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water heater lockouts caused by power failures

Our rural vacation home uses an A.O. Smith water heater for radiant heat, and is often unattended for several weeks.  Occasional power failures seem to be causing it to go into a lockout mode that is easily resettable IF somebody is there... otherwise the heat is totally lost potentially for weeks which is really bad.  The lockouts ONLY occur when there has been a power failure.  The plumber who installed the unit has checked it for all the usual lockout-provoking problems like bad sensors, gas supply, ventilation, excessive heat, etc.  Interestingly, the mere act of manually switching the power off and back on a moment later does not cause the lockout... it takes something else.

 It has been suggested maybe the power failures cause a series of veryrapid on-off fluctuations in succession, and that this is causing the lockouts.    Does anyone know what is most likely to be causing this?  More important, is there any way to overcome it?  Would a computer-style UPS of sufficient capacity smooth out the irregularities enough?  The blower motor takes 280 watts.    Thanks for any suggestions.


  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I am not a professional.

    I use computer-type UPS units on my computers. Such a unit MIGHT help with your problem. But I would be more comfortable figuring out what the problem is before investing in one for your water heater, and be real glad you do not have an electric one because the cost of that would really be something.

    The units I have have several features that would help bridge the gap if the power is flicking on and off rapidly. One is that it will power the load for a period of time after the power fails, and if the power comes back in that time (or sooner), it restores the power and the only thing the load experiences is a dropout of about one cycle (16 milliseconds) as the unit switches from the power company to the battery and as it switches back. A computer power supply can tolerate a 16 millisecond dropout. I do not know how your water heater will react to that. (These units also include a surge-protector and also switch to battery if the incoming waveform gets too noisy.)

    Another feature is really two: if the UPS has shut down the load, it will not restore power to the load until the later of two conditions are satisfied:

    1.) a certain time has expired. On my UPS units, the user can set this time, and I have mine set to 300 seconds. This handles the short-term problems as ice falls through the lines for a moment, or an automatic circuit breaker at the power company shuts off and retrys.

    2.) the battery is recharged. On my UPS units, I set this to about 50% recharged, so it has enough time left to do a normal computer shutdown if the power is cut off again soon after it is restored. I do not suppose this second feature will matter very much to you.

    These units are normally rated in VOLT-AMPERES as well as watts; the difference has to do with the power factor of the load. Now if your blower moter is 280 watts, you should probably get a 400 VA, 280 watt unit or larger. Larger because the controls will also take power and they should be supplied too. Not to mention the controls for the heating system and circulators, zone valves, etc. It all adds up.

    One of my UPS units is 2200 VA and about 1600 watts. It cost something like $900. and the batteries need replacement every 3 to 5 years, and if you forget, they will quit just when you are away for 3 weeks and the power goes out.

    My UPSs are Smart-UPS units from APC; this one is slightly improved since the one I have for my main computer. Note this: Environment: 32 - 104 °F (0 - 40 °C)

    and bear in mind that it uses lead-acid storage batteries whose capacity goes down as the unit gets colder.

  • jmanning
    jmanning Member Posts: 2
    UPS sounds very likely to be helpful

    Dear JD,

    Thanks for the very thorough and prompt response. I felt that it would be good to do a reality check on the UPS idea.  While nothing is guaranteed, it

    does sound as if my problem would be very likely to benefit from a

    UPS.  Everything you said about them sounds helpful in my situation. I agree it would be important to understand the problem better first, and indeed I intend to have a lengthy talk with A.O. Smith's tech support ASAP.

    In terms of sizing the UPS:

    Somebody suggested using an inexpensive "Kill-a-Watt" unit to

    measure the power drawn by the water heater, of which all but a tiny

    fraction is in the ventilation blower.  The circulating pumps and

    everything else is powered separately, and they are normally allowed to go off

    and on anytime they want under the control of the room thermostats with

    no adverse effect on the water heater.  So that equipment can fend for itself.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    In terms of sizing the UPS:

    While it is not a good idea to oversize a boiler, it can be a good idea to oversize a UPS (a little). Do not be ridiculous like me and put a 2200 VA UPS on a 600 watt computer though (I did that because of a large starting surge, and because my power often goes off for 20 minutes or more and that lets me run up to about 40 minutes with new batteries). Once my power went off for a couple of days and that was too much for any UPS I might afford. If it were critical, I would have a back-up generator to start up before the UPS ran out. Fortunately, for me, keeping a computer up like that is not critical.

    What is the ignitor? I do not know what power it takes for the high voltage electrozapper type, but the hot wire type ones I have seen take quite a bit to get the flame started. It lights only for a little while, but the UPS must be able to supply it. If you have a standing pilot, that is a different story.

    Using a Kill-A-Watt meter can be helpful, but reading the name plate on the heater may be just as good. It should specify the load there. I do not know if either will include the starting surge. Also, if your gas water heater is like my former electric one (I now use an indirect fired one off my boiler), it is wired in and the Kill-A-Watt meters I have seen are plug-in only.

    The main advantage of oversizing a UPS is that as they age, the run-time decreases. And you want whatever run-time you decide you need to be available when the batteries are, say, three years old (at which point you should consider replacing the batteries).

    In my opinion, there are good and bad UPS units out there. I cannot make any comparisons from direct experience because I have used only the APC (American Power Conversion) units. My current ones are all of the Smart-UPS series. You can read about them at www.apc.com . The features that I like are:

    1.) If there is a brownout, it boosts the output by diddling a tap on its transformer to raise it instead of draining the battery. If the brownout is too deep, or if there is a blackout, it switches to battery. Similarly, if the voltage is too high, it diddles the tap to lower the output voltage. If the voltage goes too high, it switches to battery. This feature could be of value to you. Perhaps other brands of UPS also have this feature.

    2.) It contains noise filtering and surge protection circuitry. I suppose most other UPSs contain this too, but you should check. It also watches the input waveform, and if it is too noisy, it switches to battery. This may be of value to you.

    3.) When power is interrupted long enough for the UPS to shut down the load, when the power is restored, it delays a while before repowering the load. Thus, if the power line is flicking up and down, the load will not see it. If that is your problem, this feature is necessary for you. If you get a different UPS, be sure it has a feature like this.

    4.) Most (if not all) Smart-UPS units produce sine-wave output. I do not know about your hot water heater, but I would expect motors like this and suppose they might run hotter with other waveforms. Cheaper UPS units put out square waves or a stepped approximation to a sine wave. I would hesitate to run a motor with those. So check for this too.

    One thing I did, when I got my new boiler with electronics instead of just relays, was to have a "whole house" surge protector installed in my power panel. It will not protect for everything, but it will clip off the peaks of things. Mine is a Square D QO2175SB secondary surge arrestor. Of course, it works only in their Series QO power panels, but you can probably get something similar for whatever you have.
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