Hoffman differental loop

Sal Vigilante

I have a boiler that needs to be replaced. There is a Hoffman differential loop on the system. So, like a good heating tech I looked it up in my very worn Lost Art of Steam Heating(thanks Dan). I get how it works, my problem is there is an extra line tied into the steam supply on the HDL that drops down into the return (see photo is the yellow line). The system is only heating the tops of the rads. I'm sure she must have some bad steam traps. I dont want to change this boiler and become the owner of the system, if ya know what I mean. So my question is this line supposed to be there or not?

Thank
Sal

Steamhead (in transit)

The short answer is \"it depends\"

that pipe is or was probably draining condensate from something at ceiling level. It looks like it joins the other drip at a point below the boiler's waterline- this is how it should be. If they join above the waterline, steam can short-circuit thru them and pressurize the dry return, which will cause the insufficient-heat problem you describe. Check to see if any of the other drips join above the waterline too.

If the piping has always been this way and the problem started recently, the traps might be the cause. This includes the crossover traps at the ends of the mains. But I'd suspect the vent on top of the Loop first. If this vent sticks closed, it cripples the whole system since it is the system's only vent. Use nothing less than a Gorton #2 as a replacement. Use more than one #2 if you can hear the air blowing out of the first one when the system is heating up.

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