Again - I really can't thank you enough for your time, patience and expertise! Truly I'm blown away at your most gracious sharing of your time. WOW! You really know your stuff. Ok - I'll try and fill-in the gaps, so that you may make more specific recommendations.
Please bear in mind that most of the house is Victorian - essentially a rectangle twin - 1875 brick/masonry construction. We did a great job on the attic above the 4th floor but there is still a lot of original window and wall upstairs.
Only the rear addition was built with wood construction. We are less concerned with the addition since we did so much spray foam on it - and it is heated/cooled by the Fujitsu dual heat pumps.
It never pays to be "conservative", always be aggressive heat load calculations, because the error creep is substantial, and oversizing creates more efficiency & comfort problems than being even 10% undersized.
To start with, the
99% design temp for Philly is +15F, not +10F. At a 70F indoor temp, +10F outdoors that's a 60F delta-T. Right off the bat that's an 8% overshoot on sizing.
>> For windows to come in with 21,756/555= 39BTU/ft-hr at a delta-T of 60F implies a U-factor of (39/60=) 0.65, which implies the crappiest possible 1970s clear glass double pane replacement windows with aluminum frames. A wood-sash double-hung with clear glass aluminum triple-track storms comes in at about U0.5, as to vinyl double pane replacement windows. Maybe you have the world's crummiest double pane windows, but if not and they're closer to U0.5, that 21,756 BTU/hr is really probably more like 16,750, a 4000BTU/hr overshoot at 60F delta, but if using the +15F outside design temp is about a 5K BTU/hr overshoot.
Most of the windows are original victorian - 23 windows in total in a 4 floor section. Most of these windows are pretty big - often 3' wide by 5' high. Perhaps 35 are even 5.5' high. Single pane with wood sashes. Nearly all of them have storm windows but they are metal frame single pane and probably leak a fair bit of air around the edges. I'm not even sure if they are better than nothing at all.
The addition on the back is a mixed bag for the windows. 5 double pane aluminum frame sliders, a few single pane wood windows - one without a storm (storm track was all messed-up). 2 high efficiency Okna windows that we put in. Some large picture windows - with a whole bunch of small sections - but they are older double pane, wood frame. We did a LOT of spray foam insulating this past spring in that section and that is heated/cooled only using the Fujitsu heat pumps (or whatever heat happens to make it's way back there).
>> To lose 10BTU/ft-hr for wall area at a 60F delta-T implies a U-factor of (10/60=) 0.17, which is a whole-wall R of (1/0.17=) R6, which implies almost no wall insulation whatsoever. A 2x4 wall with crappy fiberglass R11s or 1940s vintage rock-wool comes in at about U0.11 at a 25% framing fraction. Don't know what you've got, but if it's a timber framed wall with ANY cavity fill figure the U-factor is about 0.1, and that 27,050 BTU/hr is actually (.1/.17 x 27,050= ~16,000 BTU/hr, peeling 10K off the heat load.
House built in 1875 so some walls lack any insulation. The entire original section is all brick and masonry. Some of it is very thick - basement is below ground. 1st floor is mostly below ground and also shares 1 wall with the adjoining twin.
Upstairs apartments (2 floors, some 1100 square feet) are strictly just brick then lath and plaster. For the 2nd floor (street level - our dining room and new kitchen), we spray foamed the walls before putting up 5/8 sheetrock. It's only a .5-1" layer - which is all that would fit but should provide a good bit of air sealing for that area (500 Sq feet).
>> A ceiling losing (1920/1200=) 1.6 BTU/hr at a 60F delta-T is a U-factor of (1.6/60=) 0.027, which is (1/0.027=) R37 (a credible number!)
We air sealed all penetrations to the attic w/ fire-rated foam, then filled the attic as high as possible with blown fiberglass (mansard roof construction - just a low pitched attic area - highest point is perhaps 4'). In the middle the insulation is quite thick and much less so at the edges - but still probably 8-10". We also did have them put in a wind-powered fan up there, and they installed baffles to allow the soffits to breathe (yeah I know they aren't supposed to in this type of roof - but they have so many cracks that they might as well be modern from that viewpoint).
>> Infiltration losses of 37,875/hr at 60F imply a leakage rate of [37,875/(60 x 0.018)= 35,069 cubic feet per hour, or 584 cfm, which is an INSANE amount of air leakage for a house that has glass in the windows, and doors that close. The simple models overestimate by about a factor of two unless the leakage points are true big round unimpeded holes. Leakage thorough cracks or fiber insulation has a heat-excanger effect, which lowers the conducted losses at those points. The air doesn't enter the conditioned space at the outdoor temp, nor does he exiting air leave the sheathing at the indoor temp. Assuming you don't have multiple open undampered flues, after air sealing, probably cut that that 38K number to something under 19K using the crudest possible model, and the real number is probably more like 12K, a reduction of 26K.
Hmmm! Maybe we should have a follow-up Blower Test? We had one done about 18 months ago with an energy audit - before we started all that attic insulating and followed-up with spray foam. At that time, our place was very leaky. We are very confident that it is much improved. This summer was MUCH more comfortable.
Maybe we would benefit from a proper heat loss analysis, now that we have changed so much. The 75K btu figure I came up with made me nervous. It just seemed so LOW, you know? So I changed the design day from 15 to 10 F.
>> A floor losing 6BTU/ft implies a very cold basement or vented crawlspace with no foundation insulation and little to no insulation between the joists, but OK let's just leave it
Hmmm well much of the basement is entirely below grade. The original victorian part is about 18x40, with 18-24" thick masonry walls, coated in some kind of plaster. The floor is concrete. The 400 square foot addition basement in the rear is all block construction. There isn't any insulation on the walls or floor anywhere. I'm planning to install pieces of 2" foam board up between the joists around the perimeter - especially in the addition area, where it's more leaky, and it's exposed on 3 sides. The original basement area is only exposed on 1 side and even then not much of that.
A FEW of the heating pipes down there have aspestous wrapping (and we leave that alone). We could wrap all the other pipes but if we do that we'll have a cold basement right under the 1st floor living area - so our thinking is let it stay at about 68-70 (big heating pipes but no radiators), and it'll help keep the floor above the basement (1st floor) warmer. That's my thinking, anyway.
Plug loads, and live humans will usually take at least 1000BTU/hr of the load.
So starting with your 82K number...
... reducing by 4K for window adjustments leaves you at 78K...
...reducing by 10K for wall-insulation adjustments you're at 68K...
... reducing by 26K for infiltration adjustments gives you 42K...
... peeling off another 1K because it's an occupied house gives you 41KBTU/hr. Literally half what you estimated.
Not perfect, but that's the likely order of magnitude. Upsize it by 25% if you like "just to be sure" and you're in the low 50s, which would still be a credible number (but a number that you can cost-effectively work on.)
Given all the additional info ... would you still stand by these or am I perhaps more like 60, 65,70K?
Most tight 2x4 homes with insulation and storm windows in my neighborhood come in at about 15 BTU/ft at 0F. (Where 0F is an almost rational design temp, though the 99% design temp here is +5F.) I live in a 2400' +1500' of semi conditioned basement (never drops below 65F in winter) 2x4 framed antique with only ~R20 in the roof and triple-tracks over antique double hungs with known gaps in the wall insulation, and my heat load at the +5F design temp is under 35KBTU/hr, at 0F it's still well under 40K. When I first moved in it was closer to 50K as measured by fuel use against heating degree days, but with air sealing the grossest leaks and adding wall insulation to ~85-90% of the wall areas (some parts need to be ripped open to retrofit properly) it brought the heat load down to about 40-42K, then with further air sealing and putting 3" of reclaimed roofing foam on the foundation & band-joist it's now a bit under 35K. I'm radiation-limited to about 43-44K at the fixed temperature I'm running the system, but this place has handled -5F and slightly lower outdoor temps without losing ground.
Even at 40K for 2400' of fully-conditioned space is 17 BTU/ft, 35K for 2400' is 15 BTU/ft, but if you counted the 1500' of semi-conditioned basement thats under 10BTU/ft, and that's at +5F. With any reasonable weatherization you'd surely be between 10-15 BTU/ft @ +15F, or ~59K max. Sizing the boiler any larger than that would be a waste, and you'd probably never actually be cold with a 50K condensing boiler.
If you have a mid to late winter oil bill with a K-factor stamped on it (or gallons between exact fill up dates and a zip code for looking up weather data) it's easy napkin-math to put an upper bound on the likely heat load- it's a good stake to put in the ground that has to be explained away if it differs much from what other heat load calculations come up with. (A 3 week vacation in FL with the thermostat turned down to 50F might be one such explanation, if it comes in well under the other calculations.
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82K for 3900' is 21 BTU/ft, which is a pretty rare condition at +10F for a home with double panes and at least some insulation.[/QUOTE]
Will this help? On a really cold month we can blow through 200 gallons of oil in 3.5 to 4 weeks. But we didn't air seal and re-insulate the attic until this past March. And didn't add spray foam on 2nd floor walls and rear addition until this past May.
When I buy oil I just fill it up, via COD. There is no "K-factor", whatever that is. I just fill the darn thing and hand over a bunch of cash. Usually about $600-700.
Thanks!
Jeff