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Comparison of DHW Oil Indirect vs Electric Tank

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by velvetfoot, Sep 5, 2012.

  1. velvetfoot

    velvetfoot Minister of Fire

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    Did you replace the CC500, then?

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  2. maple1

    maple1 Minister of Fire

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    In the process of replacing. I'll be doing a report on here when I'm done - pics & everything.
  3. velvetfoot

    velvetfoot Minister of Fire

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    Fantastic. I forgot which unit you chose, or are you keeping it a suprise?

    I found some more oil bill usage, and it could be worse than I thought for the summer months where the info was there.
    (I got the house and put an insert in in 2006.)
    Should get an hourmeter for sure...


    oil usage.JPG
  4. maple1

    maple1 Minister of Fire

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    One more off-topic reply.

    Have had a Varm 40 sitting in my barn here since June. It's been a long distracting summer but time to really get down to nuts & bolts - er, pipes & fittings & stuff. It's starting to get cooler at nights - good thing I started this way back in May.
  5. velvetfoot

    velvetfoot Minister of Fire

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    Awesome!
  6. velvetfoot

    velvetfoot Minister of Fire

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    Over the past eight days I've used an average of 13.2 kwh/day, which projects out to 396 kwh a 30 day month, which is 131 kwh more than last September's number. This comes out to 1572 kwh /year for the water heater, which is considerably lower than the 4773 kwh on the yellow label. At my electric rates, that comes out to $236/year which is a savings of $800/year over oil, which I calculated at .74 gallons/day. These numbers would seem to justify the move. Hopefully they're in the ballpark.

    Did I mention that it is totally quiet? :)
  7. DBoon

    DBoon Minister of Fire

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    Hi Velvetfoot - the savings you are getting with the electric hot water heater are real. I tried to find a previous post where I worked out the math, but I think my assumptions were electricity at $0.11/kWh (overnight rate), electric hot water heater at 100% efficient (assume standby losses the same for both electric and oil) and oil heated hot water at 65% efficient. I found that ~$2.75/gal of oil was the point above which electric was cheaper. I re-calculated below.

    Calculations:
    Oil at (140,000 BTUs/gallon / $COSTOIL/gallon) x 0.65 efficiency = 91,000 BTUs/$COSTOIL
    Electric at (3414 BTUs/kWh / $0.11/kWh) x 1.00 efficiency = 31,036 BTUs/$

    (91,000 BTUs/$COSTOIL)/(31,036 BTUs/$) = $2.93 per gallon of oil as the break even point. Many have already stated that the efficiency should be in the 30 to 50% range, and if you figured that, the break even $/gal of oil would be $1.35 to $2.25/gal (assuming $0.11/kWh for electricity). If you assumed 90% efficiency (maybe possible with the most efficient and newest of systems, but not likely) for oil, then the breakeven point would be $4.05/gal - which goes to show that it is hard to imagine why anyone would heat DHW with oil unless their electric rates were sky high, and if they were, I would just encourage them to get a HPWH to halve the effective cost with electric. This doesn't count the cheaper installation and maintenance costs for electric, of course.
  8. TheMightyMoe

    TheMightyMoe Minister of Fire

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    http://nepacrossroads.com/fuel-comparison-calculator.php

    BTU calculator.

    .25 Cents per KW vs 4 per gallon oil for me.

    I'm on oil =/ Even with stand by losses oil is cheaper for me, and since the majority of the year is heating/shoulder, the standby heat just goes into the house, no real loss there.
  9. velvetfoot

    velvetfoot Minister of Fire

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    The basement gets pretty cool over the years with burning the wood insert upstairs, so I think I'm going to stick with the electric water heater over the winter too. It'd be nice if propane got cheaper around here. I have to check the price.

    Still higher than oil here, with higher efficiency for propane boiler, nevermind less maintenance. Still tempting to switch if the oil boiler craps out: cleaner, instant hot water/heat possibile (no/less standby loss), tank outside for more room inside, standby generator).

    oil: 3.92/gal
    propane: 2.96/gal
    electric (net): .15/kwh
  10. maple1

    maple1 Minister of Fire

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    Quite a bit of the standby heat when heating with oil goes right up the chimney. A cold start boiler can help with that, but there will still be some heat loss to the outdoors. I'll add to this thread on tankless coil oil vs. electric once I get another power bill or two, but for now after the first one I'm estimating I'm saving close to $100/month in the summer non-heating season with the new electric tank.
  11. velvetfoot

    velvetfoot Minister of Fire

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    I have a cold start indirect and I still figured I'm using .74 gal/day. Of course, any of my numbers could be off. :)
    I've been taking daily readings off the electric meter.
  12. JP11

    JP11 Minister of Fire

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    Your numbers are similar to what I had. I probably averaged closer to 1gal. I have an 80gal indirect.. and run a recirc line on a timer quite often. I'd rather fire the boiler once ever two and a half to three days. Armload of splits is much cheaper than 2 to 3 gallons of oil.. even if I'm running 70% homemade bio fuel.

    JP
  13. velvetfoot

    velvetfoot Minister of Fire

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  14. steam man

    steam man Minister of Fire

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    I think I did this sometime ago but here are my calculations. Correct me if any assumptions/math is wrong. I am not seing the big savings with an electric water heater at oil's current price. Here goes:

    Assume 4773 KWH yearly use for hot water as per the DOE spec. At 16.4 cents/kwh, (my rate) that is $782.00/year. For oil, 4773 kwh = 16286153 btu's. Assume #2 oil is 138,500 but/gallon. At 80% efficient (mine is higher), I get 110,800 useable btu's/gallon. 16286153 btu's/110,800 btu's gallon = 146.98 gallons oil/year. At (my rate) $3.64 gallon, that is $535.00/year, $247.00 (68 cents/day) less than electric. To be equal, oil would have to be $5.32/gallon. Another angle would be that if your oil appliance was only 55% efficient, the hot water bills would be the same.

    We assumed both heaters have the same standby losses but the boiler has some. I figure a 20 gallon boiler going from 180 deg to 100 deg once a day would lose about 12,480 btu and cost maybe 40 cents/day. Many boilers have burner dampers that close to prevent stack loss as does mine. Incidentily, I have a power venter so no chimney losses. You also have operating cost for an oil boiler which, ironically, electricity is needed. I am not sure what the consumption is but I will look into it. Maintenance/installation cost are a consideration. The only way you can compare is life cycle cost between them.

    It would seem at this hot water useage example, it is a wash between the cost of oil or electric in this situation. I do have a 100 tube evac solar array for domestic water so I can't really use a real world example.
  15. JP11

    JP11 Minister of Fire

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    Wow.. the math is so much better now. If I had read that thread when I built my house.. I would have for sure put in an electric for summer months. It just makes sense. Sure, an indirect is great when the boiler is running anyway... but in summer... It's a huge waste.

    Now.. I just need to get the wife onboard with a simple fire once every 2.5 days in summer, when I'm away that is.

    JP
  16. nrcrash

    nrcrash Member

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    Just got my Geospring up and running. Now I can take my time with the vigas install and not have to worry about getting my DHW from oil!

    Attached Files:

  17. maple1

    maple1 Minister of Fire

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    How do you heat your DHW now and what is your actual annual cost? Depending on the setup, I think oil DHW heating would be lucky to be 50% efficient in some cases.

    I'm early in my evaulation, but I know that over the last 6 years or so, I was using on average about 150 gallons (Canadian) of oil for heating DHW in the off-heating season (about 6 months). My last electric bill, for which I had my electric tank up & running for close to half of the billing period (one month out of two), was about $20 more. I had a tankless coil oil boiler that had to stay hot (140-150) all the time. I admit my math is pretty fuzzy so far - I'm expecting the next electric bill will show more of an increase. But I also have yet to add insulation to the electric water tank, which is also in my plans.
  18. velvetfoot

    velvetfoot Minister of Fire

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    That's shorter than I thought it'd be.
  19. nrcrash

    nrcrash Member

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    Only 50 gallons. But it was also FREE from Ngrid!
  20. steam man

    steam man Minister of Fire

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    The funny thing is I couldn't figure my hot water consumption expense with any accuracy. I heat DHW with a 100 tube solar array year round. I also use wood to heat DHW at times along with oil. I also have a cold start boiler with 8 gallons of water in it and no chimey so there is no heat loss per say there.

    The irony of using electric to heat hot water is that it is really only about 30% efficient by the time the power company creates it and gets it to you. What they use for fuel is a big factor along with the delivery charge. With many plants changing over to natural gas (being even cheaper than coal now), electric rates have stayed flatter over time than other fuels. Certainly there is a point on the cost graph where electric is cheaper than oil.
  21. maple1

    maple1 Minister of Fire

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    There are likely big effiency losses in oil too from the time it is found in the ground in its raw form until the time it gets in your oil tank ready to burn. I think the only 'real' way we have to try to estimate the differences is to compare actual real world monthly fuel expenditures in a before & after scenario. I don't recall reading from anyone who switched from oil to electric or vice versa that oil is cheaper - but maybe I missed it and it is in certain situations.
  22. TheMightyMoe

    TheMightyMoe Minister of Fire

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    Well I can guarantee you my electric bill went DOWN about 300-400 KWH per month, and that was around 80-100$ in monthly "savings". I can take a picture of that.

    On my boiler I have a Aquasmart aquastat with a built in history.

    IN THE SUMMER

    My average stand-by burn time is 2 minutes with a 87 minute stand-by. (140 Low-limit with 20 diff)
    Showers run it for 10-15 minutes with a 100-140 stand-by afterwards (Water is at 180 instead of low limit)

    So with my .6GPH nozzle I figure around .68 hours or $2.76 a day or 95,200 BTU

    Now if you do the math comparison...


    Electric BTU = 350 KWH x 3070BTU (3412 BEFORE 90% efficiency loss) = 1,074,500 monthly BTUs total =
    So...

    Oil .68 gallons or .68x110952x30 days (138,690 BEFORE 85% efficiency loss) = 75447.36 = monthly 2,263,420 BTUs total
    So...

    OIL- Total BTU 2,263,420 / 110952 = 20.4 x $4 = $81.6
    ELECTRIC - Total BTU 1,074,500 / 3070 = 350 x $.25= $87.5

    On a BTU basis oil is cheaper, however, I do use many more BTUs due stand-by losses and inefficiency.

    So the math adds up, and if I can get rid of stand-by times (8 months of the year during swing season / winter it rarely has to stand-by), oil is considerably cheaper. (19 minutes a day, or 9.5 hours or 5.7 gallons or $22.5 monthly savings)

    HOWEVER,

    For MOST people electric will be CHEAPER due to stand-by losses. If you can negate your stand-by losses, and have a very insulated Indirect DHW, oil maybe cheaper, especially if you have extremely high usage or are using a re-circulator hot-water pump in your home. (Cycling a 3500+ Watt heater vs firing a oil boiler...) The other benefit of a indirect water heater, is they come with a aqua stat with a adjustable differential, in the winter my differential is 20*, but in the summer it is 5*. This is because in the winter, the boiler is usually hot, in the summer it is just maintaining. In the winter I can take a shower without the DHW even coming on.

    The math adds up comparative to my usage.
  23. maple1

    maple1 Minister of Fire

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    OK - with $0.25/kwh electric and $4/gallon oil, I could see where oil might be cheaper especially with heavy usage.

    Here oil is somewhere around $1.20/litre (haven't bought any since Spring though), and electric is 0.15/kwh. I think 0.25/kwh is the highest rate I've seen posted on here.

    Also, similarly to reducing standby loses on oil, they could also be reduced on electric with extra insulation. I'll be checking that out too after a couple more months, hopefully. I guess the same as with everything, YYMV.
  24. TheMightyMoe

    TheMightyMoe Minister of Fire

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    Our power plant is run off oil... So we pay for oil... Then the power lost through transmission etc... Sucks =/

    Insulation is not what I was referring to.

    The stand by losses I am referring to are from the boiler/flue not the water tank.

    In the winter, I have little / to no oil boiler stand by losses, because I am heating the home with it.

    BTU for BTU oil is generally cheaper then electric, but since you have to maintain and heat a boiler, you have excessive stand by losses, especially in the summer, where stand-by heat is NOT WANTED.

    Make sense?

    Even at your energy rates, oil is cheaper then electric BTU for BTU.

    $4.54 a gallon and $.15 KWH

    Oil @ 38.51 million btu (85% eff)​
    Electric @ 48.85 per million btu (90%)​

    The problem is turning oil into hot water VS electric to hot water is less efficient then using electric directly due to boiler stand by losses.​
    But in large quantities / no stand by losses / certain situations oil wins out.​
  25. velvetfoot

    velvetfoot Minister of Fire

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    I'll compare electric usage on my bills as time goes by; nothing else too much is changing.

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