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Electric Water Heater question (thermostats...)

Post in 'DIY and General non-hearth advice' started by maple1, Apr 17, 2013.

  1. maple1

    maple1 Minister of Fire

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    Maybe a dumb question, but why is it so important (at least according to my manual) for the upper & lower thermostats to be set to the same setting?

    The way mine (80 gallons) gets used most of the time (heated slowly over time by my boiler via heat exchanger), I get intermittent stratification. Which is fine - and we never run out of hot water. So I'd like to turn my bottom thermostat down some so the lower element won't come on during those times, as there is still lots of hot water in the upper part. What bad things will happen if I do that?

    I'm thinking that if it is so critical, they would only have one thermostat setting on them anyway since even with best eyeballing, there is bound to be some little differences between them no matter how good you try to get them the same. Plus they're not exactly the most precise control devices around.

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  2. maple1

    maple1 Minister of Fire

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    After a bit more Internet reading, I think that is specified so that one element won't be heating more frequently than the other, thereby helping to prevent premature burnout of the overused element.

    In my case, with an inherent stratification of 20°F or so from heating from the top down with an outside heat exchanger, I think the thermostats should also be set to correspond to the same differential - say maybe 120/100 top/bottom? They will only kick in as a backup anyway if I go too long between fires in my boiler & storage cools off too much.

    Anybody else?
  3. heat seeker

    heat seeker Minister of Fire

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    I believe the reason for two elements is for quicker recovery when you run the tank "empty" of hot water. The upper element will run to heat a few gallons up top so that you have some hot water available quickly. When the upper thermostat is satisfied, only then will the lower element operate to heat the rest of the tank.

    The water will stratify in the tank with non-use. Some people have put small circulator pumps in, connected so as to circulate the water in the tank. That works, but it defeats the feature I mentioned above.
  4. Highbeam

    Highbeam Minister of Fire

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    I don't see any way that the elements will be see the same hours of use. That bottom element does most of the work since you don't often run the whole tank cold. 80 gallon tank and most of the uses will be a load of laundry at 10 gallons, washing hands at 1, etc. All those little and frequent loads will be satisfied by the bottom element.

    I don't see how it would hurt anything to have different set points but you may find that your 80 gallon tank doesn't last as long when part of it isn't as hot.

    Guys, how low can you set a standard water heater thermostat? I have an application where I will want 100 degree water.
  5. maple1

    maple1 Minister of Fire

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    I also don't see where they'd see the same amount of use - but that was the only reason I saw speculated in the searching on it I did.

    Reasons seem quite clear (although there is debate) for not lowering the temps in general - bad bugs (legionnaires) growing in there. But the top half of my tank will most often be in the 140 range, and that's where the water exits, so I'm not really worried about that. I have two temp probes & thermometers hooked up to mine, they're located where the elements are. I have an extra bottom thermostat tied very securely, and very well insulated, around the entrance pipe at the bottom fitting up tight to the tank. (That fitting is also the exit point to the exchanger when the tank is heating). It opens & closes a zone valve on the heat supply to my exchanger. I've got it turned down as low as it will go, which is 110°. Last night while I was working down there and the boiler was heating the tank, the temp at the bottom heating element was around 135 (top was heading for 160) and the zone valve hadn't cut out yet. The night before exact same conditions & it cut out at around 125. So I've concluded these thermostats aren't too overly precise to begin with. Plus they have next to no differential - when my zone valve does get closed, it opens right back up again in just a few minutes.

    I am going to replace that extra thermostat with a Johnson A419 which I ordered this morning - that should fix my control issues. And unless I read of bad things that will happen which I haven't read yet, I will most likely lower my tank thermostats to something like 120/100 (the thermostats that came on my tank go down to 90°, the extra one I had laying around that I'm using for my zone valve only goes down to 110°). For those few periods that I know there won't be any heating of it by wood, I will likely bump them back up to say 120/120 or so.
  6. woodgeek

    woodgeek Minister of Fire

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    They say legionaires (the bugs, not the old guys) can grow when the temp is below 110-115°F, many recommend 120°F to be safe. Also, takes a long time to burn yourself at 120°F, so also safe that way.
  7. Retired Guy

    Retired Guy Feeling the Heat

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    Bottom element on mine doesn't energize till the top one is satisfied. With typical usage I'd think the top one would do most of the work, Only safe way (non disease) would be with a mixing valve - like in a shower.
  8. Highbeam

    Highbeam Minister of Fire

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    My warm water application is for a closed radiant floor heat system. I don't want primary/secondary loops and pumps so the HWT contents will hit the floor at whatever temp the tank is set for and then quickly tail off. The 4500 watt element will at that time act like a low output boiler. HWTs are cheap compared to boilers. Turns out that most water heater stats can be set for under 100 degrees which is much nicer to a 40 degree slab than 120 degree water, easier on the heater, and less standby loss from the tank.

    I understand that the bottom element is the first to fire and then, if the top thermostat calls for heat then the bottom element shuts off and the top element will fire in an effort to more quickly heat that smaller top half volume. Then, once the top stat is hot enough, the top element shuts off and the bottom element comes back on to finish heating the tank. All electric water heaters are supposed to work this way.
  9. heat seeker

    heat seeker Minister of Fire

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    Is a water heater going to provide enough BTU's to heat a room(s)?
  10. maple1

    maple1 Minister of Fire

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    I have no doubt a water heater can put out enough BTUs to heat a room(s).

    The electric boiler I put in this fall for backup heat is 20kw. It has 4 x 5000 kw elements. Or about twice the heating capacity of a 'typical' water heater that might have 2 x 4500 kw elements. It only got used one day this winter, the second day of a two day trip away from home over the holidays. When we tested it after hookup, and when it was running when we got back from the trip, it was cycling on & off regularly. I.e. heating the house (2700 sq.ft. two storey) at quite a bit less than full capacity. I don't think all four elements were on all at once for any more than two minutes - they ramped up & down from 0 to 4 then back again.

    There is no way, though, that I would want to heat my house for a whole winter on that thing - the electric utility would own my hide....
  11. Highbeam

    Highbeam Minister of Fire

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    Yes. Of course. 4500 watts is more than 15000 btu and if I want to configure both elements to run simultaneously I can get 30,000 btu. I am setting up the piping so that if I want more power I can use a real mod/con propane boiler but for the desired use which is only freeze protection, I will start with the water heater.

    You might be surprised how cheap electric resistance heat really is. Our power costs 10 cents per kwh delivered which matches propane today. Lots of old myths about how much electric heat costs. In a garage, there are some advantages fireless heating.
    woodgeek likes this.
  12. heat seeker

    heat seeker Minister of Fire

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    Sounds reasonable. But, around here, electricity costs over 20¢/KWH. We're getting ***** by the power companies.
  13. woodgeek

    woodgeek Minister of Fire

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    I paid 15 cents/kWh in Chicago, back in 1988. :eek:
  14. Highbeam

    Highbeam Minister of Fire

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    I am amazed that the light from my 60 watt bulb here at my side is just as bright as yours yet it only costs me half as much to run. Living in a more populous part of the country, I would expect the economies of scale to benefit you easterners. I suppose our hydro dams have benefits.

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