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Water Heater replacement

CCI | Posted in General Discussion on November 20, 2005 07:49am

I am looking to replace my gas fired water heater.  The one I have now is a 50 gallon unit and it has provided enough hot water.  There are 5 in the house but due to schedules we never do more than 2 showers in a row.

With natural gas costs so high I was wondering if it is better to get another 50 gal unit and keep the temperature control where it is or should I get a 75 gal unit and be able to lower the temp?

I guess it comes down to: is it better to heat some water to high temp. or more water to a lower temp.  We have not run out of hot water with the existing unit but I can’t run it full hot and take shower, I have to turn on the cold to moderate the temp.  This leads me to believe that I could be just as comfortable with more “hot” water at a lower temp.

With gas prices so high I wonder if I am better off keeping 50 gals at 140 degrees or 75 gals at 120 degrees since most of the time the water is just sitting in the tank waiting to be used.

I realize point of use heaters are probably the best option but for now I am sticking with the old standby system.

Thanks.

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Replies

  1. yojimbo | Nov 20, 2005 08:38am | #1

    If you are looking to save on energy costs I suggest you look into having a tankless water heater installed.

  2. User avater
    rjw | Nov 20, 2005 03:42pm | #2

    >>With gas prices so high I wonder if I am better off keeping 50 gals at 140 degrees or 75 gals at 120 degrees since most of the time the water is just sitting in the tank waiting to be used.

    Depends on your health insurance program and tolerance for excruciating pain.

    NEVER set your water heater to 140.

    At 140, a child will get second degree burns in 0.8 seconds, third degree in 1.5

    An adult gets 2nd degree in 2.8 and 3rd in 5.4. (5.4 is a long time unless you've slipped in the tub/shower and have the 140 degree water pouring down on you while you struggle to get up and out.)

    A Second degree burn is a middle penetration burn causing blisters which often become infected when the blister breaks.

    A Third degree burn is a full skin penetration burn that destroys nerves and will require skin grafts.

    And note: when the hot water in a water heater has been drawn down, and it is reheating, the water at the top, where it gets served out to the fixtures can significantly exceed the "set" temp. So, 140 stable state can get a lot hotter when the water is being reheated.

    I've attached a .pdf file of some summary scald info I've collected


    View Image
    Sojourners: Christians for Justice and Peace



    Edited 11/20/2005 9:15 am ET by rjw

    File format
  3. WayneL5 | Nov 20, 2005 04:58pm | #3

    I don't know the answer to which would use less energy.  But are you using modern, low flow showerheads or old water hogs?  Do you turn the shower on full blast or at a reasonable level?  If you have the type of shower valve that you can't adjust the force (which thoughtless builders often install) then you can install a valve before the shower head.

    I'd go with the 50 gallon heater but buy one with fast recovery, and not set it so high that it is dangerous.  And make a rule that you get to shower first.

  4. DanH | Nov 20, 2005 06:52pm | #4

    If you set to a temp higher than 140 you need to have a tempering valve, to assure that the temp at the nearest faucet is never greater than 130-135.  Even at 140 the valve isn't a bad idea.

    The real issues in terms of efficiency are two:  Standby losses and burner/heat exchanger efficiency.  Though in general the standby losses are greater (in absolute terms) in a larger unit, whether that's true in a given case depends on insulation and other design issues.  A well-insulated 75G unit could have lower standby losses than a standard 50G unit.  (Also, you increase standby losses somewhat running the heater at a higher temperature.)

    Burner/heat exchanger efficiency (%of the gas energy "captured") will be about the same for units of similar design/quality, regardless of size.  Efficiency increases slightly at lower thermostat settings, but this can be more than offset if the burner ends up burning for shorter periods.

    In terms of comfort/capacity, in addition to gallon size you have recovery time and steady-state capacity to consider.  Generally the larger unit will have a larger burner, but if the 75G unit has only 50% more BTU input vs the 50G unit then it's recovery time is the same.  However, it can deliver 50% more flow at a steady state (with the burner on 100% of the time while delivering at the thermostat temp).  Probably a standard 75g unit is pretty close to keeping up with a single shower head.

    You can also go to a commercial unit.  These have maybe twice the BTU input of residential units.  (A 75G unit can keep up with roughly two showers.)  However, they're more expensive and are apt to have significantly worse standby losses.

    The economics/comfort situation for tankless heaters is all over the map.  One major problem with them is the "cliff" -- if you even slightly exceed the capacity of a unit then temperature falls off dramatically, whereas with a tank (where the assumption is you WILL exceed capacity in relatively short bursts) temperature changes are moderated by mixing hot and cold water in the tank.

     

    If ignorance is bliss why aren't more people

    happy?

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