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Need Help Sizing Boiler for Complex (for me) Situation in Minnesota

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sleepymcdoze
sleepymcdoze Member Posts: 10

As the title says, I need some help sizing a boiler. We are renovating our 1924 house in Minnesota, which currently has a huge old cast iron boiler (over 300k btu) and radiators. The house is around 3,600 sq ft total. We plan to replace the old cast iron boiler, with a new high efficiency boiler with a side arm water heater. I think the tricky part is we are putting in new floors in parts of the house, so we plan to do infloor heat in those areas and remove those radiators.

Here is a summary of the 3 floors (pictures attached below):

  • Basement is 1262 sq ft and has 4 radiators (plus the boiler in the mechanical room that should put off some heat)
  • Main floor has the Great room heated by 4 radiators, and is 661 sq ft, with a vaulted ceiling (little or no ceiling insulation)
  • Main floor will have staple up infloor heat for the remainder of the main floor, which is 605 sq ft. The majority of that will be white oak wood floors, but mudroom and powder room will be tile.
  • The upstairs is mostly heated by 5 radiators, which is 931 sq ft
  • The upstairs primary bathroom will be staple down infloor heat, under tile floor, and is 119 sq ft
  • We also have a 445 sq ft attached garage, which will have one radiator, and we just want to keep it in the high 40s to low 50s deg F
  • We have 4 bathrooms, 3 showers, and 2 bathtubs (one normal and one a 6' soaking tub)
  • Most windows are original, but they all have storms.
  • The big window in the kitchen is new double pane
  • The walls are 2x6 and will filled with blown in cellulose insulation
  • Attic will be blown in cellulose to code
  • The biggest loss is the Great room is a vaulted ceiling and there isn't any room for insulation. If there is any there, it's likely thin and from 1924. The roof is clay tile, so it would be absurdly expensive to add insulation from the outside.

I'm attaching drawings for the 3 floors of the house. The radiators are circled in red, and the areas of infloor heat are circled in green. I put a square footage for each window/door in blue (please note the two 20 sq ft windows in the Great room are not showing up in the drawing for some reason, but I did label the sq ft on the drawing). And note that the right side of the drawings are facing north.

Bonus question about the staple up infloor heat below the hardwood floors on the main floor. I'm thinking of using the Uponor Joist Trak because they are thicker than the normal aluminum caps, and the heat has to go through the subfloor and the wood. Do you think it's worth the extra money in this case? Also, I see most staple up installations just have a down and back in each joist cavity. With it being under hardwood floors, would it be beneficial to maybe add another row of plates/pex for each joist (down/back/down)? I know that would add another 50% cost, but I just want to make sure the space is being heated properly, now that there are no radiators in that space, and the subfloor + hardwood is quite the insulator. If someone can give me a hint on how to calculate what I actually need for infloor heat in this situation, that would be helpful.

Thanks in advance. Any help is appreciated.

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  • JR3_Home_Performance
    JR3_Home_Performance Member Posts: 23
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    You could reverse enginer the heat loss from past heating cost to see what the needed sizing would be. I've gotten similar sizes home down to 50kbtu without overly invasive work. You can't pop off roof tiles and blow some insulation down into the great room ceiling?

    sleepymcdoze
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,898
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    how much fuel did you use last winter?

  • Steamhead
    Steamhead Member Posts: 16,938
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    @sleepymcdoze , you say you have a "boiler…. and radiators", but is this a steam or hot-water system?

    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
    mattmia2sleepymcdoze
  • mattmia2
    mattmia2 Member Posts: 9,837
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    The first step is doing a heat loss calculation for the house and each area that you want to zone separately. The heat loss will tell you what output you need from your radiant in those areas which will tell you what tube layout you need (or if you can't get enough output from radiant alone and you need some supplemental heat).

    If it is hot water you will size the boiler itself to the heat loss of the house. You also need to consider the DHW load and it is possible that might be the limiting factor in the boiler size.

    If it is steam there will be some other considerations like how you make a hot water loop for it and how you size to the remaining radiation.

    sleepymcdoze
  • GroundUp
    GroundUp Member Posts: 1,938
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    A heat loss calculation should be performed to gather an idea on BTU requirements. Builditsolar.com has a pretty user friendly version on their site which I've found to be incredibly accurate when the information is entered correctly. It will take some time, but it's really the best way to get a foothold on this. I am also in MN and have done quite a few of these high efficiency retrofits, if there is anything I can assist you with please feel free to reach out.

    sleepymcdoze
  • AlaskaDick
    AlaskaDick Member Posts: 17
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  • JMWHVAC
    JMWHVAC Member Posts: 46
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    I doubt you need the third row of joist track (per joist cavity) to get the BTU you need. But my first question is, "what does your old subfloor consist of?" A

    sleepymcdoze
  • sleepymcdoze
    sleepymcdoze Member Posts: 10
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    OK thanks for the tip. I checked out Builditsolar.com and found the heat loss calculator. I'll spend some time on it tomorrow.

  • sleepymcdoze
    sleepymcdoze Member Posts: 10
    edited April 28
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    We replaced all of the old subfloor with 3/4" plywood. We'll have 3/4" hardwood on top of that for most of it (and 3/4" thinset/tile for the rest)

  • sleepymcdoze
    sleepymcdoze Member Posts: 10
    edited April 28
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    @AlaskaDick Thanks for the direct link

  • sleepymcdoze
    sleepymcdoze Member Posts: 10
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    OK. I'll try calculating the heat loss using the calculator at BuilditSolar.com, recommended by some other users

  • sleepymcdoze
    sleepymcdoze Member Posts: 10
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    I'll have to check if there is any space between the ceiling boards and the roof. If there is, maybe I can do as you say and blow in some insulation.

  • leonz
    leonz Member Posts: 1,165
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    Was your home equipped with a gravity hot water system that was converted to circulation at some time in the past??

    Is the boiler leaking? If not, have you considered just replacing the burner if it is an old burner?

    Have you considered installing hot water temperature regulating valves TRV's instead of replacing the boiler if it is not leaking? A cast iron boiler handles the heat better.

    Would you consider simply adding hot water storage for your home?? The New Horizons folks have a fully insulated rectangular hot water storage tank which is 475-495? gallons capacity with internal stays.

  • sleepymcdoze
    sleepymcdoze Member Posts: 10
    edited May 6
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    OK so I did the heat loss calc, and I think something is off. Based on our gas usage (not this past winter since it was an anomaly, but the winters before), we use somewhere around 2500-3000 therms a year for heating. The heat loss calc showed something more like 7000 therms. I'm assuming this is because our house has no insulation in the walls, so the estimated R value being wrong makes a huge difference. We have 2x6 stucco and plaster walls, and the only insulation is a 3/4" thick, waffle looking horse hair thing.

    In addition to this, we have a basement, and I would assume those walls shouldn't be included with all of the other walls, since the ground is going to be warmer than the air. But I don't see how to put this in the calculation, so I just included it with the other walls. Do you have any ideas for this?

    I did use the perimeter of the basement floor as if it was a slab, but again not sure if that's right. I did not include the garage in this at all (it is not heated).

    I'm attaching what I input, and what the result was. Also a spreadsheet of my areas and R values. Let me know if you have ideas to get it more accurate. I appreciate any help you can give. Thanks!

  • Hot_water_fan
    Hot_water_fan Member Posts: 1,898
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    These are typical heat loss results, too many assumptions in bottom up calculations. Usage = reality.

  • GGross
    GGross Member Posts: 1,072
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    You have historic usage to tell you the heat loss. Guessing at what is there now is pointless, especially if you are going to be insulating all areas possible up to code. Your calculated heat loss should be based on the space as it will be when finished, not how it is right now.

  • GroundUp
    GroundUp Member Posts: 1,938
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    I'd be very hesitant to think that your basement numbers are anywhere near accurate. You're only trying to maintain maybe a 20-30* rise rather than an 80-90* rise as you have it set up. That'd probably save you about 30k/hr on a design day. Then the great room, vaulted or not, there's no way it's being heated with those 4 rads if the R value is only 2 as you have stated. That could potentially drop the load another 5-10k/hr. The ACH thing is also likely too high, unless you can feel the wind blow while you're sitting at the kitchen table. Try to adjust settings until you get closer to your actual fuel usage, and that should get you darn close to the BTU requirement you're looking for

  • sleepymcdoze
    sleepymcdoze Member Posts: 10
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    @GroundUp thanks for the response. How would I change the basement walls to be more accurate in the Builditsolar calculator? Would I just increase the R value for those walls? What value would you use?

  • GroundUp
    GroundUp Member Posts: 1,938
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    I'd probably take the basement completely out of the equation and run a separate calc with the basement only, then add the two together. As far as value goes, I can't say for sure but a standard 8" CMU wall in a full basement to maintain 70* indoor temp typically does not exceed about 7-8 BTU/SF in our neck of the woods. Most are closer to half that when finished out.

  • sleepymcdoze
    sleepymcdoze Member Posts: 10
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  • GroundUp
    GroundUp Member Posts: 1,938
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    Yes, though outdoor temp is largely irrelevant for a basement wit heated space above. The surface area of all the concrete/CMU walls and slab in a typical fully buried basement would average maybe 50* if unheated, so the actual makeup is much less than the upper level which may be exposed to -30* outdoor temps if left unheated. The differential required to be overcome by the heat loss for your upper level is approaching 100* while the differential for the basement is only maybe 20*- theoretically only needing 20% of the amount of BTU to keep the same indoor temp. Even if the basement needed 10 BTU/SF at design, that would dictate the upper level(s) would need maybe 50, which is all but unheard of unless you can feel the wind blow through the house.

    sleepymcdoze