Hi Dana, thank you for the reply! Between 1/5 to 2/17 we used 171.5 gallons, 2/17-3/30 was 131.8. Zip code is 02474. The only heat source we have is the nearly 60 yr old oil boiler, and it is via ~100 ft of hot water baseboard all on 1 zone. A few other points of note - the basement (530 sqft) is neither heated nor insulated, but we would like to have the option to do that in the future on its own zone. Also, we plan on removing the baseboard (12.5') from the kitchen and installing a kick toe heater (or another suggested heat source, but we need the wall space where the baseboard is).
Tom Sawyer - are you suggesting we stay with oil? The current boiler needs replacing (having moved in this past fall, we were just grateful it made it through this winter!) and a gas line has been brought to the house. We are pretty set on going with gas. Thanks for the site recommendation though.
Your inputs or the U-factors in the Slantfin tool have to be WAY off, based on your fuel use numbers. If basement walls are completely uninsulated it might add 10-15K to the numbers, but unless your basement stays in the 40s all winter (not likely, if the oversized boiler is down there). With an insulated foundation (highly recommended, even if you don't fully finish the basement) the additional load is even less.
Your fuel-use numbers are credible, but to put a worst case stake in the ground I usually make the following assumptions. Just because you set back to 60F doesn't mean you should be using base 60F, especially since the house is clearly tight enough that it doesn't actually drop to that temp overnight very often, and probably never during the day (except during polar vortex events.) The 78% efficiency presumption is also probably slightly higher than reality, but it's not a worst case number. To worst case it, use base 65F and 85% efficiency, which puts a firm upper bound on it.
Assuming a source fuel energy content of about 138,000 BTU/gallon. About the best you're ever going to do with an old-school oil boiler even with the most advanced retrofit burner is 85% efficiency (that's really dreaming- reality is lower, possibly much lower), so that delivers about 117,300 BTU/gallon into the heating system (the rest went up the flue.)
Between 5 January and 30 March you used 303.3 gallons of oil, which delivered 117, 300 x 303.3= 35,577,090 BTU into the heating system
Assuming you keep the house at 68-70F, using base 65F for heating degree-day base would be about right. When it's over 65F outside you would have no heating requirements- the electricity use and body heat from occupants would keep the place at 68-70F. Taking the weather data from the Medford MA weatherstation on
degreedays.net, between 1/5 and 3/29 you experienced 3391 HDD.
So that means you used about 35,577,090 BTU/ 3391 HDD= 10,492 BTU per heating degree-day, which is 10,492 BTU/24 hours per day= 437 BTU per degree-hour.
The
99% outside design temp for Arlington is about +10F (Boston's is +12F) but for yuks let's use +5F (your approximate 99.7th percentile temperature bin) to even further worst-case it. Using 0F would be silly, since many/most years it never actually reaches that temp in Arlington, and when it does, it's not for long enough to matter. At +5F you are 60F degrees below the presumptive heating/cooling balance point base temp of 65F. That implies a whole-house heat load of:
60F x 437 BTU per degree-hour= 26,220 BTU/hr @ +5F
That's about 16 BTU/hr per square foot of conditioned space, which is a totally credible number, but given the age of the boiler it's probably even lower, since the efficiency is lower.
At your mean January temp of about 30F you're 35F below the balance point, for an average mid-winter heat load of 35F x 437F= 15, 295 BTU/hr.
With an average load that low you should rule out any modulating boiler that has a minimum fire output higher than about 15,000BTU/hr. All of those 100KBTU/hr input boiler have min-fire output about 2x that much, which is even higher than your load at +5F.)
The very smallest oil boilers out there have an output of about 60,000 BTU/hr, which is more than 2x oversized for your peak load, and 4x oversized for your mid-winter average load, which makes sticking with oil a very poor choice even if the price of oil managed to reach cost parity with natural gas over the next decade or two which is a dubious proposition at best.
There are many modulating condensing gas boilers with 50K input at high fire and ~15K output at low fire, and some with a low-fire output of less than 10,000 BTU/hr, which would be GREAT.
Assuming a 26k peak load and 100' of baseboard that's 260BTU/hr per foot, which according to
Slanfin specs takes an average water temp of about 130F, or about 135F out, 125F return. With most mod-con gas-burners an entering water temp of 125F is at the edge of condensing, delivering ~90-91% efficiency at or near it's minimum firing range.
Assuming a 15k average load that's 150BTU/ft-hr, which takes an AWT of about 110F, and you'd be running 95% efficiency or higher.
With fin tube baseboard you don't really want to go lower than about 115F for output temp when adjusting the outdoor reset curves, since the output of low-rise convectors becomes very non-linear at lower temp, but clearly you have sufficient amount of baseboard to average in the mid-90s for efficiency once you've tweaked it all in.
If you're going to cut it up into zones you will run into short-cycling on zone calls at condensing temps, since the output of the baseboard on any one zone will be less than the minimum amount of heat that the boiler can feed it. If that's part of the plan you'll be better off using massive tank-based combi boiler like the
HTP Versa. But if you continue to run it as a single zone any number of mod-con boilers would work.