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Hoffman No. 300 Boiler Return Trap and Receiver Vent

DaveW_47
DaveW_47 Member Posts: 3
Hi All,
I have a two-pipe system with a Hoffman No. 300 BRT and Vent at the end, almost identical to the image in the lost art of steam heating chapter 15, page 397 (on a kindle, apologies if different from page in printed form). I am not sure the BRT is working properly, there is a sight glass for monitoring water level, which is always empty. This is also very similar to the 'Boiler return trap' section of the online 'Dead Men's Steam School' training. All very good and useful information!!

In observing the system running, I notice that when the boiler pressure gets around 1 1/2 lbs, frequently the low water cut off triggers and the boiler shuts off. As I understand of how a BRT is supposed to work, that it is a mechanical condensate pump, and should return condensate to the boiler when the boiler pressure is greater 8 oz.

I understand it might be possible to rebuild this BRT with solenoid valves, is this something anyone has experience with troubleshooting and rebuilding ?

Thanks,
Dave

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 15,717
    Before you start tearing into this thing, do the rest of the work first.

    First, pressure. That boiler return trap is intended for -- and works well on -- a vapour system. That means low pressure. Les than 8 ounces cutout. So, get yourself a vapourstat, set it for 8 ounces cutout and 4 ounces cutin. That's step 1.

    Second, crossover traps. They will be there, or at least they were there once upon a time. Find them. Make sure they are working. If they aren't, fix them. If some Happy Harry removed them, replace them.

    Third, while you're at it, check for vents which don't belong there. There should be no vents on the system anywhere except the boiler return trap or at the dry returns where they turn down to the boiler. At the boiler.

    Fourth, check all your loop seals. They must be working properly. They may be just a loop down from a steam main and back up to a dry return, in which case low pressure is critical (your pound and a half, for instance, would require a loop seal at least 4 feet in height). They may instead consist of a drip from a steam main and, very likely, a drip from a dry return both into a wet return. That wet return must be at least a few inches -- a foot is better -- below the static cold water line in the boiler.

    Now go back to the diagram of the boiler return trap and air eliminator. It' figure 14 in Chapter 15 of the Lost Art. Study it. Study the related text. Then scrutinize your piping and valve arrangements. As Dan cheerfully notes, if it isn't piped correctly, it simply won't work -- and the odds are pretty good that if the boiler was replaced at some point, it was messed up.

    Fix the piping to be as it was intended to be.

    Now. The thing should work without problems, unless the check valves are damaged -- which is unlikely -- or the boiler return trap is shot, which also isn't all that likely.

    Now. It is NOT a mechanical condensate pump. It works by allowing steam pressure to act on the condensate when needed and push it back into the boiler. While I expect that if the float in the trap is OK you could add switches and a few solenoid valves to get it to work, after a fashion, you would be adding a lot of complexity where none is needed. If the float isn't OK, you're stuck, and will need to repair in -- or add a receiver and a boiler feed pump and at least one F&T.

    Keep it simple. Fix it first.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • DaveW_47
    DaveW_47 Member Posts: 3
    Thanks for the quick response Jamie!

    I am relatively new to the house, only 2 years in. I had the boiler replaced 5 months after move in. The only piping that was changed was a riser into the main header, the very end of the wet return (the last 2-3 feet before the Hartford loop) and the Hartford loop itself. No piping outside of these areas were touched. The installer was also very careful not to change the previous waterline, which maintain the previous dimension "A" and "B". As best I can tell, these dimensions are good.

    I have replaced the all the end of main crossover traps as well as 85% of the radiator traps in addition to removing all unnecessary air vents, the only air vent in the system is the one discussed here, the No. 300 "air vent" receiver.

    So the system had been operating for a long time with broken end of main and radiator traps, I believe some were original to the house, 1938, because they are all Hoffman 17As with a few Hoffman 17c's, but the housings are identical, not the newer Hoffman 17c housing.

    As best I can tell, the piping into and around the BRT and air eliminator are a very good match to both Figure 14 in chapter 15 as well as the description and image presented in 'Dead Men's Steam School' training. It all looks original, but I cannot tell the difference between 60 year old piping and 80 year old piping.

    The loop seals and end of dry return are as you suspected, drips down into a wet return (pitched for gravity flow of the condensate). A majority of the wet return has been relatively recently replaced, b/c its all copper, not black iron like the rest of the system. As best I can tell it is at least 1 foot below the water line of the boiler.

    So I suppose I need to replace the pressuretrol with a vaprostat. Does this really change the pressure of the boiler ? I assumed it would just stop the boiler from going above whatever the setting for cut out is, it just monitors and has little impact on the boiler pressure going up or down.

    As an FYI, the boiler only gets up to 1 1/2 lbs after a good bit of time, 20-30 min of steaming, so 45+ min from cold, most of the rads are working well at this point and seem full of steam, this is also the only time I see the LWCO kick in.

    Thanks again!
  • Jamie Hall
    Jamie Hall Member Posts: 15,717
    You're way ahead of most folks! Congratulations!

    At this point, yes, I would say a vapourstat (plus a low pressure gauge to go with it) would probably help a lot. It doesn't really change the pressure of the boiler, in that sense, but it will stop the pressure from rising beyond what it needs to be -- which is that 8 ounces or so.

    Without getting too involved, the pressure in the boiler -- and the system -- is determined by the balance between how fast the boiler can produce steam and how fast the radiators can condense it. In the best of all possible worlds (yeah, right) the two are matched. In your case -- like most of us -- the boiler can produce steam faster than the radiators can use it, so the pressure goes up -- which you don't want (or need). The vapourstat -- or whatever -- simply turns off the boiler until the radiators can catch up, and then turns it back on again.

    You probably have a significantly larger boiler than is needed, but it's usually not worth doing much about it (although you might ask your burner technician if it can be dialed back a bit -- some can).

    You may also want to check those swing checks to make sure they really are doing their thing, although they are pretty reliable.

    For what it's worth, most of the assorted whiz-bangs deigned to help a situation where a boiler pressure gets too high -- and there were a number of them -- do work, but not all that well if the pressure really gets high (relatively speaking!) -- and for a vapour system, a pound and a half is really high.

    Bringing down the cutout pressure will also help your traps last longer.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • DaveW_47
    DaveW_47 Member Posts: 3
    So if using a vapourstat with cut out to 8 oz, the boiler will likely cycle on/off more than currently, is this a safe assumption ?

    Also, if cut out at 8 oz, wont this take the BRT out of play ? Because if max is 8 oz, then theoretically the pressure will never be greater than dimension "A" and its 'weight' will be sufficient to get condensate back to the boiler and the system will not need the equalization of the BRT to return the condensate.

    A final question, it it likely that a boiler, any boiler in a 2 pipe system where everything is in working order, steams straight for 30+ min ? or is cycling when the radiators are hot normal ? I ask because I always assumed a boiler should steam until the thermostat is satisfied.

    Appreciate the help!
  • Jamie Hall
    Jamie Hall Member Posts: 15,717
    With the 8 ounces, the boiler may not run quite as long on the initial heating up phase -- though the difference may not be that much. What will happen is that it will cycle on and off more after that point -- and there isn't much to be done about that. It's not really harmful, though, and if the off part of the cycle isn't too long it doesn't even hurt the overall efficiency much (Cedric, the boiler which heats the main house I care for, cycles on for about 3 minutes and off for 1 on very long runs, for example).

    It does take the BRT out of play -- but that's not a problem. Keep in mind that the BRT (like many of the other clever widgets!) was designed to protect coal fired boilers, which could -- and did -- get overenthusiastic and raise the pressure too much.

    And on your last questions -- yes, and yes. As I said in my first note, in a perfect world the boiler would be exactly matched to the radiation, and produce exactly as much steam as the radiation could condense. Right... very rarely happens. Instead, the boiler produces too much. Now while all the radiators are heating up, the pressure will stay very low -- a few ounces, at most. Eventually, if the run is long enough, all the radiators are as full of steam as they can get, and all the traps close -- and the excess steam has nowhere to go -- so the pressure rises. Then your pressure control turns the boiler off, and the pressure falls (remarkably quickly!) as the steam condenses. And then the boiler fires back up. And so on, until the thermostat is satisfied.

    In most cases -- if the boiler was sized "by the book" -- that is, the EDR rating of the boiler matches or is slightly less than the total EDR of the radiation, it takes a longish run (your 30 minutes from steam up suggests that this is what you have) to get to the point where it needs to cycle. If the boiler is wildly oversized, though (someone decided that bigger is always better -- not that unusual, sadly) then the cycling will be much more pronounced, and in some cases people have found they need to add a deliberate time delay to the off part of the cycle. You shouldn't need that.

    There is a rather obvious related suggestion, though: steam systems really don't do well with larger setbacks, say at night or when one is away during the day. There is a rather lively debate on how much is too much, but my own feeling is that a setback of more than 3 degrees is likely to result in excessively long runs when it is trying to recover. I personally have tried Cedric with that, with longer setbacks -- and with no setback at all, and have come to the conclusion that setbacks don't save much, if anything, on fuel use (on top of which, it's a museum, and some of the contents are much better served by a constant temperature), so I don't use them in that building. They do save on buildings with forced air heat (one church I care for is forced air), but that isn't the case for steam.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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