2 pipe system, no traps

Also, make sure that there isn't anything that was under the water line of the old boiler that is no longer under the water line of the new boiler. If the old boiler was a 1930's coal boiler that had been converted, the new boiler likely has a much lower water line.

EBEBRATT-Ed said:

And post some pictures of the boiler install so we can see how it's piped

Not just the boiler, @sammytorch , but some radiators as well (without covers). This should help us ID your system.

Please tell me I didn't read correctly... 2 psi with a 1.5 psi differential...

As i said in my earlier post, that system will run beautifully piped exactly as it is -- on a maximum of about 8 ounces.-- ounces -- per square inch. At that pressure the return problem @EBEBRATT-Ed mentioned is probably correct, not a problem.

Your diagnosis of what happens to those two problem radiators is probably correct -- but, overall, you are getting steam into the dry returns, and that should never happen -- and won't happen if you get the pressure down.

When you get it down, you should also check all the radiators. It's possible that the valve settings have been monkeyed with. The valves should all be set so that, when the system is running at full song, all the radiators get nice and hot, probably all the way across or maybe not the last section -- but not all the way down on the outlet side. If they are too enthusiastic, close the valve a bit and try again.

Hmm. So pressuretrols won't work below the waterline?

My two boilers have exactly the same setup. That is the way they came from Burnham. And the pressuretrols have worked fine for for 30+ years.

The bigger problem with mounting them that way is that the pigtails tend to get gunked up from the LWCO a lot faster than they would otherwise.

@Chris_L is your pressuretrol mounted on a straight siphon tube or angled?
When was the last time you checked its calibration?
If a steam boiler sized right and running right, the pressuretrol is more or less a fixture. The system satisfies the thermostat before it makes pressure. Obviously a lot of variables change how each steam system operates.
I know I read somewhere what the recommended height of the pressuretrol should sit above the water line. I don't remember the exact measurement, because the article mentioned as long as its above the boiler jacket its (relatively) safe.
I would say on average when I'm testing the calibration of a pressuretrol, 2 out of 10 are on the money.
I see how sensitive these controls are and when I install them, I want to protect them from rusty boiler water and make them as easy as possible to clean.

This is a special type of vapor system. 2 pipe system without traps. For this system to run perfectly it needs a few things.
Keeping the pressure low, clean water and way better venting would be a start. But the next most important thing to do (if not as important) is to make sure the orifice valves are working and set properly.
2 pipe vapor systems without traps heavily rely on these valves to keep steam out of the return and to balance the flow through the system.
Steam, like everything in nature, takes the path of least resistance. With the 2 returns connecting above the water and only 1 vent, steam fills half the system, closes the vent and air locks the other half.
With working orifice valves you can fire up the system, walk around and feel what radiators are heating first and turn them down.
The name of the game is "Don't let the steam touch the last section".
@sammytorch if any of your radiator valves are leaking or not turning, they need to be replaced with another orifice valve.
Packless orifice radiator valves are pricey $ but think of all the money your saving not having to deal with steam traps.

@Jamie Hall
I understand what you are saying about the returns above the water lines. But is there any harm tying them together below the water line? Seems to me it would avoid potential issues

@EBEBRATT-Ed
The main problem is steam shouldn't even get into the return.
Even if the returns drop below the water line and are vented separately, there's still the chance for steam to short cycle through the closest radiator to the vent, closing it and air locking the radiators furtdown the line.
Most vapor systems only work with 1 vent but can rely on steam traps to stop the steam from crossing over.
With no traps, orifice valves are the only controls for balancing this system.

@AMservices, the primary pressuretrols on my boilers were mounted just as shown on this photo from the manual:



Not sure why they did it that way with an extra fitting except perhaps to keep it farther away from the boiler jacket.

In any case, street elbow at the top of the LWCO is where the pigtail would start to clog up. I never noticed gunk reaching the diaphragm of the pressuretrol.

I agree, higher is better. I removed the elbow and changed these pigtails to brass, straight-through ones a few years ago when I added a union beneath the pressuretrol.

But as for calibration, the pressuretrols are often off right out of the box. I have calibrated all four of mine, and didn't notice any difference between those that sat above or below the waterline. And they've all stayed in calibration.
(My boilers are both millivolt systems and have a secondary auxilliary pressurtrol mounted well above the waterline.)

I might add... a pressuretrol just isn't going to do it for this sytem. You have to have a vapourstat -- and an accurate low pressure gauge to check the vapourstat -- to control it. 6 to 8 ounces per square inch cutout, 4 ounce (subtractive) differential.