Two Pipe steam hoffman differential loop

Tandon153

We replaced a old steam boiler with a 5 section boiler based on radiator sizes.
The pressure trol was set at 1.5 and .5 . The system heated for a couple weeks with some banging and now only 2 of the 10 radiators heat. There is a main air vent at the end where we are not heating. Base on what I am able to understand I need to change the pressure trol to vaporstat and install a check valve on the return and remove the vent down on the end of the steam main. Can anyone shed some light on this for me.
Thanks John

EBEBRATT-Ed

Pictures will be a great help

Jamie Hall

Do you have a copy of The Lost Art? There's an excellent writeup in there on the Hoffman Equipped System.

To start off here, though...

First, the system is intended to operate on no more than 8 ounces of steam pressure. Anything more and it it operates at all, it will operate rather poorly. So start there.

Second, the system is rather particular about venting. You must have all your venting at the elbow where the dry return(s) enter the top of the Hoffman Differential Loop. There must be no vents, repeat: NO VENTS, anywhere else on the system. Misguided additional venting somewhere out there -- whether on the mains or on the dry returns -- will raise all kinds of havoc when the Differential Loop operates (which it will, at about 8 ounces, by design). So remove the vent on the end of the steam main.

Instead, what is required -- and if someone put a vent at the end of the steam main they may have also messed this up -- is that at the end(s) of any steam main(s) there must be a crossover trap -- which is just an ordinary usually 3/4 inch radiator trap -- piped as follows: a nipple vertically up from a T on the main; 90 degree over and a nipple to the inlet of the trap, then a nipple down to a T on the corresponding dry return from the outlet of the trap. This is not to pass condensate -- but acts as a king size vent for the main.

Note that if the main pitches away from the boiler, there must also be a drip at the end of the main to a wet return -- and, most likely, a drip from the end of the dry return in the same area. Make very sure that the wet return in that area is below the water line in the boiler.

There is no need for a check valve on the return. To place one there will cause problems.

Have fun! They are great systems, but they are a bit sensitive to being mistreated.

Tandon153

Tandon153

Jamie
Thanks for your advice. we will try that. They home uses a coal stove most of the time and I'm not sure how the system worked before we got involved replacing the boiler. I don't see a crossover trap and someone put that vent in a couple of years ago.
Thanks

JUGHNE

What does the rest of the boiler piping look like....header etc?

Tandon153

JUGHNE

Too close for old eyes to see, I was wondering where the equalizer connected to the header.
Steam main looks pretty big for only 1 riser from the boiler, but sometimes that is how it shakes out.

Tandon153

It is at the end of the header after the 2 mains come off and it drops into the top of the hartford loop

Tandon153

Main is 3' being feed from the 2" out of the boiler. The boiler is a 5 section 138,000 BTU 354sq ft steam

Jamie Hall

I don't think that that's going to be a happy boiler...

Steamhead

That looks like a Force boiler. IIRC only the 7 and 8 section Force boilers require two risers to the header.

The problem may be condensate collecting at the reducer in the riser, blocking the steam flow. I'd make the change from 2" to 3" where the pipe turns horizontal.