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# Differential Loop

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Member Posts: 38
Hi All - I am curious to see if someone could explain this system to me. This Hoffman Differential Loop is near the steam boiler in the utility room. It's a big 3-story 2-pipe stone house outside of Philadelphia. I am boing told that this may have been a vapor system. Dan talks about this system in his 'Lost Art of Steam' but it doesn't make sense to me. He also talks about this system in his homeowner book - which made a little more sense - but I can't find the reference to it.
Anyways, we are experiences short cycling with this system. It used to have one air vent, then we added 2 more but still short cycled. When we took the air vent out of the differential loop, the system stayed running, which is why 2 more air vent were added.
Then someone told me that this could have been a vapor system, so I changed the air vents to a vacuum vent but it still short cycle.
Thanks for any advice you may have.

Matthias

• Member Posts: 5,745
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If you are short cycling on pressure (the most likely scenario), then you have to figure out why. Short cycling on pressure is a combination of system size, compared to boiler size, with venting added in.

With that loop you do have a vapor system, all this means is it's designed to operate at very low pressure. The differential loop is there to help prevent the pressure from going to high, I can not remember what pressure it "trips" at, but perhaps @Jamie Hall can offer more input on that one.

If the short cycling is your primary concern start with the basics. Add up the EDR of all the radiation in your system, then compare that to boiler size. Most boilers have a rating plate with how many sq ft of radiation they will support, so compare your EDR number to that. With those results it can guide you to the next steps. If the boiler is drastically oversized (this is very common), then you will only get so good. short cycling will become the name of the game.

If the boiler is drastically oversized, you will need to install a vaporstat to keep the pressure at a reasonable level (you might need this no matter what). Along with that a good low pressure gauge would be a great diagnostic tool, for vapor I'd keep it low a 0-16 ounce max, but others may have a different opinion. Keep in mind this may make the short cycling worse, but the system should run better overall.

No matter the boiler size, if you post how long the mains are, the EDR of the system, someone should be able to recommend how much main venting you need. I'd suggest already you aren't even close. If I was guessing you need a Gorton #2, most likely several. Again @Jamie Hall may be able to offer some input here.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 23,524
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The Hoffman Differential Loop is a very simple device which is intended to keep the pressure differential between the steam mains and the dry returns to a low, controlled value. This provides for even, rapid distribution of steam to the various radiators.

It does this by allowing live steam from the boiler into the dry return if the pressure differential between the returns and the steam main is greater than about 7 ounces per square inch. It will reset automatically for normal system operation when the pressure differential drops.

It is part of a complete system. The rest of the system includes crossover traps at the ends of the steam mains, which allow for venting the steam mains, and one or more vents -- NOT vacuum vents -- at the Differential Loop, as they are shown in your picture. In general, it appears that Hoffman Equipped systems operate well with less main venting than some others -- but I reiterate -- all that main venting must be on the dry returns only, and located at the Differential Loop, nowhere else.

Correctly sizing the main venting is not difficult, provided you have a low pressure gauge on the boiler. Otherwise you can spend a good deal more than you need to on main vents, although you can't overvent the system.

There are several common ways in which the system can be defeated (there are, of course, uncommon ones as well). First, all the radiators must connect to the dry returns. Second, all the dry returns, if there is more than one, must connect together at the return inlet to the Differential Loop. Third, all mains must be vented to the dry returns through the crossover traps (and no vents anywhere else -- this is a common error).

It can also be defeated in operation, if the boiler pressure is allowed to go above about 7 ounces per square inch.

Now you mention short cycling. As @KC_Jones says, this is most likely caused by the boiler being significantly oversized, and has nothing to do with the fact that this is a vapour system. In any event, you will need a vapourstat to control the system, as a conventional pressurestat cannot be set low enough. If the oversize is not too great, the vapourstat may be all you need. However, if the oversize is significantly greater, there are other approaches to effectively down sizing the boiler which I can recommend.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 38
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Hey @Jamie Hall and @KC_Jones. This response is long overdue but we haven't even able to get into the house until now.
The homeowners says the system still short cycles alot and also doesn't heat even. It is a 2-pipe steam system with a Hoffman Differential loop. The vent at the loop has been off since early December.
All the pipes are covered in drywall and are really difficult to get it and look for any traps or vents. Any idea where I can start looking? Thanks for your input.

Matthias
• Member Posts: 23,524
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By saying "the vent at the loop has been off" do you mean physically off, as in removed? If so, what replaced it?

In principle, if the system is working perfectly and the pressure never ever goes of 7 ounces per square inch, you can have an open hole there. Right. Doesn't happen. You need at least one, and quite likely two, vents there.

And only there.

With everything covered up, you have a real problem on your hands, since it is very likely that you can't find and verify the operation of the crossover traps. Do what you can -- but without them, or if they aren't working correctly, it will be very hard to get the system in any kind of balance at all. Do, also, check every single radiator trap. Even one stuck open can throw off the entire system.

And check the maximum operating pressure. If that is over 7 ounces -- that's ounces -- per square inch, the system (even with the proper vents at the loop) will be very uneven, if it heats at all.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 38
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Yes, the vent has been physically off for two months with an open 3/4 pipe. I'm not sure what a crossover trap is but I found this in the garage - see attached picture. I assume this it what you meant?
As for pressure, the system has a mercury style vaporstat. Originly we installed a digital but found with the mercury one the system runs longer. It's set at 16 oz with a 2 oz Differential. See attached picture.
As far as traps are concerned, this is another problem as most of the radiators have metal cages on them that we can't remove.
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Matthias said:

Yes, the vent has been physically off for two months with an open 3/4 pipe. I'm not sure what a crossover trap is but I found this in the garage - see attached picture. I assume this it what you meant?
As for pressure, the system has a mercury style vaporstat. Originly we installed a digital but found with the mercury one the system runs longer. It's set at 16 oz with a 2 oz Differential. See attached picture.
As far as traps are concerned, this is another problem as most of the radiators have metal cages on them that we can't remove.

OK, you found one of the crossover traps. These route the air from the steam main into the dry return, where it heads back to the vent on the Differential Loop. You'll find another of these at the end of each of the other steam mains.

That trap looks like a Hoffman #8. I expect the other crossover(s) will be the same. Assuming the steam mains are all the same length and diameter, the trap of choice is the 1/2" Barnes & Jones "Big Mouth" crossover trap. It has the most throughput of any trap out there.

Then watch how well it works!
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting
• Member Posts: 23,524
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Keep that mercury vapourstat -- much more precise than the newer ones. Then set it to a 7 ounce cutout with a 3 ounce differential. Then make sure it is level.

The trap in the second picture is a crossover trap. Is it working? Are there any more like it hiding somewhere? They are essential.

I'm a little surprised that you aren't seeing steam from that open vent location at the Hoffman Differential Loop -- you should, any time the pressure gets over 7 ounces. Can you take and post a photo or two which show the loop with all three connections, and the connection to the boiler header? How are the dry returns tied into each other and the Loop?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 38
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@Jamie Hall and @Steamhead - thanks for the responses. No steam coming out of that open hole, only air. When I first took it off, I had my hand over it and it appeared to be just air. Before the removed the air vent, the system would run for >1 min on average. When I was there today, the system is running 3-6 minutes.
I did size this boiler and did measure all the radiators in the house. We have a Peerless ECT-06 boiler with a Carlin gas power venter which can make up to 1004 ft2 of steam. I calculated 764 ft2 EDR and then multipled that by 1.25 for the unwrapped pipes in the basement giving me a total of 955 ft2. We have down fired this boiler already using different gas orifices for the Carlin burner.
The crossover trap I found appears to be working as the supply line (2 or 3" pipe) is warm and the return line is cold. I also found that there appear to be 3 different steam mains and based on this discussion, should find 2 more crossover traps. Correct?

• Member Posts: 23,524
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Well, as best I can make out it looks like the Loop is piped reasonably well -- though the line from the header to the loop is longer than I'd like to see, and it would be best if it pitched down towards the loop. Does it? Do all the dry returns connect together before the loop?

I'd venture a guess that you may be running at the correct pressures, or less, which is excellent. I'd still put a couple of Gorton #2s on that vent connection (or maybe three -- that's a pretty big system), though, just in case the pressure does get too high.

Do you have a low pressure gauge to check with?

And yes -- there should be a crossover trap for each pairing of steam main and dry return. They are how air gets out of the steam main, so they're kind of useful...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 5,745
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We have a Peerless ECT-06 boiler with a Carlin gas power venter which can make up to 1004 ft2 of steam. I calculated 764 ft2 EDR and then multipled that by 1.25 for the unwrapped pipes in the basement giving me a total of 955 ft2. ”

You oversized the boiler.  The 1.25 was unnecessary.

Should have used the ECT-05-200, at most the 05-250 and then down fired it some.

2 pipe is very sensitive to pressure and over sizing the boiler is very counterproductive.

Might be a good idea to contact Peerless to see how low you can go.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 3,113
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Peace and good luck clammy
R.A. Calmbacher L.L.C. HVAC
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating
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• Member Posts: 9,828
edited October 2022
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@clammy I think there are 2 risers to the drop header, the first 2 tees connect to risers out of the boiler then the next 2 tees connect to the steam drum. it looks like 2 x 2" risers in to a 3" header is what the manual specifies.

Though it is a bit odd, unless your boiler is surging or priming because of contamination issues with the water, i don't think your problems are at the boiler(other than maybe pressure).

@Jamie Hall there is another possibility with the vaporstat settings and the pressure, that it is getting to 16 oz and the differentia loop control is frozen or clogged.
• Member Posts: 23,524
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A differential loop has no moving parts at all, and since it never sees anything but return condensate it's really really rare for them to clog up. However, they do trip at 8 ounces -- not 16 -- and if the pressure at the header goes over 8 they will allow steam at full boiler pressure into the dry returns (closing the main vent in the process) which kind of brings things to a screeching halt until the boiler shuts off and the pressure drops from condensation.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 9,828
edited October 2022
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Oh, i din't look at the vaporstat that closely. It is a great vaporstat installed poorly. The curl in the pigtail has to be perpendicular to the vaporstat because it expands and contracts with temp and the vaporstat must be level for the calibration to be correct since it is using a mercury switch. The pigtail needs to be turned 90 degrees and the piping rotated until the vaporstat is perfectly level.

Actually I'm not sure now. In the first picture it looks like the pigtail is parallel to the vaporstat and in the 4th picture it looks perpendicular.
• Member Posts: 5,745
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I'll mention it again and add some numbers. Based on the selected boiler, without down firing, the current installed boiler has a 75% pickup factor. The system is short cycling on pressure, it doesn't take much thought to realize what the issue is here. Getting it to run satisfactorily with that boiler may be simply impossible. I suggested before that the OP contact Peerless to see how low they can go, but I have my doubts if they can go low enough.

By my calculation to get it to the max it should be would require you to down fire by 24%.

The current boiler is rated input of 385k
boiler gross output is 321k
system is 764 EDR
764*240=183360
183360*1.33=243868

321,000-243868=77,132 this is the excess beyond normal pick up factor
77,132/321,000=.240 or 24% that needs taken out of the boiler capacity. Honestly on 2 pipe, I'd suggest even lower would be better given the extremely low pressures these are designed to run at.

As Jamie pointed out the vaporstat settings is kind of pointless, since the differential loop trips at 8 ounces. You need extended run times, with the burner firing, that stay below 8 ounces in order to heat the house. No way around that. I'd be cranking the vaporstat down under 8 ounces, then run the boiler, that will show you what you will get. If you can't get the run times to heat the house the only option I see is to downsize the boiler. In the picture you have it set to 16 ounces, as said, that does nothing because of the differential loop. Crank it down.

Should have installed the ECT-05-200 I don't see Peerless approving that much down firing, but I'd still call them to find out.
2014 Weil Mclain EG-40
Boiler pictures updated 2/21/15
• Member Posts: 994
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what is the boiler short cycling from. the pressurtrol, low water cutoff?

I think leaving that existing welded header in place might be a problem. the 4 risers coming out of the welded header have all been taken off between the riser tees coming from the drop header. the condensate from the risers from the welded header cannot drain into the welder header equalizer so i believe its dropping back down into the risers from drop header thereby killing the steam. there is no way that is going to make dry steam. too much exposed metal at the boiler too.
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Don’t take any of my humorous comments to heart , but as usually that system needs some real TLC and that always come w a tag\$ .
Peace and good luck clammy
R.A. Calmbacher L.L.C. HVAC
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating
• Member Posts: 9,828
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An IR camera might help you figure out where the other crossover traps are.
• Member Posts: 38
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Great comments everyone. Thank you. I thought I did a good job designing this system but it appears I may not have. Anyways, the system is going off on pressure. Finding all the cross-over traps is the first step in this finished basement. I found one trap that appears to be working. If that cross-over traps are broken, could that lead to the system shutting off on pressure? Steam leaks through those traps and gets into the dry return, right? What does it mean when the return is dry? There has to be condensation flowing back to the system, so how can it be dry?
• Member Posts: 38
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@mattmia2 the pig tail was facing the wrong back but we fixed that. I realized that once we were done. When it expanse, the vaporstat is moving front to back, not left to right. @clammy, yes the welded header does get hot. The reason we installed the dropped header is so that the steam separates from the water and dry steam enters the welded header. Our supplier also suggested it.
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My recommended piping plan would look likes this. I know the threadolet would have be a little bit of money but it might have been offset by the savings on the of all the extra piping to install a drop header. And this way you would have a better chance at dry steam. Its a vapor system and you need dry steam. Wet steam is going to kill your system and high fuel bills. If i was quoting this job out this is how i would do it. I might not get the job but i also wouldn't have to worry about a failed install if i did. Two things i learned about installing and servicing steam. Rarely is the cause of the failure at the location of the supposed failure and most failed steam systems are caused by incorrectly installed near boiler piping.

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Sorry for the terrible drawing. I need @EdTheHeaterMan help with my drawings
• Member Posts: 38
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@pedmec thanks for that drawing. Very helpful. That welded header was already on the job. Originally, I thought about replacing it but was told not to do that. I agree that dry steam is very important but I didn't realize how critical it is on a vapor system. Do you suggest redoing the entire main piping? Some of other posts have said that the system is oversized. Would repiping it this way, help with that as well?
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If it were me I would find the other crossover traps and make sure they work before I would worry about the near boiler piping.
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A failed crossover trap will have one of two effects. If it is failed open, yes steam will get into the dry return This will kill steam circulation and the heat on the affected main/return pair. If it is failed closed, heating will be slow on that main as the main is no longer vented, but it will heat.

A failed crossover trap or traps will not cause the system pressure to rise excessively.

What will do that is that the boiler is oversized -- from looking at @KC_Jones 's figures, it would appear that the boiler is oversized -- in fact, about twice as big as it needs to be. It might be possible to down fire it some, but it's very unlikely that you will be able to down fire it enough to eliminate tripping on pressure. Live with it. Whatever you do, do NOT raise the cutout pressure above 8 OUNCES per square inch. Just. Don't. Do. It.

A dry return is so called because, while it does have condensate flowing in it when the boiler is running, but no water left in it at any other time. It should never have steam in it. (A wet return always has water in it, being below the water line. A steam main and steam main extension in one pipe systems has steam in it and usually condensate).

Redoing the entire main piping would do nothing at all positive. It certainly won't help with the short cycling on pressure. You should, of course, check all the pitches on the system to make sure that condensate in the mains can make it back to the boiler or to a drip to a wet return, however.

I might also mention in passing that dry steam is not more critical in a vapour system than it is in any other steam system.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 994
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As @mattmia2 has said, the crossover traps first and see if you can get steam all around first. You might get lucky. My suspicion is that even if that happens you still going to have issues with the wet steam. the wet steam could be preventing it from reaching the end. have you disconnect the ends of the main to see if the steam will reach with it disconnected? it will act like a big mouth air vent.

As far as the boiler being oversized i would have to look at the calc's that you guys have thrown around. i have really looked into them. For me i always start at the boiler. If you fail to properly install the near boiler piping the rest is just a crap shoot. You need to start at what you know is wrong. What is the most critical part of the install is the near boiler piping. Failing at the boiler will lead you down the wrong road time and time again.
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And you don't have to remove that welded header. You have a lot of takeoffs coming off of that. use it to your advantage and leave it there. Use it as a steam separator to get drier steam. That open chamber allow any condensate to fall out of the steam. just have a threadolet installed at one end and use that to tie the 3" in just as i have it in the drawing.
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