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sizing a boiler

gypsy Member Posts: 84
can anyone tell me what happens if a boiler is too small based on btu loss? 

more specifically, if a boiler is sized correctly based on the radiators, but the heat loss due to lack of insulation isnt taken into consideration, does that matter?


  • CMadatMe
    CMadatMe Member Posts: 3,086
    It Runs and Runs

    and will only be able to deliver the btus it can make. You would see the issue more as it get colder and closer to your coldest days. You wouldn't be sizing a boiler correctly by measuring the emmitters. Emmitter measurement is giving you the capable btu output of the emmitters has nothing to do with the structure or rooms heat loss.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • gypsy
    gypsy Member Posts: 84
    thats what i thought...

    here is the problem.  i just got information about the proposed new boiler.  it is 105k btu.  i believe this must only be based on the radiator measurements because...

    I just did a heat loss calculator for the house based on room size, insulation, windows, doors, ect.. and my calculated heat loss is 190 btu.

    so, im guessing if this 105k btu boiler is installed im not going to have any heat and or hot water?
  • Robert_25
    Robert_25 Member Posts: 527
    on cold days, you will be cold

    If your house is under-radiated for design days, you will not be able to achieve a comfortable room temperature during the coldest weather. It won't do you any good to size the boiler for a greater heat load if the radiation isn't there to distribute the heat.

    With that said, 190k btu/s per hour is A LOT of heat loss. What does the home currently have for a boiler? Does the existing radiation do the job?
  • CMadatMe
    CMadatMe Member Posts: 3,086

    So this home is 6,000 plus sqft with limited insulation and crappy windows? Your adding heat emmitters to the rooms that need them added? You can only deliver to the rooms what the emmitters are capable of delivering no matter how big the boiler is.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • gypsy
    gypsy Member Posts: 84

    I currently have an old american standard arcoliner that heats the house very well and provides endless hot water-except in the warmer weather when the boiler isnt running as much. 

    i figured there was a huge heat loss due to the only insulation being what i put in the flat part of the ceiling-upstairs rooms are mostly slant ceilings-and the what i put over the stone foundation where it meets the floors.  there is no insulation in the walls and the windows are the old sash windows with storms-which surprisingly arent drafty.

    Im having a hard time deciding this because the new boiler is through a gas company program and is a huge deal, but if it isnt going to be adequate then its no deal at all!  im not sure how much leeway i have to inform them that what they are proposing will may be suffecient, and i dont want to be cold or not have enough hot water.

    what is proposed is a dunkirk dpsb-4d with a buderus indirect 40 gallon tank... i also have a 120 gallon tub and im guessing pretty hot water consumption...

    any suggestions?
  • gypsy
    gypsy Member Posts: 84
    more info...

    its about a 1200sf house with essentially no insulation, and now that im thinking of it the arcoliner ran and ran last winter...
  • icesailor
    icesailor Member Posts: 7,265
    Do The Math.

    Look at the rating plate on the boiler. See what the gross output, the net IBR output and see how many gallons per hour the boiler is rated for. Look for an old nozzle around the boiler and see what it is. Multiply that rating by 139,000 BTU's, subtract 75% or 80% from that and it gives you a third world estimate of the house or what is going into the boiler.

    In my retarded opinion, you need to upgrade your insulation before you do anything as drastic as changing to gas. The gas provider (I assume nat.gas) should be giving you a big wet kiss of gratitude for switching to them because you will be spinning that coin meter on the side of your house. And I don't care how good a deal you are getting from them.

    It's nice of them to include the indirect. I hope it is big enough to fill that 120 gallon tub you have. As a licensed Master Plumber, I see far too many undersized boilers and indirect water tanks for the real and actual load.

    What you really needed for your hot water usage in the summer was a storage tank. It also works in the winter and saves lots of money.

    I'll prove my point. If you have multiple zones (and they are wired correctly to kick the boiler to high limit), start the smallest heating zone. If there's an isolation valve on the zone, close it. Let the boiler run to "High Limit". Go fill the tub. You will fill it fine as you do in the winter. The boiler is operating in the summer on the operating control which is lower.

    Do what you want. Enjoy your gas. It makes me ill to see intelligent people buy into what some unemployed person who takes a two day course given by the gas companies, walk into homes and condemn perfectly good equipment that needs some upgrades for cheap money. Totally disregarding heating professionals with years of experience.

    If you think that the gas suppliers are giving you such a great deal, and it is going to be so cheap for you, why is it that no matter how wrong THEY are in a bill, they will spend any sum of money to deny you resolution in your favor?

    The only thing you will be getting from the gas providers  is a new pair of jeans with a hole in the back. The Vaseline is extra and can only be purchased from them at an inflated price. The Wal-Mart brand isn't approved for that use.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Heat loss

    "its about a 1200sf house with essentially no insulation, and now that im thinking of it the arcoliner ran and ran last winter..."

    I have an 1150 square foot house in New Jersey and the design temperature here is 14F. I have good insulation and good windows. I calculate the heat loss, when it is 0F outside, at between 30,000 and 35,000 BTU/hour.

    I have a mod-con rated at 80,000 BTU/hour input and the main problem I have is that it modulates down to only 16,000 BTU/hr, which is no where near low enough. Unfortunately for me, it is the smallest boiler in the manufacturer's product line. It seems to me I could better use a boiler with half the capacity.
  • gypsy
    gypsy Member Posts: 84
    thats my point... sort of...

    Its not really as cut and dry as that, and i agree.  so thank you i think? 

    here's the deal. 

    I love the old arcoliner, and hate to see it go.   Problem is, i think it has a hole in it somewhere-i forget where at the moment, somewhere that affects the hot water in the summer and drawing water which makes me have to let water our of the system sometimes-my plumber said it would be more to fix it than it was worth, and that is only if they could find the part.  grumble...

    I am low income/disabled and dont have the money to upgrade the system myself but the gas co has programs where they will do it for me.  I already have gas so its not a gimme in exchange for service...  but maybe a gimme for someone else if this proposed system is crap... excuse the language.

    I think you might have a greed with me that what they are suggesting is inadequate?  any ammo you could give me to back that up would be greatly appreciated!

    and btw... i think a lot of the reason this arcoliner is heating the house so well is because it kicks off so much extra heat into the basement which works as radiant heat to the first floor... which i would also lose wtih a new super efficient system.

    so, i guess id like to ask for some clarity if i may...  and i definietly agree that i need the insulation done first and then see what is needed for a new system if any, but unfortunately im not in a position to do that right now and they wont...  so i think my only choices right now are not upgrading, or trying to get them to put in an adequate system.  any suggestions?  my plumber said they didnt recommend the tank but an on demand water heater..

    im not sure which numbers you are saying i should use for calculations... this is all i could find on the boiler...  it is the original plate and i think its numbers for oil?  its now gas with a carlin burner.

    No. o 154 T

    series 3BT J3

    Valve Cap Min LB Per HR 132

    Max WP Steam 15 LB Water 30 LB

    Firing rate/ Hr 1.35 Gal

    Installed Radiation Steam Sq Ft 413  Water Mbh 99
  • gypsy
    gypsy Member Posts: 84
    re heat loss

    thanks jdb, but im not sure the about point you are making...
  • billtwocase
    billtwocase Member Posts: 2,385
    you say

    that there is a leak, do you see water anywhere? Is it a bad tankless coil problem that has a pinhole? I would say add an indirect, do some buttoning up on the house, and have the boiler set to warm start, as well as make sure it is properly tuned and set up. If money is an issue, that will be a better investment, as change over to gas has a hefty price tag. Sure operating cost will be less, but short term, you could get by with what you have with a smaller amount of money out of pocket. 
  • bob_46
    bob_46 Member Posts: 813

    Is this system hot water or steam?
  • gypsy
    gypsy Member Posts: 84
    pinhole leak...

    thats it, its a pinhole leak in the tankless coil... if im remebering right..

    the arcoliner is already running on gas.

    i do have a proposal to add a tankless with the new boiler so i suppose it could maybe just be added on to the arcoliner, but im not sure about it as it would be a 40 gallon buderus tank and i have a 120 gallon bathtub??

    and what is warm start?
  • billtwocase
    billtwocase Member Posts: 2,385
    if already converted

    I would definitely replace it. Gas power burners are not the thing for these, or most oil boilers. This was originally a coal fired boiler converted to oil in the late 40's. They had little to no baffling, and that gas burner is just sending heat up the chimney more the so than if it was oil fired. I've seen it too many times. 
  • gypsy
    gypsy Member Posts: 84
    steam or water...

    steam, the best! :)
  • Mark N
    Mark N Member Posts: 1,115
    Boiler size

    You have steam heat therefore the size of the boiler is determined by doing a radiation survey and figuring out how many sq ft of EDR you have. You don't do a heat loss for sizing a steam boiler. My house is about the same size as yours and I have a boiler that is 105kbtu input and the house heats fine. My attic is insulated but the walls aren't and the boiler easily heats the house even down to near zero outside.

  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666

    The only point I am making is that my house is about the same size as yours. The design day here is 14F; if yours is the same, the only difference in heat loads, I would think, would be insulation, windows, infiltration, and whatever inside temperature you wish to run at.

    My design was for 70F inside with 0 to 70F outside. That range is bigger than I need. With all these assumption, your calculated heat loss comes out far more than mine (5x). So unless your house is insulated far worse than mine, you would probably not need your calculated heat loss of 190 btu (I assume you mean 190,000 BTU/hr). Since I am not a heating professional, you may not go with my numbers, but a 5:1 difference seems a bit much to be accounted for just by insulation. What do you suppose the cause of this huge difference is?
  • gypsy
    gypsy Member Posts: 84
    no worries...

    and thanks.. im not very knowledgeable about the btu's so i wasnt sure how to judge the difference.  i just measured everything per the instructions and plugged in the numbers and thats what it clculated..  so i guess its the fact that there is no insulation, tons of windows, glass doors, and a couple rooms built over crawl spaces that might be tippig it... if that doesnt make up for the difference, im not sure...  actual figre was 191, 423 btu/hr heat loss and recommended-if im reading it correctly a 299k btu boiler... i used the pikes peak regional building departments calculator.

    where did you find a calculator that you could input desired temps and outdoor temps into?
  • Jamie Hall
    Jamie Hall Member Posts: 22,877
    Just to add to the conversation...

    First, a steam heating system is always -- always! -- sized by the emitters (radiators) which it is feeding.  No exceptions.  None.  If the system is too small to provide comfortable heat, the only way you can improve things is to add radiation -- and then get a bigger boiler to feed it.

    Don't try to short cut that series of calcuations.  You will only be miserable (and, probably more broke).

    Second, I would be much inclined to double check your heat loss calculations.  The building I superintend contains 7,000 square feet of livable, heated area on three floors.  It was built between 1810 and 1893 and, except for the attics, has no insulation to speak of.  It also has normal double hung windows (same age) with storm windows -- so it is generally regarded as being on the high end of heat loss per square foot.  The installed radiation can absorb about 200,000 BTU/hr, and this keeps the building nice and warm on a 0 degree Fahrenheit day with the wind blowing.  Now your building is a lot smaller than that -- say a fifth -- so I would be really really surprised if you needed more than somewhere around 50,000 BTU/hr.  The 105,000 BTU/hr boiler the gas company suggested may just be the correct size, if -- and only if -- they based the size on the installed radiation, even though it is smaller than the Arcoliner.

    As someone said, that Arcoliner isn't doing you any favours at all.  Again, based on the building I superintend, the original boiler was sized to the radiation, as is the present boiler.  The previous boiler burned 5.5 gallons per hour to get there; the present one burns just half that and provides more steam.

    I have never been a fan of using the heating boiler for domestic hot water -- even with a big storage tank.  My own preference would be for an independent gas fired (since you have gas) water heater.  Less hassle, in my view.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • gypsy
    gypsy Member Posts: 84

    there seems to be conflicting thoughts about how to size steam boilers...

    If its based on the radiators, it is the right size.

    if based on the btu loss it is way off.

    ... and does that take into consideration the water demand?
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    where did you find a calculato

    I used the calculator that used to be available from Slant/Fin.

    I also used one from Weil-McLain, but I do not recommend it. It may be OK for a first guess, but there are too many imponderables.


    A third "calculation" was looking at the 1/2 gallon per hour nozzle in my oil burner. That calculates out to 70,000 BTU/hr input. That was always enough to heat my house, so that was an upper limit as to what I needed.

    John Seigenthaler gives detailed procedures for this in his book that is highly worth reading if you choose to heat with hot water. For steam, Dan H. has a series of (I believe) 3 or four books that I imagine are well worth the price, but I have not read them.


    If your house is so different (I have no crawl spaces) it looks to me as though the best investment is better insulation and leak reduction.
  • gypsy
    gypsy Member Posts: 84
    thank you

    this is the one i used...  copy and google this: pikes peak regional building department heat loss calculation table  the xls spreadsheet is the first result that will show up...  apparently by the one you sent me also, steam boilers are not calculated on heat loss.  seems odd to me... but i guess that solves it.  
  • lchmb
    lchmb Member Posts: 2,997

    I dont believe anyone realized it was steam. You need to size your steam system based on your radiators not a heatloss...
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Heat loss...

    "apparently by the one you sent me also, steam boilers are not calculated on heat loss.  seems odd to me.."

    I am not a professional and have no experience with steam other than my grandparents' house was heated with steam (probably single pipe).

    If I were going to design a steam heating system, I would do a room-by-room heat loss of the building to determine the size of the radiators needed. Then I would calculate (if I did not know already) the square feet of the radiators and get the smallest boiler larger than that.

    I do not know for sure, but I suspect that the professionals here usually assume that the house already has the correct number and size radiators, so all you have to do is the second step: getting just enough steam to satisfy the radiators. This may be a good assumption for buildings that already have radiators, provided they have not been knuckleheaded since they were originally put in.
  • gypsy
    gypsy Member Posts: 84

    Good point!  happens all too often with all the old good stuff! 

    Apparently i missed those parts of the instructions that say not to use heat loss for steam. oops, lol..

    seems odd to me though.  then again it could just be that im a little freaked to knucklehead around by putting in a new boiler when this one was the first one put in the house, and based on everything else was probably well thought out and entirely appropriate.  I just hate replacing stuff.  what is made now just tends to screw with the balance of anything that was made well by hand. 

    and im afraid that losing the radiant heat from the basement is going to make this place feel freezing whether it is or not...
  • gypsy
    gypsy Member Posts: 84
    another idea for a boiler

    First off, Thank you all for your comments and suggestions! 

    Ok, so if i do indeed have to replace this boiler and i can maybe figure

    out how to swing this with the gas co., what about a combi boiler?

    and... if anyone has any ideas on how to tighten up the arcoliner if possible, please let me know(i was thinking of adding firebricks).

    It seems that the combi's have the btu rating i need, and ive read about a few

    of them that also have a small internal tank for hot water along with the on demand, and

    seemingly pretty good flow rates.  (actually they seem a

    lot like the system i have now, albeit newer and more effecient)
  • lchmb
    lchmb Member Posts: 2,997

    a combi low mass boiler is a hydronic heating system. You need a steam boiler. As far as tightening it up, if it has a leak you cant make it tight enough. The leak will only get worse and you will lose your steam up the chimney...
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    over-filling arcoliner

    can you describe the leak more accurately? is the boiler over-filling, possibly due to a bad dhw coil?

    if the boiler is losing water, then it is more serious, as the sections may be bad, but a tankless coil could be replaced.

    if you go for a new boiler, the size must equal the heating capacity of the radiators-not the heat-loss of the house. this is because the radiators must have enough steam, arriving simultaneously to be heated up, thus warming the rooms. make sure the contract describes the installation as per the manufacturers piping instructions [pipe sizes,layout] at the minimum. the existing header may not be useable.

    the relative height of the new waterline vs. the arcoliner waterline must be studied as well. plenty of main [not radiator] venting will also be needed.--nbc
  • gypsy
    gypsy Member Posts: 84
    water leak?

    I was told it was a pinhole leak somewhere because i had to let water out of the boiler every couple weeks.  I also had the low water cutoff fixed at the same time.  im not sure what was up with that because i was told it was wired backwards as a high water cut off... didnt make sense to me but apparently its fixed.  to find the hole the plumber shut off all the valves then i think opened and closed one quickly and heard a little rush of water.  he said that was the leak. 

    what do you mean check the water levels against the arcoliner and new water line?
  • Jamie Hall
    Jamie Hall Member Posts: 22,877

    If you have a leak which causes you to have to drain the boiler from time to time -- rather than having to add water to it -- the leak has to be from your domestic water supply into the boiler.  There are two ways this can happen: a faulty shutoff valve on the feed to the boiler itself or, and much more common if the boiler also supplies the hot water, a leak -- usually just a pinhole -- in the tubing inside the boiler which supplies the hot water (the "tankless coil").  Either way it needs to be fixed; leaks never get better on their own.

    I still think that Arcoliner has seen its day...  They were good boilers in their time, but so were Model T Fords.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • gypsy
    gypsy Member Posts: 84
    pinhole leak

    yeah that was it.  pinhole leak in the tankless coil.
This discussion has been closed.