40 Gal Water Heater Runs Out In 15 Min. - Plumbing - DIY Home Improvement | DIYChatroom

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Old 12-02-2011, 05:46 PM   #1
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40 gal water heater runs out in 15 min.

Problem: after about 15 minutes in teh shower my 40 gal water heater (only sevral years old) starts cool'n off, I replaced the heat'n elements & it helped just a little but not a lot, after a couple months it starts with teh cool'n off again.

The unit I replaced done the same thing, after replace'n the elements sevral times I finally broke down & bought the water heater we have now, so why would they both do the same thing ??


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Old 12-02-2011, 06:11 PM   #2
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This topic has been discussed at least three times in the last year or so on this forum, you may want to do a search. But let's start with this. You have a 40 gallon heater, assume it is at 120 degrees F. Your shower probably runs at about 2 gallons per minute, unless you have the low flow 1.5 gallons per minute showerhead (check the head to see the rating). At two gallons per minute, after 15 minutes you will have used up approximately 30 gallons of hot water, if you keep it at 120 degrees, and shower at 120 degrees (that is what mine is set at). So you will have used up 3/4 of your hot water, which would have been replaced by cold water in the tank, and due to normal mixing, the water coming out of the tank will be cooler, i.e. start to cool off, which is what you report happens. So why does this surprise you?

If you want to check the flow in the shower, get a 5 gallon bucket and time how long it takes to fill. Measure the temperature of the water at the showerhead with an accurate thermometer. This will tell you all you really need to know to determine if everything is normal, which I suspect it is.


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Old 12-02-2011, 06:34 PM   #3
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a 15 min shower on and electric water heater....no no..as dan said do the math....what i tell my customers on electric water heaters if you want a long shower get a bigger tank....50 gal or even 80 gal more storage...
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Old 12-02-2011, 07:29 PM   #4
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Also you can raise the temperature in the water heater, then for your shower, you would be using less hot water and more cold water when you adjust the shower valves. So the hot water would last longer.
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Old 12-02-2011, 07:39 PM   #5
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Typically today's shower heads generally allow for 2.5 gallons per minute. So there you are. The question has been answered above.
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Old 12-02-2011, 09:44 PM   #6
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Thanx gents, didnt realize it had been covered already, sorry about that, never done the math but was curious about flow rate into the water heater, will do a test & go from ther.

thanx again.
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Old 12-03-2011, 07:32 AM   #7
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I had a customer once with two 50 gallon heaters--I noticed they were set to 'high' so I asked ,"What's with that?"

He said that the hot water ran out after one shower---

The plumber had reversed the hot and cold pipes when the house was built---simple fix that no one thought to check---He had replaced both tanks months before---and the new tanks were piped just like the old---plumber assumed the original install was correct,I guess.
New members: Adding your location to your profile helps in many ways.--M--
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Old 12-03-2011, 02:51 PM   #8
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40 gallon tanks are what we use at the apt. complex and get that same question all the time.
Besides what has already been pointed out, I tell the tenant that you can't turn on the shower and then start dilly-dallying around while the water is running.
Also, don't be running the dishwasher when taking a shower.


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