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Down fire Weil Mclain 68 with Beckett afg

PaulFromRi Member Posts: 2
I have a 20 yr old Weil Mclain 568 boiler with a Beckett afg burner (forced hot water baseboard).

The house is about 20 yrs old and approx 3500sq ft with 6 zones of heat including the boilermate. I am guessing that the boiler is over sized because I only use 1 or 2 zones of heat (the rest are set at 50 degrees or off because those rooms are under utilized). Seems like the boiler would be the correct size if the house was on 1 zone.

The boiler seems to run for only about 2 minutes at a time (boiler temp goes up to about 190). The sticker on the boiler indicates the nozzle should be 1.5/gpm and my question is ...how much can I reduce the nozzle size to achieve longer cycle times and hopefully use less oil without hurting the boiler or anything else. ( 1/gpm ?? ....smaller ?)

I understand that the air would also have to be adjusted
Thanks for any info or experience with this issue.


  • cadec
    cadec Member Posts: 8
    I guess the question for you is what is the current nozzle size on the gun assembly now that 1.5 is on the sticker or eff tag doesn't mean that's what's in the burner. Also what is the btu rating on the boiler
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    You could go down to a 1.25, but it really isn't going to help. The real problem is all the zones. The boiler is producing a lot of btu's and the small zones are only using a little bit of that, so your boiler reaches temperature too soon.
    First make sure all the zones are purged of air.
    The best thing to do would be to install a new, properly sized/properly piped boiler to the actual heat load requirements.
    Second best would be to get a buffer tank, properly sized. This would allow longer run times for your boiler, and when only one zone is calling for heat, allow you to probably complete the call for heat in one cycle.
    If those options don't work, you can try to change the controls to allow the boiler to purge the heat down to 140 degrees before re-firing.
    If you have zone valves you could also manually open the valves for the zones you don't use, and throttle some of the flow to those unused zones, so you are using more of the btu's up in the boiler, but that's really a Band-Aid.
    Severe short cycling will make your boiler run really inefficient, and allow your heat exchanger to get sooty-decreasing efficiency even more.

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  • sdodder
    sdodder Member Posts: 4
    Hi, first post here- long time lurker. I'm not a pro, but I've got quite a few years experience with oil heat. I've got combustion test equipment and a fairly good understanding of how to use it.
    I wanted to comment on Hatterasguy's post- I have exactly the same equipment he describes, and made exactly the same changes he did, right down to the low firing rate baffle. I got very good combustion test results and quite a bit better fuel economy after the changes. My heat loss, calculated using burner run time on a design day, is approximately 60K BTUH.
    (I do have an indirect domestic hot water heater). I know most would like to forget the old Weil 68 series, and it's efficiency isn't the best. But with the modifications mentioned, it can work OK.
    I clean mine once a year and it has only a very light layer of soot.
    Thanks, Steve
  • sdodder
    sdodder Member Posts: 4
    Hi, thanks for the welcome. I have the low limit turned down fairly low (can't remember the exact temp right now). It rarely drops down that low before there's a call from the indirect. But I try to maintain enough temp to prevent condensation. Knock on wood, it seems to work OK for me. Steve
  • PaulFromRi
    PaulFromRi Member Posts: 2
    Hi, Can't thank you enough for the info so far, especially Hatterasgay. I am going to contact the tech and ask him to do the annual cleaning/maintenance (will probably do it myself starting next year) and ask him to install a .85 nozzle and to verify I have an F3 head and to install the low firing rate baffle, and to use a co2 meter to adjust the air. sound right?

    One additional question >>> I notice that the boiler stops running at about 190 degrees and starts up at about 170. Can I turn that down to make the boiler run more efficiently? I removed the cover from the grey box controller thing (picture attached) and I expecting to see 2 dials (high and low range) but there is only one.
    Thanks again