Need help sizing replacement boiler in Connecticut

Discussion in 'Boiler Forum' started by ChrisInCT, Feb 21, 2014.

  1. ChrisInCT

    ChrisInCT New Member

    Messages:
    2
    Location:
    Connecticut
    Hello All,
    Complete novice here - so please bear with my attempts at working through some of this math. I glommed on to some other threads and tried to use a yearlong fuel consumption approach to figure out what my heat load requirements are. I might be way off base here. Was hoping that someone can confirm my math or point out the error (or many errors!!!) of my ways. I am waiting for the first quotes to come in on a boiler replacement and want to be prepared to confirm or pushback on any quotes received.

    Thirty year old home in Connecticut, zip=06410, 2100 sqft with an oil fired boiler, how water baseboard system that needs to be replaced. Three zones, one for each of the two floors and a third for an indirect water heater. Home has been resided and all windows have been replaced, insulation is good. A couple of years ago we went through an energy assessment and beyond some minor weather-stripping, we got a good bill of health. The current boiler system is a "Magic Heat" - what a name! And I think the model is DM122, a bit hard to read on the placard. What I could read was a stated 89% efficiency, but I doubt it’s achieving that today. The heat exchanger is cracked and the entire unit itself is quite hot - a ton of heat loss - when it fires up.

    So, following some other threads I got the heating degree days at 6244 for my location and a 99% design temp of +6F for my location. I used 1168 gallons of oil over the past year. I used that data to calc the following:
    [TABLE="width: 667"]
    [TR]
    [TD]Item
    [/TD]
    [TD]Value
    [/TD]
    [TD][/TD]
    [TD]Source or Calculation
    [/TD]
    [/TR]
    [TR]
    [TD]Oil BTU
    [/TD]
    [TD="align: right"]138,000
    [/TD]
    [TD][/TD]
    [TD]from another thread
    [/TD]
    [/TR]
    [TR]
    [TD]Current Efficiency
    [/TD]
    [TD="align: right"]80%
    [/TD]
    [TD][/TD]
    [TD]a lower guess than what my system suggests
    [/TD]
    [/TR]
    [TR]
    [TD]Effective BTUs
    [/TD]
    [TD]
    110,400​
    [/TD]
    [TD][/TD]
    [TD] oil BTUs * .80
    [/TD]
    [/TR]
    [TR]
    [TD]Gallons Used
    [/TD]
    [TD]
    1,168​
    [/TD]
    [TD][/TD]
    [TD] actual value
    [/TD]
    [/TR]
    [TR]
    [TD]Gallons / HDD
    [/TD]
    [TD="align: right"]0.19
    [/TD]
    [TD][/TD]
    [TD]gallons / 6244 HDD
    [/TD]
    [/TR]
    [TR]
    [TD]BTU / HDD Day
    [/TD]
    [TD="align: right"]20651.38
    [/TD]
    [TD][/TD]
    [TD]above result * effective BTUs
    [/TD]
    [/TR]
    [TR]
    [TD]BTU / HDD Hour
    [/TD]
    [TD="align: right"]860.47
    [/TD]
    [TD][/TD]
    [TD]above result per hour
    [/TD]
    [/TR]
    [TR]
    [TD][/TD]
    [TD][/TD]
    [TD][/TD]
    [TD][/TD]
    [/TR]
    [TR]
    [TD]Design Temp
    [/TD]
    [TD="align: right"]6
    [/TD]
    [TD][/TD]
    [TD][/TD]
    [/TR]
    [TR]
    [TD]Heating Degrees
    [/TD]
    [TD="align: right"]59
    [/TD]
    [TD][/TD]
    [TD][/TD]
    [/TR]
    [TR]
    [TD]Heat Load
    [/TD]
    [TD]
    50,768 ​
    [/TD]
    [TD][/TD]
    [TD]BTU / HDD Hour * 59
    [/TD]
    [/TR]
    [/TABLE]

    Does this suggest that I need a unit with 51KBTUs? Any suggestions on units that would effectively and efficiently work given these figures? Appreciate any feedback at all!

    -Chris
  2. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,236
    Location:
    Maine
  3. ChrisInCT

    ChrisInCT New Member

    Messages:
    2
    Location:
    Connecticut
    Thank you Tom. Which particular unit do you recommend? Did my logic above make sense in terms of predicting my heat loss/load?
  4. Dana

    Dana In the trades

    Messages:
    2,933
    Location:
    01609
    The napkin math is about right. If you're using 0.19 gallons/HDD and netting 110,400 BTU/gallon that's 0.19 x 110,400= 20,976 BTU/HDD, not 20,651 BTU/HDD. But since that 0.19 is just a rounding up from 0.187 & change, the 20,651 number is probably the real calc.

    The real load is probably less than that since the boiler probably isn't really hitting it's AFUE numbers due to age & oversizing. Assuming it's an 85% burner but 3x oversized (do you have input/output BTU numbers on that boiler?) you're probably no hitting better than 75% as-used AFUE. (See the measured 2x & 3x oversizing derating for the systems in table 2 of this document.) A heat load of ~51K @ +6F is on the high side for a 2100' mid-1980s unless it has a lot of windows, leaks a lot of air, has no foundation insulation, or a combination thereof.

    If the 6244 HDD was local historical average it's probably better to go to degree-days.net and download a few base-65F HDD spreadsheets for the period in question from nearby weather stations. Recent years in southern New England have seen 5-10% fewer HDD than the 25 or 50 year averages, though this current heating season is tracking slightly ahead of the historical averages.

    Also, if you were heating hot water with that boiler you have to assume something between 15-25% of the annual oil use was for hot water.

    So, derating the boiler to 75% means the 51K is really more like 75/80 x 51K= ~49K, and if you were heating hot water derate that by at least another 15% to 0.85 x 49K= ~42K. For a 2100' house that's a more credible ~20BTU/foot of conditioned space.

    No matter what, barring significant building upgrades since the fuel/HDD period calculated, figure your heat load probably isn't much under 40K, or over 50K. That means pretty much ALL oil boilers out there are oversized by more than 50%.

    Of Tom's recommended series, the smallest puts out about 80K, or about 2x your actual heat load. AFUE is calculated on 1.7x oversizing, but you won't fall completely off an efficiency cliff at 2x.

    Most freebie home inspections offered up by local utilities & governments are almost worth what they cost- they're usually looking at only the lowest-possible hanging fruit ("Is there glass in all of the windows?" check. "Insulation in the attic?". check. "Do the windows & doors all shut tight?" check. etc.), and not at what's actually cost effective for a homeowner who is going to live there for a decade or more. Not that it affects your oil boiler choices, blower-door directed air sealing can be pretty cost-effective, as well as blower-door/infra-red imaging directed spot-insulation. Band joists (even on new houses) usually leak far more air than all of the windows & doors combined, and are often pretty easy to tighten up. A reasonably air-tight 2x4/R13 house with at least R30 in the attic and an insulated foundation will often come in around 15 BTU per square foot at a 60F delta-T. The same house at middle-of-the road air leakage and no foundation insulation can come in as high as 25 BTU/ft, but rarely higher than that with the windows closed. Low-E storm windows over middle-of-the-road ~U0.35 double pane replacement windows are often cost-effective at $4/gallon oil, but not a slam-dunk for better grade replacement windows under U0.30.

    Something else to put under your hat for future planning: Better grade ductless mini-split heat pumps deliver over 10,000 BTU per kwh (seasonally averaged) in a CT climate, so even at 20 cents/kwh that's over 50,000 BTU/$, substantially cheaper heat than even a 90% efficiency oil boiler @ $4/gallon oil. ($0.9 x 138,000= 124,000BTU/gallon , or 124,000/$4= 31,000BTU/$ ) Even after you've set up the oil boiler, if one of the zones can be heated primarily with a mini-split the mini-split would pay for itself in short years. A 1-ton mini-split runs about $3.5-4K installed, turnkey and will deliver over 13,000 BTU/hr @ +5F outdoor temps, and even more BTUs (at higher efficiency) at +40F, a temperature at which point it has sufficient output capacity for the entire heating load, at less than half the cost of heating with 90% efficiency $4 oil. At recent years prices they pay themselves off in under three heating seasons, and they are very efficient air conditioners to boot. If the heat load of your downstairs zone is something like 15-20K and the floor plan is fairly open, a 1-ton would about do it, since it could meet 100% of the average mid-winter load of that zone, and 100% of the whole house load during the shoulder seasons (turn it up a bit and let it convection-heat the upstairs, if possible.)
  5. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,236
    Location:
    Maine

    It does put out about 80,000 but he has oil and unless he wants to spend a wad changing to gas 80,000 is about as small as you can comfortably go with oil. The firebird does not have to condense to still get good efficiency either.
Similar Threads: Need help
Forum Title Date
Boiler Forum Need help with selecting a new thermostat Dec 19, 2013
Boiler Forum Need Help Tuning Navien CH-180 ASME Mod Con Combi Dec 12, 2013
Boiler Forum Boiler not working right, need help Jan 25, 2013
Boiler Forum Need help Wiel-McLAin Ultra 80 series 2 Jan 12, 2013
Boiler Forum Boiler help needed Mar 1, 2012

Share This Page