Electric Tankless Water Heaters

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by reb, Nov 7, 2007.

  1. reb

    reb New Member

    Messages:
    2
    Does anyone have any recent experience with electric tankless heaters? I am building a new 1800 sq. ft. house and had decided to go the tankless route for my water heater. It would have to be electric because gas is not available at the homesite. I had chosen the Stiebel Tempra 36 heater. Its capacity is large enough to handle the projected maximum flow rates. Then it was pointed out to me that this heater requires a minimum flow rate of .87 gpm to be activated. Since a lavatory faucet has a rate of about .5 to .75 gpm, it would appear that I wouldn't have any hot water at the lavatory unless I turned on some other faucet or appliance to get the required minimum flow rate. This is obviously unsatisfactory and a Stiebel technician indicated there was no realistic fix for this situation. I also have heard some talk about fluctuating water temperature at low flow rates. I would appreciate any comments or suggestions. Thanks.

    reb
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,948
    Location:
    New England
    Both of those comments are valid. Many of the heaters cannot modulate the level of heat...it is either on or off. If the thing turned on with too little flow, it would cause the water to boil, creating steam and a very unsafe situation. You could put in a point of use water heater (i.e., a small tank), but that sort of defeats the purpose. Use the search button in the blue bar for tankless and see various discussions. Some love them, many don't. They also require much more regular maintenance than most other types.

    The output temperature will vary with the incoming temperature and the volume. think waving your hand over a flame. The slower you do it, the hotter it gets. Wave it fast enough, and you'll barely notice. The things aren't magic, they can only add so much heat. If you are using a small amount, they are often plumbed with a tempering valve to cool it off to a safe temperature, but otherwise, yes, the output will vary, sometimes significantly. It depends on where you live...if you dont' have really cold winters and your incoming water is always warm (say S FL or HI or SoCal) then they are more viable.
    Last edited: Nov 7, 2007
  3. Bob NH

    Bob NH In the Trades

    Messages:
    3,317
    Location:
    New Hampshire
    The problem with electric tankless water heaters is that they require so much power to deliver hot water.

    Let's assume that you want 5 GPM with a 75 degree temperature rise, from 50 F to 125 F. You can plug in your numbers.

    5 GPM x 60 min/hr x 8.34 # per gallon x 75 BTU per pound = 187,650 BTU per hour.

    1 kilowatt = 3413 BTU per hour; so you need 187,650/3413 = 55,000 Watts.

    55,000 Watts/240 Volts = 229 Amps.

    Most houses have no more than a 200 Amp service. You would need a 400 Amp service, and the power companies HATE electric instant water heaters because they must put in a large transformer that is used very little. Also, when your water heater comes on the lights dim at all of the neighbor's places (and at yours too).

    I have taken showers at places with electric instant water heaters. They are always installed with super-low-flow shower nozzles that give you a shower that is no more than a warm mist.

    If you are not supplied with very soft water, the heaters will "lime up" like those tea kettles with 1/8" of brown crud in them, and you will need to regularly flush them out with some kind of acid circulation system that is the equivalent of, but not as convenient as, boiling some vinegar in your limed-up teakettle.

    So the questions that you must ask and/or answer:

    1.What is my incoming water temperadure in the coldest part of the winter?
    2.How many gallons per minute of water do you want; and at what temperature?
    3.What is it going to cost when you consider the added cost of electrical service and wiring to the heater?
    4.Will the power company allow you to connect that much intermittent load to their line?
    5. Is the water quality suitable for an instant water heater, and if not, how much will it cost for water treatment equipment and maintenance?

    Other points:
    The solution to the low flow problem in a lavatory faucet is to install a small point-of-use tank heater in the cabinet at the the lavatory. Google Ariston to find small heaters that operate on 120 Volts. A small point of use heater in the line after the tankless will also modulate the variations in temperature that you get with low flows.

    Of course the Instant Heater peddler doesn't want to talk about tank-type heaters because he knows that if you get all of the facts you are not likely to buy his wizardry.
    Last edited: Nov 7, 2007
  4. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    Install a smaller 120 volt unit JUST for the lav (s). They are relatively inexpensive.
  5. leejosepho

    leejosepho DIY scratch-pad engineer

    Have you considered propane? If you have a place outside to have a tank set, it would probably not take a very large one to supply the gas for a water heater.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,537
    Location:
    Cave Creek, Arizona
    heater

    Install a smaller 120 volt unit JUST for the lav (s). They are relatively inexpensive.

    And completely useless as a source of hot water. Unless you use the supplied aerators which reduce the flow to a few needle streams of water, the heater will not supply warm water, much less hot water.
    [​IMG]
  7. why not a 50 gallon electric water heater???

    your electric tankless water heater will cause you
    nothing but greif.....

    their was someone that posted stateing their unit actually
    pulled so much juice that it blew out the transformer in the
    neighborhood.........

    what do you think you are gonna save in the long run anyway???

    you will have to de lime that type of unit constantly...


    put in a 50 or 80 gallon electric and it will last you
    10-15 years trouble free....
    put a heater blanket on it and
    is probably going to be about the same cost over all... ....
  8. fortop

    fortop New Member

    Messages:
    28
    Location:
    Tucson
    I am planning on using a standard tank type electric water heater with a timer to turn it off at night when hot water is not needed- saving money and energy.

    Later, I plan to add a heat pump to the water heater so the heat pump "exhaust air" will cool the garage where it is located - supposed to save more energy.
  9. got_nailed

    got_nailed DIY Senior Member

    Messages:
    277
    Fortop
    Most of the cost in a hot water heater is to heat the water not to store it. I would think that running it at night would be more better because of the PF would be higher. Once the water is hot with the newer hot water heaters “energy star†don’t have to come on much to keep the water hot. If it’s in a heated space it will use less power to keep the water hot.
    Power companies want water heaters to run at night and not during the day to cut down on the overall peak usage when everyone else is heating and cooling there homes.
    I do see your logic but turning it off at 6:00 pm and back on at 4:00 am might be better since you will have hot water when you get up to get ready. But turning them off IMO will cost you more in the long run.
    Are you planning on using a water heater as a boiler to heat the garage? I think you need to do a good amount of research on sizing the unit before you get your tank.


    Reb
    I would see how many people in your area have tank less systems for more than 2 years and see how they like them and how much it cost to keep them running.
    Feeding the tank less into a smaller tank heater will help with the low flow issues but will still lead to cold water form time to time.

    The only saving with a tank less system I have seen so far is with the space savings they give. But at some point I bet you will end up with a tank model so plane room to put it when the time comes.

    No matter how you look at it, it will still take the same amounts of watts to heat the same amount of water a given amount of degrees. I can’t find what the calculation is but someone will give it to you.
  10. fortop

    fortop New Member

    Messages:
    28
    Location:
    Tucson
    got_nailed -

    Here is a link to the hot water heat pump:

    http://www.toolbase.org/Technology-Inventory/Appliances/heat-pump-water-heaters

    In a garage in southern Arizona the "free" cooling and dehumidifying for the garage is a big plus. Supposedly, they run at 1/3 the cost of a standard electric water heater.

    Where I live there is no "time of use" rate, but that could happen in the future.

    Based on ambient temperature, my experience is that an electric water heater will cycle on and off to keep your thermostat setting. With more insulation this happens less often. The savings may be relatively small, but why heat the water at all if you don't need to. I would consider also turning off the heater during the day if nobody is home.
  11. got_nailed

    got_nailed DIY Senior Member

    Messages:
    277
    Fortop
    I don’t know of any places that have time of use metering at this point. I do think its coming soon with the new meters they have been installing.

    I have not seen those water heat pumps because of where I live. I thought you were talking about a different product.

    I know what you’re saying about heating water when you don’t need. Depending on your location and the loads on the heater you can save or lose money with the timers.

    I would still think that with the difference in PF would change a good difference between night and day that it would be cheaper to heat the water during the night. “Hot water heat pump†your talking about who knows if you had the thermostat set right you might not even need to use your elements since the hot water heat pump heats water for free.
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