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Measuring heat loss for a properly sized Combi

Le John
Le John Member Posts: 234

I wish I had the room for a boiler and an indirect but utility closet is like 3X4

Current Combi Boiler is 175K. (Grossly oversized)

Currently there is roughly 100 feet of slant finn baseboard(The 3inch deep by 3 or 4 inches high. I think it's the smallest they used to make) over two zones. If I take this 100 X.0500 I get roughly 50K.

Also on this boiler would be 1 shower (no tubs) and a 1/2 Bath and a 1 dishwasher

Would a 60K(for heat) and 160K for hot water combi work. (3.3 GPM which I think is more than enough)

I've had three plumbing and heating guys come in and say they would go bigger and put in a 199K. I don't see the reasoning behind this. Any guidance is appreciated.

btw not one did a heat loss calculation.

Comments

  • mattmia2
    mattmia2 Member Posts: 10,856

    Is the 160kbtu for 3.3 gpm at your incoming water temp?

    The heating part is sized by doing a heat loss calculation on the part of the building to be heated but with a combi it probably is going to be the DHW that will tell you what boiler size you need. DHW is usually set up as a priority over domestic heating so it only needs to meet the DHW need, not both combined. Keep in mind you need the output, not the input to the boiler.

    Le John
  • Can you build an attached shed outside the house for a boiler and indirect?

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    Le John
  • Hot_water_fan
    Hot_water_fan Member Posts: 2,040
    edited October 1

    140kbtu or even 120kbtu would be fine too, depending on the shower head. A heat loss isn’t needed here, it’ll be oversized no matter what. Use the turndown to help with shortcycling.

    Other option would be a boiler that’s a storage tank - so like indirect + boiler all in one.

    Le John
  • hot_rod
    hot_rod Member Posts: 23,277

    The coldest incoming water temperature is the key to all this. It can be as low as 35F in cold climate, as high as 90 in southern cities.

    Well water tends to be around 50- 55 year around.

    With those numbers you size the DHW btu required.

    With a combi, the dhw load will be bigger, much bigger.

    50,000 btu/hr of fin tube output? That is probably at 180F. A heat load calc would determine how often you could run cooler, more efficient SWT.

    Our family of 2 gets by with a 120K combi on well water temperatures. A 120K combi turns down to 12K, so the heat load will be easy to accomodate.

    Most tankless size be 70- 77 degree temperature rise. So if you want 120F at the fixtures use these numbers to get your gpm. 2- 3 gpm can work for many folks.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Le John
  • Le John
    Le John Member Posts: 234
  • Le John
    Le John Member Posts: 234

    @Hot_water_fan I thought of that. I know of 2 I believe. The Laars FT Floor Standing has a small indirect ,that is only available in the 199K BTU (Min BTS 20K) and the Viessmann 222-F. The Viessmann 222F is a nice looking boiler.

  • Le John
    Le John Member Posts: 234

    @hot_rod which 120K BTU boiler are you running? Are you happy with it?

  • EzzyT
    EzzyT Member Posts: 1,342

    I just installed a Burnhma Alta 136 combi on home that has 11/2 baths. I installed a mixing valve on the domestic side which is required by most boiler manufacturers which make combi boilers. I set the dwh to 140 and mixed down to 120, have had no issues with be able to supply enough hot water.
    I personally am not a fan of combi boilers but space was limited.

    E-Travis Mechanical LLC
    Etravismechanical@gmail.com
    201-887-8856
    Le JohnAlan (California Radiant) Forbesmattmia2PC7060
  • Le John
    Le John Member Posts: 234

    Anyone ever install a cast iron boiler + 30 Gallon indirect in a 3X3X8 foot closet?

  • EzzyT
    EzzyT Member Posts: 1,342
    edited October 1

    I haven’t especially the way I plumb boilers. @Le John where are you located?

    E-Travis Mechanical LLC
    Etravismechanical@gmail.com
    201-887-8856
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,212
    edited October 1

    NTI has a 20 gallon, wall-hung indirect that would easily satisfy your DHW needs. It comes with a tempering valve.

    https://ntiboilers.com/product/s20w

    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    GroundUp
  • hot_rod
    hot_rod Member Posts: 23,277

    I replaced 16year old Laars Mascot combi, actually a Baxi, 133K I think, with a Lochinvar Nobel110. Bothe kept up with a shower and any other small draw, like a hand sink, .35- .50 gpm, or a dishwasher fill. I also had a Lochinvar combi at the main house.

    I like Lochinvar, I have had the most experience with that brand and the control settings are easy for me. Great support if you need it from reps or tech support. From my experience.

    In my current shop I have a solar tank with a small plate flor DHW, my outdoor shower.

    Or I added a plate to a Lochinvar 55K for winter use. It keeps up with a hose for squirting off the truck salt grime. Basically a homemade combi without a need fopr a diverting valve or OEM HX block inside. A pump, plate HX and .35 flow switch is all it takes. Connected to DHW demand so the boiler high fires for DHW.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SuperTech
    SuperTech Member Posts: 2,413

    It's not cast iron, it's steel and better than any cast iron boiler. Energy Kinetics makes a version of the System 2000 that has the boiler above a small storage tank. With their plate heat exchanger it produces plenty of hot water. With the Carlin EZ gas burner you will get over 85% AFUE and you don't have to deal any condensate or modulating equipment, much more appropriate for baseboard heating than a combi in my opinion. I've seen these boilers fit in tight closets nicely.

    EK also has the Resolute boiler that is designed to replace tankless coil boilers, although I am not sure if they are available with the Carlin EZ gas burner, I've only seen them with oil burners.

    Le John
  • JayMcCay
    JayMcCay Member Posts: 42
    edited October 2

    Thanks @SuperTech! All of our non-condensing boilers are fuel neutral and will operate on oil, Nat. gas or propane with a Carlin EZ gas burner.

    @Le John - Take a look at our System 2000 stackable or Ascent Combi they maybe an option

    https://energykinetics.com/wp-content/documents/system2000/system-2000-brochure.pdf

    https://energykinetics.com/wp-content/documents/ascent/ascent-brochure.pdf

    Questions feel free to reach out

    Jay

    Jay McCay
    National Sales Manager
    Energy Kinetics
    908 328-7154 cell
  • Le John
    Le John Member Posts: 234

    Hi Jay - I don't believe any of the units will fit in a 34 inch by 34 inch closet. When I look at the diagram I see that I need 49 inches from from to back with the burner door open.

  • Robertw
    Robertw Member Posts: 25
    edited October 3

    @Le John , the footprint for the EK1 model (up to 140,000 BTU/hr) is 29 ¼” wide by 30 ½” deep with the service door closed. You can usually open the swing down service door (like an oven door) through the closet door, so the open door is not usually an issue unless the doorway is less than about 24” wide. You indicated 175,000 BTU/hr is grossly oversized so maybe the EK1 is adequate (you’ll have hot water storage so you won’t need the firepower for instantaneous hot water). As a reference, the EK1 is sized properly for applications with consumption up to 2,800 therms of natural gas per year.

    Robert W.

    Energy Kinetics

  • SuperTech
    SuperTech Member Posts: 2,413

    My mistake, I got the EK Ascent and Resolute mixed up.

    Le John