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Background: We have a fairly large stone home, in suburban Philadelphia, with a two pipe Hoffman equipped gravity steam system, including a Hoffman Loop. From all appearances nothing has been tampered with or changed since the homes 1926 construction. Everyone involved with this project was very excited about finding a Hoffman Loop system intact with no evidence of being altered or disturbed. The large asbestos encased boiler was original and had always been oil fired. Everything was functioning, but we decided to switch from oil to gas. As part of the project we had to have the old boiler and asbestos removed (abatement), including the asbestos on 20 ft. of the 2 1/2 inch main steam lines, from where they attach to the boiler. One in each direction. There are two 2 1/2 inch main steam lines, each serving half the house. On one main most radiators are hot. On the other main almost all are cold. The system has two vacuum vents. A Hoffman #11 on the dry return over the Hoffman Loop, at the boiler location, and a much smaller Hoffman #6 vacuum vent, on one of the steam mains, in the vicinity of where the two steam mains and two dry returns all drop into the wet return. The house has 29 radiators; 2 ceiling mounted in the basement, 13 on the first floor, 12 on the second floor, 1 on the third floor, and 1 in the attached garage. The new boiler is a Smith 19 High Efficiency, with four sections, producing 520,000 BTU's, with a Carlin 301 gas burner. The boiler has been skimmed several times. All radiators, including one non-radiator location, had Hoffman #8 thermostatic traps. As part of this project, they have all been replaced with new Barnes & Jones #2168 cage thermostatic steam traps. All radiator Hoffman steam heat flow control valves were also checked, and left in a full open position. All returns, wet & dry, have been flushed and cleared of all mud and loose debris, including the Hoffman Loop. Some more than once.
Our problem is twofold; one is keeping the boiler running long enough to heat all the radiators and allow the Hoffman Loop to function as originally intended; the second is only some of the radiators get hot, while others are cold. When the new boiler was initially fired, on October 30th, it ran for over an hour continuously and heated every radiator. They were all quite hot. Everyone present believed "mission accomplished". The thermostat was reset for normal operation and everyone left happy. The next day the boiler was running short cycles, on a continuous basis. Shut down being caused by exceeding the pressure setting (1 to 1 1/2 lbs.), or by low water. The heating portion of the cycle lasts mostly from 1 to 2 minutes. It has continued to run in this manner, with one exception, to the present time. Continuous adjustments and changes to the pressure, water, air vents, etc., in an attempt to find the right balance (sweet spot) has not succeeded, with the exception of one recent day where the Hoffman Loop appeared to be tripping and controlling the pressure. We watched the pressure rise to a point quiver a little, and then gradually drop to 1/2 lb. pressure, and stay there for a while before gradually rising again, to repeat the cycle. It did this for an hour or more. When the Hoffman Loop was functioning in this fashion, the boiler did not shut down due to pressure or low water. Also all the radiators heated up again, as they had on the day of the initial firing of the boiler. Again we thought we solved the problem. But that night and the next morning were a repeat of our first firing, and we were basically back in the same situation that we have been in all along.
We are looking for the right adjustments and balance that will allow the system to operate as it was designed to run, on a sustained basis. Any ideas or suggestions would be most appreciated.


  • RobG
    RobG Member Posts: 1,850
    Please post some photos of the install, the more the better. Include boiler piping, vents and Rads.
  • SWEI
    SWEI Member Posts: 7,356
    Sounds like a fun project. Can you post some photos of the boiler and nearby piping?

    How much EDR is connected, and to what size boiler? Current venting?
  • It is possible that more skimming may be needed as more oil has dropped down from the new piping, and causing wet steam?
    In regard to the short-cycling, how is the air escaping from the system? you mention radiator traps, but are there any crossover traps which are not allowing the air out? A good low pressure gauge can tell you if there are any restrictions in the venting phase of the cycle.
    Or is the short-cycling beginning when the air is finally out, [oversized boiler]. looking forward to pictures.--NBC
  • Jamie Hall
    Jamie Hall Member Posts: 23,275
    Let's go back to that Hoffman Loop for a moment. It is absolutely essential to realise what it does -- and why. What it does is not so much to control the pressure, but to ensure that if the pressure rises beyond a certain point (normally right around 12 ounces) that the differential pressure (hence the name) between the dry returns and the steam mains cannot rise any farther. This ensures that water can never back out of the boiler to more than 20 inches height above the boiler water line.

    It does this in a very simple way: if the pressure differential gets any higher, it allows steam from the header connection to pass over and get into the dry return. This close that vent at the loop (which should be the only vent on the system, although it can be a cluster or antler of vents all connected at that location) and then the steam pressure pressurizes the dry return. This allows the condensate to drain back into the boiler (the pressures are now balanced). It also takes the differential pressure off all the traps, preserving them.

    OK. First, in normal operation it should not trip. You should have a vaporstat set so that the boiler shuts down before that pressure is reached. You can't get the accuracy you need with a pressuretrol.

    Second, for the system to operate properly the crossover traps must be present, and must be operating properly. You mention a vent on the dry returns -- not at the loop -- where they combine and drop to the wet return. Something is wrong with that. The dry returns should combine and go into the Hoffman Loop dry return connection, although it is perfectly acceptable to have drips to the wet return as well.

    You mention one main does not heat or heat well. Have you checked the crossover trap on the end of that main? If it is failed closed, that could be part of the problem. Alternatively (or possibly in addition!) if the return associated with that main does not go back to the Loop and its vent or vents, that could also be part of the problem.

    The low water problem may be caused by water backing out of the boiler despite the loop (which is being defeated by that out of location #6 vent).

    So... your first step is to get that pressure under control. 12 ounces max -- and you may find that your particular loop requires slightly less (actually it's not the loop; that's pretty reliable. But vapourstats vary slightly. Whatever works). Second, make sure that all the dry returns are connected to the Hoffman Loop dry return connection. Third, make sure all the crossover traps are working. Fourth, once the above is done, take that straying #6 off and put it on an antler with #11.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • gerry gill
    gerry gill Member Posts: 3,078
    Just the fact it worked the first night and not really since, still has me looking at the one and only variable that can change since that moment. The quality of the water. I think you still need water cleaning, There are other tweaks that can be done but i'm thinking your steam cleaning oil out of the metal still. The metal is like a sponge and holds oil in its pores and this will only come out by steaming it..which unfortunately leads to repeat skimmings..Then as Jamie said, you have some other issues.
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

  • gerry gill
    gerry gill Member Posts: 3,078
    Cleaning a boiler isn't quick..it isn't easy..some times its a real son of a ****..we have used wash out techniques, chemicals, washing soda, tsp, and a hefty dose of patience at times..
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.