Proper thermostat setting electric hot water tank

Discussion in 'Water Heater Forum, Tanks' started by murrayd, Aug 29, 2009.

  1. murrayd

    murrayd New Member

    Messages:
    11
    I have an electric HWT with a lower and upper element and a thermostat for each element. What is the proper way to set the two 'stats? the same? one higher that the other?
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,815
    Location:
    Cave Creek, Arizona
    t'stats

    The proper way is to have both of them the same. Normally the lower one is going to control the tank's temperature until the lower element burns out and then the upper one will be in charge until you repair the heater.
  3. murrayd

    murrayd New Member

    Messages:
    11
    Great. Thanks.
  4. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    Why not set the bottom one lower by 10c than the upper?

    My reason for this is the upper part of the tank is always hottest, and depending on the demand for hot water tank size the upper half of the tank would be enough to provide reasonable hot water supply. Thus saving electricity and wasted heat/loss from heating the whole store.

    To prevent legionella you would want the tank to heat to 60c to kill the bug, so set your upper thermostat to 60c and the lower unit to 50c. Regardless if the lower unit heats up the hot water will rise up thus the upper unit will shut off when 60 c is reached whether it is on or not. And the lower unit will shut off at 50 c you could go for 45c even. Once a month it would be worth heating the whole store to 60c.
  5. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    I've seen these 140 F legionella claims and suspicious of them for several reasons. One is that the number of people I've known who got legionella: zero.

    At an actual 120 F in the tank they can't multiply. I have read that electric water heaters there is more potential for a problem, but in the same study with gas they were not detected at all. It's not hard to propose several reasons why with electric heater's low recovery rate resulting in lower bulk temps for extended periods, plus circulation benefits of gas flues (bottom to top heating). 140 F is for instantly killing legionella. That's a far cry from preventing them from growing in the first place.

    One thing running at 140F will do besides posing a scalding risk. It will result in much higher storage losses. You can easily figure the ratio between the difference in (new temp minus ambient room)/(old temp minus ambient room) to determine how large the loss is. Storage loss is not huge anyway, but with electric the effect should add about 150 kwh/year, and with gas perhaps 15 therms.

    Another thing hither temps will do is reduce the lifetime of the tank. Higher temperatures = higher corrosion rate.

    For an electric water heater to be on the very safe side, splitting the difference and going to about 130 F might make sense. However, I can't see how running 45 C (113 F) in the bottom would make any sense. All that does is guarrantee that the water in the bottom will be in a zone where growth might occur. It won't circulate because hot water rises, cold water sinks (until you get into the mid 30 F range.) You don't want stratification in the tank.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,815
    Location:
    Cave Creek, Arizona
    tank

    If the lower thermostat is set 10 degrees below the upper one, then that is temperature you will receive for about 70% of the water you use once you use the water stored above the upper thermostat. ALL water heaters, including gas ones, have hotter water at the top of the tank, but that does not mean you reset the thermostat below the level that gives you adequate hot water.
  7. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    The presence of sediment, sludge, scale, rust, algae and organic material in
    water systems can act as a source of nutrients for Legionellae and encourage proliferation. Water temperatures in the range 20°C to 47°C also favour growth, the optimum temperature being 37°C, but it is uncommon to find proliferation below 20°C. They tolerate a range of temperatures, although below 20°C and above 50°C they are dormant and above 60°C they will not survive.
    Infection is caused by the inhalation of airborne droplets or particles
    containing viable Legionella bacteria which are small enough to pass deep into the lungs. There is no evidence that the disease is transmitted by ingestion or from person to person
    Where hot water systems have been implicated in Legionellosis outbreaks,
    the organisms have been found to be disseminated by showers and taps with spray heads. This is because the bacteria is in the form of an aerosol and breathed in when in a shower.

    Generally our water regs in the UK specify the following:

    (a) Storage calorifiers and recirculating hot water systems must store hot water at a minimum temperature of 60°C and deliver hot water of at least 50°C to the outlet. OUED is responsible for monitoring the temperature of these outlets in accordance with the regime set out in Table 1.

    Consideration of the Legionella risks associated with calorifiers and central hot water systems must be taken into account during any hot water replacement programme. Wherever practicable, hot water storage vessels should be replaced with plate heat exchange systems or direct gas fired hot water heaters.

    (b) Where thermostatic mixing valves (TMVs) are installed these should be sited as close as possible to the point of use. As a general rule TMVs should not serve multiple outlets, but where this is absolutely necessary OUED will ensure that the pipework is kept as short as possible.

    http://www.masterplumbers.com/plumbviews/2000/legionnaire2.asp in the USA it would seem similar advice is given.

    Yes there will be higher heatloss at 60c to the ambient air but proper insulation will minimise that. The risk of scalding is not relevant if thermostatic mixing of the water takes place at the shower or via thermostatic mixer to the hot tap.

    Stratification of the tank is what the principle of solar heated cylinders work on I don't see why it is a problem within a cylinder heated electrically. If the upper element is used only the upper part only will be heated. But if the lower coil is used then hot water will rise to the upper level. The water will be heated to higher than 50c at the lower coil when it is on but it will only switch off when 50c (probably 51.5 -53c given the tolerance on the thermostat) so it will heat a greater part of the store to an acceptable temperature for hot water use than the upper alone.

    As to deaths from legionella, men are 3:1 more likely to die from it than women.

    There were 387 deaths associated with legionnaires’ disease in Europe in 2006, giving a CFR of 6.2%. The majority of cases were male (61.0%).

    An estimated 8,000 to 18,000 people get legionellosis in the United States each year.[3] Some people can be infected with the Legionella bacterium and have only mild symptoms or no illness at all.
    http://en.wikipedia.org/wiki/Legionellosis

    Perhaps it would be better to have both elements set to 60c and have a timer operate both to heat the store. The bottom element can switch off before the upper element if the upper part of the store only is suitable to meet demand and on longer when the whole store is required.

    Legionellae is a real and present danger and should not be discounted.
  8. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Insulation will minimize it at 120 F as well, and 140F will still be higher heat loss in that comparison and at the same ratio as both without. Nozzles, projections and thermosiphon appear to be as large a factor as the wall in storage tank losses. Most of these losses are at the top of the tank that you suggest running at 140 F.

    It is if you don't have that; and I've yet to live in a house that had it. It is very relevant in the five homes in the last three states I've lived in.

    You suggested 45 C before for the lower, which would be counterproductive if you are concerned about legionella. If you want to produce conditions that just might make its growth favorable, then do as you suggest. You will get a hot upper tank and a cold lower. Hot water won't move down easily, it is counter to natural convection. During long draws that lower tank will cool down even more, getting you in the dangerous range. Since electric has a poor recovery rate it will stay that way longer before it gets up to an already low temperature.

    I've had to modify reactors that had stratification from the sort of natural convection problem that your suggestion will produce. The original designers did not appreciate the buoyancy and thermosiphon effects. They expected a 1 C delta, but had a delta of about 15 C because they were fighting the thermosiphon rather than employing it.

    A gas water heater has actual heating of the base of the vessel so that the coldest portions of the tank get the hottest firebox temps. This is a far superior mixing distribution profile to an insertion element. It is more logical than trying to do top down heating of water because it works with natural convection rather than against it.

    Why not just run them both at a more moderate temperature and skip the complication of the timer? Timers only make sense if your electric billing has higher rates for peak hours.

    I'll be more concerned about legionella when I meet someone who has actually contracted it.
  9. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    clearly that's something you differently but there is no reason not to fit one they're not expensive


    I said you could with a proviso


    Of course and I haven't suggested otherwise. Heating with both elements raises the temperature in the upper tank quicker. But if you only draw off the upper amount then why heat the whole store.

    which is what I said, 60 and 50. if you want to heat the store to 50 there is no problem but again it will be a demand on hot water driven setting that suits the end user best. but if you only want to heat the upper part of the store then you can with the upper element alone. There is no reason not to use timers, the tank doesn't need to be constantly heated if you know when your demand is then heat the water prior to that. It's wasteful of energy to run it 24hrs.

    That's like saying I'll wear a condom when I meet someone with chlamydiae. You know if they're not dead from legionellae their hardly going to wear a t shirt advertising that they had it.
  10. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Do the math, figure the loss rate. Then explain how many kwh/ccf that timer might save. I've done it before and it is very little, actually surprisingly so. You can add insulation to the tank and all of the hot water piping for less than the timer and save an order of magnitude more energy doing so.

    Timers make sense for getting off peak rates. They would probably make sense if your tank was cooling rapidly/completely uninsulated. Problem is, the tanks don't cool rapidly, particularly electric which has a higher EF. The change in differential temperature that would drive your econ is much smaller than the unnecessary 120 to 140 F recommendation.

    Typical of the legionella hysteria and why I don't take it seriously. The number of legionella deaths is small compared to the number of infections, which is itself pretty small. And then only a portion is coming from water heaters. And since I have a gas water heater, I'm not at risk for it. By your criteria, I'm already "wearing a condom."

    On the other hand the suggestion of running the bottom of the tank colder is equivalent to cutting off the end of your condom and thinking you've done something really clever.

    So which way are you really going with this, because from what I can see your energy saving ideas don't hold water and seem to increase the risk you are concerned about rather than reduce it.

    I'm not saying that it wouldn't be prudent for electric water heater users to take some precautions, but I am saying that what you suggest is both wasteful and ineffective.
  11. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    Well from practical experience on a tank weused to have heated by electric our energy bills were higher having the tank up to temperature than only prior to demand. A 13amp timer will cost what $5 -10??
    but they'll be cooling after the hot water is drawn off?


    Well we can agree to disagree on that. The only reason I suggest a lower temp than 50 was to assist in raising the upper store temperature quicker if that is all that is needed to heat.
    For example here I wouldn't use the element stat to regulate the temp anyway that is the thermal cut out to protect the element. I'd have a tank sensor to control both elements and use the element sensor to cut out above a limit. lets say the demand is for couple of basins and a 5 min shower in the morning but in the evening to run a bath. Then heat the upper store for the morning demand and heat the whole store for evening demand. 50c only prevents growth 60c kills. I'm only suggesting running the lower tank colder if the demand isn't there, but to take adequate precautions against prolonged use. I'm also assuming the cold supply is to the bottom and the hot is drawn from the upper part.

    Yes a gas water heater heating from the bottom will be safe, but when were we talking about that?
  12. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego

    Our pleasure to have a master from across the pond sharing the discussion. Obviously, some things are different, hence the testy discussion!


    First I would say that algae and organic material in the water supply are not an issue for most of our population. I am not a chemist, but would question whether mineral sediment and sludge are supporters of virus. Anyway, perhaps the other thing which weighs heavliy on our procedures is the well know law firm of Dewey, Cheatham, and Howe. ALL water heater manufacturers, and many of our plumbing codes, SPECIFY that water heaters should NOT be set higher than 120ºF ( about 50ºC) for the reason that if someone gets scalded, the plumber and the manufacturer will get sued.

    By the way, "masterplumbersdotcom" is a commercial site selling products and services. They are entitled to their opinion, but it is not a site that I frequent for technical advice.


    While I often do use WIKI, I also take everything with a grain of salt. Personally, except for outbreaks involving the ventilation system in a public building, I have never heard of a case of legionaires from water. But certainly it is possible. There is some movement to update codes to require higher temp settings in conjunction with tempering valves. But such changes occur slowly, and often will only apply to new construction and ALL existing buildings are grandfathered.

    [video=youtube;9E-u5YNi8B4]http://www.youtube.com/watch?v=9E-u5YNi8B4[/video]
    Last edited by a moderator: Jan 28, 2011
  13. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    From what I've read, a pre-1998 electric water heater (US anyway) is about the only time this will amount to much (other than for those who have off-peak rate charges.) With current standby losses on electric heaters being so low, I doubt you would get 10 kwh/month reduction out of a timer even doing as you suggest. My shower draws are short enough and tank large enough that there wouldn't be much theoretical advantage to it and certainly not enough that I could identify it in a monthly electric bill, even though I'm down in the 400+ kwh/month range in non-summer weather.

    You might offset some of the 140 F setpoint wastage, but if you don't really need the 140 in the first place, what have you gained? At best you've created a scald potential and added an unneeded complication. With small kids and the resultant varied water use schedules as well as scald concerns I can't see the timer and overheat making any sense.

    I had a timer on an electric water heater in my previous home. It made a lot of racket next to the dining room so I disconnected it. That saved me the juice wasted on a relic timer I wasn't using anyway. Disconnected an irrigation timer for the same basic reasons in my current home. That one used about 4 kwh/month when plugged in.

    The prices I've seen for these timers start at about $32 without tax or shipping and go up to about $60.
  14. flamefix

    flamefix New Member

    Messages:
    71
    Location:
    Exeter, England
    You credit me far more than I deserve I'm just a regular plumber sorry :)

    Lawyers ... Well yes it is an issue here but the option here is thermostatic blending valves at point of use to prevent such.
    Sorry I quickly googled to find two places that referred to it, no bias was intended.

    broadly that's true here re grandfathering.

    this supports your view
    http://www.ncbi.nlm.nih.gov/pubmed/...nel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum

    and this mine
    http://www.nature.com/bmt/journal/v30/n3/full/1703628a.html
    http://www.springerlink.com/content/n15165627254146j/
    http://www.mdconsult.com/das/citati...=MI&sp=11171286&sid=0/N/11171286/1.html?issn=

    A view of how legionellae is risk assessed in the UK
    http://www.dcswaterhygiene.co.uk/services/legionella-legal-duties.asp

    Can't finish this post as out for a job now but will get back later.
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,129
    Location:
    New England
    My city requires a tempering valve on the output of any newly permitted WH. They don't dictate how you set it, or your WH temp setting.

    [video=youtube;9E-u5YNi8B4]http://www.youtube.com/watch?v=9E-u5YNi8B4[/video]
    Last edited by a moderator: Jan 28, 2011
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