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Dan H .....\"Undersizing\" Steam boilers (continued)....Boilerpro

Boilerpro Member Posts: 410
Reread your piece on the topic. However, I believe that the system you were describing could be balanced to provide even heating with the boiler that is, by current standards, too small. Again, I think applying the concept of master venting, with a slight change, would allow boilers to be much more closely sized to the actual heating load, rather than radiation, on most systems. I've thought about this more for one pipe steam, than two pipe, but I believe the principles are the same. The basic concept to make this possible is to vent the mains very quickly and the radiators very slowly, AND IN PROPORTION TO THE LOAD THEY SERVE, NOT THE SIZE OF THE RADIATOR. I see several things happening here that make this feasible. First, if the boiler is much smaller than what the system was originally designed for, it will develop very little pressure, creating a vapor system, where steam volume is at its greatest per btu of input. Pressure drop from the beginning of the main to the end will be very low, since the main will be carrying a much lighter load than the standard 1 or 2 ounces per hundred feet pressure drop desitgn columns. If the mains are vented very quickly and the radiators very slowly, steam vapor will greatly favor the mains, since they are now effectively oversized and the volume of the vapor is greater. This should allow the main to be completely filled with vapor before vapor begins moving toward the rads. Since the radiator vents are so small, little vapor would enter the rads while the mains are filling. Once the main vents shut, the vapor will start moving towards the rad vents. By adjusting the venting rate on each rad to its proportion of the heating load, and not the radiator size, balanced heating should result. Now, assuming the radiation is oversized (much like an Orifice type vapor system), as it appears most older one pipe systems are, the vapor will continue to fill each radiator in proportion to the size of the radiator vent orifice. At some point, probably after a couple hours of continuous firing, the system will reach a steady state point, where each radiator is condensing all the vapor entering it, and no longer does any air vent from the rad vents. At this point, there is no longer any pressure in the radiators, and as vapor condenses, more vapor continues to take its place in the hot portion of the radiator. While thinking of this, I believe it is helpful to think about how that old system may have worked with a coal boiler. On start up, that coal boiler begin to make vapor very slowly as the coal and that huge volume of water began to heat up. On a cold start up, I bet it took 10 minutes or more to get the mains hot after vapor began to be produced. An "undersized" boiler would tend to mimic this slow start up of the coal fire. To help keep the system in balance, big main vents were used, and tiny little Hoffman 40's or smaller vents were used on rads. The small amount of vapor that coal boiler was making went right to the end of the mains since that was where the pressure was the lowest. Then as more vapor developed, it began to fill the rads. Now I suspect, that in warmer weather, that coal fire was kept burning fairly low all day, so the boiler spent most of its time running at a fraction of its total output..... much like an "undersized" boiler would operate that system today. What do you think?



  • To boilerpro from joe

    I missed the original article on this subject, so please fill me in. I had a problem several years ago, when we lost our power generation plant and city steam. Everyone had to convert to alternative heat sources. One account that we inherited several years later had a problem with maintaining heat in the two buildings that were now served by a single steam heat boiler. When we checked the installation, we found that the installing contractor had put the wrong burner in that boiler; half the required size; and the other customer got a burner that was twice the size needed for his installation. Burned it up several times. Whoops! But going back to the original job,we had a hell of a time maintaining heat in that building. On a moderate day it would do well, but when the temp. dropped, it did all it could do to keep up. Besides the ordinary everyday problems like being 150 years old, clogged returns, and failed traps;the steam mains seemed to lose their heat energy before they got the steam to the rads. The mains were vented, so air was not a problem. That poor boiler ran more than the offensive line of Notre Dame! Did I get Dan's attention on that quip? They spent a lot of money on trying to make the system right, but did not want to take our advise. Never put the proper burner in the damn thing; had a mild winter last year, and the problem went on the back burner. Have tried many one pipe tricks on two pipe systems; sometimes it bites you. Would love to hear from you! Happy Ramadan!
  • Under sized boilers

    Your thought process is good. The venting of the steam systems is as important as installing the right sized boiler.

    Here is the senario of what happens in a building when a boiler produces steam.

    The steam system has 1000 EDR ( square feet of steam ) worth of radiation which includes about 30% for piping pick up losses.

    This translates to 240,000 BTUH or 250lbs of evaporated water per hour (steam).

    The steam system does not care what pressure you provide so long as you can input the BTUs into each radiator.

    Assuming the steam system was designed for 1 1/2 PSIG worth of steam ( pipe size rules here ).

    For a system to operate as a vapor system the steam piping must be sized to carry the volume of steam at the lower pressure.

    If the boiler is undersized it will not evaporate the amount of water that is required to heat all the radiators.

    Pound of steam (evaporated water) translate to cubic feet of steam.

    1 lb of steam at 1 PSIG= 25 cubic feet of steam.

    If the the steam system operates at 1 PSIG and on the coldest day for the system design we boil off 250 lbs of water per hour the boiler will fill the system with 6,250 cubic feet of steam per hour.

    Let us assume that the boiler is sized at 200,000 BTUH. That translates to 200 lbs of evaporated water or steam.
    The boiler will only be able to supply 5,000 cubic feet of steam to the system.

    The steam boiler will reach set pressure, there will be enough steam to fill the system and the extremities and upper floors of the house will not heat.

    You cannot fill a 10 water bucket with 10 pounds of feathers unless you compress the feathers.

    When compress the feathers you are cheating. You cannot cheat in a steam system.

    A long time ago my good friend Dan Hollohan made a state-ment " sometimes you do something stupid and you get a reward. if you continue to do the same thing over and over again you will get bit on the rectum more times than you want."

    Some time you can increase the size of a naozzle on an oil burner and make the boiler produce more steam. I said some times, you need to check with boiler manufacturer if you can do this with model boiler you are working with.

    Undersized boilers work well most of the time, the only time there is a problem is when it gets cold.

    If the insulation was removed from the basement steam main a properly sized boiler may wind up being undersized....


  • Boilerpro
    Boilerpro Member Posts: 410
    Thanks for your extensive response

    Twas wondering though, in your thought process, if you were taking into account that if a heat loss calculation was done on that building with 1000 edr (200,000 btu/hr radiation load) and it ended up with only 125,000 btu/hr peak heat load might that 200,000 btu input boiler heat the structure? Now you only need to heat 520 edr (plus the piping)to produce enough heat for the structure. This is the scenario I am looking at, and in my parts it is a very common one. You could just change out all the radiators to the new proper size, however,I would like to establish a method to be able to leave the radiation in place, but only heat each radiator part way on that coldest day.

    By pushing less steam through the same size mains, the pipe sizes also become proportionally bigger and operating pressure is effected. 1000 edr running through a 100 ft of 3 inch main will give a pressure drop of about .8 Ounces. However with that smaller boiler now you are moving only 520 edr through that same 3 inch main, so the pressure drop across that 100 foot pipe is now only .4 Ounces(ignoring the pick up factors). This same pattern would follow for all of the run outs , rad valves, etc. So with a typical system with piping originally sized to an overall pressure drop of 8 ounces, by running only half the steam volume through the system piping that pressure drop is now only 4 ounces. This is how it appears to me that a once steam system will behave like a vapor system with a smaller boiler installed, but the large pipes remain.

    The potential key I see to making a smaller boiler work is to vent the rads so that each one will only fill with enough steam to meet its room's heat loss on that coldest day, allowing vapor to be distributed to the whole system, not just the rads closest to the boiler. Each rad will only fill with its piece of that 520 edr to heat its room properly.

    Example, A room with a 50 edr rad...3 col, 10 sections long... originaly had a load of 12,000 btu/hr. Through the years storm windows were added and the walls were insulated. Now the load is only 7,000 btu/hr and we only need about 30 edr for that room. If I install an adjustable air vent that restricts the air venting rate so that when the boiler is on for several hours at a time on that design day only the first 6 sections heat, now we only are providing 30 edr or about 7,000 btu/hr. Also, lets say the room with the thermostat was just like this one, with the same size rad, but still had no storms and insulation, it still would need that 12000 btu/hr or all 10 sections to get hot on that design day. This rad would get a bigger vent, so more air would vent, so the rad would heat all the way across. If we put that same size vent in the other room, it would greatly overheat on that design day because you would be putting into it 12,000 btu/hr when it was only losing 7,000. When putting these two rooms together, orignally 100 edr was being heated on the design day or 24000 btu/hr. Now with some improvements being made and the venting rate cut on the one radiator, only 80 edr is being heated on the design day. Now only 19,000 btu is needed to heat those two radiators on the design day, so why not cut the boiler size by 5,000 btu/hr?

    This is what I am trying to explore.

  • Under sized boiler

    The senario I used was to keep the math easy.

    The boiler BTUH capacity was what was needed in steam to heat the building.

    Unfortunatelly no matter how you run the numbers the entire piping system has to be filled with the needed cubic feet of steam.

    Even if you down size the rads you have to be able to provide the needed energy to heat the building.

    If you want to regulate the flow of steam to the over sized rads you can use Danfoss Non electric thermostatic valves on the two pipe portion and Danfoss radiator vent control valves on the one pipe side.

    These Non electric thermostatic valves are are self balancing.

    From what I read you have all your stuff togther and once you get past making an undersized boiler work you will be O.K.

    The lack of insulation is a big part of the problem. That will cost you at least 15% of the steam supplied.

    Even if the boiler has to be changed out the insulation problem needs to be addressed.

    Remember this fact. A boiler must be sized to the system as well as the standing heat load.

    Steady state is fine as long as the boiler can supply heat.

    You can get close to steady state by installing flame modulation on the burner. But in mild wheather the Temperature controller shuts the boiler down and you lose the steady state condition. All Boilers will operate at steady state whe it cold enough and the t stats are not satified.

    Even in large heating plants where tthe boilers are on line all the time boilers will shut down when the maximum operating pressure is reached or the t stats are satisfied.

  • Under sized coal boiler.

    Oh I forgot this point.

    Most coal boilers were not rated as to what edr out put they would have.

    This was because the boiler manufacturer did not know what type of coal would be burnt. Different types of coal put out different BTUH. Also if the coal was wet igave out a different BTUH.

    I found on the north shore of Long Island some real old H.B. Smith Boilers. Some of these boilers could support an oil burner firing 12 gallons of oil per hour.

    The burners used in these boilers heated the house and made hot water for Five baths these burners had nozzle firing rates from 4.5 to 6 GPH.

    These houses were equipped with Broomel systems. The only change to the system was the chains and pulleys were removed from the air damper when it was coal fired.

    There was no pressure control on the boiler. A float in the condensate accumulator had a chain and pulley set up to a on off switch that was wired to the oil burner.

    The safety worked this way. As steam pressure rose in the boiler water would be pushed out of the boiler and the water level in accumulator would rise. The weighted chain via the pulley system would cause the swithch to break and the burner would shut down. When the water went back into the boiler the float would drop and the burner switch would make and start the burner.

    The steam pressure in the system was regulated by the position (hieght) of the float ball in the accumulator.

    These systems rarely operated over 6 OZ of steam pressure. They were a work of art. I helped solve some heat problems in these buildings. The problems were not with the system but the peop[le that serviced them. No one ever taught them about how vapor systems work.

    I ran a school for the heating contractor on site and trained 6 of his mechanics.

    If anyone ever gets a chance to work in the buildings at the Pratt estates you find that the owners have the original heating plans for the system. I almost felt guilty about being paid for the work I did because I had great pleasure being in these old treasures of buildings.

  • Boilerpro
    Boilerpro Member Posts: 410
    Sounds like really neat stuff

    Had the oppurtunity to put back together a Moline vapor vacumn system a year ago....minus the vacumn. Someone had mistakenly converted it to operate as a two pipe steam system... complete with condensate pump and a bunch of F and T traps. Poor thing was running about 4 to 6 lbs of pressure. Before even knowing what it was I dropped the pressure and the system stopped banging. Found out where the main vent belonged and replaced the convector vent there with a Hoffman 75 ( now I know the vent should be bigger than that) and suddenly the second floor started heating properly for the first time in 35 years. Fortunately the boiler died a few months later and I had the opportunity to repipe it.
  • Chris NJ_2
    Chris NJ_2 Member Posts: 10
    undersized boilers

    Read this thread with great interest. Here is the background: Home is 80 years old, 3,000 SF, 2 story with single pipe system, 524 SF of radiation with a 2 1/2" main running 96 feet. Risers are 1 1/2" diameter. Boiler was replaced in 1974 with a oil fired Crown unit capable of 595 SF of steam. This original oil fired burner had been converted to gas by the prior owners, who removed the oil tank so there was no way of going back. The oil fired burner put out 265K BTU. (historical BTW: the original boiler was a coal fired American capable of 950 SF of steam.)

    When we bought the house, there was a single main vent at the end of the 96 foot main designed for coal (it stilled worked but was way too small). All the insulation had been removed from the mains and never replaced. Radiators all had Gorton "C" or "D" valves. This is what I bought into.

    Not having the $7K+ to replace the boiler with a better match and efficiency I decided to see if I could just improve this one for the time being.

    So, I have re-insulated the mains in the basement, found a "Tee" about 45 feet in the main where a vent used to be and put a Hoffman 75 there and replaced the coal fired vent at the end of the main with another Hoffman 75. Removed the jacket from the boiler and added another layer of insulation. Have installed Hoffman 40a radiator vents on all but the 2 farthest rooms in the house, Gorton #5's work best here. Found out the gas burner is putting out about 180K BTU's and creates 3 to 5 oz. of steam. I have a professional come in to adjust the burner and draft yearly.

    The house heats in about 35 minutes maintaining the house at 70 degrees on the coldest days of the year. The temperature difference between the room where the thermostat is located and the last room in the system is approximately -1 degree. All rooms heat evenly.

    After doing all this and reading Dan's Books, I realized I had (using the hand I was dealt) replicated a coal fired system using a 1974 boiler and an undersized burner that some know-nothing had installed. Now I only did this because I didn't have the money to replace the boiler, but 9 years later and it still works, so now I may just leave it in place because it works.

  • greg_7
    greg_7 Member Posts: 71

    System vents make a huge difference. Our one pipe system had none. We installed two-one for each leg of the return before the water trap. Much quicker steaming with hotter rads at he extremes.

    A hand fired coal unit creates heat that is much less easily controlled. No doubt many of the deivces and techniques on the older systems were designed to cope with the peaks and troughs of the hot fire.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    Your logic seems reasonable

    but having never worked on one, I know very little about steam systems.

    Two things strike me about this scheme:

    1) The nature of steam & vapor. As I understand it, steam and vapor are very different animals. Vapor (to my comprehension) amounts to humidity--water "dissolved" in air. The water in a boiler gives off lots of this before it boils, but it's not really steam. It does though have quite a bit of latent heat.

    The old vapor systems produced steam at lower temperatures by keeping the system in partial vacuum (thus lowering the vapor pressure/boiling point). Even though it is produced at a lower temperature, it is still steam--not air--not vapor.

    I think they might have been called "vapor" systems because once discharged in normal atmosphere you could always see it--unlike steam produced a normal atmospheric pressure which is invisible for a distance as it "shoots" out of a pipe.

    While I don't know exactly how much (if any) heat you could get from the rads from true vapor, I don't think your near suggestion that an undersized boiler is somehow turning it into a "vapor" system is correct.

    2) The extremely low steam pressure produced. Since there technically isn't enough steam the very rapid venting of the mains and slow (sized to heat requirement of the space) rad venting should allow just enough pressure to build in the mains to keep the steam expanding as required to remote locations. Likely the steam exists only for a VERY short time in the radiators and their temperature is lower than "normal" for a steam system. I can see here how you can almost think it a "vapor" system. While this may be evident, the sum of the radiation venting must be very close to the heat loss of the structure as a whole.

    The absolute "must-do" I see with this scheme is excellent insulation of the mains--and even accessible branches/risers. You're working with "barely steam" and have to keep it that way until it gets to the radiation.

    Even though less steam=less condensate in return I believe that free flow of condensate back to the boiler is doubly important.

  • Boilerpro
    Boilerpro Member Posts: 410
    Maybe I'm not so crazy after all and Twas wondering.....

    In your system on the coldest day, do all the rads get hot all the way across, or only part way? Do you setback the thermostat and then set it up? Also, in warmer weather does the house still heat evenly? One suggestion, you may want to change out that end of main Hofmann 75 to a big Gorton. It vents about 4 times faster and may improve performance even more. Found this out thanks to Steamhead and Dan H. I came across a system alot like yours and asked the owner if it heated evenly and adequetely and was also told yes... every rad had an adjustable air vent and the system could not develop more than a few ounces of pressure. The boiler was probably the original from 100 years and finally converted to gas. Interestingly the water line was only 15 inches below the end of the main, so if it ever developed pressure the sysem would have starting banging.

  • Boilerpro
    Boilerpro Member Posts: 410
    The original article

    was a small side discusion about a week ago under another thread. I think the thread was titled "System Conversion". Didn't miss much there. Another frequent visitor to this site e-mailed me and appears to also have a working one pipe steam system with a boiler that is not big enough for the radiation.

    It almost sounds like the boiler was not only "undersized" to the radiation, but also did not have enough capacity to actually meet the heat load of the stucture. From what I can see, a boiler undersized to the radiation but has enough capacity to meet the heat loss will probably provide adequete heat to the rooms near the boiler, but those further out will not heat. If you can throttle the steam flow to the rads in the rooms closest to the boiler. this will push the steam further out in the system, On one pipe, you would use smaller vents, and on two pipe, throttle the supply valves or install orifices. Just some of what I am thinking.

  • Chris NJ_2
    Chris NJ_2 Member Posts: 10
    answers to Boilerpro questions

    To answer your questions, each of the 2nd floor bedrooms has a 3 col. 13 section Peerless radiator standing 26 inches tall. At this time of year with the outside temp is around 37 degrees, these radiators will heat up about 6 sections. In January/February when the temp drops to 20 or so, they heat up all 13 sections. There are two short 3 col. radiators in the living room and dining room that are enclosed (when the house was built) with windowseats. Each of them is about 9 feet in length and they have never heated more than 6 feet. They are way oversized for these rooms.

    Secondly, yes we use a setback keeping the house at 70 degrees from 11 pm to 5 am then kick it up to 71 until 4:30 pm and then it goes up to 72 (the standard setting in 1923 and yes the house is "comfortable") Anything less and it feels cold in here (remember, no insulation). For all the readers thinking I am wasting energy sorry to inform you that boiler running time and energy usage doesn't vary whether you set the stat at 68 or 72.

    Lastly, heat stays even regardless of moderately cool or extremely cold outside temperatures. As I said, I think I have inadvertently replicated the coal fired system.
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