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Noisy Hoffman Differential Loop

Short cycling is not related to pressure settings directly, but rather to size match between boiler steam output and radiator load demand. An elephant size boiler to feed one tiny radiator would cause the problems you are afraid of. Often, you can kind of tame the elephant (and hide the problem) by giving it a wide pressure bandwidth to slosh around. I think elephants like rolling around in the mud anyway.

There is no reason to suspect this in your set-up, for now at least.

Try running your system as low as your trol can go, and you're right any vaporstat will take you the next level down.

You mention the pebble noises occurring more than it seems they should. Is it possible the sounds are coming from elsewhere? Maybe some deep puddles in the returns? Any sagging in your mains, or the radiator feeds, or the differential loop feed?

For flushing the returns

One way is to have a boiler drain type valve added up-stream of the return, bring in a garden hose, turn the water on and flush the dirt out. To let the real dirt out at the other end, the only way is to disconnect some fittings, you need to make sure you don't flush your dirt into the boiler.

You can get lots of dirt out, so much you might even cause a clog at an elbow for instance.

It's a big messy job better left for the summer. You might even find a big leak problem that can't be fixed fast enough in the middle of the heating season.

If you are worried your returns are very dirty, it is probably time to start budgeting for a brand new return main. As you drain your wet returns judge for yourself if what comes out is dirty enough.


  • Dave_4
    Dave_4 Member Posts: 1,405
    Noisy Hoffman Differential Loop

    I have a two-pipe steam system with a Hoffman Differential Loop. There is a pipe from the main into this Diff Loop and the return runs through this mechanism as well before dropping down into the lowest wet return. Between the middle and the end of the heating cycle, the differential loop makes quite a bit of noise for about 1-2 minutes. It sounds like a handful of pebbles rattling around inside the box and we can hear it up through the returns. We recently renovated much of the system: replaced all the steam traps, put in a new main vent, new operating pressuretrol set to cut out at 2 psi with a manual reset backup pressuretrol set at 5psi. The system runs most of the cycle between .5psi and 1psi and reliably cuts out at 2psi and cuts back in at .5psi. It has a Hartford Loop setup and the "B" dimension (height from water line to lowest trap -- which is at the end of the mains) is right at 5 feet.

    Question: with the set up we have, can we eliminate the Hartford Loop altogether by tapping off that feed from the main steam line and just running the return down to the low wet return near the floor?
  • Christian Egli_2
    Christian Egli_2 Member Posts: 812
    Putting a differential loop to sleep

    Did you wish to remove the Hoffman- or the Hartford loop?

    I'm guessing it is the rattling pebble noises that annoy you. However, these sounds may be the proof that your differential loop is working.

    The differential loop works by momentarily pressurizing the return with... steam. Meaning, it introduces steam into the returns that already contain liquid condensate. If the conditions are just right, this causes water hammer. Thus, the pebbles.

    In the Lost Art, Dan H. talks about a six-shooter analogy and carrying a gun in Texas. Thus, the banging.

    I don't know if you can simply remove your differential loop. The fact that it currently goes into action may mean it is necessary.

    To stop the differential action, all you need to do is lower the boiler pressure. I know you mention 1 PSI, that's already low, but going even lower might yield better operation. Hoffman loops were typically meant to kick in at 10 oz or 0.6 PSI. 14 oz if you have the big model.

    You indicate you would have enough of 5 ft B height to stack your condensate back into the boiler. A 5 ft differential comes up to about 2.2 PSI, which your system probably never sees. So, to make sure the differential loop is not needed, why not insert a valve on its steam feed. Then, when you close the valve, the loop won't come into action anymore. If the system then fails to work the way you like, reopen the valve.

    You'd have a dormant differential loop that won't cause you any problem at all.

    I would do this rather than removing the entire device. There is nothing mechanical in it, nothing that breaks. And removing a vital part would be embarrassing, no?

    Insert the valve on the highest part of the feed line so that condensate cannot accumulate anywhere. If this is not easy, install the valve so that you get no condensate pile-up on the steam side at least.

    As far as doing without the Hartford loop, it only serves to stop your boiler from bleeding to death in case of a wet return rupture. It's cheap insurance. Keep it if you have it.

    Happy to help.

  • Dave_4
    Dave_4 Member Posts: 1,405

    Thanks for your response Christian.

    It was the hoffman loop that I was suggesting eliminating. Yes: those rattling pebble noises are what wake us up at night. The noises occur even when the pressure in the system is at about 8oz. I have a couple of follow up questions if you don't mind:

    1. As far as a vaporstat, are you referring to something like the Honeywell L408A? 2. If we keep the pressure that low, are we at risk of short cycling the boiler and wasting energy?

    I like your idea of installing a valve at the steam feed. I've read before on the Heating Help site that if we keep the pressure low enough, we don't really need the differential loop anymore. But fundamentally changing a setup permanently that's been there for 80 years is a bit scary.

    A final question: people refer to "cleaning out the returns occasionally". How does one go about doing that? We have a release valve at the very bottom of the wet return that we're supposed periodically drain. Is that sufficient?

    Thanks again for your help,
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