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Short Cycling Conundrum

I replaced a 34 yr old boiler with a new one. The new one was sized down based on EDR calculation of the various radiators; total of 10, mostly Sunrads. The new boiler's output is rated at 281MBTU/hr while the old boiler's rated output was 320MBTU/hr. The system has two loops. A shorter 24' loop that feeds all the radiators on the back of my 3 story house. The second loop is about 40' and traverses an unheated crawl space and feeds all the radiators in front of the house. Both mains are 2" pipes, the returns are 1.25" and the risers are 1". All fiberglass insulated.

The system had NO main vents. The installer figured if the old boiler did not short cycle and was heating all the radiators except the last one in the front loop, he could heat everything evenly by adjusting vent sizes on the rads. Since the system was installed in Sept. everything worked fine after extensive skimming. Nevertheless, I had another boiler man look at the system and he installed 2X Gorton #1on the shorter loop and one Gorton #2 on the longer loop on antlers. The stat, a Nest set for "True Radiation" to 67 degrees at night and 70 degrees during the day.

By December temp in New York City area dropped and everything changed!

When the boiler starts up in the morning, it runs for 40mins(a little longer on colder nights) and shuts down(not on pressure). The temp rises to 68. After that the boiler cycles every 3 mins (3 mins on 1.5 mins off) on pressure and takes 2.25 hrs to reach 70 degrees. The cut-in is 0.5PSI and the cut-out is 1.7PSI on a 0-5PSI gauge. After hitting 70 degrees, on most days the stat calls for heat 1/hr and the boiler runs for 15mins and shuts down on temp. The house is reasonably well insulated. Vents do not spit but expel air for a long time and produce a low gurgling sound. I have uploaded some pictures of the system.

One of the two Gorton #1 vents on the shorter loop failed to close and has been replaced with a B&J vari vent. The #2 on the longer loop is functioning properly. The third floor is generally 2 degrees lower than the first two floors

There isn't a significant change in my heating costs based on graphs of current month's gas usage vs last year for same month on ConEd bills.

Can anyone suggest ways to limit/correct the short cycling? Thanks in advance!


  • Jamie HallJamie Hall Member Posts: 6,010
    I am slightly puzzled. You say that the boiler shuts down at 68, but not on pressure. Then you say it cycles with a 3 minutes on, 1.5 minute off cycle -- on pressure -- until the temperature reaches the set point. What shuts if off that first time? And are you sure the subsequent cycling is on pressure?

    What I am suspicious of is the Nest, and what I would do were it mine to play with is get an inexpensive digital Honeywell VisionPro and substitute that for the Nest and see what happens.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • PhilKulkarniPhilKulkarni Member Posts: 6
    The first shut down is puzzling and it occurs at 1.2PSI, that is why I noted that it is neither on temp nor pressure! The short cycling commences with the next boiler firing after 1.5 mins. and continues as described. Subsequent boiler firings (after reaching 70) do shut off on a pressure of 1.7PSI but rarely and on very cold days. They usually shut off on temp.
    Crown tech support first asked if the near boiler piping was up to spec in their installation manual, but had no comments or suggestions after they saw the pictures.

    Can definitely try the VisionPro. What CPH setting do you recommend?

    Thanks Jamie!
  • Jamie HallJamie Hall Member Posts: 6,010
    Try 1 cycle per hour. But before you go and spend the money and time, try and verify just what control is shutting off the boiler. There usually are three -- the thermostat is obvious (and Nests do do strange and wonderful things); the other two are the pressure, but it doesn't sound like that from your description, and low water -- but it probably isn't that either, although it's worth looking at.

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • AbracadabraAbracadabra Member Posts: 1,907
    I think the nest is shutting down the boiler based on it's algorithm for limiting overshooting the set temperature. How long has the nest been installed? Try installing the cheapest basic thermostat and see how things respond. Nest thermostats have been nothing but problematic is situations that i've seen for steam. Pretty sure that they were designed with FA in mind. Radiant was an afterthought.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 1,713
    how is your water level. Is it cycling on low water??

    could need more venting
  • PhilKulkarniPhilKulkarni Member Posts: 6
    Thank you all for your comments. Here are additional factors that might help to narrow down the source of short cycling:

    The Nest-until last month, it was set to work as a simple device by setting it to 'Leaf'. Because I read elsewhere on the Wall that it does not work for steam, I called up nest support who suggested I try the "True Radiant" setting which according to them works like an anticipator that some other stats have. They did not promise it was the solution, but thought it was worth a try. It made no difference. But I have left it there for now.

    Water level-When cold the water in the sight glass is slightly above the middle. When the boiler makes steam it drops to 25% of its level. It is clear and there is no surge. The level simply drops gradually without activating the auto feeder. It comes back up, but not all the way to the middle between cycles. There was some water loss when one of the Gorton #1 failed open and the feeder did bring the level up. Since it was replaced with the B&J vari vent, I have not noticed any water loss. To confirm this, I have shut both the feeder and the bypass valves for the past 10 or so days and the water level is static.

    Main vents-I have noticed that the lone Gorton #1 opens at cut-in pressure and shuts off 30 secs after the boiler fires. The B&J does not open and close while the boiler is cycling but sure enough does vent and close during longer cycles that take place after reaching 70. It does the same at start from 68 degrees. The rad vents also breathe in and out during each cycle. I haven't checked all 10 though.

    I have 2 Honeywell Chronotherm plus stats on the second and third floors that control my zoned A/C and are not doing anything now except indicate temperature. Their booklet does not indicate they have a CPH setting. I could swap the Nest with one of these for a couple of days and see if it reduces short cycling. I have Monday off so I can watch the behavior for 3 days and report back.
  • FredFred Member Posts: 4,849
    edited February 18
    Is this a one pipe or two pipe system? What size is the riser out of the boiler and the Header? I can't tell with the insulation on it but they look like they are small in diameter.
    What makes you think the boiler isn't shutting down on pressure or on a Low water check, with that Cyclegard, when it hits 68 degrees?
    I see you have a Cyclegard LWCO on that boiler. Thery will shut the boiler down every 10 or 15 minutes to check the water level/allow water to stabalize. They are a pita and most people replace them with SafeGard LWCO to eliminate the constant shut downs.
    The B&J Vari-vent should shut down on steam before any steam gets to the radiators. Why do you think oit is not closing? If it weren't you would see steam coming out of it. Do you have it fully open and the head locked down with the hex screw on the cap?
    When you say the risers are 1", do you mean the run-outs to the radiators?
    That main off of the boiler header is terribly low. How far does it run before it goes up to the ceiling? I only see one main take-off on the header but you say you have two mains. How is the second main tied in?
    I also notice that that main pitches back towards the boiler, as if it were a counter flow but it is not piped like a counter flow, no drips so condensate must be running back into the header. Not good. Do the mains ever start to pitch the other direction, away from the boiler?
    Better Pictures of the mains and the location where the vents are located and a picture of the rating plate on the boiler, please.
  • PhilKulkarniPhilKulkarni Member Posts: 6
    Thanks Fred.I will try to answer the best I can:

    1. One pipe system
    2. Boiler riser 2" dia. Header also 2" black iron. Insulation is 1/2"thick there and 1" thick in the crawl space
    3. On a 0-5 PSI gauge a reading of 1.25PSI and a reading of 1.7PSI are easily visible. Since the cut-out is at 1.7PSI, the temp is at 68, the stat is set to 70, and the boiler shuts down at 1.25PSI I am thinking that the first burn is neither shutting down on pressure or temp. It could be something that is initiated by Nest even though it is calling for heat because it has been firing for 40 mins. I am speculating here.
    4. I think the LWCO then is one path I can go down.
    5. I need to clarify that the B&J vent does not expel air with any sound during the 3min on 1.5min off cycles. It does vent very well from cold start and when the stat calls for heat after a rest period. This morning, after reaching 70 at 6.10AM the boiler cycled 3 times by 8.15AM. During these cycles the B&J vents faster than the Groton and closes on steam. I have made no attempt to balance the Main vents on the two loops so the B&J is fully open. The hex screw is taken off.
    6. I meant to say the run offs to the radiators are 1" dia.
    7. The 2" main off the boiler is 7' long and is sloped back to the boiler. I have attached a photo. It connects to a 2" tee. One side is the shorter loop and the other the longer loop. Exactly as it was with the old boiler.
    8. The pitch of the main on the longer loop is still towards the boiler while the pitch of the main of the shorter loop appears to be away from the boiler where it starts from the tee. I have attached a photo.
    9. Yes, I can see that any condensation has nowhere to go but back to the boiler. But there is no audible hammer.
  • FredFred Member Posts: 4,849
    That boiler riser and that header are very small. The main being pitched back to the boiler isn't helping either. At a minimum, if it can't be lifted so as to pitch away from the boiler, a drip leg should have been added so the condensate could drip into a nearby wet return. However, if it isn't creating a problem, so be it, at least for the moment.
    The Vari-vent will remain closed once it is hit by steam and, on short cycles, it won't open up and really doesn't need to as there is no air in the hot header, at that point.
    I'm guessing some of the shut downs are a result of that Cyclegard.
    As to the boiler shutting down at 3 minute intervals, that is definitely short cycling but that is not necessarily unusual when if the boiler is over-sized for the connected radiation and runs for expended periods recovering from set-backs. The fact that the Pressuretrol is set for 1.7 PSI Cut-out and shuts down at 1.25 PSI is also not unusual as they simply are not 100% accurate. The scales on the front have a slot and screw on them to allow you to adjust the scale to better reflect the actual Cut-out pressure. Slide that scale so that it says 1.25 where the indicator is now and then turn the indicator up to 1.7 if that is where you want cut-out to be.
    I also suspect you can virtually eliminate the short cycles by not using any set-backs at night.
  • PhilKulkarniPhilKulkarni Member Posts: 6
    Here is another update:
    For the first time today I looked at the glass gauge while the boiler was short cycling and noticed no water. So I added more water when the boiler was off until the water was just visible in the gauge. The next cycle went from 3 mins to 6mins. After 2 more cycles, I added a little more water and the cycle time hit 7 mins. Obviously the CycleGard is at least partly the problem.

    Earlier I had said the water level in the gauge goes down to 25% of the level. That was during the long boiler firings, not while short cycling. So Fred you were dead right!

    When the stat temp of 70 is reached, and the boiler has cooled, the water level is at 80% of the gauge ht.

    If I run the boiler with this water level, will it produce wet steam or any other issues?

    I also opened the Uni Feeder and noticed that the installer had left the dip switch settings for the old Mcdonnell/Miller LWCO. With the setting selected for Hydrolevel, I emptied the boiler until the feeder kicked in and filled the boiler. The water level is now higher than before.

    I want to wait and see how things work for the next 10-15 days, but should I switch the LWCO to Safegard?
  • FredFred Member Posts: 4,849
    A Safegard is the better option, in my opinion. Also, the water level in the sight glass should be no more that 2/3's full. I think there are dip switch settings on the water feeder to delay/slow the water flow into the water feeder. Based on what you are saying, however (about the boiler sight glass being empty) it soulds like the boiler needs a very good slow skim, maybe several. It sounds like the oils on the surface of the boiler water is making it difficult for the steam bubbles to break through the surface and that is causing the water to be pushed back into your wet returns, feeder is adding water, then when thermostat is satisfied and boiler shuts down and all the water returns plus the water that the feeder added, the boiler is over-filled. Do you have a skim port above the water line and are you skimming it very, very slowly? It takes several hours to do a proper skim.
  • PhilKulkarniPhilKulkarni Member Posts: 6
    Thanks Fred!

    My Uni-Match has no adjustments for controlling feed delay or rate and feed time; only 3 settings for popular LWCOs using a dip switch. But despite the low water level, which was short cycling the boiler, the Uni-match isn't filling the boiler and so I don't have the boiler overfilling problem you describe. I have turned it off for now.

    There is a 1" skimming port above the water line on the boiler and I can do a slow skim myself. Do I need to add any emulsifying agent to bond with any surface oil? There was some blue liquid chemical used when the boiler was skimmed after installation.

    The Safegard installation described on their site seems simple and I will install it myself. Since the level sensor needs a ground, teflon cannot be used, so what is used to make the connection leakproof?

    My thanks to everyone.
  • AbracadabraAbracadabra Member Posts: 1,907
    1" is too small to use as a skim port. You don't need to add anything to skim a boiler. Adding an emulsifying agent will only serve to mix the surface oils with the rest of the water. You want the oils on the surface to skim them. What was the blue liquid added? Is this blue liquid still in the boiler?
  • FredFred Member Posts: 4,849
    @Abracadabra , 1" isn't ideal but it will work if the skim is very very slow. I only have a 3/4" on my old Burnham and it works fine (again very slow). I typically heat the boiler up to almost boiling, shut the power off, open the port up, fill the boiler up slowly until the water starts to run out into a bucket and then set the flow to a dribble, hook a garden hose up to it and run that over to a floor drain and let it run like that for 5 or 6 hours. I have let it go over night.
    @PhilKulkarni , the Safegard uses the same probe as the Cyclegard so you don't need to change that out. Just use a little pipe dope on the threads if you do change it out.
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