Good info thank you! So i have two years of oil usage before we got a pellet stove....my oil co. only lists total fuel used for the year. I under stand what your saying about how to come to a actual usage but im unsure how to do the math? I get to use the eff. of my boiler and the heating degree days then.......is there a web site with a formula? If oil has about 140,000 btu per gallon and the boiler burns 1 gph and is 81% then its making 101,000 btu. But how does that play into my heat load ect? Im sure the old boiler must be sized high as to keep up with the tankless coil too.
Found a nice big crack in the back wall of the old peerless this weekend while taking the sections apart to throw out..
Going back to brands that only leaves me with the Buderus GB142 @ 77000 btu (touring Buderus factory here in Londonderry NH tomorrow)
I can't thank you guys enough for this great info by the way! If anyone on this four has a SAAB, Volvo, Mercedes Benz, BMW or Audi feel free to ask me anything
www.degreedays.net can give you the degree-days at any base temp between arbitrary dates for a weather station near you. If there are days missing (as often happens, instrumentation breaks) try to find a different weather station nearby with more complete data.
But the oil/year is good enough to ball-park it. Which years, and how much in each year?
To get from gallons/year to btu/hour at design temp, you have to first convert the gallons to BTUs out.
138,000 BTU/gal x 0.81 DOE efficiency= 111,780 (not 101K, call it 112K/gallon
Then multiply that by the number of gallons. Say it was 780 gallons for one year, that comes out to 87.36MBTU.
Then divide by the number of heating degree days (use base 65, maybe even base 60, since you're heating hot water with the beast, but start at 65 and you'll have real margin). Say degreedays.net tells you that it was a 7873 heating degree-day year:
87.36 MBTU/7873HDD =11,096 BTU per heating degree-day. Converting BTU/HDD to BTU/degree-hour is simply dividing by the 24 hours in a day:
11,096/24= 462 BTU/degree-hr
At a design temp of -2, that's 67 degrees below 65F, so multiply 67degrees x 462 BTU/deg-hr= 30,977 BTU/hr @ design temp.
That's an upper bound. If you were heating hot water with it, something like 10-25% of the annual oil use went into heating hot water (or hot-water standby) discount it by at least 10%. But even with out the hot water component, By virtue of the fact that the boiler is 3.5-4x oversized for the load the combined heating and standby losses means your AFUE was at least 10% less, maybe even 15% less than the 81% steady-state number. To be conservative, de-rate the calculated heat load by 71/81, or 0.87 x 30,977=26,950 BTU/hr, call it 27K.
But run the real numbers, see what you come up with.
Note, if your numbers are anything like the dummy example or even less, even the smallest Buderus will be 2-3oversized, and won't hit it's AFUE numbers. (It needs to be 1.7x or less oversized for those numbers to be real.)
But a TT Solo 60 would still make out OK, since it's min-mod out is still under the design-day load. If it's an all-baseboard show you may need to add some thermal mass to keep it's minimum burn long enough at condensing temps though.
Also note, if your design condition load is under 20K, that load can be met with a ~$5-6K 3-ton ductless mini-split air source heat pump, and cost less than half as much to operate as a 3x oversized Buderus, assuming $3.50 oil and 15 cent/kwh electricity. It may be operating cost-competitive with condensing gas too, but a more detailed analysis would be necessary to say for sure. But you don't already have air-conditioning, the rationale for going with a high efficency "inverter" R410A refrigerant ductless heat pump that both heats & cools goes up when the time comes. You probably won't hit the savings of
this guy on Martha's Vineyard though (his design-temps and average coefficient of performance are higher than in central NH- he'll average a COP of ~3 or better, but you'd only make 2.5-ish, maybe a bit more.)
But no matter what your mechanical systems and fuels end up being, a serious round of air-sealing (even on newer homes) usually pays back in very short years It's shorter, if oil is your fuel rather than condensing natural gas, but it's very short compare to condensing propane too. See:
http://blog.energysmiths.com/2011/03/whats-a-renter-to-do-air-sealing-can-be-low-cost-and-effective.html Until air-tightness specs are enshrined in code and verified by blower door tests, odds are you're blowing 1/4 of the fuel use or more on excessive uncontrolled ventilation, and retrofit air sealing can reduce that by half, often more. Unless your initial blower door test comes in well-under 5 air-exchanges per hour at 50 pascals (5ACH/50), odds are you can get ROI well into double-digits (after taxes, no less!) on the first $2K invested in air-sealing. While building to sub 2ACH/50 isn't hard, if the primary air-barrier on all 6 sides of the cube wasn't in the initial design and enforced during construction, you're more likely to be in the 10ACH/50 range.