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proper cycling

What is the proper length of a cycle? I am sure there are lots of variables but this is what I have. A Burnham V83S with a DOE heating capacity of 91. I have 11 radiators, 5 of them on the second floor. All the radiators heat across at about 1 PSI. When the boiler cuts out it only takes about 3 minutes for the pressure to fall to the cut in pressure of 8oz/in2. It then takes about 5 minutes to reach 1 PSI again and this repeats until the temp. at the thermostat is reached. Does this sound right?

Comments

  • short cycling

    check your main line [not rad] air venting. the cycling will be more pronounced until the air is completely out, as the air heats up and expands as steam is rising. in my view, you can never have too much venting!

    the other thing to check is whether your boiler is oversized for the radiation.--nbc
  • Jamie Hall
    Jamie Hall Member Posts: 15,334
    Pretty much

    sounds pretty normal, except that the time between cutout and cut in is perhaps a little long.



    Nicholas is right -- more venting might make a difference, but probably not.



    If this happens on every heating cycle, you may have a slightly large boiler.  However, if it only happens when you are bringing up the temperature from a setback, or on colder days, what you are probably seeing is the almost inevitable consequence of modern on/off heating, and not to worry.  Back in the days of coal, what would have happened is that as the pressure rose, the underfire damper would close and the stack damper open, reducing the draught and thus the heat from the fire -- and the rate of steam production.  Efficiency was terrible, but who was worrying?  Nowadays, the only way we can do that in most cases is to turn the burner off and on to accomplish the same result (there are burners which can be turned down, or have high/low firing).  In the early part of the cycle, the system is fulfilling the piping pickup and filling all the radiators, and needs a big fire.  Later, when the radiators are full and the piping is hot, you need that smaller fire -- hence on and off.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • short-cycling without adequate venting

    our experience was with our peerless, properly sized to the radiation; vented with new hoffman 75's on each of 6 dry returns, which short-cycled on the vaporstat a lot; until the gorton #2's came off back order, and finally arrived. with increased venting, the burner doesn't have to strain, and squeeeeeeeze the air out of constipated little openings. once the air is out, then with the radiation condensing at the same rate the boiler is steaming, there is very little pressure-cycling. this is the reason for my constant harping here, on the importance of venting and low pressure!!--nbc
  • Mark N
    Mark N Member Posts: 1,090
    heating cycle

    Are you asking how long your boiler should run to heat you house?   From what I've read it should be between 20 to 40 minutes.  I timed my system this morning after reading your post.  I'll also add I've never observed my system cycling on and off on the pressuretrol.  My themostat was set to 68 the boiler came on at 7:45am.  My system has 2 mains, I had steam to the end of each main at the same time.  The last rad on the front main had steam 10'15'' after the boiler started.  The last rad on the back main had steam 12'30'' after the boiler started.  This rad is the furthest from the boiler.  The boiler ran for a total of 20'25''.  The boiler didn't cycle on and off till the tstat was satisfied. 



    Mark
  • one more thought

    also, while i think of it, on the subject of venting and low pressure:

    heat+air+water+high pressure=carbonic acid. this a sort of acid reflux for steam systems which, over time, can erode metal pipes and boilers [do a search for  "condensate grooving"].  you can't even make a good salad dressing out of it!

    therefore, if there is very little back pressure [with good venting, and low pressure], then the acid formation is lessened.--nbc
  • Jamie Hall
    Jamie Hall Member Posts: 15,334
    Good grief!

    You're almost as obsessive about these things as I am, Mark!  My timings are about the same as yours -- a little less, on the whole (steam hits the last radiator in my system -- about 75 feet of main and 10 feet of riser -- a bit over 8 minutes after the boiler starts making steam).



    In mild weather such as we're having currently, the system never cycles on the vapourstat.  However, in the somewhat grimmer weather of the middle of winter -- near design, actually (say around 0 Fahrenheit and 20 mph wind) -- it does at the end of recovery from a setback.  In fact, I choose the setback temperature spread (5 degrees) on the basis of avoiding such cycling except in the more extreme conditions.



    For what it's worth...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Mark N
    Mark N Member Posts: 1,090
    Not really

    I wouldn't say that I'm obsessed.  But I'm quite familiar with how my system operates.  Everything in my steam system is fixed, the boiler, the piping, and the rads.  With that in mind the system should run about the same way everytime it runs.  I don't use deep setbacks 2 to 3 degrees overnight and about the same when we're at work. In my mind if your boiler is properly sized for system and your system properly vented and your pressuretrol working properly. The system should have to run for about an hour to shut off on pressure.  Your rads should be able to condense all the steam your boiler is producing and all the rad vents should be open.  Once the rad vents close then pressure should build to cause the pressuretrol to turn off the boiler.  After the pressure drops back to the cut-in the boiler should come back on if the tstat is still calling for heat.  All I know is that if my system ran for an hour my house would be so hot I'd probably be opening windows. 

    Mark
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