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  1. #1
    Peloton Dog patentcad's Avatar
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    Question for any HVAC experts here

    If an efficient oil boiler consumes an average of 10 gallons weekly in a 'shoulder' period like late October (10 year old well insulated modern house with Anderson Windows), how much more will it consume during the peak heating months of Dec/Jan/Feb. 50% more? 100% more? The house seems to retain heat quite effectively - after 9PM when the timer turns the thermostat down to 62, the house is still at 70 at 9PM. On a 27 night recently, the house only dropped to 66 by 5AM after 8 hours with zero heat from that zone. So I'm thinking that in January we'll need more oil to heat the place, but not twice as much.

    Any thoughts on this?

  2. #2
    T-Shirt Guy ehidle's Avatar
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    Well, the heat flux out of the home depends on the difference in temperature and the resistance to heat flow. The larger the difference, obviously, the faster the heat flows out.

    As the temperature falls inside the house, if the outside temp stays the same, the heat flow out of the house slows. This create a differential equation.

    If you're only losing 4 degrees overnight on a 27 degree outside temperature, your house is doing a most excellent job of keeping the heat inside.

    To really accurately calculate it, you'd need to know the total square footage of the house, the length of all the exterior walls ( basically to find the total square foot surface area through with the heat can flow out ), and the R-value of your exterior insulation.

    I would think there has to be an online tool somewhere for calculating this.

    One thing you can do is look at how many degree-days you're incurring per week right now, and figure out how many gallons per degree-day you're using. This is a very reasonable approximation and avoids a lot of math you probably don't have any interest in dealing with. So, if you're getting 50 degree-days per week at 10 gallons, then if you went to 100 degree-days per week, you'd use about twice as much.
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  3. #3
    aka Phil Jungels Wanderer's Avatar
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    You can actually break it down to "Use Per Degree Day."

    A 1 degree day is a day where the average temp is 59 degrees F. A 5 degree day would be 55 degrees F.

    Once you figure your use per degree day, it's just straight arithmetic.

    Historical degree day data is available at many places for your locale. Both real history, and average history.

  4. #4
    Peloton Dog patentcad's Avatar
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    Quote Originally Posted by Wanderer View Post
    You can actually break it down to "Use Per Degree Day."

    A 1 degree day is a day where the average temp is 59 degrees F. A 5 degree day would be 55 degrees F.

    Once you figure your use per degree day, it's just straight arithmetic.
    That's assuming a direct arithmetic relationship between oil consumption and avg. temperature, and I don't think it's nearly that simple. Regardless, your post doesn't explain how you'd calculate this. Is there some website that lays this out more completely with examples?

    For instance are you saying that if the low temp for the day is 27F and the high is 47, the average temp is 37F and that's 23 degree days? So if the average degree days over two weeks is closer to 12 degree days and in the winter it's more like 35 degree days, you'd use 3x the oil? I really don't think that's how it will work out, but we shall see, won't we? I'll be keeping very close tabs on this, I may start keeping a log of the average temps too.
    Last edited by patentcad; 11-04-08 at 03:08 PM.

  5. #5
    T-Shirt Guy ehidle's Avatar
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    Well, I believe thermal flux through a resistive barrier is approximately linear, meaning that heat leaves the house twice as fast for a 20 degree difference (indoor/outdoor) than it does for a 10 degree difference.

    Degree days do make the sometimes invalid assumption that the mean temperature is the same as the average of the high and low, and that's not always true.
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  6. #6
    Peloton Dog patentcad's Avatar
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    The next phase would be spraying closed cell foam insulation in the attic. The contractor swears that's a minimal savings of 30% lower oil consumption, but I'm skeptical. I have to go through a winter with the new boiler setup to see how much that saves us from the one it replaced first. I think I could ultimately get us down to <500 gallons of oil annually. Last winter was 1477. Hoping for 800 gals or less this winter. It's certainly starting out well, this boiler really sips the oil, throws out heat long after the burner has shut off, and burns very clean (no soot at all on the chimney).



    Ductless mini-split in basement replaced two old style casement in the wall A/C units that were bleeding cold air in the winter. I've had the thermostat on 60 and the heat zone down here has yet to kick on once. Lowest the temps have dropped at night in my finished basement is 64, and it has consistently been 27-40F at night here. So taking out those old A/C units allowed us to button it up down here. We turn the heat pump on for an hour or two in the AM, and the office warms right up to 68-70. Then we turn it off and it holds those temps all day. I'm sure we'll be using the baseboard heating once it gets colder down here, but not that much. It's not hard to heat/cool a space that's largely underground.



    What we have done is replace a boiler with a much more efficient unit and we're just about removed 1/3 of the square footage from the heating chores of the boiler most of the time. Hoping that cuts oil consumption in half.
    Last edited by patentcad; 11-04-08 at 03:27 PM.

  7. #7
    T-Shirt Guy ehidle's Avatar
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    I remember you had a detailed thread about your new system a while back. You really went all out, which is cool. I remember some 2000-gallon winters in New Paltz growing up. Our house was built in '74 and was not very economical, but it was also small - only about 1500 square feet. Oil was $0.80/gallon back then.

    Last year, I used about 250-300 gallons in my townhouse. Even at that rate, it is hard to justify the expense of upgrades because anything I do might save $100/year and cost $3000.
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  8. #8
    Peloton Dog patentcad's Avatar
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    Quote Originally Posted by ehidle View Post
    I remember you had a detailed thread about your new system a while back. You really went all out, which is cool. I remember some 2000-gallon winters in New Paltz growing up. Our house was built in '74 and was not very economical, but it was also small - only about 1500 square feet. Oil was $0.80/gallon back then.

    Last year, I used about 250-300 gallons in my townhouse. Even at that rate, it is hard to justify the expense of upgrades because anything I do might save $100/year and cost $3000.
    All out would have been the Geothermal system we priced @ $58K. That didn't make sense. If oil goes to $5+ per gallon (always possible, it was there last July) we might retrofit the upstairs with geothermal. We'd probably stay with oil heat and put in a whole house 16SEER air to air heat pump A/C unit and spray foam the attic, which could drop us to <500 gals for a 4500 sf house (1300 sf of that is a walk out basement, so it's pretty easy to heat/cool as described above).

    Man, I really got into this. It was very complex at first, but after talking to 4-5 contractors and doing a ton of research, I learned quite a bit. I certainly found out that America can heat its homes with 1/2-1/3 the fossil fuel it uses now - or ultimately use zero oil or gas to do it. Ultimately it can be geothermal, which only requires electrical power, and if home solar systems get optimized, most of that power could come from the sun. That would be very cool indeed, and would alter the energy/greenhouse gas equation in an amazingly positive way.

    The more you research this stuff, you learn that if you can get cars/home heating/cooling into the power grid, your energy options are wide open to renewable sources that can't be employed using the old gasoline engine/fossil fuel fired furnace models.

  9. #9
    T-Shirt Guy ehidle's Avatar
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    Oh I know what you're talking about. The company I work for is huge on environmental controls and energy conservation for homes and buildings.
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  10. #10
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    Just for the record, the Heating degree days (HDD) is calculated as the difference between the average high and low temperature for the day, and 65 degrees F. The house's loss of heat is linear with respect to temperature, so if the HDD double, expect about double the oil consumption....maybe more if it more windy during the colder period.

  11. #11
    gmt Grumpy McTrumpy's Avatar
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    What size nozzle does your burner have? 10 gallons per day in October seems like a lot.

    I think my riello has a .75 gph nozzle although I seem to recall that riellos operate at double the rated flow. It is not fired nearly that much in October, I'd estimate less than 3 hours total per day even on a cold fall day which would be less than 5 gallons.

  12. #12
    Peloton Dog patentcad's Avatar
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    Quote Originally Posted by Grumpy McTrumpy View Post
    What size nozzle does your burner have? 10 gallons per day in October seems like a lot.

    I think my riello has a .75 gph nozzle although I seem to recall that riellos operate at double the rated flow. It is not fired nearly that much in October, I'd estimate less than 3 hours total per day even on a cold fall day which would be less than 5 gallons.

    It's more like 10-12 gallons per WEEK so far, I don't think I typed it wrong, read it again. Should be about 48 gallons through 4 weeks. More like <2 gallons per day. 4500sf house.

  13. #13
    aka Phil Jungels Wanderer's Avatar
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    Yes,. it is really that simple. The National Weather Service keeps those records, and it is the average of the temp for a 24 hour period.

    If you know that your heating plant was turned on, and you divide the gallons used by the number of degree days for that period, you will have your use per degree day. Best results are calculated during a period cold enuf to utilize the heating plant every day. Then, it will remain pretty constant for the use per degree day.

    It is common practice for utility companies to use this calculation for estimating bills.

    If you also use that same form of energy to cook, dry clothes, heat water, you must first subtract that. The "Base Use" as it is called, is calculated by taking your normal use during summer months (no heat load) and subtracting it (or adding it) to the estimated bill.

    It is amazingly accurate, as you have to put X amount of energy into keeping your house livable (at a constant temp) no matter what extremes ar outside. You need X amount of energy to maintain 70 degrees in a house when it's 55, more when it is 30. But, the amount is the same "per degree day."

  14. #14
    gmt Grumpy McTrumpy's Avatar
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    yeah you said weekly, sorry.

    In any case, the burner burns oil at a specific rate no matter what the temperature outside is. The only important variables would be the BTUs/hr of your burner (nozzle size) and the time per day that it is actually fired.

    Petro-diesel = 130,500 Btu/gallon (36.4 MJ/liter or 42.8 GJ/t), so once you establish your consumption rate in BTUs and efficiency losses, you simply have to time your burner to figure out how much oil you are consuming.

    I replaced the old unit with a high efficiency unit when I moved in here and have been consuming somewhere between 1400-1600 gallons per year (our winters are a lot colder up here) with a 3200 sq ft house.

    I'd say 800 or less is doable if you set your thermostats to "uncomfortable". I know I cannot get that rate of consumption since I can't stand being cold. I also leave the boiler on all year for hot water.

  15. #15
    Peloton Dog patentcad's Avatar
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    Quote Originally Posted by Grumpy McTrumpy View Post

    I'd say 800 or less is doable if you set your thermostats to "uncomfortable". I know I cannot get that rate of consumption since I can't stand being cold. I also leave the boiler on all year for hot water.
    We have five zones. The basement office is one zone, I leave it on 58, I use the mini-split heat pump to take the chill off down there, in the dead of winter I'll only crank that zone up during work hours, and that doesn't take too long, the basement office is mostly underground, doesn't lose heat easily, not really exposed to wind (it's walk out, plenty of windows, but still mostly buried and wind sheltered). The upstairs zones are all on programmable thermostats and they're essentially 62 15+ hours daily, those thermostats go to 70 for a few hours in the AM and then in the PM between 3-8PM, when they go back down for when we go to bed at night. And our boiler provides hot water year-round, but one of this boiler's amazing efficiencies is that it only uses 100-150 gallons of oil annually for hot water production (40 gal tank).

    We'll see how this goes, but I'm optimistic. If oil subsequently goes to $4-$5/gallon I'll start implementing other measures to reduce our consumption further. But if I can get it from 1477 to 800 I'll feel much leaner and greener, that's for sure. From a comfort standpoint, this boiler is amazing, whisper quiet, the heat comes up very fast and it really is as close to zero emissions as an oil boiler is ever going to get. It has a control panel on it that tells me what zones are calling for heat at any given moment. What I'd really like is a master panel upstairs that lets me control all the zones from one console. Expensive boiler but worth the money.

    After all it is the Ducati of boilers.


  16. #16
    Look! My Spine! RubenX's Avatar
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    This thread gives me headache and reminds me of a course I failed at college. It was called thermo-something.

  17. #17
    Peloton Dog patentcad's Avatar
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    Quote Originally Posted by RubenX View Post
    This thread gives me headache and reminds me of a course I failed at college. It was called thermo-something.
    Then this thread will give you an aneurism.

  18. #18
    gmt Grumpy McTrumpy's Avatar
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    I would not worry about 4-5$/gal. any time in the very near future. The bubble burst and things have gotten back to the realm of sanity (almost). You have to add a percentage for retail markup but heating oil is still pretty far from $4 retail. I would not expect traders to bid the heck out of oil contracts anyway given the current state of the stock market. Too much uncertainty. Look for gold to go up.

    http://www.wtrg.com/daily/heatingoilprice.html

  19. #19
    Peloton Dog patentcad's Avatar
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    Our current oil price the last time I checked (last week) was $2.68/gallon. I do believe it continues to trend downwards. I just paid $3.10 for 120 gallons they delivered as recently as October 10th. That's less than we were paying last winter, but now we're using half the oil. That helps.

  20. #20
    Village Idiot
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    I guess you turn it off at night. But why not set your house to about 60 degrees? You could have the house retain this temperature, and it's winter so wearing a sweatshirt or a sweater indoors isn't a bad thing. It's friggin winter you don't have to be in shorts and a t-shirt indoors.
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