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Hoffman Vapor Vacuum System Help

alexd Member Posts: 19
Thanks to helpful advice from a number of folks here, I have a pretty functional two-pipe heating system in the home I've been in for 5 years now. I have a 3-zone heat-pump system that works great down to about 38 degrees outside, but this is wired to kick over to the steam boiler below that temp, and I'm in Philadelphia where that is a good portion of the winter months. There is still some room for improvement in the steam system, and I'd appreciate any additional insight.

To start, this system is very similar to the one @Jamie Hall manages, including intact Hoffman differential loop. It's a gas fired Crown boiler, 275btu, 29 rads (sorry, don't have a precise EDR). Near boiler piping is far from perfect, but not something I'm likely to fool with until boiler replacement is necessary (it's 19 years old now).

Since taking ownership, I have:

Installed a vaporstat
Remove all hoffman 40 radiator vents
Fixed radiator pitch
Replaced a few radiator traps
Insulated mains in basement
Switched ancient Hoffman 11 vapor vacuum main vent out to a Gorton #2

The system runs well enough, most rads get hot all the way across, and all the way with some fussing. The returns are slow and sometimes result in the LWCO coming on mid-cycle on a cold start, but this problem is less severe than it was prior to the adjustments I've made, and corrected through regular blow downs.

The latest issue I discovered is that a Hoffman 75H out on the end of one main was actually installed in place of a crossover trap at some point many years ago. I know this isn't the right vent for this application, but haven't changed it out since things were working well enough. Now that I can (clearly) see this should instead be a crossover trap, I have a few questions.

First, there is one other crossover trap at the end of the other main loop, right next to the differential loop. It is working (tested with IR), and is an original "Hoffman No. 8 Return Line Valve". Since I can't get one of these, I'd like to know:

1) what is a good replacement option
2) Once I get this installed, is it worth reinstalling the Hoffman 11 to restore vacuum operation?

On #1, I don't have a pipe threader, but if I got one assume I could order the right trap and take care of this install on my own.

On #2, I've read through some of the debate about the pros/cons of vacuum in a gas-fired system. I can't see how it wouldn't be a huge advantage to skip the initial 10-15 minutes of evacuating air from the system every time it fires though, so am inclined to restore the system to its original glory. If not, I do think I'll add another Gorton #2 to see if that speeds things along somewhat. I imagine switching the second main vent (75H) to a crossover trap as was intended will mean I need more capacity at the main vent anyway.




  • Jamie Hall
    Jamie Hall Member Posts: 23,324
    You can substitute a Hoffman 8C for the missing crossover trap. It will work just fine. Pipe it up and over to the inlet of the trap, then straight down into the dry return.

    The Hoffman 11 was a big vent. I would substitute a Gorton #2 (almost the same capacity) for it. Is it worth putting in a vent which seals against a vacuum? Such as the old #11 (no longer made) or three #76s (about the same capacity)? In my opinion, no. It is worth remembering that the Hoffman Equipped system is a vapour system, not, strictly speaking, a vacuum system. It is designed to operate on -- and the Differential Loop will enforce that -- a very low pressure differential between the steam mains and the dry returns. The big advantage to allowing the system as a whole to drop into a vacuum lies in taking advantage of the residual heat in a low fire, such as a coal fire going out in a very heavy boiler. But... there is remarkably little residual heat in a modern fuel fired boiler, so -- honestly -- I don't see the point.

    This is not to be confused with a vacuum system, where a vacuum is drawn on the returns and a larger pressure differential results. The Hoffman Differential Loop will quite happily defeat that differential.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,544
    Ask around . You can get pipe threaded at a big box store they have pipe machines but I question how lucky or unlucky you will be getting someone on site that knows how to run the machine.

    Call around to some of the plumbing supply houses. Some of them thread pipe and if they don't they will likely know who does.

    Philadelphia is a big steam town, you should be able to find a place to thread pipe
  • PMJ
    PMJ Member Posts: 1,265
    @alexd ,

    I see you have gotten the standard line that natural vacuum is only for coal fired systems. I disagree, and I actually run such a system. You are on the right trail on this and I hope you push further. To that end, I submit the following:

    Residual heat in the boiler is a small part of this. Steam delivery per unit of burn time is the big part. An open vented system halts delivery at the radiators the instant your burner goes off and begins filling your radiators with air immediately. A vacuum system prevents this and the flow of steam from the mains into the radiators continues without the burner. Your system then sits in low pressure for a faster time to steam on the next burn and fills faster and more efficiently because there is no air to remove. These things happen every cycle and actually do add up to something in $$ say nothing of the great things it does to even out the heat.

    Further, a two pipe system is wide open with supply mains directly connected to the dry returns through the radiators. Air removed from the dry return either mechanically or by simply not letting the air that was pushed out return gives the same result which is the entire system pressure drops. But the differential relationships inside do not change. The system simply operates the same way but at a lower absolute pressure. This does not seem to be well understood.

    The important things about vacuum either mechanically induced or natural is simply that in systems using it the steam flow from boiler to radiators is never abruptly interrupted, and there is never a large amount of air to be removed. Open vented systems do both of these things on every burn cycle, and both are in fact quite wasteful. The vacuum system will actually deliver much more steam to the radiators per unit of burn time than an open vented system will. The heat will also be much more even.

    I'm happy to help if you try these things if wish to contact me.
    1926 1000EDR Mouat 2 pipe vapor system,1957 Bryant Boiler 463,000 BTU input, Natural vacuum operation with single solenoid vent, Custom PLC control
  • Jamie Hall
    Jamie Hall Member Posts: 23,324
    edited November 2020
    Oh der. Well, standard line, yes. I will say no more, except that Hoffman Equipped systems are NOT designed to run on a vacuum. Best of luck. The above poster has all the answers, so I'm out of this, unless you wish to contact me personally.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • PMJ
    PMJ Member Posts: 1,265

    Oh der. Well, standard line, yes. I will say no more, except that Hoffman Equipped systems are NOT designed to run on a vacuum. Best of luck. I'm out of this, unless you wish to contact me personally.

    Happy to do it publicly or privately @Jamie Hall .

    I am only interested in the facts about the technology. My system is not Hoffman but I'd like to learn why a Mouat 2 pipe can run great in vacuum but a Hoffman won't if that is really the case.

    Do you wish to do this publicly, privately, or not at all?
    1926 1000EDR Mouat 2 pipe vapor system,1957 Bryant Boiler 463,000 BTU input, Natural vacuum operation with single solenoid vent, Custom PLC control
  • Steamhead
    Steamhead Member Posts: 16,843
    @alexd , I'd replace the 75H and the original crossover trap with higher-capacity traps. Are your steam mains roughly the same length and diameter?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • alexd
    alexd Member Posts: 19
    edited November 2020
    Thanks @Jamie Hall and @PMJ for your thoughts here, I promise I'm not trying to cause controversy and am genuinely interested in learning more about this. @Jamie Hall has been super patient and helpful to date in walking me through the first steps in making this system work, and I trust his advice because his system is so similar.

    I am inclined to try vacuum operation because of everything @PMJ says about how much time my boiler spends pushing air out of the system every cycle, and the potential for more even distribution. But, I don't want to cause any damage or create new problems though by trying to make it do something it's not designed for. I have a Gorton #2 on the main now, but still have the old but still functional Hoffman 11, and could experiment by switching them out to see if there's a difference.

    @Steamhead, the mains are almost identical length, both 3". @Jamie Hall recommended a Hoffman 8c for the crossover - you think something different would be better? Also, let me know if you want to road trip to Philly to have a look some time (or do a consult over FaceTime even). I have yet to find a contractor who doesn't look at me like I have three heads when I say "two-pipe steam" and have tried without success to find folks using this site.
  • PMJ
    PMJ Member Posts: 1,265

    I would ask that everyone take note of the result of your honest inquiry into vacuum technology here, that you now fear being the cause of controversy if you pursue it further. How unfortunate, and hardly what should be the result of an open exchange of honest inquiries and ideas.

    Years ago I observed the very same resistance to natural vacuum technology in residential systems here that you have just now. The group think policy was then and still is that not only did vacuum's usefulness end with the change to intermittent fire, but also that it could somehow cause damage. If those things failed to discourage people like you then that it is too complicated to achieve and not worth the trouble usually finished off the job as it likely will once again with you.

    As a mechanical engineer taking a fresh look with no previous experience with steam, I found that I really had to question the position that just because the fire was no longer continuous, all the other clearly obvious benefits of not handling all that air every cycle therefore must be lost from steam heat forever. Was it really necessary to horse all that air in and out of all the piping every time the boiler ran, only because it was no longer a coal fired boiler? That what I have found out and reported here became so upsetting to folks and controversial was a surprise to me.

    True experts welcome honest inquiry and open dialogue about subjects in their area of expertise always and from any source. They know being wrong sometimes comes with the territory. But true experts also fear not actually knowing the real truth themselves more than they do any embarrassment being wrong sometimes might cause; an experience quite inevitable for all of us occurring more often than any of us would like if we are actually doing anything.

    To be clear I am not arguing that pros should be out pressing customers to convert their systems to vacuum. I do understand how impractical that is for many reasons. I am, however, definitely questioning when they discourage someone like you who would try something on his own, and they do so not based on actual fact or personal experience with it. I should think there would be more room on this forum for open discussions about this subject.

    But cheer up, you have much company in the air processing business. And there truly is much experience here in moving lots of air around inside these systems. You will have good support in what additional venting devices to invest in to facilitate that.

    If, down the road, you somehow find yourself still interested in what life might be like without all that air handling, feel free to contact me. In two pipe it is astonishingly easy to do.

    1926 1000EDR Mouat 2 pipe vapor system,1957 Bryant Boiler 463,000 BTU input, Natural vacuum operation with single solenoid vent, Custom PLC control
  • BobC
    BobC Member Posts: 5,478
    Whenever i come across a hard to solve problem the solution is almost always the same. Start from square one and look at everything while ignoring all preconceived notions. What you previously thought was true may prove otherwise.

    Progress comes from those willing to challenge preconceived notions.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • SteamCoffee
    SteamCoffee Member Posts: 123
    I enjoy these talks a great deal, this topic comes up from time to time. Basically; Does naturally induced vacuum have a place in today’s systems. Conversely, is it only for coal? You’ll find some advocates for both, I favor “the vacuum is great” side of the discussion. If you want answers, look to the manufacturers publications. Dunham has an awesome book on the various systems the marketed..(414?). Vacuum was an important direction steam was heading but history finished it off before it could be fully developed. The Great Depression choked off nearly all R&D and the post Ww2 demand for cheap housing lead to forced air. Naturally induced vacuum was for coal and gas systems. This was clearly stated, the nuances were discussed in great detail by the manufacturers. I can only assume that the expansive Engineering Department of these manufacturers had WAAAAAYYYYY more theoretical and practical knowledge than any of us will ever know. Go to the source, vacuum and even sub atmospheric systems were the final frontier. The Dunham Handbook is a gem. Good luck either way, you’ll soon has a great system and that’s all that matters!