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heat loss question

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by verne, Mar 3, 2008.

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  1. verne

    verne Member

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    Hi all,
    How can I calculate heatloss based on my boiler size/ oil usage. I just did a heat loss online and I want to compare the two.Thank you for any info.

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  2. Nofossil

    Nofossil Moderator Emeritus

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    If you know your oil usage for some period (a heating season would be good) you can convert that to BTUs. A gallon of oil is about 143,000 BTUs, and most oil boilers are somewhere around 80% efficient. That works out to about 114,000 BTUs of heat delivered for every gallon of oil. You can do the math and divide by the number of days in the period to get BTUs per day, and then by 24 to get BTUs per hour.

    Of course some days are colder than average. In my case, my peak heat load is about twice the average.
  3. verne

    verne Member

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    OK ! glad to see you are on board nofossil. Maybe you can walk me through this . My oil fired boiler is rated at 152,000 btu/hr. I use aprox. 900 gal oil each year with hot water. I did a heat loss , and came up with 57000 btu/hr . I have been sized without the heatloss calc. for the EKO 40 and I have a 1000 gal prop. tank. so... 114000x900=102600000 / 185 days= 554594.5/24=23108 btu/hr. apox. whats the conclusion ?
  4. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    That 57k is probably pretty close. Peak heat loss is usually around twice average heat loss, so I'd say that 57k is pretty close.

    Which means that an EKO 25 is oversized for your application, let alone a 40.

    Joe
  5. verne

    verne Member

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    wow ! that sounds good to me the smaller the better. now on that note , why have the three major eko dealers suggested a 40 when I have told them the boiler size and fuel use?Also I would like to keep a garage a little warmer than before ( 38 degrees) to maybe 50 degrees . do you think 1000 gal would be ok with the 25 ? Please, I would love , any input. By the way my house is 2650 sg' with 575sq' garage. another thought, I may finish the basement someday . Thanks
  6. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    What do you have for radiation? Baseboard, radiators, fan convectors, radiant? That will have a huge effect on how big a tank you need.

    Joe
  7. verne

    verne Member

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    joe thanks for the response! I have baseboard. With the info I have so far do you think the original system is oversized?
  8. Nofossil

    Nofossil Moderator Emeritus

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    If I were a dealer, here's how I might think about it:

    If I suggest the 25 and it's not big enough, I'll have a smaller sale and an angry customer.

    If I suggest the 40 and the 25 would have worked, I'll have a bigger sale and the customer will never know. In fact, they may well be impressed by how much heat the thing puts out.

    Let me think about this.......

    It's worth mentioning that you're right on the edge. Although the 25 is rated for 80,000 BTU, it's more realistic to expect a sustained output more in the 60,000 BTU range. I think you'd be OK with it, especially with storage in the picture. I heat 3500 square feet (30,000 BTU peak demand) plus hot tub and DHW with an EKO 25 and 880 gallons - see my site in my signature below.

    You'd want to be sure you had a good supply of well seasoned hardwood laid in for the cold months. I burn a lot of junk, and it would be hard to sustain 60,000 BTU all day long with what I have. With good wood, it should be no problem. I've seen 80,000 BTU from mine.
  9. verne

    verne Member

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    so , forget about the price for a minute . would you install a 25 or 40 considering I have a 1000 gal tank?
  10. Eric Johnson

    Eric Johnson Mod Emeritus

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    With that much storage, I think it boils down more to how often you want to load it. The 40 will require less attention than the 25, simply because you have a bigger firebox. It might take two loads of wood to charge up the tank with the 40, while the 25 might require 3--just as an example.
  11. verne

    verne Member

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    Eric ,
    The least amount of effort in loading the better. I wont be home during the day so a morning nite thing should work . If I can skip days all the better.do you guys think since I am on the line with size, a larger boiler with 1000 gal storage will give me longer time between burns?
  12. Eric Johnson

    Eric Johnson Mod Emeritus

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    I wouldn't get carried away. I think a 40 should be fine. When you into the 60 range, you need more than 1" pipe to adequately dissipate the heat, and that drives the cost up pretty good (think 1 1.4-inch valves, etc.). A 25 might work just as well as a 40 once you get your system down.
  13. SE Iowa

    SE Iowa New Member

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    So help an idiot here. If I burn 150 therms of natural gas per month how much btu's/hr is that? What is a therm of nat gas equivalent to in btu's?
  14. verne

    verne Member

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    I actually wasnt thinking of any bigger than a 40 . I was just a little supprised with the results of a true heat loss.Up untill this point i was decided on the eko 40. I tink I will continue to read and look at these boilers in person in May. I have plenty to do building my barn in the time beeing.I have quite a few friends from the Adirondacks planning on attendind the show also.Should be a good time. Hope to meet some of you there
  15. SE Iowa

    SE Iowa New Member

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    Sorry, I just figured it out. 1 Therm = 100,000 btu's, so 150 therms/month = ~21000btu's/hr. So I think that means I'd need and EKO 25 right?
  16. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Just based upon your square footage, that system is oversized. You'd be hard pressed to find any residence eating that many btu's per square foot. Unless you have large windows looking over a lake, and you like keeping them open during winter windstorms, your boiler is plenty big.

    For fun, measure your baseboards, and calculate how many btu's they can actually deliver to the house. Probably nowhere near as much ability to deliver heat as that boiler produces (of course, if they are overly long, that means cooler water temperatures will work, which means that each gallon of storage will heat for longer).

    If the heat loss is accurate, I think a 25 would be fine. What sort of burn times do you get with your 25, nofossil?

    Actually, oversizing is one of the leading causes of inefficiency when it comes to oil boilers.

    The time between burns is a function of the storage volume, not the boiler. What the boiler size determines in that sort of system is the speed at which the storage can be recharged. Bigger boiler means faster recharge, but if it's too big, you will heat the storage to its full temperature before using up all the wood in the boiler. The theoretical goal is to have the boiler and storage matched such that you run out of wood just as the storage hits its peak temperature.

    Additionally, to take advantage of the larger boiler, even if the wood loading were matched right, means a larger heat exchanger for the tank. The boiler cannot load the tank any faster than the heat exchanger can transfer heat, regardless of how much heat the boiler can produce. That issue can be eliminated if the storage is part of the boiler loop, so the heat exchanger is eliminated, but there are other issues with that sort of system which cause me not to recommend it - the biggest being that having the boiler separated from the storage means the storage tank can be shut down if it needs maintenance, without having to shut the boiler down (you just lose the convenience of storage in that rare case).

    That means 25 would be plenty big, and then some.

    Joe
  17. verne

    verne Member

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    thanks Joe. I will measure the amount of baseboard . so it is possible , that I could run more efficient with a smaller oil boiler?I guess it dosnt matter now because I am putting in the wood boiler but it would be nice to know. With all that I will continue to look into sizing this the best I can . Thank for the help
  18. Nofossil

    Nofossil Moderator Emeritus

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    Historically, I've averaged 7 hours of burn time per day, and 12 hours per day during the coldest weeks. Your rule of thumb about the peak load being twice the average is almost exactly true in my case.

    I originally had only 52 feet of baseboard, though I've added another 16 feet recently.

    I've also dialed back the amount of air that the EKO blower delivers to decrease the peak output of the boiler. That has the effect of extending my burn times and better matching the output of the boiler to my heat demand.

    My burn time with a full load depends on the demand and tank temps, but about 3-4 hours of peak output is about right. You can go about 6 or seven hours and have enough coals to restart immediately.

    My normal patter is to build a fire and reload after about an hour. If it's cold, I'll reload again after another 3-4 hours. Rarely will I do a third load.
  19. verne

    verne Member

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    I just measured my baseboard. I have 175' 0n 6 zones and a 36000 btu/hr unit in the garage( used very little)and dhw. How does that compare to my boiler output? also to get back to storage sizing and eko size.what I want to to be sure of is that if I load the boiler in the morning and at night I wont run out of heat or lower the tank to much. any suggestions on whether the 25 or the 40 may be better suited for my situation? thanks
  20. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    If that's standard baseboard, then the absolute most it can absorb is about 105k btu. Add in the unit heater, and the existing boiler is still definitely oversized.

    Given your heatloss, that means you will be able to run pretty cool water through the baseboard and still get heat out of it, even on cold days. Which means you can get much more use out of your 1000-gallon tank.

    You do want to lower the tank. That's the idea. By running the tank down to 140, and then running it back up to 200 with the boiler, you get a nice long burn on the boiler and that improves efficiency.

    If you can get that sort of temperature swing (60 degrees), then your 1000-gallon tank is a "battery" that can hold half a million btu's. Compare that to your hourly heatloss, and you'll get an idea of how long you can go between recharges. The "recharge" obviously takes time, so you're going to have some overlap between the burn and tank, so I think that you'll be able to get 8 hours out of that tank (presuming the heat loss is accurate) without much trouble, even on the coldest days. When it isn't as cold, you may only be doing one burn a day, or even a burn every other day.

    Looking at that sort of storage capacity, the EKO 40 would probably be better, in terms of its ability to quickly "recharge" the tank. You may end up running smaller loads in it, to make sure that it doesn't have too much wood in it (if it finishes charging the tank and still has wood, it will idle and that's the primary cause of wasting fuel).

    Joe
  21. mtfallsmikey

    mtfallsmikey New Member

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    Do COMPLETE heat loss calculations on everything, see if you really are oversized or not. That's the best way to go about this.
  22. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    He's definitely oversized. Only question is by how much :)

    Joe
  23. verne

    verne Member

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    I did as complete heat loss as I know how to do. Also I originally planned on finishing the basement . thats 1725 sq ' with 9' ceilings if that makes a difference. thanks for the input I really appreciate it .I will start another thread when i get a chance to take on the plumbing diagram.I have one from cozy dave, but Im sure you guys will have something to add .thanks
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