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Wood boiler sizing help

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by OldStoneHouse, Mar 14, 2013.

  1. OldStoneHouse

    OldStoneHouse Member

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    I'm hoping to get some advice about boiler sizing. I had a heat loss done a couple of years ago, it came out at 130,000 - there was of course some guesswork involved in that. That includes 14,000 for the cellar which we have never done as there has not been a real need (it's cool but not cool enough to cause problems). Using their numbers and taking off the basement, that's 116,000.

    Now, the real world: using a mod/con gas boiler (155,000 BTU) with outdoor reset. The reset curve is set at 70 (low) and 135 (high) so by my calculations based on the EDR of my rads, I'm only putting out maximum 90,000 BTU at 135 degrees. This seems about right as I've not seen the boiler stay above 50% for any sustained period. Design temp is -10F. We only rarely hit design temp here, often it's much warmer, around 20 F.

    I'm expecting to have about 1300 gallons of unpressurized storage in an old cistern that I'll insulate and line. Given these numbers, what size of gasser would you put in? I'm very seriously looking at a used Tarm Excel 2000 which I believe is 30KW - I think it would handle the load okay. I'm also not averse to the gas having to kick in on a very cold night - every night on the other hand is not what I want either!

    Other possibilities are Tarm Solo Innova 30 or 50 as well as Vigas 40 - I think a Vigas 25 would be too small.

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  2. arngnick

    arngnick Member

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  3. OldStoneHouse

    OldStoneHouse Member

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    Thanks - I'll check those links out.

    Two fires would be fine, one in the morning and one in the afternoon/evening. I should have mentioned that much of the winter I can still heat the house with water at 105 or less. Yesterday for example I was running lower than 100. Do you think that makes any difference?

    The used Tarm has a tankless coil in it - given that the tank would still heat the house at 110, would it make more sense to use the tankless for domestic? Or both perhaps?
  4. arngnick

    arngnick Member

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    Being able to heat with low teperatures is a HUGE advantage when it comes to storage because you can utilize the heat for longer. I run my storage up to about 200* and can use the heat down to about 130* so being able to go down to 100* would likely give you a few hours of heat. Of course you will need to burn enough wood to bring it back to temperature. Not knowing more about the tarm I believe that you would only be able to use the tankless when you are actually burning a fire.
  5. OldStoneHouse

    OldStoneHouse Member

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    arngnick - I really like that storage calculator! If I can get my storage to 200 like you, I could have a great load off a full tank. Of course the boiler size is going to be the issue because it will take a long time to charge the tank! The Innova 50 would let me charge the tanks faster but the added cost of new vs old would give me room to do more work to the building envelope and reduce the load some.

    I'm not sure that I can get a liner for that tank that would let me go up to 200 though.

    Thanks!
  6. arngnick

    arngnick Member

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    Just remember you are figuring PEAK load and not your everyday load so most of the time your heat load will be less and last longer. The less energy you pull out of storage the less you will have to put back in the storage. Since my boiler cannot idle I batch burn and I went with a boiler that was a little oversized for quicker recovery in my storage plus I plan to add a garage in the future that will be heated.
  7. BadgerBoilerMN

    BadgerBoilerMN New Member

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    I don't know how to size a boiler without a heat load. What are you heating?
  8. OldStoneHouse

    OldStoneHouse Member

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    BadgerBoilerMN - I thought I gave the load in the first post - was there something specific that would be helpful? It's for an old stone house. Thanks.
  9. BadgerBoilerMN

    BadgerBoilerMN New Member

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    It seems a bit high. How did you come to it?
  10. OldStoneHouse

    OldStoneHouse Member

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    It was done by a local engineering firm.
  11. arngnick

    arngnick Member

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    Badger remember that he stated the design temperature was -10*
  12. BadgerBoilerMN

    BadgerBoilerMN New Member

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    Just curious about the size and quality of construction.
  13. OldStoneHouse

    OldStoneHouse Member

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    Quality is excellent because they don't build 'em like they used to. House was built about 1846, 1250 square feet per floor in the main house, 10 foot ceilings, large windows (5'x7'), plus another 1500 in an attached carriage house, now heated by the same system. Total of about 4000 square feet. Insulation in the attics is R40, walls have none and will not ever have any due to the type of construction. The walls are about 24" thick limestone and really the only available avenue for reducing heat loss is air sealing and there has been significant improvement in that but still room to go.

  14. maple1

    maple1 Minister of Fire

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    You'll get more mileage by burning one burn until storage is loaded. That might mean a re-load after the first 4 hours or so of burning. Two separate burns, morning & evening, would mean quite a bit of extra heat used in just getting the cold boiler & ceramics etc. up to temp each time. I do mine in the evenings, except for the coldest days of this winter when I was lighting my fire in early afternoon.

    From what you're saying about your heat load & loss, and the size of your storage, I think I'd be looking for something in the 40-60kw range for a boiler.
  15. OldStoneHouse

    OldStoneHouse Member

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    Thanks - I think you're probably right with the 40-60kw. But, if I can get the Excel for a good price (2-4K) I think I can deal with the few times that we ever experience design temps and just fire the propane. The extra I'll have to pay to get a new boiler would buy enough propane for my lifetime I expect. Very rarely does my load really get above 60,000 - we have some work planned which should reduce the load further too.

    Or do you think I really would not be happy with a 30kw and should go bigger?
  16. arngnick

    arngnick Member

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    I think you will be on the top end of the 30 like Maple1 said so maybe even if it turns out be be a nice unit make sure you get a GOOD deal because you will need to tend this boiler more than one in the 40-60Kw range.

    Personally since the Excel is not perfectly suited for your application I would stay on the lower end of your price range especially since in your other post you said you would likely need to put another $1K into it. Plus the units life expecancy is half over do you plan to replace the unit in 10years (or 20years if your lucky)? Then deal with all the hassles and costs associated with buying and installing another unit then? Just some thoughts I hate to pass up any GOOD DEALS but you must weigh your needs before making any decisions or offers.
  17. heaterman

    heaterman Minister of Fire

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    Bear in mind that with lower output boilers you are going to have to fire multiple times in order to raise your storage up to temp. Assuming a 130-190* temp differential on 1300 gallons that is.

    130-190 = (60* x 1300 x 8.33) or roughly 650,000btu. So a boiler rated at 30KW would need around 7-9 hours given that the 30KW is probably a peak rating. In addition you would have to add the heat being used by the structure simultaneous to the storage being charged.
    arngnick likes this.
  18. OldStoneHouse

    OldStoneHouse Member

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    Thanks - that is the information I'm looking for. There would be hassles with the used one...moving it out of where it is (1600lbs), hoping not to damage it, anything or anyone in the process. Then I have to move it into the new space (down three steps), and reconnect. It would not impress me if I did all that only to discover a leak that resulted from being moved. An Innova 50 is 400lbs lighter and shorter which I can imagine makes it quite a bit easy to move.

    Heaterman - I think I could probably get closer to 110, even maybe 100 and still be okay much of the time (this morning at about 24*, the system was were running at 104*) but if I need an EPDM liner then I shouldn't go above 170 right?
  19. heaterman

    heaterman Minister of Fire

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    I know of a person who has had an EPDM lined tank in operation for 4 years now with no issues running it at 170*. Anything above that is uncharted water. I recall speaking to the manufacturer of STSS storage systems a few years ago and he was pretty forceful about the 170* limit for his tanks.

    110*? Gotta love iron rads! :cool:
  20. OldStoneHouse

    OldStoneHouse Member

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    Still deciding what to do here. I think what I need do is try and reduce the loss (as some have suggested already) a bit more. I'm now seriously investigating spray foam for the basement for what I hope would be the biggest gains. I've always been against that with stone walls and old houses but I'll check it out. If I'm losing 20% or more (30?) out the basement then that leaves me in the position of being able to get a smaller boiler perhaps. The cash outlay for a gasser is not insignificant!
  21. mwk1000

    mwk1000 Member

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    I have an EKO 60 that is a good match for my 1500 gal tank. It needs two loads at night to recharge from 100-110 ish to 170-180 ish. ( 70 degree lift ). I have an EDPM in year 5 with no troubles ( that were not of my own making anyway ). I will try to upload my planning spreadsheet since my heat loss is close the tank size is close it's a good way to play around I fugured 80% of rated boiler output in figuring time but it's a spreadsheet you can adjust.

    Once again stumped by the web site no excel uploads ????

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