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Is boiler run time an indication of how over sized it is on a design day?

goodguy Member Posts: 12
I am going to replace my boiler this summer and I am reading everything I can here. I also have read two of Dan's books. I am a structural engineer so I understand technical stuff, but my knowledge of design and implementation for radiant systems is non-existent! I am doing a heat loss study to size a new boiler but under present conditions, on a design day (-11) SE Minnesota, My boilers run time was 55%. Radiator closest to the boiler never got above 140 degrees. Boiler has only ever shut down by the room thermostat, never by max boiler temp. Boiler usually ran about 10 min and was off for 10 to 15 min. House temp 70 degrees, and comfortable everywhere. History is 1930 2 pipe steam, converted to hot water. Present Boiler is from 1993. Is it a straight linear equation for size? Am I 45% too big. Input 250,000 btu, DOE 205,000. I will do a heat loss, but isn't this a better indication of my actual losses than a heat loss with a lot of assumptions?
I really appreciate the knowledge and respect that is shown around here. I will have lots of questions as I narrow down my choices. Hot water radiant is not common around here and what there is tends to be new in slab. So I want to be knowledgeable as I pick contractor. And I will be using a PRO, (but I'm gonna watch ;)


  • Jamie Hall
    Jamie Hall Member Posts: 21,535
    I wouldn't be a bit surprised if your present boiler was oversized. Happens all the time.

    Your best bet with a major conversion like this is to go all the way back to square one and do the heat loss workup. Use conservative but realistic assumptions.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Tinman
    Tinman Member Posts: 2,806
    By what you're describing, I'd say your boiler is way oversized. The easiest way, for me anyway, to determine how much it's oversized is by doing a Manual J Heat Loss Calculation like Jamie Hall said.
    Steve Minnich
  • Snowmelt
    Snowmelt Member Posts: 1,394
    Minnesota is John Barba neck of the woods, anyway do you know how to do a heat lost calculation. Taco has a nice program that I use. I also bought the book with u-factors to do it manually.
  • bob_46
    bob_46 Member Posts: 813
    You are on the right track. First clock your gas meter so you know exactly your firing rate . When you are at outdoor design conditions and 70º inside time how many minutes your burner runs in an hour and you will know exactly what your heat loss is. If your firing rate is 200,000 BTUH and the burner runs 30 min. an hour your heat loss is 100,000 BTUH. I use to teach load calculation . Give 20 guys the same blueprints and the same u-factors and you ill get 20 different answers .
  • goodguy
    goodguy Member Posts: 12
    Bob-- Oh no, my gas meter is 400' ft into the woods and under a blanket of snow! Design temp will probably be at night, this will be fun! but your insight on the heat loss calc and how that relates to what I actually see happening is exactly what I was looking for. Thank you! I'll do the heat loss calc as well, but in this 90 year old beauty there will be a lot of educated guesses.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,575
    Your converted steam system has apparently passed the first two tests:- no leaks, and lower water temperatures!
    Slantfin has a good heatloss app, and you can compare the EDR of your radiators with that as well.
    Cell phones, and Wilkie talkies were made for just such a meter clocking.
    I am surprised that the system does not shut down from the aquastat, given the possible oversizing.--NBC
  • GW
    GW Member Posts: 4,609
    There are a number of factors heating guys use, manual J is the official program as mentioned. Sizing according to your existing radiators would give you a second giant boiler. We just did a whole house conversion this week, pulled out a 1.5 gallon steamer and installed a 100k wall hanger. Thats less that half the size. Another thought for your list of things to think about: be agressive with your downsizing, if you are too slight on btu, simply reduce your heat loss! Isn't that a marvelous concept?

    I did a rather huge renovation two,three years ago. New radiant in entire house, and new geo system. I "almost" went to a 4 ton system for simple ability to heat in mid winter effectively. I installed a 3 ton. Well to my hearts delight the home keeps 70 at zero outside with 3 tons. This is a 2800 plus house, (I use the radiant mostly of course)

    So, pay attention to your heating but also pay attention to your house's losses.

    Good luck!
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    [email protected]
  • goodguy
    goodguy Member Posts: 12
    Nicholas-- I am surprised too. I have never seen temp above 150. I am tempted to turn up the heat to 80 after a set back and see what happens. The main loop is 3" pipe, maybe all the mass.
    JUGHNE Member Posts: 10,790
    You only have to clock the meter once and at any time of day (I would go for daylight) as long as it set high to stay on constant operation. And of course that being the only user of gas at the time.
    I go for several cubic feet to pass while timing, thinking there is more precision in the measurement.
    Once you have that factor, then it is just timing for one hour of run time during your design temp. Does that make sense??
  • goodguy
    goodguy Member Posts: 12
    Juggle-- Makes good sense! Thank you!
    Hatterasguy-- After all my reading, mostly on this sight, your description is exactly the system I want with an indirect thrown in. Thank you for the recommendation. They are about 80 miles away, think that's too far for them or me?
  • goodguy
    goodguy Member Posts: 12
    thanks Hatterasguy! Summer is the plan at this point.