Hoffman differential loop

gcp13

Hoffman differential loop #2
I have to replace the vent on top ,wondering if it's a good idea to add more vents to the top or add them to the dry return.
It's a Hoffman 75 ,could I add Gorton # twos at least. It need much more venting
Six inch mains about 50" then tee with 4" about 20"
It's a two pipe steam with traps and thermostatic radiator valves

nicholas bonham-carter

I think state supply has a rebuild kit for these.--NBC

gcp13

What's to rebuild other than cleaning it
I don't think there's any moving parts inside
I tried to get the half-inch plugs out of the bottom but they're frozen

Jamie Hall

There are no moving parts in a differential loop. However, if you want it to keep operating the way it was meant to -- and I highly recommend you do -- you can replace the vent at the top with an antler and as many Gortons or whatever as you please.

Do NOT, repeat NOT, put any vents on the mains or on the dry returns ANYWHERE ELSE in the system. The mains must vent into the dry returns through standard thermostatic traps (called crossover traps in this application) -- standard radiator traps will do very nicely. Then the dry returns vent at the differential loop.

Then, to continue with the good work, you should be using a vapourstat to control the boiler, and the cutout should not be more than 8 to 12 ounces; you ,may have to fiddle a little bit to get it right. The vapourstat should cut out the burner BEFORE the differential loop trips.

Doing all that will ensure that all the radiator and crossover traps and the vents in your system have a long and happy life; I've seen a number which have gone 80 seasons with never a problem.

gcp13

Thanks that's the answer I'm looking for.
I replaced the main vent with another Hoffman for today but will talk to the owner about adding more.
When the system is running the air coming out of the vent is louder than the oil burner.I probably need at least 4 to 5 number twos to help it out
Have you ever had to clean the differential loop at the bottom from the plugs?
They have a regular pressuretrol,couldn't get it to trip below 2 pounds need to talk to the owner about switching it to a vapor stat. I know the differential loop operates on 10 ounces correct?
One other thing the Hoffman loop is higher than the water level of the boiler it looks like the old Boiler may have had a higher water line. where should it be?
I know.....
page 233
Thanks again for the info

nicholas bonham-carter

there is probably a float, and vacuum valve in that box, but if you keep your pressure low, it wont need the float to operate.
there are some excellent Hoffman books in the library here, and I believe the height should be at least 24 in above the waterline.--NBC

gcp13

Page 233 of dans book has a cut away.
No mechanical device inside at all
It just uses a tube inside the water seal on the right side and as pressure pushes down the condensate drops below the tube and the steam pushes it back to equalize. I think
My main question was whether I could add more vents to the top for the whole system
I need to relay all of this info to the owner hopefully he wants to improve it

Jamie Hall

So long as the loop is well above the water line -- your's is -- that's not a problem. And no, you don't have to take it apart and clean it from time to time. Even if you could... !

10 ounces is about right for the trip point. The reason I say you may have to fiddle a bit to get it right is that it is fussy, and so are vapourstats, and what it thinks is 10 ounces and what the 'stat thinks is 10 ounces may not agree. It rules...

The reason you don't want it tripping is that when it does, the first thing that happens is that it sends live steam into the return. This closes the vents and pressurizes the return, which reduces the steam flow rather drastically -- particularly if the radiators aren't full yet -- until the steam condenses and the vents cool. This sounds as though it might be a bad thing -- but it actually isn't; if you can control the pressure so it doesn't trip, it has no effect, of course, but when it does trip it prevents any traps which are already closed from being over-pressured, since it equalizes the pressures on both sides of the trap. This is why traps last so long!

Your client may find that using a vapourstat will save some fuel. I'm not going to say that it will save the cost of the 'stat -- they aren't cheap -- but between the fuel savings and the longer life on the traps, it probably works out as a plus.

gcp13

With only a three-quarter hole for venting that only leaves me four Gorton twos for the whole system seems Insufficient but better than what he has now
I wish vaporstats were cheaper I put them on every boiler I work on

Jamie Hall

Oddly enough, you should be fine with the four Gortons. The system I take care of has one Gorton 2 and one Hoffman 76 for the whole show. I dare say it could be quicker -- but it works, and no pressure problems.

gcp13

Thanks for the info I'll see what I can do to help things out
I also had both low-water cut off's clogged up Freed one up but need to replace both of them

SWEI

Jamie Hall said:

There are no moving parts in a differential loop. However, if you want it to keep operating the way it was meant to -- and I highly recommend you do -- you can replace the vent at the top with an antler and as many Gortons or whatever as you please.

So can we build a modern equivalent out of standard parts?

gcp13

Thinking the same thing

Jamie Hall

I'll have to think about that. My first answer is probably yes -- at least the sort that gcp13 has (there were two different models).

Considering the level of steam knowledge and pipe spinning capability we have around here, we should be able to build just about anything1

Let me think about it...

SWEI

I'm going to repeat my suggestion that we ought to have a recipe for converting a two-pipe system to vapor/vacuum operation using currently available hardware.

Michael Welch

Ran into a situation a few years ago where a new President moved into his college home and shortly there after when winter hit his wife was complaining there was not enough heat in parts of the house. The first heating company they had looked at it failed miserably. There were a lot of problems with the 10 year old boiler including the steam piping to the Hoffman differential. So after sitting in that boiler room for 6 hours taking in the entire system I came up with a plan which included re-piping a lot of the near boiler piping and return legs which we found to be clogged with mud. Fortunately the previous tech had thrown the original air vent on the floor which also was in good working order. But it is amazing how this device with no working parts is really the heart of the system.