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Another Boiler Sizing Question...Free Boiler?

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mdmccool
mdmccool Member Posts: 13
edited February 2018 in THE MAIN WALL
Hi Guys-

New to the forum and I have a question about boiler sizing. I have been reading posts from this forum for a few days now and I am impressed with the knowledge of the members here.

I have an old farmhouse built about 1850, about 2000 sq/ft. The heating system is an old oil fired American Standard Arcoleader boiler, 100,000 btu from about 1960 feeding cast iron radiators (hot water). The boiler works great, but I think I can do much better in the efficiency department.

As luck would have it, it turns out that a friend who recently purchased a house just converted from oil heat to natural gas and offered me his 10 year old oil boiler for free, it's 100,000 btu (gross), perfect match to replace my old American Standard boiler...

Before going to pick-up a 400+ pound boiler I thought it might be a good idea to calculate the heating load of the radiators in my house, as I learned from posts on this forum it's not a good idea to assume that the size of the existing boiler is necessarily correct.

After counting tubes and sections and applying some simple math, I calculate a heating load of about 110,000 btu. This is 10,000 btu less than the existing boiler that has been serving the house for 50+ years.

I understand the pros and cons of a boiler that is too small or too large, but is there any allowable tolerance when sizing boilers? Obviously by my calculations the current boiler is about 10% undersized, as is the "free" boiler I have been offered. I have no problem obtaining a correctly sized boiler but I also don't want to discount the perfectly good working free boiler I have been offered if the slight under-size won't be too detrimental.

Any thoughts would be greatly appreciated.

Thanks,

Matt
NJ

PS - I am also amazed that my old boiler is rated at 100k btu @ 1.3gph nozzle and the potential "new" boiler is rated at the same 100k btu @ 0.85gph nozzle.

Comments

  • Eastman
    Eastman Member Posts: 927
    edited February 2018
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    It's the heat loss of the house that is of primary concern, not the potential output of the radiators. You're thinking of a steam system where boiler sizing is dictated by EDR.
  • mdmccool
    mdmccool Member Posts: 13
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    Eastman - Thanks very much for your reply. Maybe I have misunderstood what I have been reading? From what I understand EDR calc is still applicable with a hot water cast iron radiator system, just lower output due to lower water temp versus steam.
  • Fred
    Fred Member Posts: 8,542
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    On a hot water system, you want to size your boiler to the heat loss of the total space, then you can size (or check the amount of connected radiation) to ensure you have enough radiation to heat each room @ Design day.
    Solid_Fuel_Mandelta T
  • Eastman
    Eastman Member Posts: 927
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    An EDR calc would tell you what your radiators are capable of providing, but there's no law that demands that you use all of their capacity. The goal is to match supply (boiler output) with demand (heat loss of house). What is it that you are reading?
  • Jamie Hall
    Jamie Hall Member Posts: 23,428
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    Yes and no. For a hot water system, it really is the heat loss of the house you need to worry about, rather than the EDR of the radiators. However... the EDR of the radiators is still significant, since if the house is under-radiated, you can't get it warm enough, and that's not good.

    However, in your situation, you know you can get it warm enough with that radiation and that size boiler. Nothing like actual experience! The question then becomes -- can you run the boiler at a lower temperature water supply and stay warm? Or even better, can you run the house circulation at a constant lower temperature and just run the boiler from time to time to kick some heat into the system? And the chances are you probably can.

    Were I in your position, I'd go pick up that new boiler and then study it some and find out how low a temperature on the return water it can take, and then pipe it and control it so the house stays circulating most or all of the time (that's where outdoor reset comes in, with various possible control schemes).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Solid_Fuel_Man
  • nicholas bonham-carter
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    Some makes of oil fired boilers have not been approved for firing with gas, so that may have been the reason for the replacement.
    SlantFin provide an easy to use heatloss program for phones and tablets, which will give you a reasonably accurate figure for your house, (which must be fairly large).
    You want to avoid oversizing the new boiler, if possible, so as to prevent short-cycling. This may be an opportunity to correct any piping deficiencies at the same time. Post some pictures here of your boiler, and piping.—NBC
  • Steamhead
    Steamhead Member Posts: 16,889
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    Fred said:

    On a hot water system, you want to size your boiler to the heat loss of the total space, then you can size (or check the amount of connected radiation) to ensure you have enough radiation to heat each room @ Design day.

    This.

    Your friend could donate his old boiler to someplace like Habitat for Humanity if you don't use it. What make and model is this boiler?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
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    "...old boiler is rated at 100k btu @ 1.3gph nozzle and the potential "new" boiler is rated at the same 100k btu @ 0.85gph nozzle."

    Probably the pump pressure is turned up to 140 psi. You'll still burn the same amount of fuel, most likely better combustion which means better efficiency, when set up properly, which would save you some money.

    There was an error rendering this rich post.

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,613
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    Heat loss of the building (not EDR) + 15% for piping and pick-up is what you should e looking for
  • mdmccool
    mdmccool Member Posts: 13
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    Hi Guys - Thanks very much for all the comments, much appreciated. It looks like a heat loss calc is in order.
    I'll see what I can come up with and report back. (NBC - Thanks for the Slant/Fin app info, I will give it a shot).

    "New" boiler in question is a Burnham 100mbh DOE / 87mbh net with tankless coil.

    I'm sure I'll be back with more questions...

    Thanks again,
    Matt
  • mdmccool
    mdmccool Member Posts: 13
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    OK - I performed a heat loss calculation using the Slant/Fin program as suggested. After reading through the short manual the application is quite easy to use. It took a few hours to accurately (as accurately as possible) complete but I am shocked at the results: 65,285 btu. I thought it would be much more. (I calculated indoor temp at 72 deg with an outdoor temp at 20 deg. Let me know if I should amend these temps).

    So...current 100k boiler....potential free "new" 100k boiler....about 110k load of cast iron radiators.....

    What say the experts?
  • Eastman
    Eastman Member Posts: 927
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    20 degrees is your design temperature?
  • Brewbeer
    Brewbeer Member Posts: 616
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    How much oil do you use per year?

    That old arcoliner might last just as long as the 10 year old boiler you are considering.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread: http://forum.heatinghelp.com/discussion/154385
    System Photo: https://us.v-cdn.net/5021738/uploads/FileUpload/79/451e1f19a1e5b345e0951fbe1ff6ca.jpg
  • the_donut
    the_donut Member Posts: 374
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    Design temp is based on location. Recommendations can be made by zip code.
  • mdmccool
    mdmccool Member Posts: 13
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    the_donut said:

    Design temp is based on location. Recommendations can be made by zip code.

    OK, thanks - should have read to the end of the software manual... Heat loss revised to 70,306btu @ 16 deg.
  • Fred
    Fred Member Posts: 8,542
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    With a little smaller nozzle you may be able to get the new boiler down where you need it. As @Brewbeer said, make sure you really feel the new boiler has more life left in it than the old one. It really is the luck of the draw, sometimes.
  • mdmccool
    mdmccool Member Posts: 13
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    Finally getting to the point of replacing this old boiler. As requested, attached is a photo of the old beast that has served well. This is a cast iron radiator mono-flow system fed with 1 1/4 copper, 1/2 inch to radiators. A quasi-single zone system for two floors with the exception of a portion of the house that was renovated in the 90's. Five radiators were piped into a separate "loop" and relay installed that controls just a separate circulator pump.

    I would like to change this to a 2 zone system, first and second floor. Maybe it's wise to start a new thread with questions about keeping the mono-flow system in place or replacing it with a manifold-type individual feed (maybe PEX)?

    Also, as the new boiler might be slightly oversized, what duration would be considered a "short-cycle?"


  • HVACNUT
    HVACNUT Member Posts: 5,861
    edited August 2018
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    Did you ever do a proper heat loss?
    What make and model is the new boiler and burner?
    A single zone monoflo loop can be zoned by "home runs" back to the boiler on all 2nd floor convectors. Branch manifolds for the supply and return.
    You can't just cap off the old monoflo branches. They must be piped back into each other.
  • Ironman
    Ironman Member Posts: 7,379
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    One of the best things you could do for efficiency is get rid of the tankless coil setup and use an indirect to heat domestic. We typically see about a 30% drop in fuel consumption doing this.

    IDK who's gonna do the installation, but if you're paying a true pro, I wouldn't recommend wasting you $$ on that 10 year old boiler. If it was Buderus that had been well maintained, maybe.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    delta TSTEVEusaPA
  • mdmccool
    mdmccool Member Posts: 13
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    HVACNUT - Heat loss calc was done as per above. About 70k BTU. Thanks for the info about the monofolw branches. My thoughts were to remove the monoflow system entirely, leaving the 1/2" feed and returns from each radiator and piping them each to a supply and return branch manifold system like you mentioned - hopefully one for first floor and one for second floor to create 2 separate zones. If this would be an appropriate "retrofit" I am wondering about zone activation - I have seen systems use zone valves with one circulator and systems that use a separate circulator for each zone with no valve. Boiler is a "standard" Burnham with Beckett burner with a tankless for domestic.

    Ironman - Thanks also for your comments. I am going to do the install myself so no $$ loss, just my time. If the "new" boiler lasts for a few years I'll be happy as there might be some larger scale renovations that might warrant a larger/different system. Interesting on the tankless inefficiency - I do have a Buderus S120 indirect tank I could use instead. (my previous home had a natural gas boiler with an indirect so this oil boiler and tankless setup is new to me). The tankless on the old existing boiler is disabled/bypassed with an electric water heater. It looks as if electric might have been used in the non-heating months, not sure.
  • hot_rod
    hot_rod Member Posts: 22,254
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    You could add a TRV on each radiator and have room by room temperature control. TRVs are non electric and work great with delta P circulators.

    This corner radiator feeds in one bottom end, out the other with 1/2"pex
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream