Comparison of DHW Oil Indirect vs Electric Tank

velvetfoot Posted By velvetfoot, Sep 5, 2012 at 6:22 PM

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  1. avc8130

    avc8130
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    I know beyond a shadow of a doubt that my oil burner ran .4 hours/day to make DHW this summer. My nozzle is 1.25 gph. That means I burned 1/2 gallon of oil daily for DHW ONLY with an indirect setup. I paid $3.34 for the oil back in the spring. That means it cost me $1.67/day for DHW this summer. Over a 30-day month, $50.

    ac
     
  2. TheMightyMoe

    TheMightyMoe
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    But the question is:

    How much of this is direct heating? VS Maintaining the boiler?

    Do you use a low limit on the boiler? Is it a cold start?

    Etc...
     
  3. R Mannino

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    1.25 GPH........do you live in a mansion? My 2000 sq ft ranch has a 0.4 GPH nozzle (I have an indirect water heater too). It's supposed to be a 0.5 GPH. I'll wager with the correct boiler size and firing rate your DHW and heating use of oil would be drastically reduced.
     
  4. TheMightyMoe

    TheMightyMoe
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    With a 1.25 GPH nozzle, depending on flow rates, the DHW will still run the boiler at max without short cycling in most cases. Most indirect WANT a 1+ GPH nozzle though, per specs.

    Mannino with your .4 GPH nozzle, do you take short showers or run out of hot water? With a nozzle that small you are underfiring the DHW. (I am underfiring mine as well @ .6, and only run out of water if we are all taking consecutive showers)
     
  5. R Mannino

    R Mannino
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    I haven't run my 42 gallon indirect out of hot water yet. I also don't have a Jacuzzi to fill either..................
    I have the DHW on priority, I'm sure that helps;) FYI, cold start.
     
  6. TheMightyMoe

    TheMightyMoe
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    Yeah, we have 4 people that shower in the morning in a row. =/ Mine is a 50, on priority, not cold start however, I want to but I don't want new controls. I've thought of up-sizing, but .6 is perfectly rated for the house.
     
  7. avc8130

    avc8130
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    It is NOT a cold start, low limit is set at 130.

    I have exact data for start and stop times of the boiler motor. Generally the wife and I shower in the morning and then the indirect is just maintained at 120F.

    Here is Aug 30 for an example:
    8/30/2012 6:59:11 AM Turned ON
    8/30/2012 7:06:00 AM Turned OFF
    8/30/2012 7:57:04 AM Turned ON
    8/30/2012 8:00:52 AM Turned OFF
    8/30/2012 3:21:27 PM Turned ON
    8/30/2012 3:27:56 PM Turned OFF
    8/30/2012 10:58:03 PM Turned ON
    8/30/2012 11:04:08 PM Turned OFF

    For whatever it is worth, I shower at 6:50AM and the wife showers about 7:45AM.

    As you can see, it runs 7 mins when I shower and only 4 when she does. This is most likely because the oil burner is just about at low limit when I shower and it is probably close to temp when she does. Then it is 7 hours until the next 6 min firings.

    ac
     
  8. TheMightyMoe

    TheMightyMoe
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    With your nozzle, you are gonna have very good recovery, and if the boiler is low mass you could probably cold start it and save another 20-40 a month.
     
  9. velvetfoot

    velvetfoot
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    Thing is, if anybody uses the hot water during the day, the whole boiler has to come back up to temperature, more like 190 on priority vs. the resultant 120 hot water. Then it just sits there and cools off til the next time.
     
  10. R Mannino

    R Mannino
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    Once we accept that the boiler will always be above the temperature of what it is heating, it shouldn't bother us as much. The key it to eliminate the standby losses as much as possible. Whenever your heating water for storage standby losses are inevitable. The key to heating with oil is to have the lowest mass as possible, wood boilers different story. Control is the other big issue, wrong control strategy can cost a lot of BTU$.
     
  11. avc8130

    avc8130
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    But won't that cause the boiler to condense?

    ac
     
  12. velvetfoot

    velvetfoot
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    It bothers me that I have to heat up more mass and probably the same amount of water as the indirect tank and just let it cool its sorry self off between firings. I don't know, but I wouldn't call that standby loss, and it does bother me. Lowest mass as possible would be a tankless. I have a Burnham V8 operated as a cold start, with a now-inactivated and drained Amtrol BoilerMate. I'm using an outdoor reset for some setback (since it's not a mod con and I guess more importantly, the emitters are common baseboards, can't go too low I guess), but for dhw, I think it's gotta be at 190 on priority.
     
  13. TheMightyMoe

    TheMightyMoe
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    Your boiler manual will tell if you IF it can be a cold start. A low mass boiler should heat up so fast, that it will not condense long enough to be an issue or in the manual it may call for a system bypass or plumbing primary/secondary etc...

    If you want to lessen the calls on the DWH, you can increase the size of the differential on the Indirect Water Heater aquastat, so hand washing etc, will not cause the boiler to run.

    It is more efficient to heat water up from 80 to 180 while providing energy to your water heater then to reheat water from 100 to 140 for no reason.

    Either way you will have stand by losses, and you have to figure out what is better for your house hold. Most people use a low limit because it is just more fool proof.

    Velvet in your case, electric sounds like the way to go =) With baseboards 160 hi-limit is about as low as it should go for COMFORT, you can go lower, but usually that effects the comfort level of the home more then anything. 180-190 is typical priority hi-limit, and the control should by pass the setback hi-limit.
     
  14. BoilerMan

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    AVC, is there a way to tell how much 120F water you in those two showers? What we need to do this totally right is a totalizer on the DHW pipe of the water heater/indirect. avc has the oil numbers now if we can get the total gallons of actual hot water used. Anyone have an old water meter kicking around????? It would be as simple at calculating the needed BTUs for the termp rise of the water actually consumed divided by the oil used (timer on burner calculation. All the rest is standby loss. Boiler drafting heat up the chimney, jacket loss of boiler, jacket loss of indirect, loss from associated piping etc. etc. etc.

    AVC, I'd do a heat loss calc on your house, I've followed your threads on backup, type of house you have, and the rest. 1.25gph is alot of heat, you do not need it for DHW as you have proven.

    An underfired indirect is almost a given, as they spec them at riduclous flows and BTU inputs. Here in the real world, we use hot water at a given rate, and if the boiler and stored hot water in the indirect can keep up, it is a non issue. Look at heat pump water heaters, they make something like 2,500 BTU/hr but have large tanks to make up the difference, and electric elements for supplement. Take a typical electric water heater at 4,500 watts, that = .135 gallons of oil at 80% eff.

    TS
     
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  15. avc8130

    avc8130
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    Unfortunately I do not know how much hot water is used. I do know my shower is 7 minutes long on average. My wife's is similar. We have our indirect set to 120F. Shower head is a modern fixture. That is about as far as I know for fact.

    Monitoring water volume is much more difficult than monitoring oil burner run time.

    ac
     
  16. TheMightyMoe

    TheMightyMoe
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    Well said Taylor.

    Electric water heaters have the benefit of having no-negatives to cycling off/on to maintain temperature, recovery begin instantly. Where as on a indirect, recovery begins at the call for heat (A 5* diff at minimum), and the boiler reaching 140*+.

    Easiest way to monitor a shower, would be run it a minute into a 5 gallon bucket, and then you know your GPMs.
     
  17. BoilerMan

    BoilerMan
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    But we do not know how much actual hot water is used..... There is cold mixed in too. Just trying tobe as scientific as we can here.

    This is very true, as we are all nerdy on here, someone needs to come up with a community water meter we can UPS it to each other to find out our actual useages.

    Modern shower heads are spec'd at 2.5 gpm, but that is not real life either, as it's just an orface that flows 2.5 gpm at a given pressure, are all houses the same pressure?? Nope. For that matter, is the pressure consistent throughout the shower for us who have wells?? Nope.

    I'm still for electric in the summer just for the sake of saving on the entire boiler system short cycleing, and extra soot associated with those 5-10 min runs. Drain the indirect in the summer and fill the electric and flop on the 30A breaker, enjoy not hearing the oil cycle in the SUMMER!

    The thread quoted back on page 2 has lots of info on electric water heaters and their almost non-existent losses. New electrics are supposedly .94% efficient, only 6% jacket loss. I maintain, there is sugnificant loss from the top piping arrangement, olb boilermantes had bottom connections to minimize loss, but plumbers hated running their copper to the floor to hook them up ( I don't blame them, as a tradesman) but it was more efficient.

    TS
     
  18. avc8130

    avc8130
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    For what it is worth, I believe my indirect is a priority zone.

    ac
     
  19. velvetfoot

    velvetfoot
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    That was my point, about cycling the boiler.

    My water softener has a flow meter built in and I made an assumption of what percent would be hot water.
     
  20. velvetfoot

    velvetfoot
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    That was my point, about cycling the boiler.

    My water softener has a flow meter built in and I made an assumption of what percent would be hot water.
     
  21. fire_man

    fire_man
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    According to this calculator and my electric/oil costs in MA, Oil is slightly cheaper.
    I currently have a dedicated oil fired hot water heater.


    Cost per Million BTUs
    upload_2014-2-19_19-24-26.png
     
  22. BoilerMan

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    I think your 65% number may be high for a Bock water heater. If you go back to the first page Dick Hill found an indirect setup to be 33% efficient. Not sure of a direct fired, with that 6" flue right up through the tank..........

    TS
     
  23. fire_man

    fire_man
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    This may be just advertising, but it's right off Bock's tech sheet:

    "The popular Bock® 32E leads the industry in Energy Efficiency with a .63 EF rating and 170 gallons first hour delivery."
     
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