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Cost to heat DHW with oil (tankless)... update 1 month later..

Post in 'The Green Room' started by WES999, Jul 9, 2009.

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  1. WES999

    WES999 Minister of Fire

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    Loc:
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    Cost to heat DHW with oil


    A few months ago, I was reading about how inefficient it was to use a tank-less oil hot water heater, in the past I did not give it that much thought. I did seem to be using a fair amount of oil during the summer just for hot water. It was something around 100 gal every 3 months.

    I wanted to get a more accurate measure of how much oil I was using. I picked a cheap hour meter (120V) and hooked it up to the furnace, so it would read the hours the burner was on. You can see form the spreadsheet that it takes 1.08 gallons of oil for DHW (no heating). This is and older model furnace, probably not the most efficient. I have a small single family ranch, no dish washer, wash laundry in cold water, just me living there (teenage daughter stays here on weekends). Hot water requirement should be fairly modest.

    I installed an 80 gal electric water heater I picked up on CL for $50, so far it is working well. The water temperature is much more consistent, neither too hot nor too cold.
    In the future I am considering adding solar collectors.

    I plan to monitor the on time for the electric elements; I have ordered a current switch from E-bay which will have the wire to the heater pass through it, and will turn on the hour meter when power is on to the heating elements. This should give me the information to see if the electric heater is more economical to run than the oil furnace.

    Oil used to heat DHW

    burner on time
    date burner on time hours current oil price
    5/9/2009 8:37 AM 0 $2.50 gal
    5/18/2009 5:50 AM 11.9
    5/19/2009 5:48 AM 13.1
    5/20/2009 5:54 AM 14.4
    5/21/2009 5:57 AM 15.6
    5/23/2009 6:51 AM 17.8
    5/24/2009 8:30 AM 19.2
    5/31/2009 7:22 AM 28.6
    6/1/2009 6:00 AM 29.2
    6/2/2009 5:57 AM 30.4
    6/3/2009 6:01 AM 31.6
    6/5/2009 5:59 AM 33.9
    6/16/2009 5:54 AM 45.7
    6/18/2009 5:56 AM 48.1
    6/21/2009 11:39 AM 51.8
    6/23/2009 5:55 AM 54.0
    6/25/2009 6:00 AM 56.2
    6/26/2009 5:54 AM 57.3
    6/27/2009 9:00 AM 58.5
    7/3/2009 10:00 AM 65.7


    total days 55.00 ave burner on time 1.19
    hrs/day

    nozzle 0.90
    GPM

    oil use 1.08 cost/day $2.69
    gal/day

    oil use 32.68 cost/month $81.71
    gal/month

    oil use 392.19 cost/year $980.48
    gal/year

    Attached Files:

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  2. missrobo

    missrobo Member

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    Very interesting, I always wondered the same thing since I have a tankless oil system and it is older. In the near future I'll need to replace it and wondered how I should heat my hot water: tankless, separate oil hot water tank, or electric. No gas option where I live. Let me know what you end up doing?
  3. solarguy

    solarguy New Member

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    The average family of four will spend roughly 25% of their fuel consumption heating DHW.

    Tankless heaters really are very inefficent. You would be better off to install an indirect hot water heater (superstore)
    set up as a zone off your heating system.

    Then there's always solar. At 1000.00 a year for dhw, w/ the tax incentives you're looking at a 7 year payback period @ todays price of fuel.

    Bob
  4. mayhem

    mayhem Minister of Fire

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    I have never once heard this before, can you explain why this might be because I'm not sure I understand.

    If I have a tankless water heater, I'm only running the burner when I'm using hot water and only heating as much hot water as I use. If I have a hot water tank I keep a large volume of water at a desired temperature and am stuck with the boiler cycling at least once per day when there is zero water consumption (I know that my boiler runs at least once per night in the wee hours of the morning), just to keep that volume of unused wter hot and ready for me to use.

    Not saying you're wrong, just that I don't follow you.

    I don;t have good hard data available to me, but I can tell you that I use a good full tank of oil in a year for hot water. 3 peoplein the house, but I'm the only one who takes a daily shower (wife and kid take a shower or bath every other day), dishwasher and laundry is usually cold only, but sometimes you just need hot water. We don't use warm water to wash our hands because even just 30 seconds of bathroom faucet usage is enough to make the boiler fire.

    Hot water consumption is one of my main energy consumption points, but until I have several thousand dollars to spare, I don't think I can do much about it except try to limiit my usage.
  5. stejus

    stejus Minister of Fire

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    If you think about it, DHW is free in the core winter months for those who heat their homes with oil or gas and have a tankless water heater. RE: The furnace is running several hours during the day so the water is heated as a result. The real costs are in the months where you are not using the furnace to heat the house. I estimate I use about 30 gallons a month with all sorts of use. Three baths, He Washer Machine, Dishwasher and 3 females in the house. The appliances are Energy Star rated, the Front loader washing machine uses very little water and we installed low flow shower heads. I guess every bit helps to reduce the amount of hot water used. As painful as it is, I try to teach my kids not to run the hot water like it's free. I think it's starting to sink in!

    That said, about a gallon of oil a day for hot water at today's market price, 365x$2.25= $821.25. Some may say this is high, but I'm not sure how you're going to lower this dramatically by investing in alternative options. If the cost of oil doubles, then that’s a different story.

    Maybe there's someone out here that has converted to a external tank fired by gas or oil or electric so they can give us some numbers to compare with.
  6. WES999

    WES999 Minister of Fire

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    I just did a rough estimate on the cost to run a electric hot water heater in place of the tank less oil fired one.

    est hot water per day GAL 30
    est efficiency % 95
    BTU to heat 1 gal water 553
    KWH to heat 1 gal water 0.162
    KWH to heat 30 gal water 4.86
    cost to heat 30 gal .12/KWA $0.58 per day
    cost for electric water heater $17.40 per month

    If my numbers are correct (feel free to point out any errors you see) the electric is considerable cheaper.

    $82/ month oil :ahhh: vs $17.50 month electric. :)

    comments?
  7. WES999

    WES999 Minister of Fire

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    It's not free for me, I leave the furnace thermostat set at about 55-58 and usually burn the wood stove when I am home.
    According to my estimate the electric should me significantly cheaper. When I get my poor mans "data logger" setup I will be able to make a better comparison.
  8. Gooserider

    Gooserider Mod Emeritus

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    I think the argument is that with a tankless, you are doing a lot of short cycles, each of which takes a certain minimum amount of fuel for the startup and shutdown, plus the need to heat up the non-water parts of the unit, and the fact that it is probably burning more fuel than it actually needs to heat the amount of water running through the unit at the time...

    With an indirect setup, you fire the burner ONCE to heat a large tank of water - presumably at the optimal efficiency for the burner - thus getting rid of the inefficiency of multiple short cycles, and the fact that the tankless is probably running a bigger flame than needed if you aren't drawing water at the units maximum rate... Downside might be the loss from the heated tank, but that can be minimized by good insulation practices, etc.

    Gooserider
  9. solarguy

    solarguy New Member

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    If we think about a tankless coil, basicly you're using your boiler as an on demand hot water heater. Most coils are rated for 3 GPM & they are immersed in your boilers water jacket. Everytime you use hot water your pushing cold water thru the coil which cools the boilers' water jacket & your burner fires. Also, your boiler is maintaning temperature 24/7 because in it's "mind" it needs to stay hot in order to provide DHW. Additionally, the coils plug up, the amount of hot water decreases, you complain to the oil company, they in turn increase the temperature in the boiler which all leads to more fuel consumption.
    With fuel oil hovering in the 2+ per gallon range, most folks have trouble justifing the money for an indirect hot water heater or a solar system but what the hell, that's the American mindset....

    The thought of "free" hot water in the winter doesn't wash. There are no "free lunches"

    The 25% figure applies to oil fired systems only. Electricity & natural gas for DHW are cheaper to operate than oil.

    Bob
  10. woodgeek

    woodgeek Minister of Fire

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    I have always wanted to wire a cheap AC clock to the oil boiler--never got to it. But I did time the cycle during summer periods when I wasn't calling for DHW. I figured my standby at 1.3 gal/day! After an investigation, I discovered I had a thermosiphon--hot water gravity circulating slowly through my upstairs hydronic baseboards. After closing a manual cutoff valve on that loop, my standby dropped to 0.4 gallons per day. Turning down the aquastat got my to 0.3 gal/day, which is the rated standby for my oversized 15 yo, 140kBTU/hr slantfin boiler.

    So, I save close to 200 gal oil per year by closing a valve in the spring, and opening it in the fall. The system does have a checkvalve to prevent thermosiphon, but I guess it doesn't work, and I was quoted out $400 to replace it. I think I'll use the cutoff a while longer.
  11. DBoon

    DBoon Minister of Fire

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    Your numbers seem in the ballpark, though I would guess that your incoming water temperature probably averages 45 degrees year round, and if you heat your water to 130 degrees, then you need (85 degrees F)*(8 lbs/gallon) = 680 BTUs/gallon, or 1/5 of a kWh/gallon. 30 gallons a day = 6 kWh/day, 180 kWh/month, and at $0.12/kWh, a little closer to $20 than $17. That is about what my wife and I use in a month with all the energy saving washers, shower fixtures, etc.

    Is your kWh rate a special night rate, or is it the day rate? Your utility may give a big discount to night meter your electric DHW tank, and if so, you can put a timer on it and only heat water overnight. It takes some discipline (and lack of teenagers) to spread the loads out throughout the week, but you can save a a hundred dollars or more a year by doing that, depending on how much hot water you use.
  12. mbcijim

    mbcijim Member

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    Wes999 - FANTASTIC posts. TONS of information.
    I would like to buy your meter for your 80 Gallon electric hot water tank when you are done with it. Even if it is a few months till you're done. Or I could rent it and send it back when done.
    I have an 80 gallon electric hooked up to my desuperheater on the geothermal. No clue if I am saving money or not. Just my wife & I too, and probably using about the same for HW daily consumption.

    My electric bills are averaging 30-40KW/Day for the last few months, but I suspect a lot of it is the geothermal.
  13. jebatty

    jebatty Minister of Fire

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    Pretty much a repeat of what I posted before for electric DHW: 2 person household, large soaking tub which wife likes to use every day (but she doesn't fill to the brim), plus ordinary other uses.

    Typically monthly usage: 80-100 kwh, separately metered, off-peak rate of $0.042/kwh = about $4.00/month. Two electric hot water heaters, 80 gal + 50 gal; wrapped with 6" fiberglass sides, top and bottom; all hot water lines insulated; heat traps on cold and hot water pipes to hot water tanks.

    On-demand hot water heaters have the potential to save over other kinds, but often not realized because of now "endless" supply of hot water and therefore more hot water usage than before.
  14. WES999

    WES999 Minister of Fire

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    Quick up date. I installed the current switch, ( but not the hour meter) it's very simple, just run one wire from the electric water heater through the hole in the switch. The unit is self powered, when current flows through the wire led blinks and the switch contacts close. I will be wiring the switch up to my hour meter, then I can see how often the water heater turn on.

    Attached Files:

  15. mbcijim

    mbcijim Member

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    Wes999, I don't know crap about wiring.

    1. Are the wires just tightened into the breaker with a screw, like a wall outlet? So just unscrew insert current switch, and re-insert?
    2. Can you post a picture of your hour meter?
  16. Gooserider

    Gooserider Mod Emeritus

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    Not intended as a put-down, but if you don't know electrics, you have NO BUSINESS going into the breaker panel box! The open panel box has a lot of exposed conductors in it, some of which are ALWAYS LIVE, w/ serious currents involved... IOW, MAJOR SHOCK HAZARD!!! Working on a panel is something even an expert electrician puts in the "scary" category as slips can be seriously injuring or even fatal, although proper procedures will minimize the risks...

    That said, a standard breaker will have a hole that you insert ONE wire into, with a captive screw that gets tightened down to clamp it in place (and there is a torque spec involved)

    The best approach if you don't know electrics is to pay a pro, or if you can find one willing to teach you how to do it right, get some lessons, IMHO this isn't something that can be taught safely via message posts...

    Gooserider
  17. WES999

    WES999 Minister of Fire

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    Well I got the hour meter hooked up yesterday. It works very well, when the hot water heater is on the hour meter will run.
    In the first 24 hrs, the heater was on for a total of 42 min, if my math is correct, that comes out to a whopping .38 cents per day.
    Looks like it should be a lot cheaper than oil.

    Attached Files:

  18. mbcijim

    mbcijim Member

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    Goose-
    no offense taken
    and no I don't plan on doing it myself.

    Have some good electrician friends. if i get the stuff here and show them what I want and have the materials here I am sure they will help me out!
  19. WES999

    WES999 Minister of Fire

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    Well, it's been exactly one month (31 days) since I installed a electric water to replace the tank less oil fired water heater.
    In 31 days the heater has been on for 32.5 hrs, 1.05 hr/day, 4.725 KWH/day, $.55/day, $17.00/ month. Exactly what I estimated.
    So it looks like I went from about $80/ mo to $17/mo. :cheese: A saving of $63/ mo. Not bad for a $50 water heater from CL.
    Now I just need to make some solar panels and I will be golden.
  20. Gooserider

    Gooserider Mod Emeritus

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    Sounds like a good improvement for sure... As to solar panels, Build It Solar seems to have a lot of good info on doing home brew solar stuff - including a panel design that is supposedly within a couple percentage points of being as efficient as the commercial panels for a LOT less money - The difference between the two mostly being in stuff that we can't economically get on the small builder scale, like the solar glass and selectively absorbent coatings...

    Gooserider
  21. DBoon

    DBoon Minister of Fire

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    Glad to hear that you finished the experiment and got the result you expected. I'm not surprised. My 200 kWh/month for hot water is all usage at the night rate, so this is an overestimation of what I use just for DHW, for sure.

    Your experiment has helped put some facts behind the theory that a household is better of with electric hot water versus an oil-fired system (which is pretty inefficient, overall). Thanks.
  22. newstove

    newstove Member

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    I have discussed this before but haven't gotten around to doing it yet. But this experiment leads me to want to do it more. ;-)

    I have an oil fired boiler which heats our 50 gallon hot water tank. I am not sure how much it costs exactly but it appears to be on the order of 30-35 gallons per month of oil in the summer. Now my electric rate on this area is $0.168 per kWh.

    What I want to do is simply get a 50 gallon electric hot water tank and replace my existing 50 gallon tank. But, one thing I was considering was using the electric tank as a pre-heating tank to the indirect oil tank. We have an 80 gallon jacuzzi tub that we like to use every now and then and his would give us enough hot water to actually fill it with hot water ;-)

    I figure it will also only use the oil to maintain the tank temp in the indirect hot water tank.

    Then I was also considering possibly getting a heat pump hot water heater for the electric tank. That would let me also dehumidify the basement and I could shut down the seperate dehumidifier I have to run as well.
  23. DBoon

    DBoon Minister of Fire

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    I would suggest that you do an honest assessment of how much hot water you really use in a month. If you use a lot - why is that? If it is for things like clothes washing, purchase a more efficient washer first. Consider efficiency in usage first, then consider purchasing new hot water heating equipment.

    If you can get your hot water usage down to where you are only using 35 gallons or so a day, the just to with 1 50 gallon electric tank, and put it on a day/night timer using your utilities off-peak electric rates or programs.

    It it is for luxuries (that you don't want to discontinue) and/or you just need a lot of hot water, then a 1+1 water heater system may be a good idea. Have the air-source heat pump hot water heater feed an electric resistance heater. I saw that Stiebel Eltron started selling a combined system (no heat pump bolt-on) - just saw it in Solar Today, no experience with it. Then, put both hot water heaters on a day/night timer (your utility probably offers a day night rate for electricity, and it can save you a lot of money). Or, have a solar heating system feed the pre-heater tank (this will be the most costly option with the longest payback).

    In any case, using oil boilers to make hot water is, in my opinion, the most inefficient way to get this done, especially in the summer months. I can't believe how much oil my father-in-law burns, and most of it is just for hot water since he heats his house nearly 24/7 with wood.
  24. newstove

    newstove Member

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    One reason for lots of hot water usage - teenagers. ;-)

    Our utility does not offer off-peak rates unless you use more than 2,500 KWh per month. We don't. ;-) Our current average rate is $0.147/KWh.

    We have a very high efficiency front loader washing machine. We wash clothes with cold water anyway, so that's not a factor.

    I have been figuring that we use about a gallon of oil per day for hot water heating based on our average fall oil delivery and dividing that up over the summer months - as to how much hot water that turns out to be, I'm not sure, I don't have a meter on the hot water (though that would be an interesting experiment.)

    All my calculations (below) show that at 50% efficiency for the oil heating of water, and at my current electric rates, it is a wash (see calculations below.) Note that based on this, it shows about 100 gallons of hot water usage per day to get to the magic 1 gallon of oil per day that appears to be consumed. I think that may be high, I don't think we use nearly that much hot water per day. But, in any case, the number doesn't matter, that just raises the scale of savings, the percentage of savings between oil and electric is the same.

    Any ideas?

    BTU to heat 1 gallon 1 degree = 8.34
    Starting Temp = 45
    Desired Temp = 130
    Total Temp Rise = 85
    Total BTUs per gallon to heat = 708.9

    Gallons used per day = 100
    BTUs used per day for DHW = 70890

    BTUs per gallon of oil @ 50% eff = 69,345
    BTUs per KWH = 3,413

    Gallons of oil needed per day = 1.022
    KWH needed per day = 20.771

    Cost per gallon = $2.790
    Cost per KWH = $0.147

    Cost of oil per day = $2.85
    Cost of electric per day = $3.05
  25. DBoon

    DBoon Minister of Fire

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    All your calculations look correct, and if your usage is as efficient as it can be, then the best strategy is probably to do nothing, at least right now.

    I think 50% oil DHW efficiency is about the right estimate (i.e. it's not as good as people think). Most boilers have a fairly large boiler vessel, it takes time to heat the water in it up before that can be used to heat your HW, then a fair amount of heated water in the boiler vessel is left in it when it when they shut down, which is wasted.

    I did a lot of research a year ago as I was considering replacing my oil boiler. I don't heat HW with my boiler (it's electric HW), but I was trying to understand if I could get some savings by doing it this way. I concluded that even if I bought the most efficient boiler possible, it didn't make any economic sense to change what I had. My off-peak electric rate is 0.10/kWh, but I don't think a nickel more would have changed things - payback was just too long.

    But if my boiler did fail and need replacing, I have my eye on a System 2000 boiler that seems to do about the best job of capturing the waste heat, regardless of whether I have oil DHW or not. Problem is, there is no dealer in my area for this, and you can only get these through a specialized dealer. But I think that any boiler with a small, non-cast iron boiler vessel will by nature be more efficient at heating HW out of season, and that would be the thing to look for in a new boiler if and when you need to buy a new boiler anyways. If your oil boiler is old, you may want to check out this system. The marketing seems a little over the top at times, but the engineering passes my sniff test.

    Another thing I considered would be to use a 1+1 tank solution and use the first tank as a pre-heater for a 2nd tank with the first tank heated by a long-cycle oil boiler burn to avoid the short-cycling inefficiencies. Again, the payback is not so good. I wouldn't recommend this.

    Solar DHW for me is not such a good payback - I don't use enough HW to ever make it economical (and I really would like it to be). But, you may find this to be a good choice. I think a professionally installed system would run you about $5000 to save you ~80% of your hot water. That would be ~250 gallons of oil/year @$3/gal (conservative) and that is $750/year. Again, just a rough estimate. This may make sense for you.

    Lastly, I've been pretty intrigued by the air-source DHW heat pumps that some other people in the Green Room have been talking about. The people who have these have used them with one large tank, or with a pre-heater tank that the air-source DHW heat pump heats first. The price is pretty reasonable. I'm thinking about taking the plunge on one of these. Still, the payback for me is long since I have minimized my usage a lot already, but I like the basement dehumidification aspect of them. For your larger usage, these might be a good idea.

    And if money and payback time is not an issue, and you just want to reduce oil usage, most of what I say doesn't apply. In that case, avoid pure electric - MA burns more coal for electricity than other northeastern states, so you're probably not doing the planet a favor with that option. If you have a good site, go solar and use an air-source DHW heat pump as a backup to your solar.

    Also, I'd take a look at one of the Marathon polybutylene electric DHW tanks if you need to replace yours. They are really well-insulated, will never leak since they have no glass-lining and metal construction, and can handle temperatures up to 180 degrees F. These are great storage vessels for solar. Expensive, but good.
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