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  1. #11
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    Default Re: Tankless Hot Water Heaters

    I built a house about 2 years ago which had 2 of the largest electric tankless units made. When they operated, the voltage drop caused all sorts of electrical problems. They didn't operate as desired or expected. Repeated calls to tech support, swapping out the units, nothing worked until they were removed and a standard 50 gallon HWH was installed. No problems reported since it went in.

    Those two electric on demands are setting in my garage. I can't give them away.

  2. #12
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    Default Re: Tankless Hot Water Heaters

    What was the reason why they were causing issues?
    Did your electrical calculations for the home include these two units?
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #13
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    Default Re: Tankless Hot Water Heaters

    Inner city Houston, Plenty of power from the transformers, worked the the power company for whatever that was worth

  4. #14
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    Tennessee
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    Default Re: Tankless Hot Water Heaters

    Quote Originally Posted by canuk View Post
    Curious -- the water heater is a 240 volt appliance as is your electric clothes dryer , range , air conditioner , heat pumps , etc.
    I'm trying to understand how the demand water heater will create the issues you mentioned ?
    There may be a need to have a 200 amp service to your home but that's not uncommon these days.
    You would definitely need a 200 amp service, but that has nothing to do with the transformer. Transformer size is expressed in kVA or kiloVolt Amps. Thats amps times volts, same formula as watts, but watts express power consumed, kVA expresses capacity.

    A typical residential transformer is 10-50 kVA. A 15 kVA is typical when the transformer only serves one residence, 25 and 50 kVA units usually serve multiple residents.

    A 15 kVA puts out 62.5 amps at 240 volts. That is not its maximum load limit however. It can easily withstand 2x to 3x of its rated capacity for a short time. The rating comes from the oil temp rise over 24 hours at that load. The standard for temp rise is 65 C over ambient. 20C is the standard ambient temp. So a 15 kVa transformer would see its oil temperature rise to 85C 24 hours after being hooked up to a load that draws 62.5 amps.

    Where there is heat being generated, that means there is resistance or impedance. The primary side of the transformer is a load to the distribution system, and it is the source to the load on the secondary. Now we can get into about a semesters worth of theory here, but lets just boil this down to a Thevenins analysis.

    http://www.electronics-tutorials.ws/...its/dcp_7.html

    If you went to the tutorial I posted as a link here, you will see that all sources, generators, batteries, transformer secondaries etc. have an internal resistance (Rs) that is in the loop. As you increase the load attached to the transformer, that is decrease the loads resistance, then more current flows in the circuit. The transformers resistance (Rs) does not change so with greater current flowing through Rs, it drops more voltage, making less voltage available to the external circuit. The load then sees less voltage.

    If you ever get up close to a transformer nameplate, there is a piece of information there called %IZ. This is the percent of applied voltage to the primary that will be dropped by the transformer itself at rated current. That voltage drop will not be available to the secondary, therefore the secondary voltage will drop too.

    Typically the %IZ for a residential transformer is 1.5 to 3.5%. Lets say that the %IZ is 2%. That means that if you measure the output voltage at no load and then again at rated load, there will be about a 2% difference. That 2% is at rated, as the load increases from there, the %IZ goes up. At double the rated load, the %IZ would be 4% or more. The resistance within the transformer goes up with heat, but that can take time for the heat factor to kick in. The rise to 4% would be immediate, any further rise would come as the coils begin to heat up more.

    In addition to the transformers %IZ, there is the resistance of the wires going from the pole to the service panels and resistance of the house wiring. All of this adds to the voltage loss as well. It actually will probably account for more voltage drop that the transformer itself. Throw in a loose connection and voltage will really drop from there.
    Last edited by keith3267; 04-16-2012 at 08:11 PM. Reason: Change RL to Rs

  5. #15
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    Default Re: Tankless Hot Water Heaters

    Keith --- thanks but I have a very good knowledge of electrical principals and theory.
    The thing that I'm wondering is the service transformer. I can't say what goes on down there but urban residential feeds have it's own transformer ?

    Around here there only couple are so transformers for the whole street. In the past they calculated loads for each residence to be 100 amps ( even though many were only 60 at the time ) and those transformers servicing x amount of homes were sized accordingly. In the recent years supply loads have increased in many homes to 200 amps in which case those transformers have been replaced with larger capactiy units. We never experience brownouts or surges.

    Regardless , whatever the calculated load of the residence should have the appropriate service supply from the utility.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  6. #16
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    Tennessee
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    Default Re: Tankless Hot Water Heaters

    canuk, In my immediate area, each house has its own distribution transformer, but I live in a rural area. In the cities or suburban areas, one transformer will serve several residences, especially in areas with underground cabling and pad mount or underground transformers. The size of the service does not dictate to the utility what size transformer will be used.

    I have a brother in law who has a 400 amp service, but he and his neighbor are serviced by a single 25 kVA transformer. The transformer is rated at just over 100 amps. That transformer should be able to provide 300 amps for about an hour and a half to two hours before the internal load sensing fuse blows.

  7. #17
    Join Date
    Apr 2012
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    203

    Smile Re: Tankless Hot Water Heaters

    Quote Originally Posted by HoustonRemodeler View Post
    I When they operated, the voltage drop caused all sorts of electrical problems. They didn't operate as desired or expected. Repeated calls to tech support, swapping out the units, nothing worked until they were removed and a standard 50 gallon HWH was installed.
    I was looking into buying a tankless last year. I heard stories like yours from MANY tankless owners. When I heard that Loews was dropping all tankless from their stores, I knew tankless was not all it was hyped up to be. If I had the money I would get a stainless steel tank and be done with it

  8. #18
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    Default Re: Tankless Hot Water Heaters

    The natural gas tankless ones work flawlessly

  9. #19
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    Tennessee
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    Default Re: Tankless Hot Water Heaters

    Quote Originally Posted by HoustonRemodeler View Post
    The natural gas tankless ones work flawlessly
    For the most part that is probably true, but you do need to work with the utility to insure that gas meter can handle the extra flow needed if the tankless water heater and the house heater come on at the same time. Around here, they have a larger capacity residential meter for homes with these units. Once the new meter is installed, then no problem.

    I still question the economics of these though. A well insulated tank is not going to cost much to maintain the desired temperature, so if a tankless unit cost a significant amount more, you may not ever recover that extra cost in savings on gas.

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