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Thread: Hot water tank, whats cheaper to run?

  1. #1
    DIY Member yngwie_69's Avatar
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    Default Hot water tank, whats cheaper to run?

    I live alone and have a bath everyday, I have my tank set so I only need to turn the hot water one to fill the tub which uses 3/4s of the tanks water.

    Would it be cheaper to turn up the setting so i use 1/2 cold and 1/2 hot when filling the tub which im assuming would use 1/2 the tanks water.

    Like I said i live alone and its a 40gal

  2. #2
    In the Trades SacCity's Avatar
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    From an engineering point of view, it is all about heat transfer rates.
    So for this case the energy loss is from the heating the water, and the water giving off heat to the surrounding environment.
    in both cases heat transfer is a function of delta T, in which case the lower the water temperature the better the delta T for energy conservation.

    So to keep the water heater set at the lowest possible setting will provide the better energy savings

    Mike
    Last edited by Terry; 04-06-2011 at 09:08 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Lower temp means less energy, but then you have to consider potential health issues. Depending on who you want to believe, running the water at less than say 120-degrees means that there is the potential to grow nasty stuff in there. Some say you need even higher. Running it at say 105-110 which, depending on how far from the heater, might be a nice bath temp by the time it get to the showerhead might be putting your health at risk. Then there are things like a dishwasher that really doesn't work well with cool water. Much laundry can be done with cold, but there is the need for hot once in awhile.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Nuclear Engineer nukeman's Avatar
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    While lower temperatures do mean less heat loss, i don't think that you will ever notice on your bill one way or the other. By far the majority of the cost of heating water is in how much HW you actually use. The loss out of the tank to surroundings is generally quite small. I don't know if you have gas or electric, how new the WH is, or where it is located (garage, basement, closet, crawl space, etc.), but I can tell you that a newer electric WH costs about $20-$30 a year in standby losses. Moving the storage temp up/down a few degrees isn't going to make much difference.

    You would probably be better off running it a bit warmer (in case you are running at a low temperature for reasons that Jim points out). If you want to save $$, start by either reducing HW use (low flow faucets, shower heads, etc.) and insulating the hot water pipes near the tank (and any others that you can get to). This is especially important if the WH is in a space like a garage, crawl space, or attic, but should be done even if the WH is inside the house.

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    General Engineering Contractor ballvalve's Avatar
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    Here is an interesting article in which bacteria was found in larger concentrations in HOTTER water heaters. So go figure.

    http://aem.asm.org/cgi/reprint/25/1/72.pdf


    And here is an interesting one about garden hoses. Looks like life is a real mine field.

    http://www.cleanwatertesting.com/bull_heater-bugs.htm
    Last edited by ballvalve; 04-07-2011 at 10:37 AM.

  6. #6
    In the trades Dana's Avatar
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    "...While lower temperatures do mean less heat loss, i don't think that you will ever notice on your bill one way or the other. By far the majority of the cost of heating water is in how much HW you actually use."

    True, for electric tanks, but for gas-fired tanks with low volume use patterns, not so much. The standby losses are higher in gas-fired tanks than with electric, and grow even faster with rising temp due to center-flue convection losses. It would be pretty easy to measure the difference in standby loss between a 115F tank and a 140F tank (required installation setting by code in some places) in the bill. But a 1-bath/day 1-person household is using about half the volume at which HW heaters are efficiency-rated, and a 0.55EF gas-fired tank would be running an effective ~0.30EF at the DOE EF-test temperature of 135F, but probably ~0.50EF effective at 110-115F bath-draw temps.

    Would raising the temp by 20F break the bank due to the higher standby loss? Not at all, but you'd be able to measure it in your summertime non-heating season gas bills. With an elecric tank the difference in standaby loss would be small, and "in the noise" of all electricity use.

    But at any storage temperature or energy source, the standby losses can be further mitigated with R4 closed-cell foam pipe insulation on both hot & cold plumbing (including the temperature & pressure valve & outlet piping), within a couple meters of the tank. Doing the same for all accessable HW distribution pluming is also cost-effective as a DIY project. See:http://www.leaningpinesoftware.com/h...er_pipes.shtml

  7. #7
    In the trades Dana's Avatar
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    Quote Originally Posted by ballvalve View Post
    Here is an interesting article in which bacteria was found in larger concentrations in HOTTER water heaters. So go figure.

    http://aem.asm.org/cgi/reprint/25/1/72.pdf


    And here is an interesting one about garden hoses. Looks like life is a real mine field.

    http://www.cleanwatertesting.com/bull_heater-bugs.htm
    Thermopyllic bacteria are less likely to be human pathogens, and are of much less concern than those bacteria that thrive in temps near human body temp, which is a necessary aspect for being a human pathogen. (The legionalla sweet spot is from 85F to 115F and does really fine at both body and skin-surface temps.) Something that doesn't thrive below 125F just isn't very likely going to cause human health issues.

    OTOH Garden hose temps can spend many hours "in the zone" of human tissue temps for much of the summer and grow all sorts of stuff that you might not want to injest.

  8. #8
    General Engineering Contractor ballvalve's Avatar
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    Why are we alive when we spent our youth drinking from garden hoses while atomic bombs were being detonated monthly in the air in Nevada?

    And American Motors paint shop dumped its acres of excess in plain sight in a gully next to the plant [where a school is now built]

    And the earlier generation used DDT like talcum powder.

    Recent article says that automatic sensing faucets have a 75% higher chance of Leg. disease than a hands on. One hospital is removing hundreds of hands free units.
    Last edited by ballvalve; 04-07-2011 at 12:58 PM.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    Quote Originally Posted by ballvalve View Post
    Recent article says that automatic sensing faucets have a 75% higher chance of Leg. disease than a hands on. One hospital is removing hundreds of hands free units.
    I'd guess that's because this type of faucet has a tempering valve where warm water is fed up to the faucet and sits in the supply lines regardless of how hot the water coming in from the WH is. And, depending on how hot that is, there may not be much hot flow compared to a 'normal' faucet to flush it out (typical flow on that type of faucet is often less than a g/m.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    Between Ralph Nader on the East coast and California on the West, they keep us advised of ALL the hazards to our health and why we are doomed unless we follow their recommendations.

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