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Thread: Hot Water Heater Questions

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  1. #1
    DIY Junior Member stanh's Avatar
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    Jun 2011

    Default Hot Water Heater Questions

    I have a question I am hoping someone can help me with. I had a 50 gallon (40K BTU) AO Smith water heater for about 10 years. In the past few months it was not producing enough how water for more than one person, but, before that we had no issues with my wife and I taking back to back showers. So, I decided to replace the tank with a new one that I bought from Home Depot. It is a GE 50 gallon tank (made by Rheem - GG50T06AVH00 - 38K BTH 50). However, I am finding that the tank is acting in a very similar way in the sense that it cannot produce enough hot water for back to back showers.

    I called GE and they told me my expectations are too high? According to them a hot water tank is only partially hot water (which I get) and I forget the amount quoted, but I think the hot water content was something like 35 gallons (so, say 70% of the total). So, by their calc with my showhead putting out about 3 gpm. I could only count on 10 to 12 minutes of hot water. Of course this has to be wrong give the 35 gallons are heated well beyond what anyone would shower at (we set the dial between A and B - I forget the exact temp range but the water at the highest temp is very hot).

    One other thing I had heard from GE is that our groundwater is cold here (just outside of Chicago). But we have the issue regardless of outside temp.

    GE has offered to send someone out (at a cost if they find no issue) but they assure me that nothing is wrong.

    I am wondering if anyone can help me with the following questions:

    1. Could anything outside of the hot water heater be at play? We did a bathroom remodel and replaced the shower hardware (including a new value etc). There was no immediate issue after the remodel, but, I thought I would throw that out there as we had seen something similar with both water heaters. We did check both the kitchen and shower and temps are the same.

    2. Is there a real calculation that I can use to figure out how many minutes of hot water I can expect? If not, does anyone have any real world experience with this water heater or any 50 gallon water heater that they can share? Given my experience with the AO Smith this water heater is not working correctly.


    Last edited by stanh; 02-19-2012 at 08:58 AM.

  2. #2
    In the Trades liquidplumber's Avatar
    Join Date
    Feb 2012
    Gastonia NC


    Back to back showers should be no issue IF your taking a shower to get clean and not to hang out until you are pruned....lol Seriously tho, incoming cold water is a big issue that folks dont think about. Lets say your heater is heating water to 140 degrees ( which is pretty hot!) normal shower temp is more or less give or take 100 degrees and incoming cold water (in wintertime) is 40 degrees If we take your 50 gallon water heater and deplete it by half, now its half full of hot water (140) and half full of cold water (40) which would make the water inside maybe 90 degrees.. which is not as hot as you are calling for in your 100 degree shower. So your 3 gpm showerhead would run out your usable quantity of water pretty quick ( here we base hot /cold water usage in shower at 70% hot 30% cold) With these numbers 3 gpm in 10 minutes would equal 30 gallons, 70% of which is hot. Whats 70% of 30 gallons? 21? So in 10 minutes youre almost down to that halfway point where half the heater is 140 water and half is 40 water.. again 90 degrees.. At 90 degrees in your shower you are probably already cranking the hot valve up a little higher and the cold down a little which is changing the ratio of hot/cold even further making you use even more hot.
    Most folks dont have their water heaters turned up to 140 either. More often would be 125 to 130
    In the summer when your incoming water is already 75 degrees you barely use any hot to reach the 100 degree mark in the shower PLUS 75 degree and 140 degree water mixed together in your water heater would leave you with 100 degree water for a much longer period of time.
    In short, IMHO this is probably a seasonal issue that should clear up when things warm up outside.

    yes I do realize that the hot water in the heater rises (toward the outlet) and the cold comes in and is directed to the bottom... but they are still going to mix and lower the overall temperature

  3. #3
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    There is a "70%" rule on water heaters, because the instant you start drawing hot water out, fresh cold water is coming in mixing into the tank. Of course there are variables, such as what temp. the tank is at and the temp of the incoming cold water, and even what temp you are showering at

    The 70% rule is supposed to approximate how many gallons you can get that are hot enough to shower in. That is in itself a variable....do you shower at 100 or at 108??. There are guidlines about what percent of the water coming out of the shower head, and of course that is subject to the variables discussed.

    The difference in BTU is slight and should not be that noticeable. Hard to explain what you used to get, but the new unit should giver about 10 minutes worth of showers, not much more.

    If you take 10 minute showers, you won't get two of those back to back.

  4. #4
    In the Trades Plumber111's Avatar
    Join Date
    Feb 2012
    North Carolina


    Call the plumber back. He can probably give you better performance. I could.

  5. #5
    DIY Senior Member SteveW's Avatar
    Join Date
    Feb 2005
    Omaha, NE


    Simple thing to check would be the actual temperature coming out of the showerhead when shower valve set for full hot. On my WHs the "A" and "B" settings are not at all reliable and I have had to crank up the WH temp setting to get hot enough water for showers, not to mention for the dishwasher.

  6. #6
    Join Date
    Jan 2010
    Blog Entries


    With a standard btu input residential water heater you can draw 70% of the tank and still be within 20 degrees of your initial temp. For example if your water is 140 degrees and you have a 40 gal water heater you will be able to draw 70% (28 gal.) and have no less than 120 degree water.

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