We have single-pipe steam in our four unit building. We installed Danfoss RA 2000 valves in 3 of the 4 apartments but they seemed not to work too well and also tended to spit water from the air valves.

The system was running around 3-5 PSI. I turned it down to 0-2 PSI. After doing that however I noticed the water level in the boiler slowly rising and now it flooded. Any idea why that would happen? Am I stuck with higher pressure?


I have the Danfoss controls for 1 pipe steam. (My radiators stopped heating consistentenly, so I put a Hoffman vent on one of the radiators.) The rest are Danfoss vents.

As for the boiler flooding... looks like my Pressuretrol tripped? The mercury bulb tipped over and opened the circuit.

I drained the boiler until the sight glass was 1/2 way and pressed the lever and the boiler turned on again. But it keeps tripping. Sorry if I'm not using the right terminology.

The water in the glass moved higher and seems to be hovering around 3/4 full. (It visibly oscillates up and down slightly when the boiler is on but I assume that's normal?)

I am attaching picture(s) of my boiler controls maybe you guys can see what's wrong...

I don't know how to adjust these controls to try to keep the pressure below 2-3 PSI.

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  • Sounds to me like it added more water to compensate for the pressure loss, or if a sealed system, the pressure drop causes the water to expand into the tank more. That's why the water would come out. No pressure to hold it back. You need pressure to force it thru the pipes, unless you are on a circ pump setup.
    – Jeff Cates
    Commented Feb 19, 2017 at 18:06

1 Answer 1


Here is the real answer to your heating system problem. I worked on these for 40 years. 1 pipe steam systems are very critical of any thing you or anyone has changed in that system since the time it was first installed. Now; exactly what valve did you install? Did the valve replace the steam vent in the last section at the end of the radiator? I hope so, since this is the only Danfoss valve that will work on a 1 pipe steam system. You will also have to install the steam vent that came with or was an optional part of that valve. A Hoffman steam vent #43 will work with Danfoss valves. The steam pressure should never exceed 3 psig. All the manual inlet steam radiator valves must be fully open or fully closed; never close a valve part way to try and control steam flow. After you read this respond with what you have done so far. You may have to add a boiler control system (not just a wall thermostat) since the boiler has to cycle off to get these valves to work properly.

  • Hi. Thanks. Yes--I used the one-pipe steam version of the Danfoss valves. The inlet valves are fully open. (In fact they can't be closed completely--I assume they're worn out? Do they get pitted or something?) When I first checked the pressure on the system I was up to around 5 PSI--which I think was the reason that my Danfoss weren't functioning properly, hence me mucking with any of this in the first place.
    – nielsbot
    Commented Feb 20, 2017 at 2:15
  • Is the heating system working OKAY now?
    – d.george
    Commented Feb 20, 2017 at 11:27
  • In the bottom picture of your post there is a brass looking pressure reducing valve just above the top of the boiler, in the copper feed water line. The handle is upright or horizontal when you look at it. If you have water pressure above about 50# in your building then the handle on that valve needs to be moved down to a vertical position so it can regulate pressure to the feed water control. If it does not control the pressure it needs to be replaced. Excess water pressure can cause the boiler to flood. Did the boiler work ok before you installed the Danfoss valves?
    – d.george
    Commented Feb 20, 2017 at 12:38

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