two pipe returns

Paul_87

I recently did some re-piping of a steam system. The whole system was one pipe convectors originally. Someone added an addition to the house and put two convectors that are two pipe with traps on them. We ran the return from the the main that feeds that section of the house and the return from the two convectors independently to the wet return instead of putting a trap at the end of the main and we have a Gorton number one on each of them. I expected this to work and to simplify things by getting rid of a trap. The vent at the end of that main starts spitting after about 20 minutes. I think it needs to be skimmed more but want to make sure I'm not missing something. We have already done a lot of skimming and it hasn't helped I have attached a drawing of the system.

Jamie Hall

Two questions: first, what is the cutout pressure on the boiler? and second, are the radiators and convectors pretty well hot across when you start getting the spitting?

JUGHNE

Does each convector without a trap have a separate return line that drops below the boiler water line?

I am thinking of a 2 pipe emitter/radiator with it's own air vent and a separate return that goes below the water line.
If you think this would be the case, you can look here on the main site for info on 2 pipe air vented systems.

nicholas bonham-carter

The main vents are probably too small to let the air out quickly with no resistance. The rule of thumb is one Gorton 2 for each 20 feet of 2 inch line.—NBC

Paul_87

Cutout is set at 1.5 but is shutting off at 3. The convectors on that part of the system are hot when it starts spitting.
Definitely not two pipe air vent. The convectors have traps on them and no vents.
I don’t think the main vent is too small but I can try a bigger one. This particular main is about 12’ long and it’s only 1 1/2”. Then the return is about another 12’ of 1”. The other one pipe mai s have bigger vents on them. Hoffman that we reused. Not sure of model number.

Steamhead

How much height do you have between the boiler's water line and the spitting vent? And which main is the vent on- one of the originals, or the return from the two-pipe convectors?

Jamie Hall

If the radiators are hot pretty well across, that tells you that you have generated enough steam and the boiler should shut off. If the pressuretrol is set at 1.5 but doesn't quit until 3, that's part of your problem -- if not most of it. The pressuretrol may be out of calibration, but I'd start by simply taking the pigtail off and making sure it, the opening into the boiler, and the opening into the pressuretrol are completely clear.

Paul_87

Height from the soitting vent to the water line is around 38”-42”. Not there right now to measure. The vent that is spitting is on the return from the 2 pipe convectors. Not the original mains.
Pigtail is clean. We have already done that.

Steamhead

There's your problem. That return line is a "B" dimension, not an "A" dimension. This is explained in detail in @DanHolohan 's excellent book "The Lost Art of Steam Heating", but here is the condensed version:

The original return lines run straight from the ends of the steam mains back to the vents and return lines with no obstructions in them. This means that there is steam pressure in them- not quite the full pressure that's entering from the boiler, but enough to keep the water in the returns from backing up by exposing it to this "leftover" pressure. We call this the "A" dimension. It is usually 28 inches or so, and the slight difference in pressure between the supply and return ends of the main stays constant no matter what pressure is in the boiler.

OTOH, the 2-pipe convectors have steam traps. Since these traps close when steam reaches them, there is no leftover steam pressure in the return. Therefore, as the boiler pressure rises, the water in the returns backs up into the vent and makes it spit. This is the "B" dimension, and it needs to be 30 inches per pound.

Now you see it: If the pressuretrol doesn't cut out until 3 pounds, the pressure can push the water in the return up 90 inches! No wonder the vent spits in protest.

Replace the Pressuretrol with a Vaporstat that cannot be set higher than 1 pound, and I'll bet the problem disappears.

EBEBRATT-Ed

As @steamhead and others have mentioned you pressure is too high. 1 psi will raise water 28" so with your vents at 38-42" so at 1.4 psi the vents will spit.

Raising the vents up on long nipples will help keep the water out but will not fix the problem. Lower the pressure to 1 psi max and see what happens

Paul_87

I will install vaporstat. I was thinking I needed 28". Thanks guys.