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System sizing and actual demand

hvac-tech Member Posts: 36
1,000 sq. ft 2x4 walls 1964 home 12yr. old windows
north MN. ELE. furn. 20KW. Will hold 70F down to
-15F. Running on only 10KW. And paying 3.7 cent/ KW.


  • S Ebels
    S Ebels Member Posts: 2,322
    OK gang here's the deal

    Probably not telling much that a great majority of you don't already know but here's some more details re; a two boiler system we installed a couple years ago. The system is just going into it's third year so it now has two complete seasons on it.

    We removed a heating plant that fired at around 800,000 roughly clocking the meter and installed two boilers, staged, each firing at 300,000. Here's the first telling fact of the matter. Of the current run time total on the burners, the second stage ran for less than 10%. That's right, a building that had an 800K unit in it is now heated 90% of the time with only 300K. Total burner run time is 2,874 and of that the second stage only fired 236 hours. I would dare bet that the single boiler would heat the place under any normal temperatures we have here in Michigan. (-20 and up)

    The first season showed an actual cubic ft reduction of 43% running 6* setback for the whole building via the main control for 9 hours a day. Not to shabby. I installed a setback stat for each of the three zones at the begining of last season just to play around with it some more and now have a total for last season's gas usage. I was surprised to see an additional 8% reduction through the use of the programmable stats but it's there on the gas bills as big as life. I haven't balanced the reduction vs degree days yet but I know last winter was not 8% milder than the previous one.

    Moral of the story: A properly sized and intelligently controlled system, even one with roughly the same AFUE as the previous one, can save you some major $$.$$
  • Ken_40
    Ken_40 Member Posts: 1,320

    No heat load calcs.?

    Who was responsible for the oversizing by 40+%?

    Will they need an experrt witness to testify?

    Hopefully, the orange boilers were not the chosen brand...

    Let me know who the attorney's are. Retirement is boring. (;-o)


  • S Ebels
    S Ebels Member Posts: 2,322
    My main man Ken

    The heatloss I ran on the building (13,000+ sq ft) indicated around 560-580K IIRC. The combined DOE output of the two boilers we installed is only 500K. More proof that heatloss calcs have a lot of CYA factor in them.

    So sorry to disappoint you but the boilers are Silver and Orange. (newer version of the Euro marketing BS) :)

    The occupants of the building are quite happy wiith the 45%+ reduction in their natural gas usage, thank you very much.

    Enjoy your retirement my friend, it should allow you lot's of spare time to pick on mouthy, young whippersnappers like me. ;) Have to admit I deserve it sometimes.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    No surprise about the difference between calculated and actual load...

    The setback interests me GREATLY...

    Do I have this right?

    For first partial and next full year, simple "dumb" setback of 6° for 9 hours.

    Last year you changed to "smart" setback with the same amount and duration of setback.

    And the last year with "smart" setback seems to have reduced gas consumption significantly.

  • S Ebels
    S Ebels Member Posts: 2,322

    We setback the temp of the boilers using the Vitotronic 200 time schedule only the first year. Last season, I went back and installed 3 Honeywell Vision Pro stats and set them up to kill the circs in addition to also keeping the water temp setback on the Vitotronic. The Vito tronic is still setting back the water temp for the same length of time as before but the stat's will now kill the circs and are programed for a greater time period than the boiler control is. Building is HEAVY brick and block masonry construction, old windows in half newer in the other half. Not much in the line of R-value anywehre but literally tons of mass. I found out last season that the building hardly drops in temp for 4-5 hours after setback unless it is <10* outside with a stiff wind blowing. The stat's are basically providing a longer setback time while allowing the system water temp to come up before the building goes back to normal setpoints. Faster recovery while still maintaining a long setback.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Forgive me for Steve but:

    So...initially your setback came ONLY from the reduced supply temperature set back by 6° (C? or F?) Thermostats stayed at normal setting--calling for heat and circulation--and you starved them?
  • ABD
    ABD Member Posts: 13

    > the second stage ran for less than

    > 10%. That's right, a building that had an 800K

    > unit in it is now heated 90% of the time with

    > only 300K. Total burner run time is 2,874 and of

    > that the second stage only fired 236 hours. I

    Am I missing something here? The heating system has to be fully capable of providing enough heat on extremely cold days. Such days are few and far between. So of course the second stage will fire less than 10%.

    Many 2-stage hot air furnaces have a 60% first stage and a 100% second stage. The 60% stage will do the job always except (a) on days close to or beyond the design day, and (b) during recovery after setback.

    This doesn't mean the system is oversized. It's just how winter is. Your customers don't want to be cold even one day in winter.
  • Weezbo
    Weezbo Member Posts: 6,232
    i like what you are saying :)

    remember the buildings with the triangle tube Prestige that were built new that had Foley and crew:) written up in the Mag :) ?

    i wonder what last years useage might have been ....i think the modulation part of the condensing boilers is doing a heck of a lot dialing the fuel bills down...the set back savings is ... is... remarkable *~/:)

    that must calculate into a Bright Penny every year. :)

    a building with 2 meggers we replaced with less than 600K maybe i should shoot back over there and see if i can exact some spare change outta them and sell them some programmable set back t stats :)
  • hvac-tech
    hvac-tech Member Posts: 36
    under sized units

    Have u-all found a unit under sized and not
    heating the home to the set tep. on cold days or less?
  • Constantin
    Constantin Member Posts: 3,796
    Hear Hear!

    First of all, congrats on saving your clients 45% on their gas consumption. Tracking this sort of info can be hard, but it is worth it! That the T-stat controls save that much extra is interesting. I suppose there must be some pretty massive loss between the boiler and the emitters.

    With all that mass, body heat during the day, and some uptake via insolation, I could see why the building can "coast" w/o issues from day to day and still feel warm and cozy. Most outdoor temperature distributions have a thin "tail", so that would only contribute to the lagging boiler not firing much.

    One day I hope that more residential oil burners will feature at least high-low operation for the same reason.
  • S Ebels
    S Ebels Member Posts: 2,322
    Not quite

    The system as originally installed was set up with 4 zone pump, all running constant circ. TRV's on all the rads (22 of 'em) The boiler was programmed to setback for the afore mentioned time period. Everything ran as normal but with reduced water temps.

    Just for kicks I went back last year and installed an SR 503 Taco zone panel which is "lit up" by the Vitotronic. I left one zone running "wild" with just the TRV's in control. The other three still get reduced water temp as per the original program but in addition to that, now have the programmable stat killing the zone circ according to stat programming.

    The stat's stay in setback longer than the boiler reduces the water temp so in effect the boiler comes out of setback before the stat's start brining the circ's back on. This seems to give much faster return to normal temperatures in the building.

    There weren't any problems with the way it ran originally, I just like to tinker with things and the job is virtually in my backyard.
  • S Ebels
    S Ebels Member Posts: 2,322

    I have never seen that and I can't recall that I ever heard of it either. The urge to oversize equipment apparently overcomes all logic. It's like the urge to purge "air" out of a hot water system.
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