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Need some expert input

I have another question regarding stack temp. Going with the G115/21. Do I have to worry about the stack temp and what can be the lowerest safe stack temp for my existing chimney?


  • Hi,

    I have a small ranch 1200 sqft and a finished basement 900 sqft heated. I did a heat loss and came up with 43000 BTU. I will be looking to add radiant heat in the kitchen which is 230 sqft.

    I had an installer come over and he recommended the G115/28 with the S-120 over the G115/21 with S-120. He said he likes to add for the in-direct and the G115/21 would cut it to close.

    He explained, the in-direct water storage will have priority and in the winter when your taking a long shower. The larger boiler will be needed to heat the water up faster so if there is a call for heat it will be ready.

    Does this sound correct and would going with the larger boiler he recommends effect the efficiency for my application?

    Thanks in advance.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    You don't need a bigger boiler

    since you never use its full capacity to heat the house for more than one or two days a year. The indirect takes advantage of the extra capacity that is almost always available.

    The G1165/28 is too big. Don't waste the money. The G115/21 is fine.

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  • Hi Steamhead,

    Thanks for your quick reply. You mentioned wasting money. Do you mean the initial cost of the boiler or do you mean by over sizing the boiler will hurt the efficiency?
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688

    An oversized boiler will short-cycle, reducing its efficiency, and costs more to buy.

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  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    If you get below 325 or so

    you're in trouble, as the flue gases will condense in the chimney.

    The smaller boiler should actually help prevent this, since it will run with longer "on" periods and the chimney will warm up better as a result.

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  • Thank you for your quick responses to my questions. You have been a great help. Just one other question if you don't mind. Can radiant heating be added to this boiler easily or do you have to use a different system?
  • ScottMP
    ScottMP Member Posts: 5,884

    Everyone has differnt opinions. I would go for the larger boiler. To get good recovery from your system I would go with the larger boiler. You'll have faster recovery for the indirect.

    What about an addition ? Ever think you might be adding on. I sure would be a pain to find out the boiler you put in three years ago is too small.

    Any time a boiler is under 75,000 I always go alittle bigger to get good results from the indirect. I don't hink an extra 24,000 is so much that you'll get short cycling.

    Just my opinion.


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  • D107
    D107 Member Posts: 1,849
    A bigger indirect maybe ST-150 should help

    HO here. We had a G-115/21, Logomatic R2107 control/outdoor reset and ST-150 HWH installed in the LI family home last year. 47Kbtu heat loss, convector capacity about 100K. (good to know emitter output.) Many wallies use the DOE Gross output not the net output to size boilers since they believe the heat loss calc programs themselves overstate the loss, so then if you're oversizing above that, short cycling certainly seems likely.

    So the DOE output for the G-115/21 is about 74K, already 42% oversized for your 43K loss. If you set the indirect for 140 degrees with a good mixing valve like the TACO 5000 and mix down to 120 or so, you get even more HW capacity while protecting yourself against the Legionella virus.

    Many contractors will say --correctly--that the HW demands exceed heating in the small homes. But the preponderance of posts that I've seen here is that to solve that you use more storage not boiler btus unless the house is supersmall or HW demands are extraordinary. You can calculate the btus of your actual hot water peak demand and figure out how large a tank you need.

    Hopefully you will keep the night setback no more than 3-5 degrees so morning recovery won't take too long. If day temp activates at 5am, then at 4:30, the HW priority will activate and bring tank up to temp within only 15 minutes or so. In many cases, the morning water usage comes after most of the heat recovery. If you have a 10 degree nighttime setback, then heat recovery could take a longer time than you may want. It is unfortunate that the R2107 does not have a morning boost that lets the boiler go all out until setpoint is reached, and then return to the default heating curve.

    A year later, I can only say that I wish Buderus had a smaller boiler; it's 36% oversized and cycles are on the short side. As is it is, looks like savings were about 35% from the prior boiler. Used about 775 gallons of oil annually. We also had a new 5" stainless steel chimney liner installed -sized to Buderus specs.

    All this assumes you have done an accurate heat loss; I had a pro do mine and I did my own afterwards only as an exercise to learn more about heat loss. It also assumes your installer knows how to install and setup the boiler and burner properly and can recognize any inherent piping or other problems with your existing system.

    Good luck,

  • Paul Pollets
    Paul Pollets Member Posts: 3,656

    when sizing an oil-fired boiler. I've had better luck with firing rates over .8gph. A 120g indirect takes time to recover when the input is 78K and the incoming cold water temp is 35 degrees. I always size for the indirect recovery, and use a motorized mixing valve to reduce short cycles.

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  • Thanks everyone for your responses.

    I guess there are 2 opinions on this matter. Some say larger boiler and others say larger in-direct tank.

    I'm wonders what would be more efficient. Does short cycling really kill the efficiency of the boiler?

    When choosing this boiler (Buderus with Logicmatic) and with todays rising fuel cost. I am looking for efficiency in this system but don't want to wait 20 mins between showers. We are a small family of 3 but hope to add 1 more shortly.

    As Dave has stated, I'm 42% oversized regarding my heat loss with the G115/21. I'm not sure if I'll have a hot water issue with the G115/21 and the S-120. I have read some add BTU's for the indirect and some don't.

    I just want to say that I appreciate everyones input. I consider this a very important decision and I'm gald there is a place like this to go and ask questions.


  • ScottMP
    ScottMP Member Posts: 5,884
    Your quite welcome Carter

    feel free to continue to ask away.


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  • Glenn Sossin_2
    Glenn Sossin_2 Member Posts: 592
    Large boiler is less efficient

    The boiler is a dumb device for most installations. It is told to turn on and off and it responds just like a light switch. The problem is, it has no idea how much heat you really need. It fires full blast when called upon - the same way whether its 10F or 45F outside.

    Common sense tells you, I don't need as much heat when its 45F outside. But ..... we have the boiler, firing full bore - but it's only 45F outside and your heat load at 45F isn't the 43,000 btu's you calculated at design conditions (which might happen 1 or 2 days ) - it's approx 1/3 of that - probably closer to 15,000 - 18,000 btu's (depends on what design temperature you used). The bigger the boiler, the more on/off cycles - the more heat $$ get sent up the chimney - heat/$ that you can't utilize. The more cycling there is, the shorter the life span of the equipment you just spent thousands of dollars for.

    Most indirect heaters will probably lose approx 1/2 -1 F per hour. It may turn your boiler on 2-3 times a day for a few minutes if no demands are made (nobody home taking showers). There would be minimal energy loss over the course of 24 hours when compared to the too large, short cycling boiler.

    In my opinion, most times, you don't need to be concerned with recovery rates if the tank is sized adequately unless you have a large family (several teens taking showers one after the other for their Friday night dates) or expect multiple large draws for some reason. You stated you have a small ranch. 2-3 people?? I don't think you would have a large multiple event draw that would require high recovery rates.

    Given these factors, I wouldn't think it would make a difference if the tank took 20 minutes or 2 hours to recover if you sized it to your short term load. I would opt for the large indirect to insure I had adequate hot water, and choose a smaller boiler, closer to my heat load at design conditions. You will increase the life span of your equipment, save on your fuel costs in increased system efficiencies, and likely always have the hot water you need even if you spend an extra couple of minutes in the shower. Buy the smaller boiler, use that savings towards a large indirect.

    If you really want to save money on fuel, get an indoor/outdoor reset control - give your dumb boiler a little brains :)

  • I'm curious, because I hear this all the time; who cares about recovery time?

    If you have sized a tank/boiler combo for the peak demand (storage temp, volume, and input temp), then how fast does it really have to recharge?

    Turn up it's temp and/or get a bigger tank to store more heat to make it through that peak demand. But try like heck not to upsize the boiler... that's my two cents. Remember you're adding half the guy's peak heating demand to the boiler here...

  • Thanks Glenn,

    Ok, so I'll go with the G115/21.

    What is the best way to determine my HW usages. I see buderus has a S-120 32Gal, St-150 40Gal and a ST-200 53Gal. We are a family of 3 and trying for a second child, 2 bathrooms and dishwisher.

    I will be going with the logicmatic contorls.
  • Tony_23
    Tony_23 Member Posts: 1,033
    My 2 cents

    I usually install 28 gallon tanks, sometimes a 36 if requested. I have gotten so I always install a mixing valve, which adds about 50% to it's sensed capacity. Keep the tank at 140-150 and mix it down to 120-130. Very consistent temps at the faucets and no complaints about longetivity of hot water. In fact, we just finished a job today that has a W-M Plus 30 (28 gal) and a mixing valve fired by a Baxi 310Fi and FOUR bathrooms. It had an indirect coil. They may never leave the shower this week :)
  • Glen
    Glen Member Posts: 855
    mine two -

    last year I attended a "Siggy" lecture in Seattle. While all the info was superb - it is his opinion that boilers are now more than ever - sized for the dhw load to achieve correct dhw recovery. That made my ears perk up as we use priority strategies when ever possible - with long post purging on the pump. Consequently - we now carefully look at the performance of the indirect and its requirements. It is not to be overlooked.
  • D107
    D107 Member Posts: 1,849
    Maybe a specific example would help...

    Say Carter gets an ST-200, a 53 gallon Buderus tank with his G-115/21 with its 74Kbtu DOE Output. The literature says:
    With 149Kbtu, 90 degree rise (50-140deg), boiler water 194deg, Boiler flow rate of 17.6 gpm and coil pressure drop of 4.3ft of head will get 199 gallons per hour continuous. (But the first hour would be 252?)

    Of course HIS DOE is half the 149K btu, and I don't know what flow rate he can get. And perhaps he only gets 180 deg not 194 deg boiler water, but then again he can mix down from 140 to 120. Maybe someone here can figure this out. Seems like Carter would want to be able to have enough hot water for 4 consecutive showers or a bath and two showers.

    Note Carter that many favor the ST series for its easy-access top cleanout cover for maintenance/repair.

  • Eric_8
    Eric_8 Member Posts: 66

    I have a Weil McClain steam boiler on a 3 flat apt building now the 3rd and 2nd floors are converted to Forced air independent , the problems is the boiler is working very often for short periods, how to control this behavior?

  • Eric_8
    Eric_8 Member Posts: 66

    I have a Weil McClain steam boiler on a 3 flat apt building now the 3rd and 2nd floors are converted to Forced air independent , the problems is the boiler is working very often for short periods, how to control this behavior?

  • Timco
    Timco Member Posts: 3,040

    You should start a new thread...that will get you the help you need...

    Just a guy running some pipes.
This discussion has been closed.