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what is the proper way to size a replacement boiler

jonny88 Member Posts: 1,139
the company i work for doesnt do a heat loss calculation when installing new boilers.our head guy just says he knows how to size through experience.when replacing steam boilers we usually go back with a boiler with the same btu's.is this something i can learn or is it for an engineer to figure out,thanks in advance.


  • meplumber
    meplumber Member Posts: 678
    Opening yourself up for liability.

    In many states, a heat loss calculation is required to be kept on file for each new or replacement boiler installation. 

    It isn't rocket science either.  There are loads of reliable software available that makes it easy.  You just load in basic information (windows, doors, insulation, etc...).  Most of the programs will even give you a room by room heat loss.  Also, most of the supply houses here in Maine will do it for you if you are buying the equipment from them. 

    Any other way and you are just leaving it up to chance.  Most of us that have been doing this for a while can estimate about what a structure needs for heat, but it is purely a guess.  In the litigous society that we find ourselves in, the heat loss calculation is cheap insurance when the attorneys get involved.
  • Jamie Hall
    Jamie Hall Member Posts: 22,934
    It isn't rocket science for steam, either --

    just add up the EDR ratings of all the radiators in the system (there are sources here on this site to help with that), add in a pickup factor for the steam piping (sometimes built into the boiler rating -- need to check that) and there you are.  Note that the heat loss of the building itself has nothing to do with the size of the boiler needed -- it's the amount of installed radiation.  But you do need to do it; a steam boiler which is significantly oversized is going to be wasteful, and one which is significantly undersized may not heat the building properly at all.  Customers don't like that...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • icesailor
    icesailor Member Posts: 7,265
    Sizing Boilers:

    Maybe someone in your company is more experienced than I. Honestly, I don't know that much. I need a crutch.

    Me personally, I first, before anything, do a heat loss of the total building. That's room by room. Then, I do each room and the installed radiation. Then, I compare the installed radiation with what the room needs. This is 99% lower than what is installed. Usually. Then, what is the heat. Steam or forced hot water. Gravity hot water is the same. If it is FHW, I size the boiler for the load. Plus 1/2 again for domestic hot water. Or the not the smallest boiler that will fit because that one may run like crap and the next one up will run better. I hate explainations if things don't go well.

    Steam is another matter. I add up all the heat emitting units (radiators and size the boiler to that. It is my understanding that a boiler with not enough nuts to fill them with steam could be unacceptable. A 50.000 BTU house with 100,000BTU's of radiation will no work with 60,000BTU's of boiler nuts. Well.

    I don't do steam. so I don't need to worry.

    With FHW though, insulation and other factors may have been added since the origonal install. Not to worry. That means you have pleanty in reserve. Besides, the domestic hot water load is the major load. It is a peak load, 365 days of the year. It may often, in normal residences be much higher than the heating load on the worst design load day.

    Take the IBR course if they still have them or one like it. It's fun and easy to learn. And you sure will know what the heck you are doing.

    That guy that figures it "from experience" will hate your guts because YOU know something that he doesn't and doesn't care enough to learn it. You do.

    Go out and have fun. You won't regret it for asking, trying and learning. Like those of us here who gave enough about a rats red rectum to learn it.

    If you get the Slant Fin free program, that is the easiest to learn. An idiot can use it. Learning the IBR thing will explain it.

    If you get the IBR/GAMA H-22 heat loss guide and the #250 advanced design manual, and teacjh them to yourself, you will know more than most guys who work in the supply house. And your competition.

    That's what I did.
  • Ironman
    Ironman Member Posts: 7,350
    edited November 2010
    Boiler Sizing

    Simply put:

     Size hot water based on a load calc. Don't worry about the EDR unless it's under sized.

    Size steam according to EDR, But still do the load calc. and read what Dan says about "Greening Steam".

    Your "head guy" probably doesn't know how to do a heat loss or how to size a boiler.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    next one up will run better

    I am not a pro, but I am getting a lot of experience with my new mod|con. When I decided to get it, the nearest thing the contractor did to a heat load was to pace around the outside of the house. I had read John Siegenthaler's Hydronic Heating book by that point, and calculated the heat loss, which was about 35,000 BTU/hr if both zones were running. The contractor suggested a 105,000 BTU/hr to provide a margin of safety. I said I did not see how that could work efficiently as it was too big. He said it would modulate down to 35,000 BTU/hr, which is true; actually, it would go down to 21,000 BTU/hr. But my heat load can go down to less than 6000 BTU/hr. So I opted for the  80,000 BTU/hr model (the smallest in the product line). But if W-M made an Ultra-3 40,000 BTU/hr model, it would have been better for me.

    As far as domestic hot water, I have an indirect-fired hot water heater. I made no allowance for it in sizing the boiler. The indirect runs less than 10 minutes at a time, two (perhaps three) times a day with top priority,  and that is not enough to disturb the heating of the house. Of course I do not run a commercial laundry or have a lot of teenage girls in the house.
  • Gordy
    Gordy Member Posts: 9,546

     A little confused. Why would number of zones running play a role into a heat load calc.

    You do a room by room load for a set point, and design day. This dictates emitter sizing, water delivery temps, and boiler sizing.

  • SpeyFitter
    SpeyFitter Member Posts: 422
    Triple it and you're good to go

    I always thought the best way was to double or triple whatever was there to make sure they got enough heat? Oh wait, that's how they size furnaces....whoops, sorry
    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Heat Loss

    Doing a heat loss is the proper way to size a boiler. Other than giving your customer the correct piece of equipment for the job there are many other reasons from a contactor point of view of the value of a heat loss.

    Gives you the tools needed to sell. Yup sell. We are in the selling business are we not? How would you explain trying to sell add on outdoor reset controls without a heat loss? How would you explain the possible use of a Grundoss Alpha, Taco Delta-T punp without the heat loss? How about boiler choices, Wouln't you like to possibly upsell a job? You can't do that without that heat loss either. Then my favorite, you get the job because the boiler you quoted which is sized correctly is one or maybe two sizes smaller then the boiler everyone else is quoted.

    What does your boss do when he runs into replacement job that has radiant heat and hydro air? I know, he makes the same mistake the original installer made, Oversize the boiler.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I am confused, too.

    I did not think I said the number of zones would affect the heat load of a system or the boiler size. If I did, I mistyped because I do not believe it. I agree that calculating the heat load of each room, and depending on how the building is zoned, aggregating the rooms in one zone together. That is what I did, since my original contractor did not do a heat loss calculation, so I had to do it.

    The thing I was trying to address was the problem that the modulation range with mod|cons can be too small. My house has two zones. One will take abour 25,000 or so BTU/hr when it is 0F outside. The other one will take only about 6500. As design day around here is only 14F, even these numbers are a little high.

    It can happen on very cold days that both zones run at once, and I suppose the indirect could try to run at the same time. (Actually not, since if the indirect runs, the house heating goes off.) So the calculation would come out to 31,500 BTU/hr, and the number of zones has nothing to do with that. But if it is a moderately warm day and only the small zone is running and needs only 3000 BTU/hr, my mod|con will not go that low (only down to 16,000 BTU/hr), so instead of modulating, it goes into bang-bang mode that is less efficeint and can even rapid cycle.

    So when you do heat load, you certainly should do at least a zone-by-zone calculation and add them up to get the size of the boiler at maximum firing rate. But you also want to know the load of the smallest zone. And ideally get a boiler that will modulate down to that. My guess is that there are none. And because of this, it is very important not to have too large a mod|con boiler because it unnecessarily restricts how low the modulation can go at the low end.
  • icesailor
    icesailor Member Posts: 7,265
    Outdoor re-set


    Here is my conundrum on outdoor reset. You are at whatever you have "set" the temp. curve. It is a 20 degrees outside. The brains have decided you need 140 degree water to maintain the inside temperature at 70 degrees. Life is good. Susie teenager and her little sister get in the shower for their 1 hour each morning shower. The tiny "effecient" hot water indirect comes on to heat the domestic hot water. The priority takes over. You have no heat in the house until the priority is satisfied. As the house cools to 67 degrees or less, it equals lowering the outside temperature to 17 degrees or more. But the brain trust can't figure that out. When the priority is done and the brain trust comes back to heat the house, the water temperature drops to 140 degrees but it should go to maybe 145 degrees or more. Meanwhile, you have a cold house. Unless you set the thermostat up and the brain trust knows what you did, it will take more time to heat the house back up. If they made inexpensive potentiometer thermostat, outdoor reset could work. I don't know of any. I'd go for it in a minute. A flaw in the ointment IMO.

    Think it doesn't happen, it does. I've seen it more times than you can imagine. If you ask the client, they may not realize it is happening. Ask the right questions and you will find out.

    That is why you can not use set back thermostats with outdoor reset. You are manipulating the outside temperature with the inside temperature.

    My home has setback thermostats. I get up at 4:30 AM. Four zones are set at 55 at night. 3 come on at 4:00 AM. I get up at 4:30 AM. They shut off again 8:00 AM until 5:00 PM and shut off at 9:00 PM  My high limit is set for 165 degrees. It is warm in the bathroom when I get in the shower. It wouldn't be if I used re-set. I know. I've done that before. It wasn't satisfactory. The one with the longer hair than mine made an untimatum. "Turn that thing back up. It's cold when I get up.  Or when I come in from outside." I did.

    If clients don't understand this concept, they will think you screwed up. And blame you for a poor job. The next person along may know less than them and agree. There goes your reputation.

    Been there, done that.

    It's no different than with excessive infiltration in ceilings with recessed lights and soffit vents. It's cold.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Why Would You Set Back

    Why would you use set back on an outdoor reset job anyway? The whole purpose of reset is to maximize comfort while providing fuel savinngs.

    I'm a Viessmann guy and when we run into customers that want to setback we strongly encourge the use of their Vitorol or Como-OT thermostats/remote controls. They are not real pricey either. They provided indoor feedback to the boiler in it's calculation of the curve. The other nice thing about them is when they get turned up let's say to 70 the boiler recalculates it's heating curve based on the indoor feedback/outdoor temp/supply water temp. Turn them up to 72 boiler recalculates, turn them down to 65 the curve recalculates. So we can react to changing indoor requirements on the fly and not at a verly costly price.

    For the domestic load if we use a good indirect the problem your stating could be overcome very easily without having to oversize my boiler. Storage and changing those heads out to 1.6gpm heads could be one solution. A Vitocell CVA-53 (not the stainless tank) 50 gallon tank would take care of that demand very easily without effecting the heat side.

    Problems begin at the design stage. This stage in my opinion is where most contractors stumble. Some, simply and truly do not know the science or the math, others don't care to know. They are too busy worrying about the dead presidents they get once the job is finished or they don't want their pride hurt.  
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • icesailor
    icesailor Member Posts: 7,265


    You forget that your boiler is always oversized until you reach design outdoor temperature. You are doing what I do by manipulating the outdoor temperature with boiler water temperature.

    If someone doesn't have $10,000 in spare cash hanging around and they want to improve their system to save ca$h, they will go with what they can afford.

    I get the majority of my work by giving customers choices. From doing nothing to the most. Somewhere in the middle, I will find a good place. In your comment, I see the words "not much more" over and over. It's like the person who goes into a kitchen and bath store to re-do their kitchen. They start with all they want. Then work their way DOWN to what they can afford to live with. All those "not much more's" will add up quickly.

    Don't think I am opposed to technology for technology's sake. I'm putting a Veissman Vitodens 100 in next week. The ONLY thing the customer will accept for heat emmitters is in the floor powered fan coils units. There isn't enough floor space for radiant floor. But you dance with the one who brung you.

    Did you ever hear of a Cost/Benifit analysis? Everything I do uses them. I've never seen these "add-ons" help that much. When I might loose the job to someone who doesn't do anything high tech and makes a mess. In which case, I may be back to try to fix it.

    Then, I see a lot of the ones that are hopelessly messed up. Like the one on the board here with the exhaust vent in a corner with an entrance door on one corner and a window on the other. Totally illegal. I KNOW that that is in the install manual under "venting". On hat alone, the CEA should have failed the install. Where I work, the chances are zero to none of ever getting that passed. Nor filled by the LP provider.

    You can't fix stupid.
  • Gordy
    Gordy Member Posts: 9,546

    My bad I kind of miss interpreted the sentence. My appologies

  • CMadatMe
    CMadatMe Member Posts: 3,086

    For the reasons you stated that is always why we give choices. I am not numb to knowing that there are customers that cannot afford certain "add-on's"or higher efficient heating systems. I'm also of the belief that they should be educated on the "add ons" that can be installed or added down the rode. Maybe they can afford the 87% oil boiler today but have to wait to add the out-door reset but shouldn't I be the one to install it instead of them calling someone else. These decisions are also not for any of us to make. It's the consumers decision. With that said, it is our responsibility to educate customers on the choices. You seem to bring this to your customers which is great. Unfortunately there are many out there that do not. They out number contractors such as yourself.

    As for the price of "add-on's."  A Vitotrol for Vitodens 200 or Como-OT for the Vitodens 100 is not that much money (can' talk price but less than 400 clams)and requires the same labor as installing the thermostat on the wall. 
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • icesailor
    icesailor Member Posts: 7,265
    Sizing Boilers:

    When I was starting out, I was told that the "Engineer/Salesman" would use this formula. No insulation, cover all the walls, 1/2 insulation (whatever that was) cover two walls. Full insulation, cover one wall. Size the boiler accordingly. Double that for the tankless.

    Easy as that.
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