set at 5 psi. I changed out the control to a low pressure stat, set at 1.5 psi, with a 10 oz differential. I will lower this to7 oz CO/2 oz CI. There is but one vent, on the condensate return, just before the Hoffman. I know that it vents, but would the 1.5 psi system pressure still keep air in the system? Thanks for all the help.

Jamie Hall

I can't think of an easy way to check the crossover which may -- or may not! -- be in the soffit. However, that said -- if steam gets to the end of that main quickly, it's probably open. If steam does NOT get into that dry return, it's probably closing on steam, and thus OK. That's kind of rough.

You may want to add venting capacity where that one vent is. In fact, it would probably be a very good idea to do so. As I said above, don't even think about adding vents anywhere else!

The 1.5 psi pressure will cause the Differential Loop to trip. Once it does, that vent will close -- that's what it's designed to do. At which point any remaining air in the system will stay there, and that's not good. So yes, the 1.5 psi pressure will keep air in the system, much to the detriment of your heating. In fact, any pressure over the 7 ounce trip will do that, and waste a lot of fuel and money. Once you get the cutout down to 7 ounces, you will find that the differential loop doesn't trip any more, and your heating will be quicker and more even and you will use a good bit less fuel.

WayneMech

Thank you, Jamie. This makes sense. This is my first Hoffman DL system, and I'm still getting used to incorporating the DL into my thought process. It's like the "turn it down!" lesson, on steroids. If I am thinking about this correctly, the boiler may seem to short-cycle on startup, since the air will "push back" until it is gone, after which the radiators can condense the steam at about the same rate the boiler produces it. This will then keep the boiler from short-cycling for the rest of the cycle. Would installing a vacuum vent (Hoffman #76) help?
Thanks,
-Dale

Jamie Hall

You don't really need a vacuum vent, although it won't hurt anything. Ideally, when the system is running as it should, the vents will never close anyway. The system in the place in my signature uses one Gorton #2 which supplements the original Hoffman 76, and is fine -- operates at just a bit over 1 ounce until all the radiators are full.

As you say, the Hoffman DL is sort of a strange critter, and teaches that turn it down mantra with a vengeance!

WayneMech

Okay, I have recovered from pneumonia, and am back to work. The lone vent is a #76, but looks pretty bad. The vent action goes like this: The boiler starts, and builds up to 6 oz. cycles off, drops to 2 oz, restarts (at about .5 oz, after waiting for the flue damper to cycle open). It is only when the pressure is on the decrease cycle that the vent opens. It opens at about 3 oz, the air comes out, and no more comes out until the next decrease. The house is still not comfortable, as the system cannot seem to keep up. The radiation adds up to 140,820 btu. Adding a 33% pickup load, at 80 efficiency, the system can support a boiler supply of 234,113.25 btu. The boiler is a Hydrotherm VGA-300-BSV. This does not help the situation. The Main Riser from the boiler is an ugly mess of nipple fitting, in 2". After it makes the turn to horizontal, it increases to 3", in a concentric reducer. I believe there is a little condensate sitting there, but I wonder how much effect it is having on the system. It is getting replaced, anyway.
But I digress. Why is the vent only venting on the "Down stroke"? Is this normal? My guess is "no", but what should this symptom be telling me?

Jamie Hall

Is that vent where it should be, on the dry return above the Hoffman Differential Loop? If so, it isn't working the way it's supposed to. It almost sounds as though steam is hitting it... and I just sent you a reply to your PM!

WayneMech

I shot the vent with the thermal camera, which indicated 111*F. The condensate piping was about the same. The vent is pretty ugly/a little corroded, but appears to be in the correct place, and is the only one in the system. I will get some pics tomorrow morning. See you then. -Dale

Steamhead

The near-boiler piping is at least part of the problem. That boiler looks like it has a 3-inch steam outlet- here's the piping diagram:

https://www.manualslib.com/manual/972203/Hydrotherm-Vga-175b.html?page=6#manual

WayneMech

Here are the pictures:






Sorry they are not better, but there is very little room in there.

Jamie Hall

Didn't give you much room to play around with in there, did they? Do I take it that the lead to the odd fitting on the leg of the differential loop assembly goes up beside the square box, then over, then back down behind to below that king size gate valve, then ties into a T which goes to the steam main on its leg, and on down to what passes for an equalizer on the other end of the run?

If so it is at least hooked up to the steam line properly...

I must say that the boiler piping is just a bit unusual...

The vent is more or less correctly placed. However, judging by your description of its behaviour, it may be not working right.

You don't have a whole lot of room, though, for options. I'd consider replacing it in kind -- that is, with another Hoffman 76. Oddly enough, given the emphasis on massive venting, that may be adequate -- but only real way to tell is with a low pressure (0 to 3) psi gauge.

I am curious about crossover traps. A Hoffman system has to have them, one at the end of each steam main going over to the associated dry return. Are they there? Are they working? If they are there but failed closed that may explain the more rapid pressure rise than I would have expected (or if they aren't there at all!).

Steamhead

That vent is too small compared to what was there originally. You need at least one Gorton #2. I would not use a Big Mouth there because it does not have a float that closes against water.

And you can bet on there being another crossover trap. Open the soffit and you should find it.

Jamie Hall

I suppose a Gorton #2 would fit -- that certainly would be better.

Fred

I don't know??? A Gorton 2 is about an inch taller than the Hoffman 75/76 plus that 3/4" tapping will have to be bushed down to 1/2" for the "2.

WayneMech

If all goes well, I will get the 3" pipe installed today, as well as the #76.

WayneMech

Okay, so I didn't get the pipe replaced, but I did replace the vent. Yes, the old vent was bad. although I don't know how it vented at all. On the system, it vented on the "down stroke", as described above. Off of the system, I cannot get air to pass through it at all. I have never had a vent fail in this way before. It also occurred to me that, since it is a Hoffman system, it should work all right with a Hoffman vent. I know, they don't make the same vents they did back then. At any rate, the new vent is working well, and the system seems to be working better. It is 45 degrees outside today, so the system won't get much of a test, but I believe this was near the heart of the problem. Additionally, the boiler does not cycle on pressure nearly as much, after the vent replacement. The 2" pipe will get replaced next week, which, I hope, will yield even better performance.
-Dale

Jamie Hall

I'm delighted! I love Hoffman equipped systems -- so simple and they work so well -- and I'm pleased that yours is coming back to life!

WayneMech

I must admit that I am excited! Earlier this week, we replaced the 2" supply, from the steam chest, with 3" iron. Oh, Mama! What a difference.
As we broke the seal on the system, which had been cycled off long enough to just leave the pipes pleasantly warm, the vacuum in the system said "Hi, guys! Good to see you again. I'm feeling much better these days. Almost like I did in my 20's. What's happening today?".
When we took the horizontal run loose, we looked inside, and saw the water line left by the constant puddle. That is what made the pressure gauge needle go crazy. On startup, everything was very quiet, less the noise of the burners. Then the vent started, very smoothly, with a constant, steady flow. We shot the radiators with a Flir C2 thermal camera. I giggled out loud. Nice and even, slow steady heat came across the top of those gorgeous Rococos. Back at the boiler, the needle was just barely off the pin. Enter the client. I explained how the lower the pressure, the lower the bills. He's lovin' it.
Today, I am going back with a pressure/vacuum gauge, and taking some follow-up pictures, to be posted this afternoon. It's a bit colder today, so I am hoping to get some operational data.
In the words of Hannibal Smith, "I love it when a plan comes together!".

Jamie Hall

Thank you! And keep it up! They do work well, don't they?

Steamhead

WayneMech said:

...... Nice and even, slow steady heat came across the top of those gorgeous Rococos.......

A Hoffman system, having the later style Differential Loop, with Rococo radiators?

That tells me the Hoffman system was a retrofit to an earlier installation.

American Radiator had stopped making Rococos by about 1920. That type of Differential Loop came out in the very late 1920s or early 1930s. So there are four possibilities as to what the original installation might have been:

1- One-pipe steam;
2- Two-pipe air-vent steam;
3- An earlier version of Vapor, like a Broomell or Tudor system;
or 4- hot-water.

The simplest way to tell would be to look at the radiators and see if any air vent holes have been plugged off. This would be obvious. When you go back, let us know what you find.

"Heating Archaeology", indeed.

WayneMech

In using the term "Rococo", I am using Poetic License, because it rolls off the tongue well. Everything but the boiler, trap guts, and vent, is original.

WayneMech

I finally got some pictures, and some data.



I had to replace the fiber gasket with a rubber one, since the flanges are slightly misaligned. The needle danced a lot. It was from the center gasket hole is slightly smaller than the center flange hole. This creates a puddle in the horizontal run. A little trim work on the gasket, and voi-la!, smooth action. It runs at about 2-1/2 oz pressure now. Over the summer, the boiler is getting a thorough cleaning. It has to wait, to accommodate the owners cash flow, but at least it is getting done..


This is quite frustrating. I "copy and paste"d the picture, as in previous posts, but it does not seem to be working. I will keep trying.

WayneMech

WayneMech