Advice on converting oil boiler to gas, adding indirect water heater

Discussion in 'Boiler Forum' started by embepe, Mar 1, 2013.

  1. embepe

    embepe New Member

    Messages:
    4
    Location:
    Quincy
    Greetings all,

    I am new to this forum, looks like a wealth of info.

    General question - any opinions on this approach to improve enviro-friendliness and energy efficiency of boiler and water heater?
    -- Convert our 10 year old Smith 8 Series boiler from oil to gas; measuring at 83% efficiency (When new, rating was 86%) By conversion, I mean literally keeping the boiler but changing the fuel mechanism. We are told this is possible with this model, but don't know if it would be more effective long-term to replace the boiler altogether. Wish we never used oil, but we did.
    -- Replace 9 year old traditional gas water heater with an indirect-fired SuperStor Ultra (45), connected to boiler
    -- Include a hydrostat / outdoor reset control to modulate water temps with outdoor temps
    -- A few details: We have hot water, radiator heat, which we love. Fairly low energy use, heat partly with a wood-burning insert with radiator heat as needed, and to keep the house at least at 50.
    -- For our 2-adult, 1800sf conditioned area, our energy use is currently 145 therms for gas (water heating, clothes dryer, cooking), and about 250 gal, of oil per average year.

    Specific questions -
    -- Does anyone have a way to estimate the CAE (combined appliance efficiency rating) of the converted gas boiler/indirect water heater combo? I am assuming the gas boiler will run more efficiently than the oil, we are told 20-30%, but that sounds high. No idea how to estimate EF or efficiency of the indirect, except that is supposed to lose less than half a degree an hour such that the boiler may only come on once a day to heat the water in non-heating months.
    -- Based on some energy-use charts, it doesn't look like a new energy-efficient standalone gas water heater would be more efficient than the indirect/gas boiler arrangement, even at the highest .7 EF. Am I wrong?

    Thank you all in advance for any comments of guidance.
    Last edited: Mar 1, 2013
  2. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,231
    Location:
    Maine
    The conversion is simple and straight forward but if you have a choice of gas burners I would go with a carlin EZ. Adding outdoor reset and an indirect will bring your system up to much better efficiencies and be a lot less expensive than changing it out.
  3. embepe

    embepe New Member

    Messages:
    4
    Location:
    Quincy
    Oh, good to know... that's what was recommended. Are you saying that using the converted boiler with an indirect would be more efficient than a new, separate gas burner and new, standalone/traditional gas water heater? Thanks.
  4. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,231
    Location:
    Maine
    Not more efficient but for the dollar amount saved by changing the boiler the payback would be many many years if at all. You can go here www.nora.com and find free software that will compare various units, efficiencies and costs but having run the numbers and if your estimates are in line the most cost efficient thing would be to convert your boiler especially if its in good shape. For that matter, simply adding outdoor reset and an indirect would most likely be your most cost/efficiency option keeping your oil burner. Gas is less expensive for now but the price will increase sooner or later and gas is always less efficient than oil because gas burns at lower temperature with less btu's than fuel oil. Add the cost of removing your oil tank, and running a new gas service and our payback time increases substantially.
  5. embepe

    embepe New Member

    Messages:
    4
    Location:
    Quincy
    Thank you - gas service already in place nearby, so hopefully that helps our bottom line.
  6. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    Your original information is correct.

    Fuel cannot be defined by "efficiency" though choice of fuels can have an effect on the ultimate efficiency of your heating system. For instance, 96% combustion efficiency can easily be achieved while burning gas in one of more of a dozen condensing gas boilers on the market today, all of which feature outdoor reset as standard equipment. Oil boilers, by contrast, and with few exceptions, will fire at about 86% combustion efficiency, if installed, tuned and serviced properly. The operating cost of any boiler is determined by the cost of fuel per therm and the efficiency of the burner plus the control and distribution strategies.

    With a conversion from oil to gas you will enjoy cleaner burning and perhaps longer life of your existing boiler, but you will still have a hot chimney and be drawing conditioned (air you paid to heat up) from the house and mixing it with the byproducts of combustion thence to the atmosphere to condense in clouds...acid rain.

    Your current boiler may last another 20 years with some care, but the combustion efficiency will be in the low 80's. This can and should be confirmed by the technician when he installs the factory approved gas conversion gun. The only way to confirm proper efficiency is with a combustion analyzer. This test will reveal a stack temperature of 350°F, about a hundred degrees under the old oil burner and saving about 1% of the fuel.

    Once you have the best boiler you can afford an indirect-fired water heater is the only rational choice. Unlike a conventional tank-type water heater, the typical indirect-fired water heater is simply a storage tank with a heating loop or coil coming off the boiler. Since there is no stack coming off the top of the unit, there is no parasitic draft to cool the heated water between burn cycles. Most are stainless steel and feature foam insulation and as you suggest operate once day or two. Even better, the average indirect will last 30 years--about three times as long as a standard glass-lined tank water heater with standard 6 year warranty. If you don't have soft water you will want a model with a serviceable coil/heat exchanger such as those offered by Crown-MegaStore, NTI-TrinStor and Buderus-ST.

    You may want to consider a new NG condensing boiler with indirect or a combi condensing boiler with built-in domestic water heater such as the Bosch GreenStar or NTI Ti.

    With your low usage, justifying a large investment in upgraded equipment is questionable from a purely financial perspective, but with comfort, fuel usage, longevity and ecology factored in, the choices are not so straight forward. Have your contractor quote the conversion, with ODR and an indirect, but insist on two other options as well. Only the smart guys can give you choices, so which ever you choose, you will generally end up with a better technician.

    "Gas is less expensive for now but the price will increase sooner or later and gas is always less efficient than oil because gas burns at lower temperature with less Btu's than fuel oil."

    Whereas it is true that there is more energy in a gallon of fuel oil than a gallon of propane (an important factor in judging between the two), converting from either to natural gas is rarely a hard choice since natural gas has historically (last 20 years) been a good value and I see no convincing evidence that this trend will change in the next 20.


    http://www.businessweek.com/articles/2013-01-10/why-natural-gas-will-stay-cheap-in-2013
    Last edited: Mar 2, 2013
  7. Dana

    Dana In the trades

    Messages:
    2,924
    Location:
    01609
    There is even LESS evidence of oil returning to it's pre Y2005 levels, which is what it would take to make gas more expensive, even with a substantial run up in natural gas pricing! The last time 86% efficiency oil was a cheaper way to heat a house than gas (even 80% efficiency, which isn't even legal to install for a new boiler in MA) in New England was in 2002, more than a decade ago.

    At current oil prices it's substantially cheaper even to heat with 25 cent per kilowatt-hour electricity (2x the national average) if you use better-class ductless heat pumps (mini-splits.) There's a chance you'll see $3 oil again (perhaps a fleeting glimpse, if the economy re-crashes) but gas would have to roughly double it's current retail-residential price levels to be more expensive than $3 oil, but the fundamentals that would bring oil that low would also set a new (perhaps insanely) low price point for natural gas.

    No matter who is blowing smoke about oil prices falling simultaneously with rising gas you'd do well to not inhale. While predicting future energy prices is always a fools errand and fraught with error, the price of oil is set by the world demand, whereas natural gas tracks local & regional sources and demand- the cost of liquifying gas for export is a huge cost-adder, which makes gas markets mostly local to the pipeline infrastructure, which is being built up regionally in the northeastern US to get the Marcellus and Utica shale plays to the NY/NE market more cheaply.

    The existing pipelines into southern New England have for decades lacked the capacity for the wintertime peak demands, so half or more of the gas in the Boston area is already the more expensive sort. Most of the fossil-fired electricity in New England is also from natural gas, so in Quincy MA, the retail price of natural gas is ALREADY maximized relative to the rest of the country, and even so it's well under half the per-BTU-delivered cost of heating with oil

    The NORA site indeed has a lot of useful efficiency information on it, but at any efficiency you can't make a rational case for NOT converting to natural gas, if there's a main on your street. The cost of converting a high-mass boiler (even a steam boiler) from oil to gas is paid for in lower heating cost within the first 1 to 1.5 heating seasons almost anywhere in New England. (I have personaly seen multiple real-world examples of exactly that which have occurred within one city block of my house in Worcester MA within the past 6 years.

    Also, by definition all oil boilers are oversized for the heat loads of small or medium sized existing houses in New England, which has efficiency consequences well documented on the NORA site. Without retrofitting heat-purging controls or buying a new oil boiler with those functions built-in, there's no way to actually hit near the tuned-up 86% steady state efficiency as an as-used AFUE. Even breaking 80% isn't in the cards for the typical 3-5x oversizing I see in my neighborhood. See Table 3. You'll note in Table 3 that even at 2x oversizing an 88% AFUE mod-con gas boiler (system #11) hits 85% annual efficiency. A right-sized 95% mod-con will pretty much hit it's numbers if the system designer doesn't bring it to it's knees by micro-zoning the hell out of it, and in the hands of a good designer with more low-temp radiation it can even beat it's AFUE tested & labeled numbers.
  8. embepe

    embepe New Member

    Messages:
    4
    Location:
    Quincy
    Thanks to all who answered. (And hello to MN! We used to live in S. Mpls. ... with a giant old beast of a boiler, I might add!)

    I think our approach is going to be to go with the conversion to gas boiler (Carlin EZ) with an indirect water tank attached. The existing Smith boiler has a lifetime warranty and seems to be in good shape. The water heater needs to be replaced anyway.

    Thinking that in 10-15 years we can spring for the best mod-con boiler available then if budget and circumstances allow. The extra $ to fully replace a respectably functional boiler now could be better used (we think) toward other energy improvements that will make the whole house "work" better with the heating energy it uses - i.e., air sealing. The house is already well insulated and our basement pipes all have (encased) asbestos wrap.

    Any pro's / con's on that approach?

    Thanks again.
  9. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,231
    Location:
    Maine
    i think thats the best approach but then again I said that earlier. Be sure to add outdoor reset to the conversion. I like the Taco PC700
  10. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    We replaced your old boiler in Minneapolis with a Bosch combi Greenstar last week!

    Every system is limited by funds. A new gas gun and indirect will do a good job as Tom suggests.

    Attached Files:

    Last edited: Mar 8, 2013
  11. jefferson17

    jefferson17 New Member

    Messages:
    34
    Location:
    Bristol, PA
    Advice would be appreciated for reasonable estimates etc

    We're in PA - about 25 miles North of Philly and are in a similar situation. We've got a 10 year old Weil Mclain boiler that is way oversized - 240K. It was there when we bought 4.5 years ago. We can finally get gas now and I've placed the order w/ PECO.

    I've done several detailed heat loss analysis workups and I get about 82000, while trying to be at least a bit conservative. We did a lot of air sealing and insulating this year, starting in the spring. The summer was much more comfortable than prior years. Total sq footage is 2950, 4 levels and 3 zones (zone a is front of house, zone b is middle of house and zone c is rear - only 2 floors not 4). Rear of house also has very new fujitsu split heat pumps, that kept it comfortable solo (that zone was drained out for last 18 months).

    I'm trying to get some quotes from local guys here and would appreciate some basic figures, parts and labor separately - if we go with the following:
    - Carlin EZ conversion
    - Outdoor reset
    - indirect water heater, perhaps a 40 or 50 Amtrol? They seem to be less expensive and ppl say good things about them (current boiler doesn't seem to have integrated connections for this so if we do go w/ one vs. a standalone gas water heater we'd need a small separate circuit built for that). There is plenty of room next to the boiler.

    Also - does anyone have really good figures on recovery value for an indirect water heater vs a high recovery 50 gallon stand-alone gas water heater? We can get a Rheem 42vr50-40f for under $600 delivered.

    Thanks very much in advance, for your assistance!

    Jeff
    Last edited: Sep 18, 2013
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,153
    Location:
    New England
    I'm not sure I believe your heat load analysis! Keep in mind there is absolutely no reason to make it bigger to heat water when you use an indirect; plus, using the design day temperature will keep you warm, especially since two things: the house doesn't immediately become a freezer if it is that coldest day in 100-years, and as long as the boiler you choose is large enough to do it the other 10,000 days, it will likely have some extra capacity anyways, since they are hard to buy at exactly the size you want anyways.

    An indirect will outperform almost any standalone WH (except maybe a few commercial units) since when it does need heat, it gets the full capacity of the boiler (when installed in the normal manner as a primary zone) which is bigger than most standalone burners for WH. Size the thing for your biggest anticipated draw, then let it reheat when it can. Bigger is better if you have say a large soaking tub, or you'll have multiple people showering at the same time, or are using a car wash for a shower (i.e., multiple showerheads).

    If I were a moderator, I'd split this off into your own thread...it's not the best idea to piggy-back others! You may not get the attention since the previous info doesn't apply, people may never get to your question as they get bored.
  13. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,231
    Location:
    Maine
    I thought you were a moderator
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,153
    Location:
    New England
    Lost that somewhere along the line...
  15. jefferson17

    jefferson17 New Member

    Messages:
    34
    Location:
    Bristol, PA
    I wasn't adding anything to the heat loss calc for an indirect water heater. Those are simply the #s, with some wiggle room that came out of a detailed excel spreadsheet. I didn't trust the "totally accurate" figures I punched in, so I gave the house in the end a design temp of +10 (ODT conversion of .86 instead of +15 (ODT of .79 for 74,735 heat loss). And that gave me the 81356 BTU. Maybe the 74735 is totally accurate but it just seemed a touch low to me, and I was anxious about the possibility of undersizing instead of "1 size up".

    Calculations
    (Factors found in "Heat Loss Factors" Tab at Bottom)
    Multiply x Factor = BTU/Hr
    (A) Window and Doors: 555 Sq. Ft. 39.2 21756
    (B) Net Wall: 2705 Sq. Ft. 10 27050
    (C) Cold Ceiling 1200 Sq. Ft. 1.6 1920
    (D) Infiltration 30300 Cub. Ft. 1.25 37875
    (E) Cold Floor 1000 Sq. Ft. 6 6000
    and/or 0
    (E) Cold Floor 0 Lin. Ft. 0
    BTU per hour heat loss at 0 degrees Outside Design Temp 94601
    ODT Conversion (if other than 0 degrees): x 0.86
    Adjusted BTU / HR 81356.86

    So you are probably correct that my 82K (rounded up) is still high, but unless someone can convince me that sizing to about 75K is still safe, I wouldn't think that I'd use so much more fuel and would sleep better.
  16. Dana

    Dana In the trades

    Messages:
    2,924
    Location:
    01609
    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.

    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.

    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!)

    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.

    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

    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.)

    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. :) )

    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.
  17. Dana

    Dana In the trades

    Messages:
    2,924
    Location:
    01609
    Just saw your post indicating that it's "mostly brick". What is the wall stackup on the brick, and what U-factor did you use?
  18. jefferson17

    jefferson17 New Member

    Messages:
    34
    Location:
    Bristol, PA
    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.

    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.


    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. :) )

    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
    Last edited: Sep 25, 2013
  19. Dana

    Dana In the trades

    Messages:
    2,924
    Location:
    01609
    I'd have to run the real numbers on the real construction to come up with a sort of real heat load number. The type of brick matters when it's 18-24" thick- something difficult assign a U-factor to, or even a range, but it's probably at least R5 (U0.20) from a thermal mass dynamic modeling point of view, and could be much higher. I assume this is a town house, and the other side of the common wall with the "twin" is a heated space (U-factor = 0.)

    But a couple of comments:

    Any single pane windows with leaky or absent storm windows can cost-effectively be upgraded with tight-fitting low-E storm windows. Even though they are more expensive than clear glass windows, the payback is 5 years, not 10 due to the higher performance. The as-is clear glass storms+ single panes have a U-factor in the U0.5-U0.6 range. Tight low-E storms over those antiques would deliver U0.31-U0.34. Harvey makes the tightest storm windows in the biz, and have a low-E glazing option. The Larson low-E storms sold through the box store chains don't suck if you spring for at least the "Silver" version (the low end "Bronze" series leak a lot more air.)

    In the basement it's far better to put the foam on the exterior foundation walls that to cut'n'cobble between joists. This is for several reasons: Putting it on the foundation walls brings the joist edges and boiler completely inside the conditoned space, which means the wood stays warmer (= drier), and the standby losses of the boiler accrue to the conditioned space. Furthermore, it's damned near impossible (even with copioius amounts of spray foam) to really air seal at the basement-ceiling, but fairly straightforward at the foundation wall.

    Unless you're using fire-rated Thermax you'll be required to put in a thermal barrier like half-inch gypsum over any wall -foam. An inch of foil faced iso and a 2x4 studwall with unfaced R13s might be cheaper, and would deliver ~R16 whole-wall (U0.06) after factoring in the thermal bridging of the studs. Keep the bottom edge of any polyiso off the slab, as well as the bottom plate of the studwall by putting it on an inch of EPS as a thermal & capillary break. (Polyiso can wick ground moisture, as does wood. EPS won't.)

    200 gallons on 3.5-4 weeks of "...really cold month..." isn't enough to go by, unless you have the EXACT fill up dates and a complete fill-up volume, so we can look up the heating degree-days for your zip code. But let's play the game anyway. The binned hourly mean temp for January in Bristol is ~32F according to weatherspark.com data, so lets assume a really cold month averages about 30F. That means that your heating degree day averaged (base 65F) 65F-30F= 35 HDD/day. You're burning something like 200gallons/25 days or 4 gallons per day.

    In an 85% burner that 4 gallons is delivering 0.85 x 4 gal x 138,000 BTU/gal= 469,200 BTU per day, or an average of 469,200 BTU/24= 19,550 BTU/hr at 35F below the presumed 65F heating/cooling balance point (it'll be close enough).

    That's 19,550/35F= 559 BTU per hour per heating-degree.

    At an outside design temp of +15F you have 65F-15F= 50 heating degrees, and a heat load of 50F x 559= 27,950 BTU/hr

    At an outside design temp of +10F it's 55 heating-degrees, and an implied heat load of 30,745 BTU/hr.

    In order for the heat load to be 2x that would have requires an average outdoor temp below 0F, which clearly didn't happen, or you were really looking at 2-weeks of oil use, not 3.5-4 weeks. No matter what your heat load for the zones heated by oil is probably under 50K, and is likely under 35K, and may even be under 30K. The smallest of the line ~50-60K mod-cons or or semi-smart 3-plate cast iron boilers are probably your best bet. But see if you can't nail down some exact dates & volumes on the oil fill-ups base on the real hdd for those dates at a weather station near you. In gross terms, if you're using "only" 700-800 gallons/year or even 1000 gallons/year your heat load at +15F simply can't be 75K or anything close to that:

    An average year in Philly sees fewer than 5000 HDD. At 1000 gallons and 5000 HDD that would be 0.2 gallons/HDD which is about 980 BTU/degree-hour. 980 BTU/degree-hour x 50 heating-degrees is 49,000BTU/hr, the +15F heat load. 980 BTU/degree-hour x 55 heating-degrees is 54,000BTU/hr, the +10F heat load.

    Are you still thinking 75K is "low" heat load number?

    If you can, add up the total fuel use in the past year. If it's under 800 gallons, any 50K boiler (condensing or otherwise) will cover your actual loads, assuming you didn't spend January in Belize with the thermostat turned down to 45F.
  20. Dana

    Dana In the trades

    Messages:
    2,924
    Location:
    01609
    OK, now that I've had my PM coffee I'm allowed to do math in my head, unlike when I wrote: "...200 gallons/25 days or 4 gallons per day."

    Uh... make that 8 gallons/day not 4. (DOH! ;))

    Which makes the +15F heat load number 55,900 BTU/hr...

    ...and the +10F number 61,490 BTU/hr.

    That's still 3-plate Burnham ESC3 (60K DOE output) territory, but I'd want better fuel use data before saying for sure. If you insulate the foundation (and not the basement ceiling) you'll likely have margin with the ESC3.

    Don't bump up to the ESC4 (90K DOE output) "just to be sure" rather, instead BE DEAD-SURE that the ESC3 won't actually cut it after a more careful analysis. If running a heat load calc on the construction's U-factors, run it on the "after" picture. The uninsulated foundation is likely to be on the order of 10K of load, a load that drops to 1K or less after insulating.

    So if fuel use analysis is telling you it was ~60K in a less-insulated condition, it'll be ~50K after just insulating the foundation walls & band joist, which is worth doing on a number of grounds other than mere heat load reduction.

    If you punted and went with the ESC4 and your post-insulation heat load turns out to actually be under 45K (a realistic possibility), you'd be more than 2x oversized, the condition you're trying to get away from. (AFUE testing is done at 1.7x oversizing for the load, which is the maximum you'd ever want to be.)
    Last edited: Sep 25, 2013

Share This Page