Which boiler to choose...

Discussion in 'Boiler Forum' started by kenbo90, Feb 14, 2012.

  1. kenbo90

    kenbo90 New Member

    Messages:
    3
    Location:
    Northport NY
    Greetings, I figured I'd start asking around to get some help on the matter. I'm looking to covert to gas from oil after my home's 14yr Burnham had 2 delayed ignitions and practically blew itself apart (bad transformer). It's all bandaided up at the moment, but I'm burning off my oil steadily. I have one zone currently, would like to go three (2 story with basement). I have wall vectors in most of the rooms except for 20' of baseboard in the master-upstairs. Plan on putting baseboard downstairs to get some heat there. I brought in two guys, both suggested atmoshperic (chimney). Both said the ES2-4. One did also recommend the weil-mcClain CGA-4, because me no happy... Now they both look good. There's 1% eff. better than the other. One is a good deal cheaper. The money is not a deal breaker, which model would you guys suggest? I have toddlers, so having a reliable heat source is a must.
  2. kenbo90

    kenbo90 New Member

    Messages:
    3
    Location:
    Northport NY
    No opinions? :-(
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,315
    Location:
    New England
    The efficiency ratings are theoretical, so 1% difference is in the noise. Personally, I've had one installed that wouldn't install the Weil-mclain with the alumunium heat exchanger. that was several years ago, and they may very well have resolved any issues there may have been. I like the flexibility of a mod-con to adjust itself to the needed load. This does still mean that it needs to be sized properly as the range of modulation isn't infinite. But, these things are more complex, and the quality of the install is critical for proper operation and efficency and longevity. After looking around, for several reasons, I chose a Buderus mod-con. Not the cheapest, not the most expensive. Several things that swayed me: the installer had one in his home, the US headquarters is an 1/2-hour away, and it got good references. I looked at the W-M unit and the Viessman, but liked the Buderus for the price point.
  4. Dana

    Dana In the trades

    Messages:
    3,027
    Location:
    01609
    The per-delivered BTU cost of oil is more than 2x that of natural gas, and the longer term prospects for each seem to be diverging even further, so this is a good move.

    Even though neither of those are monster-sized boilers, it's more important to "right size" a cast-iron boiler than a mod-con to get the best efficiency out of it. Either one of those has more 2x the output of my design day heat load, and my design temps are more than 5F colder than yours. (1920s 2x4 timber framed house, ~2200' of fully condioned space + 1500' of insulated basement kept at 65F.)

    At 2x+ oversizing a boiler will perform LOWER than it's AFUE rating (the testing of which presumes no more than 1.6x oversizing), so unless your house is VERY large or has very low insulation levels, leaks air like crazy, only single pane windows or something, the odds are pretty good that these models are at least 1 size larger than necessary, and possibly 2 sizes too big, and going smaller would make a bigger difference in as-used efficiency than any difference in side-by-side AFUE tests.

    To get an upper bound on your true design day heat load, how much oil did/do you use per year, or better yet, if they stamp a "K-factor" on your oil bills, what's the K-factor on a mid to late winter oil fill-up?
    Last edited: Feb 15, 2012
  5. kenbo90

    kenbo90 New Member

    Messages:
    3
    Location:
    Northport NY
    First of all thanks for the input. The house is a 1955 cape, I insulated the hell out of the 'cubby holes' R32 on the floor and R19 on all the knee walls. Unfortunately I don't know how the sealed walls are and the rafters above. I was thinking of different methods to shove insulation up there without ripping down the sheet rock or the roof (any ideas are appreciated). The windows are all new andersons and as for oil consumption, I can usually go a month and half with approx 240gallons (I filled up on jan.15 and it's only on half). The old boiler is an RSA-125 which I feel is a little oversized DOE 144 and IBR 125 vs. 88 & 77 for the new guys. So I'm feeling the contractors did at least some of their homework.:confused:
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,315
    Location:
    New England
    There are ways to KNOW what your heat load is, and until you figure that out, any speculation as to what size boiler is best for you is a (bad) guess. Each time a boiler starts and then shuts down, there are periods of inefficiency. The bigger the boiler, the more of those short cycles it has, the less efficient it is. That's why a 'just-right' size is so important; not only will it have fewer start/stop cycles that lose efficiency, but it will last longer, too. A well-designed system will have long burn times and few cycles on the cold design day. Ideally, it would run constantly, just keeping up, then, as well. A constant lower level of heat out of the system is much more comfortable than one that cycles on and then off. It is more efficient as well.
  7. Dana

    Dana In the trades

    Messages:
    3,027
    Location:
    01609
    Unless they have you a room-by-room heat loss calculation based on a realistic outside design temp I'm betting they didn't do much homework at all- just eyeballed it, took a WAG, then upsized it slightly "just to be sure" they never get the 5AM call from the now-cold client. Even downsizing 20% from a Manual-J type heat loss calc RARELY falls behind the true heat load, and most contractors UPSIZE at least 20% from there.

    The K-factor (= heating degree days per gallon of fuel use) on an oil bill tells a lot, and establishes a firm upper bound on the boiler sizing.

    Blower door testing and air sealing can take the heat load down a LOT, if you think the place is a bit leaky. A well insulated wind tunnel is still expensive to heat. Your place is better insulated than this, but could easily be as air leaky.

    Without exact dates and fuel use quantities to work from the error bars are huge. WITH exact dates you can look up base 65F heating degree days from a nearby weather station from degreedays.net , and use the boiler's efficiency to come up with a BTUs per degree-day number. Divide that by 24 to come up with BTUs per degree hour, then multiply it by the difference between your outside design temp (about +15F in Northport) and +65F to come up with the BTU/hour output. To have any accuracy you have to look it up.

    As an example, say you used 240 gallons over a period that added up to 900 heating degree days, and your boiler tested out at 85% for a DOE output number. That's 240/900= 0.27 gallons per heating degree-day. There are 138,000BTU/gallon, and at 85% you're getting 0.85 x 138,000= 117,300/btu per gallon, so at 0.27 gallons/HDD, that's 31,671 BTU/HDD. Per degree-hour that's 431,671/24= 1320 BTU degree-hour. At at design temp of +15, at design temp the delta is 65-15= 50F degrees, so your heat load at design temp is then AT MOST, 1320 x 50= 66,000 BTU/hr.

    But if you're heating domestic hot water with that boiler, knock off 10% from that number. If the existing boiler is more than 2x oversized (=yes), knock of another 10-15%. If the place has never been tightened up with blower door verification, knock off another 15% (and do the air-sealing work- it's not huge money, and a better investment than a high-efficiency boiler.) I would be shocked if your house had a true heat load over 50K @ 15F, and I wouldn't be too surprised if it were under 30K @ 15F. But run the real numbers, see what you come up with.
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