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Short-Cycling After 30 minutes

Mark Gibson
Mark Gibson Member Posts: 21
Jim, thanks very much for that. I've got a few corrections: My main is 2" pipe, and the whole loop is only 40' long. Does that change the size/number of vents to use? I guess I can just call Gorton and ask.

I've attached a picture for reference. At the far left you can see the pipe that drops down to the Hartford loop.

Thanks again!
Mark

Comments

  • Mark Gibson
    Mark Gibson Member Posts: 21


    Hi all...wonder if some of you who know better than me can check my thinking.

    Ever since I replaced the clogged pigtail on my pressuretrol, my burner short-cycles.

    When the burner first fires, it runs for 32 minutes (I timed it several times). All the radiators get hot all the way across during this time. However, if the thermostat isn't satisfied by 32 minutes, pressure starts to build until the pressuretrol shuts down the boiler at 1.5 psi. (You can see this on the gauge as well.) It will stay off for 1-2 minutes, then fire again for 2-3 minutes, then shut down again. It will repeat this cycle until the thermostat is satisfied. These rapid on/off cycles seem bad to me. Are they?

    I've got one huge radiator that's turned off. It's a 19-section guy that comes out to 71.25 square feet according to 'Lost Art...'. Which is 24% of my entire house's EDR. So my guess is that by turning it off, I've effectively made my boiler too big. Does that make sense?

    If I know I'm going to leave that big radiator off forever, and my boiler is in fact too big, I'm sure the "right" solution is to get a smaller boiler. But can I fix this problem by underfiring the boiler I have? I can calculate the gal/hr I'd need to feed the remaining radiators (and all of that pipe). Or do I run the risk of the "boil-a-gallon-of-water-with-a-lighter" problem?

    Thanks very much for your time...

    Mark
  • Brad White
    Brad White Member Posts: 2,398
    Seems reasonable to me

    but others with more direct experience may have other opinions which I am eager to hear myself.

    (I owned several steam systems years ago and which I knew then what I know now. Still, that is the foundation of my perspective.)

    Your on-time (2 to 3 minutes) versus off-time (1-2 minutes) seems OK to me. Having an on-time longer than 2 minutes means your combustion products are becoming more stable beyond the initial spike so that is good right there.

    That it holds pressure for 1-2 minutes before firing again seems OK but might be improved by insulating any bare piping expecially not in the heated space.

    If you open up that radiator I imagine your off-time will decrease (shorter time to re-start) but then you will reach setpoint faster. Always a paradox because over-sized equipment tends to cycle more.

    I cannot speak to the notion of changing the fire rate. Any thoughts of getting a vaporstat? Lowering the pressure just a wee bit?

    Does the "32 minutes to setpoint" time happen in all weather? Is it always from the same setback? I am just curious. Seems it might change depending on the "climb".
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • jim_110
    jim_110 Member Posts: 24


    Mark seems you have oversized the system by shutting the large rad. Did your system not short cycle prior to closing the rad?If you can live with it open do so and see if it stops the short cycle.If it does but creates uncomfortable heat try adding a trv to it.If it must be turned off you can have a pro lower your firing rate . Most boiler manufactures will tell you your system is firing properly, but we all are in search of "STREAM HEAVEN" were our system fires for one long burn ,never producing more then a few ozs of pressure and shutting off at the stat rather the on pressure. Good Luck
  • Mark Gibson
    Mark Gibson Member Posts: 21


    Thanks for your thoughts.

    So it sounds like you're saying that while one long burn is ideal, a few short ones at the end of the firing cycle isn't a big deal. Good to know.

    I probably should have mentioned that the radiator in question is in a second-floor enclosed porch (nowhere near the thermostat, which is on the first floor). I don't use the room often, so I figured I could save oil by shutting it off. Obviously if I'm using more oil due to short-cycling, that would defeat the purpose, but that doesn't seem to be what's happening.

    To answer your last question, in warmer weather (we've had a few warm days in MA lately), or if I don't use a setback, the themostat is satisfied before 32 minutes have passed. So the cycling doesn't happen. Before that, since my pressuretrol's pigtail was clogged the burner was continuing to run regardless of pressure. In fact, I became aware of the pigtail problem when the boiler's safety valve vented one cold day. That was exciting. Instant sauna.

    M
  • Mark Gibson
    Mark Gibson Member Posts: 21


    It didn't short-cycle before, but then the pressuretrol wasn't shutting off the boiler on pressure no matter what (see "instant sauna" above). Also, when I moved in (9 months ago), the pressuretrol was set around 5 psi cut-in, and all of the radiators were open.

    Seems like I've done a lot of things to contribute to this:

    -- Installed setback thermostat
    -- Insulated near-boiler piping
    -- Closed large radiator in unused room.
    -- Replaced clogged pressuretrol pigtail
    -- Lowered pressuretrol to 0.5 psi cut-in and 1 psi diff.

    Maybe I should be thankful it's not worse!

    My burner needs to be serviced anyway, so I'll ask the tech to install a lower-flow nozzle (based on the connected load).

    Thanks,
    Mark
  • D107
    D107 Member Posts: 1,849
    you're probably already saving money

    HO here who used to help run an apt steam system. We also lowered pressuretrol from 5psi to 1.5psi. I knew little about boiler sizing in those days so I'll assume the landlord oversized it. But we experienced the recycling as saving money, since the steam would continue to heat 'for free'. Putting a TRV on that giant rad as suggested would allow you to keep it at 60 or so if you liked and possibly prevent the room's cold wall from conducting into one of the adjacent rooms.

    I'd be interested to hear from the pros on this, but even with optimum rad and boiler sizing I'd think on very cold days there would be some cycling on pressure.

    David
  • jim_110
    jim_110 Member Posts: 24
    Cycles

    Since we are entering the peek heating months, given the info you have, I would surmise that on a cold day with that rad open, coming out of setback you will continue to have those cycles until the stat is satisfied.But during the day to maintain a steady temp you will have single cycles eg. 15-30 minutes, how many and how often depends on the tightness of your home. This is perfectly acceptable. But as you said you can have the pro adjust the nozzle in an effort to further smooth your system and your wallet. Good Luck
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    Mark, turn the big rad back on

    and see if it still short-cycles. If so, have it downfired slightly.

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  • Christian Egli_2
    Christian Egli_2 Member Posts: 812
    The nicest boilers are the off boilers?

    I don’t know exactly if the 32 minutes come long after most of the radiators are mostly hot, but if they don’t, we could be observing a situation with not enough main venting capacity (this is one pipe, air vents on the radiators, right?) Check to see that you have plenty end of main vents and check that all your radiators get boiling at the same time and the same speed. If all this happens real quick, then you’re system is ideal.

    If the boiler were monstrously oversized, we’d expect reaching cycling phase somewhat faster. Note how a system does not have to be fully hot to go into crazy cycling - air is just as happily compressed as steam is, they’re both gasses. These horrible scenarios, caused by a gross lack of air vents are the bad, heatless and costly cycling you seem worried about. These systems boringly spend their winters cooped in continuous burn cycles without the fun of pouring out the heat (and this could equally happen on forced air with closed registers or hot water with air locks, nobody’s immune).

    Clearly, from your descriptions this is not the case in your new home. That’s good. Steam heat is good.

    Cycling on for 2.5 minutes and off for 1.5 indicates roughly that your boiler is modulating at about 60%. Considering the pipe warm up job is completed by then, this is not wrong. Less firing time would indicates a bit of oversizing. It would be interesting to observe the different pattern with the porch radiator open, but indeed, this would cost you in steam usage. Shutting radiators saves fuel just like not turning the heat on does; you won’t loose much system efficiency, if any, by closing just one.

    At least, if you plan on down firing the oil burner, you’ll know not to down fire by more than the one radiator output. The next risk after that, is that you may possibly not be able to heat all of the radiators all of the way, but you have insulated mains and the odds should not be unfavorable. You’d then burn continuously, no seemingly inefficient cycling, but you’d burn longer to satisfy the thermostat, and this may not be more efficient on the whole as reaching full home heat the quickest. But all this can go in many different directions.

    Lastly, if the current 60% cycling bugs you, try fiddling with the differential pressure. Increasing it will lengthen the on and off time, though without changing the firing ratio.

    Also, for preventing long heat demand periods, you can fiddle with the heat anticipator in the thermostat and shorten the cycle length (a little bit). But, you say nothing about temperature swings in your home, so without your complaint, I wouldn’t really change anything.

    Except for making triple sure the system is well vented. Main vents.
  • wondering

    After reading the posts, you said the plms happened after u replaced the plugged pig tail, did ya install new one properly? The "donut" hole and presuretrol should not be facing the same way....
  • Mark Gibson
    Mark Gibson Member Posts: 21


    Thanks Jim...my goals are basically a) maximum fuel efficiency and b) not breaking anything. So if the cycling isn't hurting my burner or using excess fuel, it doesn't bother me. The house basically heats fine.
  • Mark Gibson
    Mark Gibson Member Posts: 21


    Thanks Steamhead. Will do...I'll let you know what happens.
  • Mark Gibson
    Mark Gibson Member Posts: 21
    Main Vents

    After reading "The Lost Art..", I was dismayed to look at my mains and find exactly zero venting. Not even a plug where an old main vent might have been. Yet all of my rads heat all the way across before the pressuretrol shuts off. As for how quickly this happens, I'm not sure -- 15-20 minutes maybe? I'll time it next time. Since I never short-cycle early in the burn, any thoughts on how much benefit I'd actually get from installing main vents on this particular system?

    The cycling itself doesn't bother me at all, as long as it's not wearing out my burner or wasting fuel. From the various replies it sounds like it's not hurting anything, so it's just a question of whether I'm wasting fuel or not.

    Thanks!
    Mark
  • Mark Gibson
    Mark Gibson Member Posts: 21


    Yep, checked this. Although my pressuretrol doesn't have a mercury bulb anyway. (My understanding is that the issue you refer to is caused by the pigtail unwinding when it gets hot which tilts the pressuretrol away from vertical, "fooling" the mercury switch inside.)

    In any event, the pressuretrol appears to be working exactly right -- when the pressure gauge hits 1.5 PSI, the burner shuts down, and when the pressure drops to 0.5 PSI, the burner fires again (assuming the thermostat is still calling for heat).

    Thanks,
    Mark
  • jim_110
    jim_110 Member Posts: 24
    main vents

    Mark main vents are a must,they will balance you system .How many mains do you have-what size pipe- how long is each main.Do you have wet or dry returns . Main vents should be placed at the end of each main some 15" after the last supply where the return heads down. Tell us your end of main config and answer above questions. Adding main vents providing there is room is not a big job , what is BIG is the benefits they will add to your system's performance.
  • Mark Gibson
    Mark Gibson Member Posts: 21
    My Mains

    I have one main, which is basically a square-ish loop of 2.5" pipes running around my basement, with risers coming off every so often to feed the various rads. The main ends back at the boiler where it drops down, I beleive into the Hartford loop. I think this means that condensate travels in the same direction as the steam. I'll measure the total length when I get home tonight, but my very rough guess is 60'.

    The main runs about 12" below the basement ceiling, which I guess doesn't bode well for a big vent on the end of a 12" nipple. 90% of the main is covered in asbestos. (The remaining 10% is covered in Home-Depot-quality fiberglas pipe insulation.)

    Let me know if I've left anything out (other than exact length figure).

    Thanks,
    Mark
  • jim_110
    jim_110 Member Posts: 24
    main vents

    Mark what you or someone capable needs to do is drill a half inch tap on the side of the elbow fitting that turns the main down to the hartfort,and install two #2 gorton air eliminators
    vents teed together on top a 6" nipple.I do not have any photo's , but others on this site do. To make life simple call Gorton at 908 276 1323 they are super at resolving venting problems.They have a brochure that shows the exact vent makeup you need tap position and all.I am sure they will send it to you or fax a copy, also ask them if my suggestion of two Gorton #2 vents is proper. Good Luck
  • jim_110
    jim_110 Member Posts: 24
    vents

    Mark your main contains approx .86 sq ft of air . I think the Gorton #2 vents approx .7 sfm. So your ok with one,but if you need more venting you can always tee a additional #1 Gorton to increase things. I say this because the #1 Gorton is much less expensive then the #2. Mark you can't have too much venting on the mains call Gorton. Good Luck
  • Mark Gibson
    Mark Gibson Member Posts: 21


    Thanks Jim...I called Gorton today, and they recommended one #2 for my main, and said it should be mounted more than 6" back from the elbow that drops to the Hartford loop (saw this in "Lost Art..." as well). So far so good.

    But:

    1) Gorton said I could just drill and tap the main pipe itself. Is this legit? I've got a fair amount of experience drilling and tapping pipe thread, but is it okay to do so into the side of a pipe (versus using a T).

    2) There happens to be a straight coupling in roughly the right spot (see picture in an earlier post). Should I drill into that instead?

    3) Since I don't have a lot of space above the main and a #2 is so tall, Gorton suggested a horizontal nipple coming out of the side of the main. Is this okay? Should I pitch the nipple into the main to let condensate flow back?, or do I need to try to go into the top of the pipe with a close nipple, then an elbow, then a horizontal nipple from there? (see attached picture)

    Thanks again!
    Mark
  • brucewo1b
    brucewo1b Member Posts: 638
    Mark

    Method 1 is fine just give it some pitch so the condensate can run back, the coupling would be better than the pipe but the pipe will work, just make it as high as you can
  • jim_119
    jim_119 Member Posts: 2


    Mark #1 is fine. Tapping into the coupling gives you more meat but either way is fine. As far as pitching the horz nipple You don't need much you want the vent as vertical as possible. When you have the 1/2" nipple in the main attach 1/2"elbow then put a 1/2" coupling on the bottom of the vent and measure how long a vertical nipple you can fit between the elbow and the vent with coupling. You be good to go . Good Luck
  • Mark Gibson
    Mark Gibson Member Posts: 21


    Steamhead: Per your suggestion, with all radiators on (including the big one, which I verified does get hot all the way across), the burner still short-cycles after 32 minutes. However, once the short-cycling begins, the duty cycle is higher -- more like 4 minutes on and 1.5 minutes off.

    For what it's worth, here are some more numbers:

    EDR: 303.25 including the big rad, but no pickup.
    Boiler: 162 MBH (gross)
    Nozzle: 1.25 gph

    According to my (quite possibly wrong) calculations, my boiler is a bit oversized, but the 1.25 nozzle underfires it to pretty much exactly my EDR + pickup (with the big rad on).

    Thoughts?
  • Mark Gibson
    Mark Gibson Member Posts: 21


    Thanks very much Jim and Bruce...looks like I'm off to buy a Gorton #2!

    Thanks again,
    Mark
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    Mark, you may find

    that since the boiler's entire steam output goes into just one main, that one Gorton #2 might not be enough. In these cases I usually figure the vent size in the usual way, then double it. This seems to do the job well.

    So you'd need TWO Gorton #2 vents for that main.

    I've used your Method 1 more times than I can count, with zero problems. Just make sure, as Jim and Bruce said, that the horizontal pipe slopes back toward the steam main. Also, I'll second drilling into the side of a fitting (the last takeoff tee is a good place) since the walls of the fittings are a bit thicker and you can get more threads that way.

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