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Boiler doesn’t shut off when radiators full

Have to replace my steam gas boiler and this is my only real issue with the old one.  Trying to see if there is a fix. Thanks in advance!

So, I have an old home with huge oversized radiators.  I like to set the temp back at night or when I’m gone and then wake or come home to full, cozy radiators.  The problem is that it appears my boiler never shuts off due to pressure.  After about an hour of running, the radiators all feel fully hot, the radiator vents start hissing, and the boiler never shuts off.  One of the return pipes then starts a small water  leak, the hissing is loud, and it feels like a waste of fuel.

I had an aquastat installed, but it never hits pressure.  This was an old coal boiler system, and my understanding is that it’s hard to generate any pressure in these big systems using natural gas.

Is there an easy solution that I’m missing?  I’ve looked for thermostats that let me set a max cycle time, but I can’t find one. Is there something I can do  at the boiler end?  My radiator vents are pretty new, so I don’t think it’s them.


  • Zman
    Zman Member Posts: 7,446
    Do you have a reliable pressure gauge on the boiler? What is it reading and what is the pressuretrol set to?

    The does boiler eventually turn off when the t-stat is satisfied?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    EBEBRATT-Ed Member Posts: 13,427

    I would suspect a plugged pigtail under the pressure control. Has it ever been cleaned?

    You need a good steam guy. Post where you are located and check "find a contractor" on this site
  • Jamie Hall
    Jamie Hall Member Posts: 21,096
    "I had an aquastat installed, but it never hits pressure. This was an old coal boiler system, and my understanding is that it’s hard to generate any pressure in these big systems using natural gas."

    First off, an aquastat has absolutely nothing to do with pressure. It is sensitive to temperature only. So that was a complete waste of someone's time and money.

    Second, what the boiler is fired with has nothing to do with whether it will generate pressure or not. What matters is how powerful the fire is. So saying "it's hard to generate any pressure in these big systems using natural gas" is simply not true.

    Now. First, you need an accurate pressure gauge on the steam side. 0 to 3 psi. Depending on the boiler, there are a number of ways to hook it up. Second, you need an accurate pressuretrol also on the steam side. The pressure gauge is there to tell you the pressuretrol is accurate. The pressuretrol is there to shut the boiler off it and when the pressure rises to about 1.6 psi or so.

    A few other thoughts.

    First, it is possible that your particular system may be matched well enough that the power delivered by the boiler is very closely matched by the power required by the radiators. If this is the case -- and it does happen -- it's very desirable! However, it can be confusing, as if it is the case the system will never build much pressure -- the steam is being condensed in the radiators exactly as fast as it is being produced in the boiler.

    Second, against this hopeful thought you mention that the vents start hissing. This suggests very much that the system is building pressure, and that your pressuretrol is either set incorrectly (quite likely), the pipe, called a pigtail, leading to it is clogged (also quite likely) or a combination of those -- or that it is either missing completely or not wired correctly; both unlikely but quite possible.

    Third, you mention that you like to turn the thermostat down at night or when you leave, and come back to nice full cozy radiators. I can understand that, but most steam systems do not take kindly to setbacks more than 3 to 5 degrees F. It simply takes too long to heat back up, and wastes fuel. It's quite common -- particularly in larger systems -- for it to take as much as an hour for all the radiators to be heated all the way across (this usually does not happen when the system is just maintaining temperature, except very close to design conditions) and if you want full cozy radiators at some point you first need to be sure that the whole house cools down enough so that when you start it up again it needs to have the radiators full -- and then you need to set the thermostat so that the system has time to get there; as I said, an hour ahead of when you want that to happen is not too much. If your schedule is sufficiently regular, a good programmable thermostat can be set to do just that. A word of caution: you will find, if you do do this, that the house temperature has a tendency to overshoot, as all those nice hot radiators will just keep right on heating when the thermostat does shut the system off. Some, but not all, programmable thermostats are moderately clever at avoiding this problem by shutting the system off early to allow for it.

    So, bottom line, yes indeed there is something you can -- and must -- do at the boiler end. Make sure that you have an accurate, working pressuretrol wired in correctly and set it for a cutout of 1.6 psi and 1 1 psi differential (if it is the subtractive type -- most are) or a cutin of 0.6 psi and a differential of 1 psi, if it is the additive type. Lose the aquastat. Don't fiddle with the thermostat for the moment -- leave it set at 1 cycle per hour (if it is... ).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • neilc
    neilc Member Posts: 2,196
    well I'll say it , , ,
    Let's see some pictures,
    the aquastat,
    and any other controls,
    the water level sight glass,
    and one, or more, general large view of the entire boiler, floor to ceiling, showing the piping above
    known to beat dead horses
  • jacobloftin
    jacobloftin Member Posts: 6
    edited September 2021
    Sorry I have a VAPORstat not AQUAstat. Confusing the terms. I had it installed specifically because I thought it might solve this issue, but it has not.

    I tend to think it's not clogged because it was installed by a great steam boiler contractor (from this board) and then subsequently cleaned by another great steam boiler contractor (also from this board) that I saw clean out all the parts. So, I don't think it's a cleaning/clogged issue. The boiler has just never cut-off from pressure.

    To be clear, I do turn it off as soon as I hear the vents hissing, so maybe if I ran it for longer for the vents to close completely it will trigger? But I would rather not wait... if that makes sense. The radiators are warm enough already - the hissing and other issues aren't worth it to me.

    Here's a pic. Boiler is dirty because I haven't yet had it cleaned for the season.
  • motoguy128
    motoguy128 Member Posts: 394
    The vents hissing can sometimes just be damp air vapor blowing through them. I also found that is the main vents are too small, and the radiator vents too big, steam rushes to the vents and makes them noisy. Slower vents are often a lot quieter. Seems counter intuitive. Also if you have wet steam it too can make a system noisier and heat less evenly. Some boiler treatment can help. 8 Way can work wonders to reduce surging.

    Also, I gave up on the vapor stats, mine became unreliable. If the system drops into a deep vacuum on shift down I found, it inverts the disc in the vapor stat and wont work correctly the next cycle. Better to than use a pressuretrol for a safety limit and use a temp limit snap disc on the last radiator on the main, or whichever one heats up last.

    The purpose is to heat the system not generate pressure. A pressure control is better as a safety limit not a operating control IMO. A snap disc can easily be hidden behind a radiator on the 1st floor and 2 wire thermostat wire run back to the boiler.
  • jacobloftin
    jacobloftin Member Posts: 6
    edited September 2021
    @motoguy128 Thank you! That sounds like a good option for me, and there is definitely a radiator I can hide it. Now that I think about it, I may be able use a remote thermostat sensor for this purpose.
  • Jamie Hall
    Jamie Hall Member Posts: 21,096
    Couple of thoughts here.

    A minor -- or maybe not -- aside. @motoguy128 's comment on vapourstats having difficulties when the system to which they are attached go into significant vacuum is quite correct. However, to be blunt, that's not the fault of the vapourstat, but the application. A vapourstat (and to a lesser extent a pressuretrol) is designed and intended to work within a very small range of pressure differentials, in which the pressure side of the sensing diaphragm is always at a slightly greater pressure than the external, atmospheric side. Any other application, whether having the internal pressure beyond the rated pressure range in the positive direction, or at any value in the negative direction, is a misuse, plain and simple. If the device fails to work properly, or fails to work entirely, in such an application, it is the fault of the installer or the operator.

    Unfortunately, there is no good way for a manufacturer to keep people from misusing equipment. Equally unfortunately, it is normal for people to blame the manufacturer for problems or damage which they brought on themselves.

    End of rant...

    For @jacobloftin . From your description, my present feeling is that first, your radiator vents are probably being asked to do the job which generous main venting should be doing. They will hiss when that happens. They will -- unless they are damaged or very cheap -- stop when steam finally gets to them and they close. However, I get a sense that you feel -- and may very well be correct -- that if you do allow the radiators to get to that point the structure will overshoot the desired temperature and you would like the system to shut off earlier in the cycle. Solving this problem, however, does require one further bit of information: does this happen when the thermostat is attempting to hold a constant temperature, or does it happen only when the system is attempting raise the temperature of the structure -- coming out of a setback, for instance. If it's the former, it's really a thermostat problem. Unfortunately, most modern digital thermostats do not have a reliable anticipator function, unlike the old mercury or straight bimetal dry contact thermostats. While they were a little tedious to adjust, they could be adjusted so that for any given system the problem simply didn't happen. Some of the better modern digital thermostats do have a more or less reliable ability to "learn" how much in advance to shut off the system to avoid overshoot. Unfortunately (there I go again) it is based on time, not on the actual relationship of the rate of change of temperature to temperature error which it should be -- but which requires a modicum of computing power. The problem is that since it is based on time it is confounded by the difference in time when coming out a setback vs. that required to maintain temperature.

    Think of the blessed little thing this way: Suppose that it has been running for a few days or weeks always maintaining temperature. It will "learn" that if it shuts the boiler off let's say 5 minutes before the actual space temperature hits the setpoint, the setpoint is not exceeded. And it's very happy. However, we now introduce coming out of a setback, which gets much more of the radiation up to temperature. It shuts off that 5 minutes early -- but all that radiation just keeps going and the temperature overshoots. Rats, it says to itself, I goofed. So now it resets itself for a longer delay. Now it's holding, but with a longer delay -- and it doesn't reach the set point. So it gets confused.... in a setting where it is alternating between coming out of a setback and holding, it will never be able to determine what that time delay should be -- and the operator (you) will be unhappy with it.

    @motoguy128 's suggestion of cutting off the system when steam reaches a certain location in the system does work, after a fashion. In some systems it works rather well -- and in large systems, with varying loads, such as may be found in apartment buildings or office buildings etc., it may be by far the best approach. However, it does place a somewhat arbitrary and quite inflexible limit on the system run time (or, to put it another way, firing cycle length, as you mentioned in an earlier post). In some situations this may be satisfactory. In other situations, such as where the actual heat load varies widely (some days it's 10 below outside with wind, others it is still air and 55), the cycle will be much too short or much too long.

    So what do we do? First, go and revisit your main venting. It really is critical. Second, try to avoid using setbacks, or limit them to no more than a few degrees. Third, get that low pressure gauge I mentioned and use it to set your vapourstat. This is not hard, but there is no one number which is correct for every system.

    What you do is this. Start the system from cold and observe the pressure gauge. As steam starts to rise in the boiler, you will observe that the pressure gauge will rise to a few ounces. Now one of two things will happen: either the pressure will hold at that level, give or take an ounce or so, or it will continue to rise. If the latter is the case, stop and go back and add main venting. You don't have enough. If the former -- it holds at a reasonably constant level -- eventually it will start to rise again (sometimes remarkably quickly). This tells you that all the vents are closed and the system really is full of steam. No point in making more for a bit. So, set the cutout pressure so that the system cuts out at perhaps two ounces above that more or less constant plateau level -- and the differential so that it cuts back in at a couple of ounces below that more or less constant level.

    Depending on how well the boiler is matched to the system, reaching that point where it starts to rise again may take an astonishingly long time (one system I'm aware of can take well over an hour).

    Now go back and revisit the thermostat problem, if you are still getting overshoots.

    I apologize if this is a bit long-winded, but it's actually harder to describe this stuff than to do it...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • wlgann
    wlgann Member Posts: 14
    I feel like this is almost certainly a venting problem not a pressure problem. Pressure won't build as long the main is venting, and if the radiators are vented too fast they will often hiss and get too hot, especially if the main isn't vented fast enough. Most likely you want a faster main vent and slower radiator vents.
  • motoguy128
    motoguy128 Member Posts: 394
    To expand on that, if radiators are vented too fast, the steam will tend to rush across it rather than evenly push out air and full heat all the metal surfaces. A radiator that doesn’t heat fully consumes less steam and causes the system pressure to rise. SO ideally, the main and laterals and risers vent very fast, then the radiators heat up very slowly.
  • wlgann
    wlgann Member Posts: 14
    You should probably post a pic of the main vent, approximate size and length of the mains, approximate square footage of the building, number of floors, and radiator count per floor. Maybe sample pics of a few of the radiator vents.

    Some specific troubleshooting steps you can take include:
    1. Close the valves at all radiators, crank the thermostat, wait by the pressure gauge and see if the pressuretrol is working. Obviously, kill power to the boiler and open the radiator valves if pressure exceeds the cutoff on the pressuretrol for more than a couple minutes and more than one or two PSI. I'm assuming here that the pressuretrol is set to sane levels, which it looks to be in the picture.
    2. With the radiator valves open and the system cold, crank the thermostat and start a stopwatch. After maybe 10 minutes the header should get hot and then the main should start to heat immediately--like seconds later. Hold a thread or a bit of down by the outlet on the main vent (if there are no convenient nearby cobwebs!) and you should be able to detect air movement--you may not be able to feel it. Within another 10 minutes--unless your house is HUGE--the fittings near the main vent should start to warm up. My house has about 50 feet of main pipe and the main vent (a B&J Big Mouth, which is probably way bigger than I need) closes after 15 minutes. At that point I can visit the first and last radiator and in both places I'll find that the valve is hot and the first section is warming.
    3. You state, if I'm reading it right, in the original post that the radiator vents hiss after about an hour of the boiler firing. Do they not hiss before that? Do they all hiss? At about the same time? As another poster said your boiler might be making really wet steam due to contamination. But also be sure to lay a level on each radiator and do what you can to eliminate poor drainage as a possibility. If you change out one of the vents that hisses for a much smaller vent, does the smaller vent also hiss? If not, I stand by my original post and you might want to do the math on whether the main is vented enough.