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Thread: What size boiler?

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  1. #1
    DIY Junior Member fredfons's Avatar
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    Question What size boiler?

    I bought a 2300 square foot bi-level in Philly suburbs built in 1964 for investment/rehab and am converting from oil heat to gas. I am a carpenter and figured I could handle doing a heat loss calculation to size a new gas boiler using the calculator at www.crownboiler.com/Support/Education/Boiler-sizing. It calculated 56,000 BTU heat loss but the existing oil boiler a New Yorker APU 690U bt4 has a DOE heating capacity 129 MBH. Does my BTU calculation sound too low for this house or is the existing boiler oversized? Was it oversized due to DHW? Or am I misreading the rating numbers? The house is 4 bedroom 2 1/2 bath, 25' x 47', lower level uninsulated brick veneered conc. block and upper level is frame with 2" batts in walls and 5 1/2" loose fill in attic. All new double glazed mid quality windows and doors. ODT= 13. I am looking at Crown mod con BWC070 rated 70,000 BTU. Any thoughts on if this might do the job?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Most older boilers were grossly oversized. Many calculators are as much as 100% over the 'real' need. A better solution for this house would be to compare the actual oil used verses the heating degree-day for your location. No idea if that brand and model is good or not...

    If a lot of the insulation/window upgrades were made after the last heating season, your new calcs may no longer be accurate.

    Generally, there's no need to oversize a boiler to provide DHW unless it is a commercial situation where there's significant hot water useage all day long. It's generally run on a priority zone, and uses all of the boiler's output while heating the DHW, then reverts back to space heating when that's done. The homeowner will rarely notice that short gap.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    A general rule of thumb is 35 BTU's per square foot; 80,500 for your home. However, since you have uninsulated walls I feel that this ROT is useless for you.

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    In the Trades Tom Sawyer's Avatar
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    Gary, there are no rule's of thumb for BTU's/sq' It's that kind of thinking that has screwed up the heating business for generations.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    I'm a seasoned DIYer with a Modcon that I'm using for the first time this season. I basically installed it myself after firing 2 plumbers I hired to do the job. I've spent 100s of hours reading and expanding on my plumbing knowledge during the process. Here is what I recommend based on my experiences:

    Experiences

    The first plumber I hired picked out a unit that would allow for my future expansion plans. This resulted in a unit that is around double the size i need for my house/level of insulation. I'm lucky that with the amount of research I've done and the unit I have (Burnham Alpine) that I can modulate down to my needs and avoid short cycling.

    Mounting the unit on the wall looks better but I've had a few people tell me it creates more noise on the first floor when running than their floor mounted models. I may add some insulation to the basement ceiling to see if that helps. The Bimini Buddy looks great though. I would definitely get that if you go wall mount. That would make for a really clean install.

    Larger zones are better. I have 4 small zones in my house that I'll be combining into 2 once the heating season is over.

    IHW tank is highly recommended. Mine only runs a few times a day and has maintained temp without running for up to 3 days if I was away and didn't use any hot water.


    Pretty much everyone else will back me up in saying that the prework for a Modcon is the hardest and most important part. If I had picked the right professional to install it and do the right heat loss and pick the right sized unit, it would have saved me money and I wouldn't have to spend time tweaking the system all the time. Mod cons break almost every norm that has been accepted in the heating world. Everything changes and you need to hire someone who understands that and can do the job right.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, the Buderus wall-hung unit I have is totally inaudible (to me at least) if you are more than 3' away from it or you have the cover off. It is mounted on the concrete basement wall, though, so hard to vibrate anything. the only internal moving part is really the fan, if you discount the gas valve.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    Quote Originally Posted by Tom Sawyer View Post
    Gary, there are no rule's of thumb for BTU's/sq' It's that kind of thinking that has screwed up the heating business for generations.
    Are you kidding? There are LOTS of rules of thumb for BTU per square foot...


























    ...so WHAT if they're all dead wrong? ;-)

    If you have fuel use from billing between dates where we can look up the heating degree day data for those dates it's pretty easy to calculate a solid upper bound for the BTU/hr at the outside design temperature. But if this is a rehab with no history you probably don't have sufficient data to work from.

    But from experience, a reasonably tight 2300' house with double-panes it would have to have almost no insulation to hit as high 56KBTU/hr at a 13F percentile design temp, and you have at least some. If you were planning to live in it or rent it to a friend it's worth blowing cellulose into the cavities that currently have the econobatts, and blowing over the existing fill to boosting the attic R to something more like code which will bring the peak heat load down considerably.

    Even if you're planning to flip it, there are probably subsidies available that can often make it cheaper to hire a pro than insulate it as a DIY. If you can get the heat load under 50K (likely- since it may already be less than that) for cheap you can then go with a smaller cheaper mod-con. (Is the BWC070 even a modulating boiler? The specs only have one input BTU number not a max & min.) Even if it's the same total cash outlay, what you end up with is a tighter more comfortable & efficient place.

    I have a similar sized 1-1/2 story house with all framed construction and a 0F design temp, and even before I insulated the walls (it previously had paper-clad half-inch horsehair for wall cavity insulation- maybe R1 on it's best day) the measured heat load (measured by the fuel use & DOE efficiency of the boiler) at 0F was less than 50K. Uninsulated brick is probably similar, and the econobatts slightly better than where I started. The ratio of exterior surface area to floor area may be comparable (or not) and your total glazed area may be comparable (or not), but your design temp is 13F warmer. My gut tells me 56K is the right range, but an upper bound, but that insulating it and air-sealing the attic floor/ceiling interface would likely bring you down to under 40K. Most Manual-J or IBR type heat loss calculators hit ~25% above measured reality, so if you filled in the Crown Boiler spreadsheet model diligently and it says 56K, odds are pretty good it's under 50k already. And $500 in cellulose could easily knock that down another 8-10K putting you in the range of a Munchkin T-50/ Pinnacle T50 or Triangle Tube Solo-60 or something.

    What type/how much radiator or baseboard do you have? It makes a difference on how the boiler needs to be set up. Most of these are not great for DIY installation, but if you're determined, see if you can find a manufacturer's installation class/seminar before diving in, or it may end up costing you more than letting a pro do it.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Dana View Post
    If you have fuel use from billing between dates where we can look up the heating degree day data for those dates it's pretty easy to calculate a solid upper bound for the BTU/hr at the outside design temperature. But if this is a rehab with no history you probably don't have sufficient data to work from.
    Yep, and if one is really wanting a precise number that beats any estimate (such as Manual J), record daily fuel use for a season apply the efficiencies, and calculate btu's req. vs. average daily outdoor temp (from a local weather data logger.) That's what I did to size my furnace and it gave a nice linear fit to the plot. With that you can pick any design temp you want to size the unit. In my case it was overkill because the AC air handler frame size limited how low I could go on furnace size. At least I was able to use a two stage furnace, so it almost never runs in high fire mode.

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