Venting Hoffman Differential Loop and other issues

JimA

I have an old Peerless Boiler that has a Hoffman Differential Loop. It is a hot water closed system and the most remote radiators lately have been only partially heating. My plumber told me to fill the boiler and turn up the heat to create steam and use up the air that must be in the boilers. There is a vent on the Differential Loop that is a Dole that I need to replace. Any suggestions on whether the Dole is the best vent or if there are other approaches to ridding the air? The house is 85 years old and the radiators are Hoffman which I have been unable to find any valve replacements for. Any wisdom is appreciated

KC_Jones

If you have a Hoffman Differential Loop and dole vents you have a steam system not a hot water system.

Post some pictures of your system so we can take a look at what you have.

Jamie Hall

First off, your plumber is, clearly, clueless. He may well do far more harm than good. Find someone who knows and understands steam.

Do NOT fill the boiler past three quarters up the sight gauge. If the water level is higher than that, drain it down to that.

Now. Second. The Hoffman Differential Loop is an amazingly simple arrangement. However, it must, repeat must, not be operated above about 8 ounces per square inch pressure. That means that you must have a vapourstat to control the maximum pressure on your boiler -- which is likely to be oversized, but that's a separate topic (if it were right sized, it wouldn't reach the 8 ounce system trip in the first place, except on very long runs).

Third, the vent at the top of the loop assembly is the only vent for the entire system. The Dole was probably OK for when it was coal, if it ever was, but isn't now. You need at least one, and preferably two, Gorton #2 vents at that location.

OK. Deep breath. Now go out and look around all your piping. At the far end of each steam main, you will find -- or should find -- a crossover trap. This trap will be piped up from the steam main , then over, then down into the associated dry return. You need to make sure that these traps are present and are working properly. There must be no vents at those locations -- one of the more common knucklehead revisions is to put vents there, and they must be removed for the system to operate properly -- and the crossover traps reinstalled if they were removed.

There should be no need to replace the Hoffman radiator valves, which is probably a good thing as there are no new manufactured replacements available. However, they aren't all that hard to repair if you need to -- but they are bullet proof, and except to turn a radiator on or off you should never need to touch them. I would note that they can be exceedingly stiff to turn, if they haven't been operated for a decade or two, but that doesn't mean they are broken. Patience and more patience and you can get them to operate.

At the outlet end of the radiator, though, you will find traps. These do sometimes fail -- particularly if someone has cranked up the pressure. Fortunately, they are very easy to replace, and replacements are readily available. Unfortunately (or fortunately?) a failed open trap -- which is the more common failure -- in a properly adjusted Hoffman system can be rather hard to detect (it also, in a properly adjusted system, doesn't hurt operation much if it's failed -- that's the good news). Failed closed is easy to detect -- the radiator doesn't heat even with the valve open.

On your symptom of distant radiators not heating, or partially heating. The first place I'd look is a failed closed crossover trap -- or one which is missing completely. The second thing I'd do is replace that Dole with the Gortons. The third thing I'd do is get the pressure down to where it belongs. With a Hoffman system, if the pressure goes much over 8 ounces, the system pretty well stops operating, even if the burner is merrily burning away (it will still provide some heat -- but any radiator which wasn't hot across won't get any farther until the pressure is allowed to drop again).

Last, I might note that there are a number of possible knucklehead changes on a Hoffman equipped system -- or any vapour system, for that matter -- which can make it perform poorly. Take some pictures of the near boiler piping and the piping around the Differential Loop, and let's have a look.

Jamie Hall

I just noticed that you have listed this thread under "Domestic Hot Water". Ah... no. It should be under "Steam Heat".

I'm glad I saw it -- I look at all the threads -- but not everyone does.

@Erin Holohan Haskell -- is there any way to move this person's thread to where it belongs?

DanHolohan

I moved it, Jamie. Thanks. Great advice, as always.

JimA

Thanks, Jamie. I had just purchased a Gorton#2 and have replaced the Dole. I have attached pictures of the boiler, the differential loop and what I assume are the traps. Take a look and give me your final thoughts. I am in Chicago and appreciate any referral.

Steamhead

Yes, those are the crossover traps. They look like Hoffman #8.

The steam piping above the boiler can best be described as pathetic. The risers from the boiler look like 2-inch, the problem is that all the rest of the piping is too, and it's configured completely wrong. I don't know the model of the boiler but the main header should probably be at least 3-inch, and the connections to the steam mains should be full-size.

This boiler needs to be completely repiped.

Try getting in touch with @The Steam Whisperer - he's located in Oak Park.

the_donut

Water line looks way too high from picture.

Jamie Hall

My final thoughts? What @Steamhead said, redoubled. Plus -- the water line inside the boiler is too high. The pressure is too high. However -- I'm going to bet that boiler design water line is much too low, and that somewhere out in the wilds of the basement there is a wet return and water seal which is dry. Bottom line -- the whole boiler installation is a catastrophe.

Do see if you can get hold of @The Steam Whisperer (Formerly Boilerpro) !

Mike Cascio

That near boiler piping is a disaster. Also you need a vaporstat on that system, not a pressuretrol. This system needs to run under 8 oz. Not a few pounds like it is now. Please find a steam professional in your area that can help.

Erin Holohan Haskell

Jamie Hall said:

I just noticed that you have listed this thread under "Domestic Hot Water". Ah... no. It should be under "Steam Heat".

I'm glad I saw it -- I look at all the threads -- but not everyone does.

@Erin Holohan Haskell -- is there any way to move this person's thread to where it belongs?

Thanks, @Jamie Hall.

DanHolohan

@Mike Casio, there's not limit on the system pressure with the Differential Loop. It will create a differential pressure from supply to return under any conditions and that will allow the condensate to return. There's a good explanation here. Scroll down a few pages.

https://heatinghelp.com/assets/documents/89.pdf

nicholas bonham-carter

The problem with the piping on the boiler is the connection between the boiler risers to the mains. This will blow up a lot of water into the system. In addition to that, the chemicals in the water may result in a more violent boil throwing more water up into the system.
Many people here have been able to remedy their own piping, (in the summertime), so if you are handy, with advice from here, you could do it.
Maybe the next course of action will be to check the operation of the various traps, and drain and refill the boiler to get back to clean water.
Can you list any symptoms you have had with the system, including the end radiators slow to heat? The end result of these changes will be a reduction of fuel usage, and even, quiet and comfortable heat.—NBC

JimA

NBC,
I really haven’t had other problems except a large heating bill. The water level is only that high from what I explained earlier and the coloring is from 8 Way in the boiler. The piping has been the same for the 20 years I’ve lived in the home and this is my first year with this problem. Do I disconnect each of the traps or just those on the side of the House impacted?
Jim A

nicholas bonham-carter

Find the traps whose outlets are steam hot, and order new inside cages from Barnes and Jones, and many problems will improve. Proper piping down the line will make things even better!—NBC

JUGHNE

I don't mean to hijack this posting.
But, seeing how there are some old pros tuned in to this discussion I thought I would pose my dumb question for the day.

Cross over traps vent air from the main directly into the dry return and close when steam hits them.
Good large air vents such as G-2 or Big Mouth vent air into the basement and then close when steam hits them.

Why must the cross over traps not be replaced with air vents.
It seems the air vents would pass more air quicker than the traps.

I assume the traps nor the air vents would reopen until cooling down.

Does the Differential Loop need the added air pressure venting from the traps. It would get air from the dry return, would it not?

Jamie Hall

The Differential Loop (and some others -- Webster, for instance) really has two functions: one is to limit the pressure difference across the radiators (and, not incidentally, the traps!) and the other is to limit the pressure difference between the boiler pressure and the returning water, thus allowing the returning water to return. The Differential Loop does this by limiting the pressure difference to 8 ounces per square inch, thus a maximum of 14" stack of water in the drips to the wet returns.

It does this by a startling simple approach: if the pressure difference between the steam main or header and the dry return rises above that figure, the Loop immediately "trips" (no moving parts!) and allows steam into the dry return -- and closes the only vent on the whole system, which is right at the loop. This pressurizes the dry return and thus the wet returns and the water gurgles back into the boiler and the traps are spared the pressure differential. As the steam in the dry return cools and condenses, eventually the loop resets, the vent opens again, and off you go.

If there is a vent anywhere else on the system this action may be defeated.

A crossover trap will pass a lot of air -- in fact, a B&J Big Mouth has a lot in common with a crossover trap -- so the question really becomes how big a vent do you need at the loop?

DanHolohan

The Loop needs to shut it's main vent very quickly to establish the proper differential pressure, so I think that's the reason why the used the crossover traps and the single main vent at the Loop.

DanHolohan

Jamie said it better than I could, and at the same time. Thanks, Jamie!

JUGHNE

So this is really to cover the situation on perhaps start up before steam gets to the crossover traps. We work with steam from the boiler to close that vent on the loop? If our "after market" vents had not closed yet, then no pressure would build up in the dry returns, and no pressure applied to the condensate return water to push it into the boiler?

If the condition of high pressure existed after steam had got to the end of the main, closing the crossovers or air vents then either would work. If not then we have a "leak" of pressure at the end of main??

I knew this was the right time to pose this question, thank you.
For me and any one else who wondered, if any.
I have been accused of being the only one at a training/continuing ed class to ask questions, it causes some eye rolls, but later someone might say "I was wondering about that".

Jamie Hall

Bit of a history lesson here. If you look at the advertising and flyers for the Hoffman Differential Loop you will see that it was referred to as "The Watchman of the Water Line". There's your clue. Back in the bad old days of coal, it was difficult to control the fire in the boiler sometimes. Even with some rather ingenious damper systems, the fire would flare up rather badly -- and the pressure in the boiler and steam mains would rise remarkably fast. The whole idea behind the Differential Loop was -- as it's name implies -- to ensure that the pressure in the dry and wet returns was always within a certain rather narrow margin less than that in the steam mains (for Hoffman, 8 ounces per square inch). The hope was that then a flaring fire wouldn't raise the steam main pressure so far that water was forced out of the boiler into the drips and even the dry returns, leaving the boiler dry (remember that many systems had the dry returns open to the atmosphere). Dry boilers are not happy campers, and there were no low water cut offs on coal boilers which worked fast enough to prevent damage (if there were any cut offs at all!).

So the idea was to measure that pressure difference with a manometer (which is really what the loop is): you attached one leg to the boiler's header, and the other to the dry return. If the differential got too great, the steam pressure would force the water in that manometer gadget down so far that steam would escape into the leg attached to the dry return. Then what? Well, you simply place a thermostatic vent at the top of that dry return leg. Normally that vent would be open at all times -- there's not supposed to be steam in the dry return -- but if steam got around that manometer affair (what I call "tripping" the loop) it would hit that vent -- which is sitting right there -- almost immediately and close it -- and you then got header pressure, or very close to it, throughout the system. Water couldn't back out of the boiler since the pressure on the wet returns was now the same as the boiler pressure, and you solved your problem. Eventually the fire would relax (or the building super would put down his newspaper and investigate!) and things would calm down. The vent would reopen, the loop would refill with water coming back from the dry return, and all would be well. As @Dan Holohan noted, because of the way the system worked, it would tend to maintain that 8 ounce differential regardless of boiler pressure, and the system would continue to operate -- although not as well.

No moving parts except in the vent. Nothing to fail (those Hoffman 75 and 76 vents are pretty sturdy beasts!). Lovely system.

It turned out there was a side benefit which is still with us today: the pressure across the various system traps could never rise over that same differential. This prolongs the life of the traps quite a bit (like almost indefinitely...).

There is a third pipe connecting to the loop, by the way -- it goes to the wet return, and is connected so that condensate from the dry return overflows and goes back to the boiler when the loop manometer is filled to the correct elevation.

You still need an LWCO -- there are reasons for low water other than high pressure! -- and for efficiency and protection from overall high pressure you need pressure control if your system is not perfectly matched, but the loop still works, decades after it was installed, and watches the water line and protects your traps. What's not to like?

DanHolohan

In one of Hoffman's old pieces of literature, they likened their Differential Loop to a six-shooter in the days of the Old West. They commented, "You don't always need it, but when you do, it's sure good to know that it's there."

That has always made me smile.

JimA

Hi,
I am the guy who started this post and now have a new question. I am obviously a novice and the stream has been terribly informative. I bought the replacement cages that NBC recommended, but after I replaced the one trap cage that had the symptoms NBC described, the outlet pipes are equally hot. The cage seemed oversized for the Hoffman 8 trap, but it has a spring on top and I felt I was really forcing it down. Also, the base was flat rather than pointed as the original and I was concerned that the seating wasn't proper, though the entire old cage lifted out and I know there was nothing in the base. Could I have done something wrong?

Jim

Jamie Hall

The question is -- how hot is hot? Keep in mind that the condensate from a radiator which is full of steam will be near, but not at, steam temperature. This is rather hard to judge by hand... you best bet is to get and IR thermometer (they're cheap) at your local Big Box, and compare the inlet and outlet temperatures of the trap. The outlet should be several degrees cooler. Anything under 200 or so is probably OK.

New England SteamWorks

Sounds like you either have the wrong size cage unit, or the correct replacement cage unit also requires a new cover (not uncommon).

JimA

One last shot. I went to Barnes & Jones and identified a replacement for the Hoffman 8 cage trap. I purchased their item # 3494, but there was also an item #2168 listed. One was with an inverter(?) the other without. I am concerned that I bought the wrong one and thus the seating and size issue? any help?
(BTW, there were any number of #8s (8C,etc) listed but mine was the original #8). Sorry to be such a poor student...

Sailah

There are a number of Hoffman #8 traps. A plain old #8 will have an integral seat. In which case 2168 is correct cage unit.

A 3494 would be for a #8 with CL-1 or CL-II on it. These have a removable seat. Look down in trap body. Is there a hex there for the seat (removable?) If so you need the part number you bought. And also to remove the seat.

There should be firm pressure with the spring but nothing you can't easily push the cover down with.

A 3494 will have a larger brass seat to fill the opening when the old seat has been removed, which is always recommended. I suspect you have a regular #8 in which case the seat will just be the hole in bottom of trap.

Which also would create the added spring pressure since your cage unit is not fitting down in the hole. Which would allow steam to pass since it's not sealing on the gasket. Which would create elevated outlet temps you describe.

If in doubt take pics of trap body from side, cover from top and a pic looking down in trap and I'll give you exact advice.

-Used to work at Barnes & Jones

DanHolohan

You left a very large hole behind when you moved on.

JimA

Thanks, Sailah. No hex, no seat; I need the 2168 but they seem harder to find. Got the 3494 from Zoro, but they did not offer the 2168. Any recommendation on where to buy or best price on internet.

Double D

You could try here.https://www.statesupply.com/barnes-and-jones/steam-trap/cage-unit/hoffman

Sailah

JimA said:

Thanks, Sailah. No hex, no seat; I need the 2168 but they seem harder to find. Got the 3494 from Zoro, but they did not offer the 2168. Any recommendation on where to buy or best price on internet.

Where are you located? I'll tell you the rep to call.

State Supply is a good one.
Thuemling Industrial in WI almost def has them on hand.
FW Webb in New England & NY probably has them.

Zoro gets them from Grainger and Grainger buys them from Thuemling so I'd try them.

B&J will be closed tomorrow due to storm so if none of those I listed above have them you'll have to wait. The factory has them, that I know, you could always call them and ask if they'd make an exception.

Sailah

DanHolohan said:

You left a very large hole behind when you moved on.

I do miss being there. I went back a couple months ago to volunteer to work again when the staff was at AHR. Funny I got calls from some Heating Help guys saying "I thought you left!"

I do very much enjoy what I'm doing now though. Which involves tweeting lol

DanHolohan

Nice!

New England SteamWorks

Whatever @Sailah does, he is going to be a success.

I think we all just wanted to share in it...

JimA

I got the trap cage from State street and it appears to be working. I still have an issue though. I got the IR gun and it showed me that the half inch pipe from the steam line to the next trap seems to be blocked as it is very hot at one end and is not heating the trap at the other end. Does that sound like the need simply to replace the connecting half inch pipe?
Jim A

BobC

Maybe the trap that line feeds has failed closed so nothing can move.

Bob

Sailah

Bob has a point. Was the trap in question repaired? I can't recall if the Hoffman 8 fails open or closed. Could be a blockage in the return pipe as well.

You can remove bellows from Hoffman 8 and reinstall cover. If nothing changes you probably have a blockage. I would suspect return line before supply.

With steam off I have found a rubber tip air blow gun a good diagnostic tool, not sure if that's available to you or not.

While not exactly "safe", I have also removed cover from trap to see if you are getting steam to trap. That would allows you to diagnose if the supply line is good or not. If you do this, be careful although you are probably running low enough pressure it shouldn't be an issue. Close valve if needed.