We are aggressively setting the temperature back, so that might be it. The morning/evening temps are 68*, during the day is 65*, and night is 58*. I don't believe we are very well insulated (if nothing else, double-hung windows with no weatherstripping and uninsulated sash pockets). Does it make sense that the setback has that much of an effect, or am I working from bad inputs?
In any case, I'll definitely have a heat load calculation performed. I would like to know if my "So here's what I worked out..." has any basis in reality, though.
With reasonably tight storms the empty sash weight pockets and lack of weatherstripping can be of remarkably minimal consequence. Stack effect infiltration from leaks at the attic & basement are much bigger contributors to heat loss than air leaks that happen in-between. The former are a 24/7 drive whereas lateral leakage rates are mostly wind driven.
As for the "basis in reality", for a rough cut, what's the size & shape of the house, and how many windows & doors? Even if the nightime setback is 58F, I assume it only gets cool enough to trip that setpoint on the coldest nights of the year (?). Setbacks of that magnitude would only result in single-digit percentage savings, whereas your total annual domestic hot water that uses the same fuel would usually be into double-digits. If you were regularly keeping the place at 50F for most of the day you'd be well into double-digit annual savings though. If you can verify the gas company data using your own heating degree day data from a third party source against fuel use during a winter billing period you can find out pretty easily if their numbers are WAY off. No matter what they're not going to be 2x-3x off, which is what it would have to be to rationalize installing a 110KBTU/hr mod-con.
Blown cellulose tightens up walls by quite a bit- something on the order of 90% tighter than low-density batts or low density blown fiberglass, and the installation process fills in all voids & anomalies, and plugging crack-leaks with cellulose. The only way to get tighter in a retrofit is with ultra-fine new-school blown fiberglass dense-packed to 1.8lbs/ft^3 or higher, or to use expanding polyurethane foams (or non expanding injection foam.) It's better than you might think, independent of absolute R value.
When having the heat loss calc done, insist on using 68F for the interior temp, and -6F as the design temp. If they use a different number, make them explain why. I've had heating professionals tell me with a straight face to use values that were more than 10F lower than my local 99th percentile design temp, which is something I'm very loathe to do. Since heat load tools tend to overshoot by double-digit percentages even using the correct inputs, adding another ~15% error to the high side is a ridiculous thing to do.
1600 square feet, 2 stories. There's a bump-out that's about 150 square feet per floor, the rest is a cube shape.
4-6 windows per room, all double-hung. The storms aren't great. I definitely don't feel drafts in the winter, though.
Blown-in insulation in walls and ceiling, plaster walls.
I don't think the boiler ever runs in the middle of the night. It can take a while to get the house back up to temperature, though, and we'd like to keep it warmer if possible because we've got kids now.
An 1800' mostly-cube, reasonably tight, reasonably insulated would almost always come in under 50KBTU/hr @ -6F, and could easily be under 35K.
How many windows total? Typical double-hungs are about 10 square feet, and w/storms have a U-factor of about 0.4-0.5 (R2-ish). The cellulose insulated walls run about U0.1, or about 1/4 to 1/5 the heat loss per square foot of the windows, so the actual window count/size matters.
Assuming you have 20 windows total @ 10' per, that's 200 square feet of window. Assuming they're U0.5 that's (200' x 0.5=) 100 BTU/hr per degree-F, and about 70F delta-T at design condition, for 7000 BTU/hr for windows. (In zero wind they will perform better than U0.5, and with blinds, shutters or curtains, better still.)
Assuming have at least 7" of cellulose in the attic the U-factor for the attic is less than 0.05, but let's call it U0.05. and 900 square feet of attic floor gives you 45BTU/hr per degree F. Times 70F gives you ~3000 BTU/hr out the attic.
With the bump-out on the footprint you probably have a total wall perimeter of ~150', give or take, maybe 25' tall, for 3750' of area, less 200' of window & door, call it 3500'. At an average of U0.1 that's 350 BTU/hr per degree-F, x 70F comes to 24,500 BTU/hr for wall area.
Assuming you have 2' of exposed uninsulated foundation for that 150', for 300' of U1.0 assuming it's about 55-60F down there, not 65F+, call it 60F delta -T tops, for 60x 300 = 18,000 BTU/hr.
Add it all up and you have 52,500 BTU/hr, at design condition, assuming minimal air infiltration (add 3-5K, just to be sure- more if you have known drafts), and an interior temp of 66F or greater. Subtract 250 BTU/hr per adult human, maybe another 1000BTU/hr for background electricity use (that can be calculated if you know your active plug load at night.) It's under 60K, and probably in the neighborhood of 50K that needs to be supplied by the boiler to keep it at 65F at night.
This is probably worse than worst-case, if you have any confidence in your annual therms/HDD numbers, but it's roughly how heat loss calculations work. If you're holding the line at 58F instead of 68F at night, that means your delta-Ts are about 60F, not 70F, and the 52.5K number drops to 45K. If the attic is ~ R40-ish rather than ~R20-ish your attic losses are ~1500BTU/hr lower. But whatever the true number is, your as-operated heat load at your setback temp is well under the output of the Solo 60.
And, if you put 2" (R8) of rigid EPS foam up against the foundation walls,cutting & cobbling some over the foundation sill & band joist, foam-sealing the seams & edges for good air-tightness then trap the wall foam in place with a 2x4 studwall with UNFACED batts, your basement losses will be cut by at least 90%, from ~18K down to less than 2K, cutting your whole house heat load to less than 40K @ design temp even with a 70F interior. The basement stays warmer & drier, and the first-floor floors would be noticeably warmer in winter too. Uninsulated basements & band joists can be more than 25% of the total heat loss on houses that shape, and are often the easiest most direct big efficiency upgrade to retrofit, after attics.
This last one is a very valid observation. Though windows are often the first thing to be replaced, they are just as often in last place for return on investment. More often a new properly sized condensing boiler can save double-digit percentages in fuel bills, something new windows will very rarely accomplish.
Windows... let's see. I'm going to categorize since they vary.
"Classic" double-hung (uninsulated sash pocket, no weatherstripping): 12.
"Upgraded" double-hung (1" foam in the sash pocket, nylon pile weatherstripping): 5
1 vinyl slider, 1 vinyl awning, 5 modern casements, 6 modern double-hungs.
It's a very window-ful house. I love the light, but it does make it hard to find room for bookshelves :)
All of the classic/upgraded have metal-framed storms in good shape. The rest don't, but are double-glazed at least. We've primarily been replacing the casements which _were_ wind tunnels; I expect to put in modern double-hungs only where the wood frames are really rotted out.
Oh, and heavy curtains, but hey.
WRT the foundation, there is actually no exposed uninsulated foundation. There's 5' of buried concrete wall (well, 6" or so showing) and then stud wall on top of that; the stud wall is insulated.
So. I just talked with the heating guy and he's happy going to a 60K boiler; but it's clear that he has no interest in running a manual-J and I'd like the insurance. Anyone want to run one for me? I assume this is something where I can do the measurements and so on?
Your 100 year design frost depth in Madison is probably something between 54-60", and you are still losing a large amount of heat out the foundation.
The band joist + foundation sill is probably about 150 square feet of ~R2/U0.5, for a heat loss of about 4500BTU/hr @ 60F delta-T (assumes a cool basement.) The 6" of bare concrete is ~75' of R1/U1 for another ~4500BTU/hr. So it may be closer to 9K than 18K, it's still a pretty big number, a number that is fixable on a cost-effective basis without tearing your house apart.
From the "better than just a WAG" heat loss estimator adjustment point of view, the windows in excess of the 20 I used in my quick example adds another ~ 3K, but the reduction in above grade exposed foundation area probably knocks off ~6-9K, so the net is still at 6K lower than 52.5K I came up with previously, for something on the order of 46.5K, and it might well be under 40K.
That makes going with the solo-60 even more of a slam-dunk.
If the slab is 5' below grade, fine, it may not be worth insulating the floor unless you're finishing it as a room, but the foundation walls are DEFINITELY worth insulating and air sealing. Losses to the cold earth with is more of a seasonal steady-drain than a peak-air temp drain. Crudely modeling the below grade 5' as 55F dirt, and the foundation as R1 for a 10F delta-T, that's still 750 square feet of sub-grade R1 at 10F, for 7500 BTU/hr. Soil types vary- it could be more, could be somewhat less but it's not much less half that- maybe 12K rather than 9K would be the better guesstimate to use on your basement load (ignoring the slab for the time being.) It's still well-worth air-sealing and insulating the foundation a number reasons beyond the (still cost-effective) fuel savings aspects, comfort and indoor air quality among them. Foundation sills & band joists are often the single largest air leak in the house, and being at the bottom of the stackup, contributes a gia-normous fraction of the stack-effect driven infiltration.
BTW: Storm windows tighten up the air leakage of old un-weatherstripped double-hungs by a LOT. But if you can still detect air leaking at the sash pulleys it's worth buying a sack o' these. The installed view looks something like this:
If you want to do your own heat loss calc, you might try running Harold Kestenholtz's I=B=R tool:
http://www.livepast60.com/heatpro/heatfla/flashpro.html (<online, 1-room version)
Or download a standalone to play with: http://www.livepast60.com/heatpro/es...n_programs.htm
Watch out for excessive air-change numbers- the defaults of this (and many tools) is much higher than reality. A pretty-tight house can have 0.1 air exchanges per hour or less when not actively ventilating, but play around with it.
(Kestenholtz wrote one of the very first heat loss tools for computers some 30 years ago. It's not much different from Manual-J methods.)
I'll definitely look into sealing the basement. The band joist is plastered over (did I mention plaster, plaster everywhere??) so getting in there hasn't been a priority. Should be one, though.
And the problem with the double-hungs is air coming through the access ports as well as the pulleys. It's okay - I'm fairly handy, and putting in weatherstripping is easy. The only part that takes a long time is stripping off the old paint and reglazing.
Okay, one estimate coming up, and a 60K.
Thanks everyone for all their suggestions!
If you tighten the place sealing every obvious air leak, it's often worth hiring a pro to run a blower door test with or without infra-red imaging to nail down the rest of it. Around here (central & eastern MA), that service runs north of $500, but in combination with expert air-sealing the comfort level go up while the heating loads come down.
If your place is tight enough that it stays north of 35% RH @ 68F indoor temps throughout the winter you may need to actively ventilate, but that would generally be a GOOD problem to have.
With the band joist plastered over, if the plaster is in good shape it may be doing a decent job of air sealing (better than 95% of homes that age out there) but if it has cracks, maybe not. Either way, installing rigid foam and foam-sealing the edges (or giving it a 1-2" shot of closed cell spray foam) would guarantee it. Insulating at least the upper half of the foundation to R10 or better has pretty good payback in your climate zone. R5 XPS (1" blue or pink board) and an unfaced-batt insulated studwall trapping it in place is about an R15 wall, after the thermal bridging is factored in, and still reasonably cost effective.
This is a good point. After I insulate an attic, the next target is the rim joist where enormous infiltration is a given in vintage homes. Only after these are addressed, the frost line is the enemy of comfort and economy.
if you use gas for heating domestic hot water, you have to deduct domestic hot water portion of gas usage from your gas bill.