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Hoffman Differential Loop
Jamie Hall
Member Posts: 24,529
Someone awhile back asked if it would be possible to create a Hoffman Differential Loop. The answer is yes -- and I think the patent has expired!
One question before you begin spinning pipe, though -- is it worth the bother? And -- although I wouldn't take one off if I had one on, on a modern system with a vapourstat control, I'm not at all sure.
Basically, it accomplishes two things: first, it ensures that the pressure differential across any traps or radiators (or orifices!) is never going to be greater than a certain amount -- typically a bit over 8 ounces per square inch. This has its points, as it means that the traps will never be damaged by excessive pressure. Second, and perhaps more important, it also ensures that the pressure differential between the dry returns and the boiler is also never greater than about 8 ounces -- and that therefore excess pressure can't back water out of the boiler into a dry return.
Both of these Good Things were of considerable value when you had a coal burning boiler, or an unreliable pressure control. With a decent, properly set vapourstat... meh. Maybe not.
That said, if you want to make one they are pretty simple, really. You will need five T's, three plugs, a 2" long nipple, a close nipple, an 18" long pipe and a 16" long pipe. I'd suggest the whole assembly in 1 1/2 inch, unless your dry return is smaller -- then go with the dry return size. Also a reducing bushing and a nipple to go up to you main vent(s). The arrangement is: Take one T. Put a plug in one end of the run (for a cleanout) and connect a line to the boiler header to the bull. Take the 18" long pipe and connect it to the other end of the run. Take another T and connect one end of the run to that pipe and put a plug in the other end. Put the close nipple in the bull. Take another T and connect the bull to the close nipple. Orient it parallel to the T with the 18" pipe. Put a plug in one end of the run and the 16" pipe, running parallel and alongside the 18" pipe, to the other end of the run. At the other end of the 16" pipe, take another T and screw one end of the run onto the 16" pipe and the 2" nipple to the other end of the run. Connect the bull to the wet return at the boiler with suitable elbows and what have you. At the other end of the 2" nipple, put one end of the run of the last T. Connect the dry returns to the bull, and a reducer and a nipple and whatever you need up to your main vents. Hang the whole thing so the pipes are vertical.
You're done.
Note: thinking about it, it may be easier -- depending on the length of your close nipple and the T's -- to assemble the 16" pipe, the T, and the 2" nipple first... sort of working backwards from the above.
Venting when using one. In order to make the whole thing work as it should, you must have NO vents anywhere else in the system. Connect the ends of the steam mains to the dry returns with crossover traps. You may have as many main vents as your heart desires, but all located at the loop -- and I strongly suggest that at least one of them be a Hoffman 75. The others can be Gortons. Why the 75? Because it will reopen at a higher temperature, which is important for the system to recover if the differential loop trips.
Set your vapourstat to no more than 6 to 8 ounces cutout.
One question before you begin spinning pipe, though -- is it worth the bother? And -- although I wouldn't take one off if I had one on, on a modern system with a vapourstat control, I'm not at all sure.
Basically, it accomplishes two things: first, it ensures that the pressure differential across any traps or radiators (or orifices!) is never going to be greater than a certain amount -- typically a bit over 8 ounces per square inch. This has its points, as it means that the traps will never be damaged by excessive pressure. Second, and perhaps more important, it also ensures that the pressure differential between the dry returns and the boiler is also never greater than about 8 ounces -- and that therefore excess pressure can't back water out of the boiler into a dry return.
Both of these Good Things were of considerable value when you had a coal burning boiler, or an unreliable pressure control. With a decent, properly set vapourstat... meh. Maybe not.
That said, if you want to make one they are pretty simple, really. You will need five T's, three plugs, a 2" long nipple, a close nipple, an 18" long pipe and a 16" long pipe. I'd suggest the whole assembly in 1 1/2 inch, unless your dry return is smaller -- then go with the dry return size. Also a reducing bushing and a nipple to go up to you main vent(s). The arrangement is: Take one T. Put a plug in one end of the run (for a cleanout) and connect a line to the boiler header to the bull. Take the 18" long pipe and connect it to the other end of the run. Take another T and connect one end of the run to that pipe and put a plug in the other end. Put the close nipple in the bull. Take another T and connect the bull to the close nipple. Orient it parallel to the T with the 18" pipe. Put a plug in one end of the run and the 16" pipe, running parallel and alongside the 18" pipe, to the other end of the run. At the other end of the 16" pipe, take another T and screw one end of the run onto the 16" pipe and the 2" nipple to the other end of the run. Connect the bull to the wet return at the boiler with suitable elbows and what have you. At the other end of the 2" nipple, put one end of the run of the last T. Connect the dry returns to the bull, and a reducer and a nipple and whatever you need up to your main vents. Hang the whole thing so the pipes are vertical.
You're done.
Note: thinking about it, it may be easier -- depending on the length of your close nipple and the T's -- to assemble the 16" pipe, the T, and the 2" nipple first... sort of working backwards from the above.
Venting when using one. In order to make the whole thing work as it should, you must have NO vents anywhere else in the system. Connect the ends of the steam mains to the dry returns with crossover traps. You may have as many main vents as your heart desires, but all located at the loop -- and I strongly suggest that at least one of them be a Hoffman 75. The others can be Gortons. Why the 75? Because it will reopen at a higher temperature, which is important for the system to recover if the differential loop trips.
Set your vapourstat to no more than 6 to 8 ounces cutout.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
0
Comments
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I have installed one on a system that had the old one removed. I find if the system is kept fired close to the EDR it does not trip.Cost is what you spend , value is what you get.
cell # 413-841-6726
https://heatinghelp.com/find-a-contractor/detail/charles-garrity-plumbing-and-heating0 -
It shouldn't! Mine almost never does. Well, at 10 below with a howling wind it thinks about it. But then the vapourstat shuts it down (in fact -- that's how I got the vapourstat setting right...)Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Did your boiler stop the miss fire? I will have time this coming week if it needs attention still.Cost is what you spend , value is what you get.
cell # 413-841-6726
https://heatinghelp.com/find-a-contractor/detail/charles-garrity-plumbing-and-heating0 -
It does still misfire once in a while -- but always (so far... better not say that) fires and runs on the second try. Always welcome to come by, though! And we do have to look at it when you do the regular maintenance...Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Jaimie, how about a picture of this contraption for those who may not have a grasp of the terminology yet, eg. Bull, Run, etc.
Maybe this has a use, in that it would prevent an added cycle of the burner, on occasional long runs, where the pressure was creeping up.
As I understand it, the burner will take some time to stabilize the flame, after it has been cut off, and relit.--NBC0 -
I'm the guy who asked the question, and I want to tank you for taking the time to answer. I'll need a bit more time to sketch it out and get a better picture.
Good points about the better control options we now have. I'm still fascinated by some of these simple solutions that came before and would like to make sure we don't lose them completely. Carrington Event, anyone?0 -
"Carrington event"??
There must be a drawing of the HDL in the museum, so maybe I will look there, and post a link here.--NBC0 -
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