Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

One-pipe system control

In a one pipe system I know that the air vent is used to control (increase) heat load by exhausting air from the radiator.
However when we need to decrease the load we need air to go back into the radiator. From what I understand it's done either by shutting down the boiler or a zone valve (when there is one). But in that case, all the radiators loses their steam supply
Is there a rule thomb about the required frequency of this shutdown to make the system work properly or is this controlled by a thermostat somewhere in the building?
I'm not sure exactly how this "breathing" action is controlled
Thank you


  • ethicalpaul
    ethicalpaul Member Posts: 4,667
    edited January 2020
    I get the sense that you're not thinking about it quite right. I'm nervous about your words "I know that the air vent is used to control (increase) heat load by exhausting air from the radiator."

    Slowing the radiator by using a smaller capacity vent is a kind of one-way thing--once the radiator has no air in it, it will continue to pull new steam into itself even if the air vent never reopens. Maybe you know this but I couldn't tell.

    If a room gets too hot compared to other rooms, you put a slower vent on it in order to delay the air escaping/steam incoming. If a room doesn't get hot enough you can make its radiator get heat sooner in a heating cycle by using a larger capacity vent which lets the steam get to that radiator sooner (relative to the other radiators).

    The heating cycle is indeed controlled by a thermostat. That makes the "breathing" you refer to. Once a radiator is full of steam, the only way it gets rid of the steam is for the burn cycle (steam production) to end by a satisfied thermostat (or other condition), and for the existing steam to condense which will pull air into the system until the next burn cycle starts.

    From what I understand it's done either by shutting down the boiler or a zone valve (when there is one). But in that case, all the radiators loses their steam supply

    I agree with this.

    Did I help with your question? I can't tell what your question is, but may have missed it.
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • retiredguy
    retiredguy Member Posts: 753
    Anytime I had an area that would overheat and it could not be controlled sufficiently by venting I would install a "Danfoss Thermostatic Valve" in that room only. If it is a 1 pipe steam system, you can not install a zone valve on the supply piping since it may also be part of the return. A lot of people do not like those valves but they can fix an otherwise unfixable situation.
  • simonverville
    simonverville Member Posts: 8
    Thanks for the answer. If The heating cycle is controlled by ONE thermostat, where this thermostat is normally located when you have multiple rooms (thus, different heat loads)

    I understand that I can use smaller or bigger air vent to better adjust the heat capacity of each radiator but heat load can vary differently in each room during the day, so sometimes we just need to put some air back in the radiator to reduce the load. The only way I see is to shutdown the steam production, but what do I do if one room needs heat (so steam) and the other don't need it anymore but this radiator already purged all it's air

    Thus, I understand (or I think I do) how a one-pipe radiator work but I am still not sure how the hole system (boiler or zone valve) are controlled.
    Just one thermostat somewhere in the building seems a really bad design to me.

    Thanks again

  • ethicalpaul
    ethicalpaul Member Posts: 4,667
    edited January 2020
    You can have as many thermostats as you want, but in the end, they will have to decide when to fire your one boiler.

    There is no "adding air into a radiator" without ending the burn cycle.

    If a radiator is too hot for a room, there's nothing you can do at that moment. All you can do is put a slower vent on it so that in the next heat cycle(s) it will hopefully not be too hot any more.

    All these statements are for 1 pipe steam which you indicated. With two pipe steam you can partially close a valve and immediately have less steam going to a radiator.
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,573
    If the radiators are original to the house, the chances are that the sizing in EDR of each radiator was correct for the room.
    In the days of coal, burning constantly, the air would have been pushed out when winter started, and finally let out in early spring.
    Now with on-off burners, the air is let back in many times a day.
    This makes it so important to have generous main vents whose back pressure is less than the agrégate of all the radiator vents. In this case the pipes will fill with steam first, followed by the radiators simultaneously.
    Otherwise some radiators close to the boiler will get steam first, and overheat the occupants.
    Only use TRV’s to reduce individual heat output as a last resort, after main venting, and thermostat placement.—NBC
  • mattmia2
    mattmia2 Member Posts: 7,769
    In one pipe steam the TRV (the danfoss or other manufacturer's thermostatic valve) is part of the vent. It can't stop the radiator from heating once it has started, but it can close off the vent and keep it from heating on the next cycle. That is the practical way to make a 1 pipe steam system compensate for loads that vary from room to room.
  • gfrbrookline
    gfrbrookline Member Posts: 727
    I agree with @nicholas bonham-carter about radiator sizing to a point. If you added thermal windows and insulation to the wall they could be over sized for the heat loss of the space. Also if your building was built in the early teen like mine the radiator were sized to heat the space with the windows open to vent out the germs associated with the spanish flu.

    I have Danfoss TRV's in the top floor unit on the boiler side of the building and they work well. The trick is finding the vent that keeps them from overheating, I have Hoffman 41's on them, others have had good luck with Gorton 4's or 5 straight vents.
  • mikeg2015
    mikeg2015 Member Posts: 1,187
    You are thinking of this backwards. The air vents let out air which lets in steam. Letting air back in is only nessesary because the radiator goes into vacuum when the steam condenses and no additional steam is being supplied by the main. You don’t “put” air back in. Air fills the entire system after it shuts down to fill the vacuum left behind by steam that has condensed.

    Vents only control the rate that air leaves the radiator which allow for balancing and the RATE that they heat up. How fast the entire system heats up depends on the boiler size vs. load.

    As mentioned a TRV can be used on radiators that overheat compared to the main room or where the thermostat is located. On that note, the venting rate of the radiator(s) near where the thermostat are located become the default baseline heating rate that all others are balanced against.

    If you put a slow #4 vent on a radiator twice as large as another one, the large one will only be heated 1/2 way across when the other is fully heated. This can be a problem, because the boiler is still making steam as though both are heating at full rate, so the excess steam will get divided to all other radiators and so on making them heat faster.
  • gfrbrookline
    gfrbrookline Member Posts: 727
    @mikeg2015 I agree to a point. My 3rd floor gets enough convection heat from the units below to way over heat it with the connected radiators if I size the vents to the gill/pajek formula. Slowing them down with 4's, or hoffman 41's in my case, makes a big difference in stopping the overshoot. It also allows the steam to redistribute to the cooler side of the building evening everything out.
  • gfrbrookline
    gfrbrookline Member Posts: 727
    The radiators don't need to heat fully on each cycle to maintain a constant temperature.
  • Jamie Hall
    Jamie Hall Member Posts: 21,117
    The various formulae for vent sizing are a decent starting point. However, every building is different, and what works well in one application doesn't work in another. You have to experiment with varying your vent sizes until you get what you want, and this takes time and patience. Nice variable vents help a lot -- you don't have to keep unscrewing things.

    The procedure is to start with main venting. Until that is really adequate, you can't expect to get the rest of it right. Then put slightly faster vents on spaces you expect to need more heat, and slower on others. However, don't overdo that. Then run it and observe it. Start adjusting with spaces that are getting too warm, and slow them down. Don't start with spaces that are too cool and try to speed them up, at least at first -- that almost never works.

    Keep in mind that any change in venting will affect all of the radiators, although if your main venting is adequate the effect will be small.

    TRVs controlling vents do work, after a fashion. It is better to use slower vents with them, because, as noted, they only work by stopping steam flow when the radiator isn't full of steam -- and if the vent is too fast, they can't do anything. But they are a last ditch solution. Get the venting balanced to be as close to what you want first.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • retiredguy
    retiredguy Member Posts: 753
    edited January 2020
    Most everyone that monitors this site gives very good information as to the things that have worked for them. Most of these guys have tons of experience in the heating business and in steam boilers. They know what works and what doesn't. Since all buildings heat differently, you have to experiment with vents etc. to see what works for you. Most 1 pipe steam systems were installed when coal was the only heat source. The radiators were sized for the heat load in each room for the supposed coldest design day temperature in that area. Around Pittsburgh, Pa. that design temperature was at one time -5 degrees F. I have no idea what the design temp is today. The coal boiler was lit and the coal boiler produced steam till spring. There was very little temp control. You just changed the amount of coal you shoveled into the boiler. And as @gfrbrookline said the windows were open slightly to vent out the "bad bugs" that would attack you while you were sleeping. All houses had double hung windows so you could open the top and bottom panes to get good air flow circulation. What this all means is that 1 pipe steam systems were not usually designed to be turned on/off as we can do now by replacing the coal and shovel with a gas/oil burner and a thermostat. Today, to get an old steam system to heat evenly main vents and other different variable type vents are used to try to control the distribution of steam to each room to satisfy the home owners comfort. That said, this is what I have done in some smaller buildings in my service area when all else failed to satisfied the building/home occupants. If you can't get your residence to the comfort level you desire, you could add TRV'S, like Danfoss or others, to the radiators in all but the coldest rooms. Move the controlling thermostat to the coldest room. This will allow the boiler to cycle on/off to maintain the house temp based on the coldest rooms temp and the TRV'S will control the temp in all the other rooms. It's like having a thermostat in every room. A better system would be to re-pipe , resize and install a completely new 2 pipe steam system with all the bells and whistles with every room zoned, which would never be cost effective. These TRV'S work by controlling the flow of steam into the rad and stop the flow when the room temp is satisfied. The boiler continues to fire until the coldest room temp is satisfied and shuts the boiler down. As the condensate flows out the air flows in and fills the rads and piping with air until the boiler re-fires for the next cycle. For these TRV'S to work properly the boiler must cycle on and off to allow the system to fill with air. when the boiler again fires, the TRV'S control the air bleed at each rad to control the amount of heat from that rad and so on. Hope this helps.
  • mikeg2015
    mikeg2015 Member Posts: 1,187

    The radiators don't need to heat fully on each cycle to maintain a constant temperature.

    Absolutely. Actually my boiler is too small to fully heat all of my radiators. I use TRV’s for 2 rooms that are unused and for four room that tend to overheat. I balance the rest with Ventrites.

    However, fully heating a radiator is nice, as the meta stores some energy between cycles. However, with a smaller boiler, it runs longer anyway and you can use 2 CPH without short cycling and control temperature tighter.
  • dopey27177
    dopey27177 Member Posts: 885
    Two things not spoken about.

    Are all the radiators cast iron?
    Are the radiators steel fin tube.
    Is there a mix of steel and cast iron in the building.

    How does a vent valve work?
    Most manufactured vent close when heat is applied to a bimetal spring or a bellows. They close at about 180 + degrees.
    All vent valves have whats called a drop back pressure.

    Drop back pressure is where the vent valve will open and let some air in but in reality it is the point where it allows condensate out of the radiator which then causes the pressure in the radiator to drop. Hopefully at that juncture the condensate temperature is below the flash point of water at the lower pressure in the radiator.

    During the heating cycle the vent valve will open and close many times.

    Why does this happen?

    The barrel of the vent valve cools and so does the bimetal spring and the fluid filled bellows,

    This continual opening and closing ( a slight lowering of pressure in the radiator ) allows the condensate to leave the radiator.

    What is drop back pressure?

    It is a pressure that is reached where a vent valve will open.

    Usually you would not worry about this in steam systems that operate at higher pressures.


    The Hoffman # 1 will operate in systems up to 10 pisig.
    The drop back pressure is 1.5 psig. that means the valve will not open until the system pressure drops back to 1.5 psig or less.

    If the valve does not open steam can not enter the radiator. As the condensate cools in the radiator a vacuum can form and cause banging either from flashing or steam entering the radiator because it is in a vacuum.

    Remember the vent valve makes one pipe steam a living and breathing thing or sometimes a monster.