How long 30 gal tank to give shower


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Old 12-02-07, 05:14 AM
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How long 30 gal tank to give shower

Is there a rule of thumb a person can use for how many minites of shower time a person can get from a full tank of hot water.
I know that there are variables such as shower temperature and tank size and tank setpoint and shower head restriction

Just a ball park for say a 151 liter gas fired hotwater tank with just a normal shower head. Shower at "medium temp"
 
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Old 12-02-07, 08:46 AM
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frankee, there are several variables, but here's the basic math.

How much of the flow comes from the hot side and how much from the cold? Say 90% hot... this depends on how hot your water is. With any scalding control, the % hot gets high since it isn't as hot to begin with.

How much does the showerhead flow in total? Say 2.5 GPM or 10 litres per minute.

What's the usable capacity of your water heater. Let's say 75% - once the cold starts mixing in, the end of the tank gets iffy as far as maintaining temps are concerned.

So 90% x 10 litres is 9 litres per minute.
(or .9 x 2 = 2 gpm)

And 75% of 150 litres is 112 litres.
(or .75 x 40 = 30)


112 / 9 = 12 minutes
or
30 / 2 = 13 minutes (everything is bigger in the U.S.)

Assuming you are starting with a full tank. Plug in your own assumptions.

You can measure your showerhead by holding a 5 gallon pail in front of it for a minute and measuring. Sadly it's a 2 person job and SWMBOs think we're nuts when we do things like this.
 
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Old 12-02-07, 02:13 PM
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That 75% figure I've seen everywhere, so that's what I use as well.

Only thing is, this is all assuming that the heater is switched off, isn't it? Or doesn't it matter? The math is spot on but that result of 13 minutes seems extremely short to me, based on just my own usage. (I used 80% hot in my calcs cause 90% seems way too high but that's still only 15 minutes)

I've got a 30 gal heater, have removed the flow restrictor in my detachable showerhead, and am easily able to get at _least_ 20+ minutes before I have to start slowly increasing the hot ratio.

The heater seems to take about 20 minutes to heat the entire tank. That seems pretty quick, so I would think that would have some effect on the minutes you get out of a tank.

Well heck I've got nothing to do today, think I'll measure the showerhead output as suggested, temperatures, and minutes. Might be interesting to see where this really ends up compared to the theory.
 
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Old 12-02-07, 03:48 PM
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Yes core, I don't have any recovery in my calculations because the calculations get obscured and I figured if the recovery was good this question wouldn't be asked.

If your tank is 140 and your shower is 110 and the cold is 55 then the ratio of hot will only be 65%, and you'll get a longer run than assuming 90% is from the hot side... I was just trying to show the math.

If you want to factor recovery in, then the math gets far more complex.

A BTU is the amount of energy required to raise a pound of water one degree. An electric water has elements that are 3, 3.8 or 4.5 KWHs. If 3 KWH then that times 3413 gives you the 10,000 BTUs. Divide that by 8.33 to go from pounds to gallons and then divide by hot temp minus ground temp and you'll have your recovery time.
 
 

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