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the shortest burner cycle

mattyc Member Posts: 44
I know I cannot be the only boiler tech whos experienced this phenomenon. It doesn't happen often, but it drives me nuts when it does. Here's what happens:

-The thermostat calls for heat,
-The cast iron boiler is within its high limit range so it doesn't fire for a while.
-the boiler finally cools down just enough to trigger the high limit differential and the burner starts to run.
-Just as the burner starts running the thermostat has reached temp and turns off, thus turning off the burner (boiler is now around its low limit temp range so it has no need to get hotter)
-The burner has now run for approximately two seconds

Drives me crazy sitting on my couch at night hearing this occur beneath my feet lol

Usually when this happens, the thermostat has been on for about 15 minutes or so, the burner has already gone through a usual run cycle. The boiler has turned off on high limit while the thermostat is still on. Then the thermostat runs just long enough afterwards to cause this "issue".

In regards to the boiler in my own home, the boiler is oversized and definitely in need of some updated controls/piping. But it came with the house and I haven't gotten around to making any changes yet. Just curious if any others out there notice this happen and have it drive them crazy as well.

Also for reference, in my experience this usually occurs with oil boilers that utilize a tankless coil for domestic hot water. Typical temp settings or 180/160 or 160/140. somewhere in that range to keep the coil warm.


  • hot_rod
    hot_rod Member Posts: 22,341
    edited December 2022
    It’s the basis mismatch between the load, whatever that may be, and the boiler output. Maybe widening the differential, if possible will help?

    Seems like in the process to keep a tankless coil warm, it is the jacket loss , flue loss, from the boiler causing the cycling, if no hot water is being drawn? Or some ghosting into heat circuits.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    It happened all the time at my house until I got a buffer (Oil-4 radiant zones, 2 high temp zones, indirect). I'd even hear the burner start pre-purge and either stop, or fire for a second and go into post purge.
    In my case, it was the boiler off on high limit (from outdoor reset), water still circulating and the call for heat ending shortly after the boiler temp dropped low enough to fire the burner.
    A smaller buffer sized to keep your smallest load running for 10 minutes may do the trick.
    Or mimicking the manager from an EK boiler may help (they won't just sell you one).

    There was an error rendering this rich post.

  • Jamie Hall
    Jamie Hall Member Posts: 23,536
    Happens to steam boilers too. nice long run (2 degree setback) of maybe 45 minutes to an hour. Boiler finally starts to build pressure (6 ounces) and shuts off. Post purge 30 seconds. Thermostat still calling. Pre purge another 30 seconds... burner fires. Lovely. Thermostat shuts off. Mumble mumble. Not much you can do about it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Dave Carpentier
    Dave Carpentier Member Posts: 603
    An "always hot" boiler never gets an abandoned call shutdown because it always operates fully between its low temp and its high temp.
    More stack loss during shoulder seasons though.

    I wonder if anyone has done an apples-to-apples compare of always-hot vs TT-controlled ?
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • fentonc
    fentonc Member Posts: 240
    I have a (4x-oversized) cold-start boiler and have monitored every burn cycle so far this season - it looks like 1.4% of total burns were cut short by a call for heat being satisfied.
    EBEBRATT-Ed Member Posts: 15,737
    Get a smaller boiler or reduce the firing rate with combustion testing if you can. There is nothing else you can do except add a buffer tank if it is hot water
  • mattyc
    mattyc Member Posts: 44
    @fentonc where did you get a chart like that from?? was that something you installed on your burner?
  • fentonc
    fentonc Member Posts: 240
    @mattyc - I built a little system that monitors the supply and return temps on all three zones of my house, as well as whenever the circulator is running, when the burner is firing, and what the outdoor temperature is. It then tracks how many times each zone called for heat and run time for each zone, estimates BTUs delivered to each zone, tracks burn counts and durations, estimates net efficiency (fuel burned vs delivered BTUs) and calculates BTU/HDD-hour for each day. My computer logs all of the data and then I can post-process it to generate plots like that (just in excel).

    @EBEBRATT-Ed - In addition to my boiler being 4x bigger than needed, I have enough fin tube that I could probably run at 110F at my design temp - I would probably save a ton of fuel from having a 50K BTUh modcon instead of a 140K BTUh CI boiler (or putting in a buffer tank, but my mechanical room doesn't have a lot of space for one at the moment). Re: reducing firing rate - everything I've read here says 'Don't plug burners on your CI boiler!!!!!!'. I have a WM CGA-5-PIDN gas boiler that has 8 burner tubes. If I plugged 6 of them it would be perfect! All that being said, I only burned about 400 therms of gas last year (~$1k), so it's hard to justify doing anything given that it would probably cost me $10k-$15k to get a new boiler put in. I'm outside NYC and the area has very high cost labor. Maybe one of these days I will get more adventurous with learning some basic plumbing skills.
  • gmcinnes
    gmcinnes Member Posts: 118
    Interesting! I have this same issue. About once a week I get a short burn like that. Nice to know the reason. It's annoying to the perfectionist in me, but not annoying enough to add a buffer tank :)

    @fentonc I'd love to know more about the setup of your system and how you're collecting the data. I'm building the same thing.
  • Long Beach Ed
    Long Beach Ed Member Posts: 1,213
    If it really bothers you, a time delay relay could be wired into the thing to hold off firing for a certain length of time after the thing shuts down. Or to delay a shut down.

    Really don't see any advantage, and the added parts just impact reliability, if anyone still cares about that.