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Hoffman Differential Loop

Doug Oest
Doug Oest Member Posts: 34
Hello fellow Wallies! We have a job here in PA that has a Burnham steam boiler installed in a light commercial type building. It has a Hoffman Differential Loop. Dan has it covered on pages 232 in Lost Art. It has to do with getting water back into the boiler. To paraphrase Dan: If the pressure is low, the loop does nothing, just sits there. But if the pressure builds up beyond the "B" deminsion (from the trap to the NWL) the loop goes to work. In short, it allows steam to enter into the loop and shut the air vent. When this happens, it puts pressure on the air trapped in the dry return- enough do it equals the pressure in the boiler. Now, the static height of the water that is stacked up in the wet return can push the water back into the boiler.

Like Dan says, typically, you'd leave this thing alone. When changing the boiler, just be sure that the new boilers water line is close to where the old boilers water line was. The new boilers water line can't be higher- highly unlikely considering the size of todays boilers.

Now, getting to the problem with this job... a motorized valve was installed on this system a while ago, when the old boiler was still there. It was installed between the supply main and the differential loop. What do you think that did to this steam system? I'm guessing that when the motorized valve was closed, first of all, it rendered the loop useless. Second, when this valve was closed, the pressure on the radiator side of the valve went to nothing and the radiators would cool down eventually. On the boiler side of the valve would have pressure and when the valve opened- whoosh, high pressure went to low pressure and the water left the boiler. Which is exactly what is happening with this job. Once the water leaves the boiler and gets up into the system- it starts getting thrown around by steam and yeah, we got heavy duty water hammer.

I put this on the wall to try and explain a little about differential loops cause you see them every once in a while. I hope Dan jumps in here too in case I didn't say it quite right.

So, for this job, we're gonna wire the zone valve so it stays open and we're gonna turn down the pressuretrol. My only question to Dan is, why didn't it cause a problem with the old boiler? I beleive it was happening but they just didn't realize it. My guess is that the old boiler probably had a lot more water in it and was a larger type- providing more room for the water to move around in and less chance for it to find it's way up the return. Dan?

Comments would sure be welcome here. Thanks guys.

Comments

  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,487
    Sounds

    good to me, Doug.
    Retired and loving it.
  • Doug Oest
    Doug Oest Member Posts: 34
    Hoffman Differential Loop

    Jason/Ron- did you see this yet?
  • Doug Oest
    Doug Oest Member Posts: 34
    Hoffman Differential Loop

    Jason/Ron- did you see this yet?
  • Steamhead
    Steamhead Member Posts: 16,710
    No, but I did

    and I think you're on the right track. Just one question- what is the motorized valve wired to, and what makes it open and close?

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  • Doug Oest
    Doug Oest Member Posts: 34
    Zone Valve Wiring

    Hey Steamhead, thanks for responding! I beleive the T'stat is wired to the zone valve and the zone valve end switch makes the boiler. I'm thinking we should leave the zone valve open all the time and re-wire the T'stat down to the boiler direct. What do you think?
  • Steamhead
    Steamhead Member Posts: 16,710
    That's what I'd do

    also get the pressure down as low as you can. And don't forget two things: 1- the vent on the Loop is usually the only vent in the entire system, so it must be a BIG one, and 2- the steam mains are probably vented into the dry return thru radiator traps. If these traps fail, the mains won't vent well or steam will get into the return.

    I think that ZV might have been put in to mask some other problem in that system. Watch for possible problems when you rewire it.

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  • I spoke

    with Ron today about this and the homeowner had the Motorized Globe Valve put in when he also had a zone of hot water heat connected to the boiler. I assume his reasoning was to prevent the boiler from making steam when the boiler was operating off the aquastat for the Vacuum Loop. As far as I can figure, this guy was an engineer and probably intended to have the valve reverse-acting which should have had it opening when the house tstat called in.

    Glenn
  • gustheplumber
    gustheplumber Member Posts: 1
    Hoffman diff. loop

     I am a master plumber working at a job where we have an old vacuum steam system and they installed this Hoffman differential loop. We are changing out the boiler and installing a burnham in-7 in place. I read some of the replies here and am a little concerned. We set the boiler so the condensate lines would work on the boiler (set the boiler lower). We piped the boiler then i read Dan's book and saw the illustration about the loop. We cut the old 21/2" nipple out extended the old brass lines down to be below the new water level, is there any reason why this won't work?

    Thank-you all for your help.
  • Steamhead
    Steamhead Member Posts: 16,710
    edited August 2010
    You didn't need to go that far

    since the new boiler's waterline is lower than the old one's, you have even more "B" dimension so there's more static height available to put the water back into the boiler.



    How much height do you now have between the boiler's waterline and the lowest dry return?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
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