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Tankless water heater

Dave_22
Dave_22 Member Posts: 232
what about a combo-boiler?? Like the Veimann with a built-in DHW HX...

Comments

  • Dave_13
    Dave_13 Member Posts: 110
    tankless water heater

    The following is copied and pasted from a different forum:

    I figured out what my gas consumption is with my tankless unit.
    40 gallon 40,000 btu heater runs about 4 hours a day to keep the water hot, about 160,000 BTU's a day.

    Tankless 100,000 BTU heater runs about 20 minutes a day, 33,000 BTU's used.

    Thats around 127,000 BTU's a day not being used.

    Does this seem right that a water heater would run 10 minutes an hour to maintain temperature? If it is, it seems like a tankless WH is the way to go. I am considering an Indirect tank off a boiler. How do you think that would compare BTU wise in a situation like this? Any comments? Thanks.
  • ScottMP
    ScottMP Member Posts: 5,884
    Dave

    "Tankless 100,000 BTU heater runs about 20 minutes a day, 33,000 BTU's used."

    A tankless coil has no idea when you want hot water therefor it must maintain that 100,000 btu heater around 120 degress just to be able to supply you with hot water when you want it. Also a boiler with a tankless coil has a minimul insulation so it has a high stand by loss. I have no idea how it could only run 20 minutes a day. Unless you shut the unit off and then turned it on when you wanted hot water and then waited 5 minutes while the boiler heated up to temp to deliver your hot water.

    I also believe there is more to delivering hot water than btu used. There is the longevity of the equipment. Most tankless coils become clogged with mineral deposits after a few years and reduce the transfer of heat so as to reduce its efficency.

    I am a big fan of indirect hot water heaters, which would use the same 100,000 btu's from the boiler that a tankless would use. The difference would be that the indirect can store that temperature in a tank that commonly has a 1/2 degree per hour standby loss. Alot better than a boiler with a inch on insulation and a flue damper that is open to the atmosphere.

    I would wait and get others responses before you make a decision. I think your question may be related to a post that talks about practices many of us do not believe in.

    Good Luck

    Scott






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  • ScottMP
    ScottMP Member Posts: 5,884


  • At a dollar per therm,

    that's a savings of $127 per day if all the figures are correct.

    I'm duly impressed.

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  • Dave_13
    Dave_13 Member Posts: 110
    OK

    I am talking about a gas hot water tank and a instanteous water heater. It sounds like there might have been some confusion...
  • Joe_30
    Joe_30 Member Posts: 85
    Indirect or ordinary DHW?

    When the ice drops the power line, or the storm or a squirrel blows the transformer, the boiler goes down. But,with a $300 regular gas water heater and a standing pilot, you still have hot water for showering, washing stuff, and even for filling the tub as a big "radiator".

    But, with indirect, you pay $900 to a $1000,[ though you get great hot water], and you lose it in a storm. Unless you also pay a lot for a standby generator. Add to which, there is the concern when your suburb has heavily mineraliized well water befouling a very expensive water heater.

    Opinions?
  • Tony_8
    Tony_8 Member Posts: 608
    a therm

    is 100,000 BTU's. That should help with the math for operating cost comparison. We are currently paying $1.50/therm in WNY. Instant WH are better than common tanks for production and economy of operation. Indirect tanks for heating boilers are much better than indirect coils, but you didn't ask about those, did you ?
    GOOD instant units aren't troubled as much by hard water as others (including common tanks), and are cleanable if need be. I like Bosch.
    The electricity issue might not be one if you have your own well anyway, you wouldn't have ANY water at all!
    I guess if the purchase price is less an issue than operating cost, instant is the way to go, in my opinion.
    Tony
  • Oops

    Thanks for the correction, Tony. That changes things a bit; $1.27 savings per day for a "flash" heater vs. a conventional 40 gallon water heater. Still, that's almost $500 per year.

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  • Dave_22
    Dave_22 Member Posts: 232
    Hello

    I am the one who wrote the original post. I was questioning if anyone thought that a normal gas water heater would run 4 hours a day? That seems excessive. Unless it is old and scaled up pretty bad. To me, the numbers look impressive. I just wonder if you can get the quantity out of an instantaneous heater. And I wonder what kind of numbers a indirect off a boiler would run (BTU wise). I do have well water, so if I loose power- I loose water anyway!!! I will be putting in a boiler next year and probably an indirect also. Though I like the principle of only heating water when you need it.
  • Heatermon
    Heatermon Member Posts: 119
    Water heaters

    A "normal" gas water heater running at 40,000 b.t.u.s for 4 hours per day would produce approximately 160 gallons of useable hot water (40 degrees in and 130 degrees out). An instantaneous heater would have to run water out of it at 8 gallons per minute to get the same quantity out of it in the 20 minutes you suggested. So, either your gas water heater time estimate of 4 hours is high, or your instantaneuos estimate of 20 minutes is low. I believe both estimates are incorrect. The "normal" usage of hot water for a 40-50 gallon natural gas should be around 60-80 gallons per day (let's just say 80 to cut your estimate in 1/2). To get the same 80 gallons out of an instantaneous you would run approximately 40 minutes at 2 gallons per minute. If the unit averaged 100,000 b.t.u. per minute, you would be at 66,667 b.t.u.'s compared to 80,000 b.t.u.s for tank type. This would produce a savings per day of 13,333 b.t.u.s, which I believe is closer to the truth (of couse, your milage may vary). Personally,I sell tankless technology for numerous other reasons than fuel savings. I even warn potential customers about how instantaneous can actually increase the monthly gas bill (If you can't run out of hot water in the shower, you, or you children, could stay in the shower longer and run the gas bill higher than if you were to chase out the "water hogs" by running out of hot water). Either way, good luck with your assesments and choose the best type for your situation.

    Heatermon

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  • Dave_22
    Dave_22 Member Posts: 232
    Hello again

    I am assuming the person who wrote the original post that I cut and pasted meant that he used hot water for 20 minutes a day. The time the HWH ran for was to keep what was in the tank heated all day long. I have never paid attention to how long my HWH runs per day. 4 hours seems like a lot of run time when he was only using 20 minutes of hot water during the day???
  • Jack Smith
    Jack Smith Member Posts: 53
    2 gallons a minute ?

    Just wondering where that number comes from.
  • Heatermon
    Heatermon Member Posts: 119
    2 gallons per minute of hot water

    Comes from a 3 gallons per minute shower head (1 gallon per minute of cold @ 40 degrees with 2 gallons per minute of hot @ 130 = 3 gallons per minute of 100 degree water).

    Heatermon

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  • hot_rod
    hot_rod Member Posts: 21,835
    The run time of an unused

    tank type water heater will depend on. The size of the tank, how well the tank is insulated, the temperature of the water in the tank, the temperature of the room the tank is located in, the draft in the flue, any thermosipnoning, etc.


    These are all considered standby losses. It would be hard to put an exact number on fuel used just to maintain the tank temperature. Four hours a day seems pretty excessive, unless the tank is outside:) I would guess a 40 gallon tank, located in a warm basement,unused, may fire 4- 6 times a day for 10- 20 minutes to maintain 130 degrees?? Again, it depends!

    You could spend a day camped out next to your own heater, or get a data logger and track this for a week or so, to get a closer number.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Tony_8
    Tony_8 Member Posts: 608
    4 hours

    seems like a lot to me for standby. I've left mine (a 94 model) on pilot for 3 days while gone, came back, forgot it was on pilot, got a 10 minute shower before noticing. Of course, a woman would only get 1-1/2 minutes :)! Instants definitely use less fuel, but how much less could probably be debated for days. If you're going to have a boiler installed anyway, just get an indirect w/it. They only lose about 1/2 a degree/hour according to specs.
  • Tony_8
    Tony_8 Member Posts: 608
    combos

    are great. I am not familiar w/Viessman. Use Teledyne Endurance a lot. Has a plate HX. Can't run 'em out of HW! Non-condensing 86% eff., good price. Vent can be a PITA. Concentric SS thru-wall (sometimes roof). They are also 50-100% modulating and quite quiet. Easily converted to LP. I have a customer w/5200 sq ft heated space, using rad slab, staple-up tiled, HW fan coils (2). On top of possibly the windiest hill, around 2000ft ASL, using an EBP175 (load is 120 @ 80f diff,other size is 110 in) @ 1.50/therm he burned $308 of NG for his highest month this past winter. I couldn't tell him I was surprised, just thanked him for referrals. He came from DC area to WNY and was shivering in the rented house for a year until new was done. Rental was 1/2 as big and twice as much to heat(80% FA). Needless to say, he's very pleased.
  • munchkin-man
    munchkin-man Member Posts: 247
    Superstor Ultra indirects

    Heat Transfer Products mfg's high quility stainless steel indirects. See www.htproducts.com click on the Superstor ultra and go to literature you can see the recovery rates and contact us through the page or [email protected] or call our technical support department 1-800-323-9651 ext.925,942, or 957 for technical information.
  • Jack Smith
    Jack Smith Member Posts: 53
    You must be fortunate,

    you only have one fixture running at a time. Somtimes, it so happens that at my house, I'm showering, the wife has laundry in the wash machine, and is cleaning dishes at the sink. Laundry, she tells me is on average 3 loads/day, dishes-3 times a day, showers-4 to 5 a day. Soon, those showers could be simultaneous( almost done adding a second full bath), could we then get near that 8 gal/minute flow rate ?
  • Greg Wallace
    Greg Wallace Member Posts: 3
    The BAXI Luna 310

    is a great combination boiler. Fully modulating from 105,000 down to 35,000 btuh. Seperate temperature controls for heating and DHW built in. It even has a place to plug an outdoor sensor directly into the board. DHW is handled by plate to plate heat exchanger. 3.3 gpm of DHW output...by the time you mix that with cold, you've got enough to run two showers simultaneously. The max heating btuh can even be limited by a potentiometer to match the load...perfect for smaller buildings, so you get extended run times. Clean coaxial venting. Lots of installations. Good reliability. H stamp. Check out www.wallhungboilers.com
  • munchkin-man
    munchkin-man Member Posts: 247
    HTP also manfg's the Munchkin Boiler

    Stainless steel fully modulating condensing system. Digital display and out door reset control with domestic priority mode see the Munchkin at www.htproducts.com click on the three D Munchkin and have a go at it. Any questions E me at [email protected] or call us at 800-323-9651 ext.925,942,957 our technical support line. Mention the wall and this post.
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