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New Boiler Sizing

I'm replacing a 80,000 BTU Viessmann cast iron boiler that sooted up in a 1,200 square foot house which is built slab-on-grade, flat roof, single-glazed windows with very little wall insulation; one thermostat; six loops of 1/2" copper. It's a copy-cat Eichler.

I sorely want to install a Lochinvar KHB055 which has a heating capacity of 51,000 BTU and a AHRI rating of 44,000 BTU (less 15% for a piping and pickup factor). The heat loss calc's. come in at 50,000 BTU and I feel pretty good about my numbers.

The next size up boiler is a KHB085, but my supplier is out of stock and doesn't expect any for another month. I can't go with a wall-mounted boiler because it's an alcove installation and the on-demand water heater sits directly above along with other reasons. It's a very small space and the floor mounting is ideal.

What would you do?

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

Comments

  • Larry Weingarten
    Larry Weingarten Member Posts: 3,247
    edited January 2022
    Hi Alan, I’d do just what you’re planning with the right sized boiler and tell the client that there is little margin for error. Then help them make a plan for air sealing, wall insulation and windows. In the meantime, suggest some small electric space heaters, just in case.

    Yours, Larry
    Alan (California Radiant) Forbesrick in Alaska
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,344
    heat loss calculations always have some guesstimating. And the home owners life style plas a part as @BC_20 mentioned. Seems a little risky.

    Can you clean the old boiler and get it running while you wait for a larger boiler?
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,344
    How about a wall mount boiler (are they available) on a free standing strut rack? I have done that and it works fine
  • hot_rod
    hot_rod Member Posts: 21,871
    The wiggle room in the load calc should cover it.
    Upgrades to the shell will save them money and lower the load also 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HVACNUT
    HVACNUT Member Posts: 5,752
    edited January 2022
    If it were my own, I'd go with the 085. I'd want the peace of mind knowing those extra BTU's are there if I need them. It's not as if its grossly oversized, especially since it modulates.
    The boiler might be a big bite out of the budget and air sealing might be an expense that can't be done at the same time.
    If they're monetarily blessed and can do both at the same time, then I'd go with the 055.
    What's the max SWT and turn down ratio on these boilers?
  • hot_rod
    hot_rod Member Posts: 21,871
    42 btu/ sq ft load?  Those floors must run smoking hot on design day? Or is there some supplemental heat? Typically mid to high 20s is doable from a bare slab.

    Any input from the owner how much that boiler runs on cold days?  If it cycles off, it’s oversized and the smaller Loch should put out what that floor can emit?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,799
    How much gas did the existing boiler use per year? I suspect 51,000 is 2x too big. 
  • What gives me hope is that there are only 6 loops. At a 20 degree delta and 1/2 gpm, that’s 30,000 BTU’s. Even a 30 degree  delta grabs the ring on the merry-go-round. 

    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
    rick in Alaskakcopp
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,344
    @Alan (California Radiant) Forbes

    True. Useless to put in a boiler larger that the radiation can output.
    ethicalpaul
  • How about a wall mount boiler (are they available) on a free standing strut rack? I have done that and it works fine


    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
  • hot_rod
    hot_rod Member Posts: 21,871
    I just installed a 55 Lochinvar knight wall mount in my shop. It came with top and bottom connections , and top vent, so it would fit in that space 

    The floor mount you mentioned us 16” wide, 0 clearance on the right side, 24 on the left for piping access, I think the vent is out the back. A strut rack would hold it up and give you mounting for piping and pump. Check the pump spec it comes with, possibly a direct connection to the loops if they are one manifold without zone valves
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • JUGHNE
    JUGHNE Member Posts: 11,021
    Did that CI boiler suffer from lack of combustion air?

    I shoehorned a boiler into a closet that was smaller btu than the original.
    But there was only so much BB in the house so that sized the new one.

    2 days of well below zero and no phone calls yet.

    Couple in their 80's want it warm.
    kcopp
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,942
    edited January 2022
    @hot_rod There's a shelf about 18" above the boiler with an on-demand against the wall. I should have sold them a combi.

    Did that CI boiler suffer from lack of combustion air?
    Choose your poison: lack of combustion air, short cycling, condensing or a combination of two or three. It was installed before we got smarter about these things.
    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
  • hot_rod
    hot_rod Member Posts: 21,871
    Yes, a job like that cries out for a combi. You’ll work it out with what is available.

    Headed you way for some Pace training, talking about electrification of hydronics . I’ll split the day with Nico from Thermo 2000, talking about their offerings and the AltSource tank that may skirt around Title 24
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,942
    edited January 2022
    Finished! Satisfying to get the job done, but I will never again do an alcove install.




    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
  • hot_rod
    hot_rod Member Posts: 21,871
    nice! 100 lbs potatoes in a 50 lb sack
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,344
    Maybe 200lb in a 50 pound sack.

    Very tedious and slow, very difficult to make progress.

    It's one of those job where you get up off you knees after working all day and wonder why it took so long
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,942
    edited January 2022
    It took half a day and 7 hole saws just to drill two holes in the roof for the vent and air. Three inches of foam, two previous tar and gravel roofs, multiple layers of plywood and surprisingly, a 12 gauge layer of copper.

    That's what I like about our work. Although not always a pleasant task, the work is always different and never boring.
    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
    Le John Member Posts: 222
    Finished! Satisfying to get the job done, but I will never again do an alcove install.
    Wow. Nice job! Talk about working in tight spaces!  The Knight is a nice looking boiler!