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  1. #1
    Silver Member
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    Aug 2004
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    Southern Wisconsin
    Tractor
    New Holland TC33DA hydro w/supersteer

    Default Water heater - how hot is too hot?

    I have an outdoor wood burning furnace that provides heat for my house. I want to expand the system to also heat my domestic hot water.

    I have a 50 gallon propane water heater, and constructed a heat exchanger that transfers heat from the woodburner water loop to the heater tank via thermal siphoning. It works VERY well, which is actually what concerns me.

    The temperature of the water in my woodburner water loop is about 180 F. I can get the temperature of the water in the heater tank to within a few degrees of that. Is that too hot for my water heater tank? The relief valve on the tank is rated at 150 PSI and 210 F, so I'm below that, but how accurate are those things anyway? Am I running a risk of popping the relief valve?

    I also wonder about the high temperature cutoff in the regulator itself. I can't find any documentation of what mine is set for. It's a Robertshaw propane regulator, and have read that these don't self reset once they trip. So, I want to avoid activating that safety device by having the water in the tank too hot.

    I'll be using a tempering valve to keep the water going to the faucets at a safe temperature. That doesn't concern me. The effect of the higher temperature on the water heater itself is what I'd like to know.

    I've also read about magnesium and other minerals coming out of solution in the tank if the temperature is above 140 F. I guess I'm willing to sacrifice some tank life if I save on energy costs, but would like to know how significant this problem really is.

    Thanks,

    Tom

  2. #2
    Elite Member RonMar's Avatar
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    Port Angeles WA
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    Jinma 284 delivered 06/28/05

    Default Re: Water heater - how hot is too hot?

    I wouldn't take it over the max tmp that the hot water heater can currently deliver/regulate to, which is probably in the vicinity of 130-140F, for the reasons you just mentioned.
    Ron

  3. #3
    Elite Member
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    Mar 2005
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    Bedford, VA
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    John Deere 2320

    Default Re: Water heater - how hot is too hot?

    the water heater mfgs recommend keeping the hot water under ~120 degrees to keep from scalding someone..

    Brian
    Bedford, VA
    2320 w/ 62D MMM, 200CX FEL, Pats EZ Change, LX4 Cutter
    Co-owner (with my father) of John Deere 790, 30 HP, 4x4, 513 cutter, 70 FEL
    2012 Nissan Xterra Pro-4x

  4. #4
    Super Star Member Egon's Avatar
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    Nova Scotia, Canada

    Default Re: Water heater - how hot is too hot?

    Best be talking to the manufacturer of your hot water tank.
    Egon
    50 years behind the times
    Livin in a
    Worn out skin bag filled with rattlin bones

  5. #5
    Platinum Member davitk's Avatar
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    Oct 2007
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    South Central Wisconsin

    Default Re: Water heater - how hot is too hot?

    Quote Originally Posted by adlertom
    The relief valve on the tank is rated at 150 PSI and 210 F, so I'm below that, but how accurate are those things anyway? Am I running a risk of popping the relief valve?

    Thanks,

    Tom

    I would be more concerned with the possibility of creating a bomb in your own basement - those TPR (temp/pressure relief) valves sometimes stick - and it is there for a reason. Water heaters tanks are relatively whimpy.

  6. #6
    Silver Member
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    Southern Wisconsin
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    New Holland TC33DA hydro w/supersteer

    Default Re: Water heater - how hot is too hot?

    Quote Originally Posted by thatguy
    the water heater mfgs recommend keeping the hot water under ~120 degrees to keep from scalding someone..

    Brian
    I will address that issue with a tempering valve. That is not what concerns me. The temperature at the faucets will be controlled to be less than scalding.

    What concerns me is the effect of the higher temperature on the hot water heater itself.

  7. #7
    Silver Member
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    Southern Wisconsin
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    New Holland TC33DA hydro w/supersteer

    Default Re: Water heater - how hot is too hot?

    Quote Originally Posted by davitk
    I would be more concerned with the possibility of creating a bomb in your own basement - those TPR (temp/pressure relief) valves sometimes stick - and it is there for a reason. Water heaters tanks are relatively whimpy.
    I certainly don't want the tank to explode. If however the relief valve did stick for some reason, my "backup" safety relief device would be my PEX tubing. That would rupture before the tank exploded. The PEX tubing PSI/temp rating is only 100 PSI at 180 F, and 80 PSI at 200 F.

    A rupture would certainly create a watery mess, and I don't want that. I don't however think a tank explosion is very likely in my situation. A lot of things would have to go very wrong before that happened.

    My concerns center around those stated in my original post.

  8. #8
    Platinum Member
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    Jul 2007
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    Hagerstown, MD
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    '05 Kubota BX1830

    Default Re: Water heater - how hot is too hot?

    I know I'm not answering your question. But.... After reading the scalding chart, if you don't have children, or very elderly, I would have the water heater temp at 140. While you can submerge your body in 120 deg water for 5 minutes and not get scalded, it isn't hot enough to kill much of any bacteria that may be in the water.

    Here's a link to the scalding chart Hot Water Burn & Consumer Safety: Chart - Accurate Building Inspectors ® | 1-800-640-8285 |
    '05 Kubota 1830
    HST, FEL, 54" MMM, Canopy,
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  9. #9
    Gold Member
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    Southern NY

    Default Re: Water heater - how hot is too hot?

    I personally wouldn't rely on the pex for safety. I have seen pictures (at the supply house) of hot water heaters that exploded and it ain't pretty. They literally can take off like rockets and end up in your neighbors yard. Install a secondary pressure relief valve. I usually get 12yrs plus on my hot water heaters by keeping the temp as low as possible. The hot water heater is a steel tank coated with glass to prevent rusting out. The glass does crack from thermal expansion or poor handling and installation and that's the purpose of the anode in the tank. -Ed

  10. #10
    Elite Member RonMar's Avatar
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    Port Angeles WA
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    Jinma 284 delivered 06/28/05

    Default Re: Water heater - how hot is too hot?

    Quote Originally Posted by Red55
    I personally wouldn't rely on the pex for safety. I have seen pictures (at the supply house) of hot water heaters that exploded and it ain't pretty. They literally can take off like rockets and end up in your neighbors yard. Install a secondary pressure relief valve. I usually get 12yrs plus on my hot water heaters by keeping the temp as low as possible. The hot water heater is a steel tank coated with glass to prevent rusting out. The glass does crack from thermal expansion or poor handling and installation and that's the purpose of the anode in the tank. -Ed
    They did this on Mythbusters... Awesom explosion!. Their best sent the tank shell probably more than 100' in the air and pretty much demolished the test house structure they built around the tank.
    Ron

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