The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 04-06-2007, 12:43 PM
Sofaspud Sofaspud is offline
Guest
 
Join Date: Jun 2006
How fast should a water heater heat water?

I have a (fairly new) water heater. Of course I know how long this particular heater takes to recover from a dishwasher/laundry/shower Trifecta of Doom that uses all the hot water, but if I hadn't timed it, how could I make a good estimate?

All I see on the label is marketing bunk -- basically, that THIS heater is better than the OTHER BRAND because it uses X less kw/h per year. Bah. This tells me nothing useful. I want to know how long I have to wait to avoid frostbite after Junior uses all the water, dangit.

I would guess that it's dependent, in part, on size. Is there a quick formula I can use to get in the right ballpark?
Reply With Quote
Advertisements  
  #2  
Old 04-06-2007, 01:00 PM
Zeldar Zeldar is online now
Guest
 
Join Date: Jun 2003
This is just from experience so it could be particular to my situation and not indicative of other heaters. After our "hot" water is reduced to cold from evacuating the contents of the heater, it usually takes a half hour or so to get enough hot water for a shower. Ours is 50 gallons, so you could extrapolate to 100 gallons per hour and adjust for that by whatever size yours is. Ours is electric if that should matter.
Reply With Quote
  #3  
Old 04-06-2007, 01:34 PM
Cheesesteak Cheesesteak is online now
Guest
 
Join Date: Jan 2001
There should be something on the label called FHR (first hour rating) that is theoretically how much water it can heat up to full temp in one hour.
Reply With Quote
  #4  
Old 04-06-2007, 01:44 PM
gotpasswords gotpasswords is offline
Charter Member
 
Join Date: Mar 1999
Location: San Francisco area
Posts: 14,706
Something else to consider is the consumption of hot water.

If you've got an old high-flow shower head, someone could run you out of hot water in 10-15 minutes. Replacing with a new low-flow head (Oh, stop whining - there are very good low-flow heads now that don't feel like someone's misting you with a plant sprayer) will work wonders at keeping hot water in the tank, rather than down the drain.
Reply With Quote
  #5  
Old 04-06-2007, 02:15 PM
Tuckerfan Tuckerfan is offline
Guest
 
Join Date: Jan 2001
Another thing to consider is how far your sink/shower is from the water heater. The water in the pipes leading to the faucet could contain water that has cooled down and thus skew your results.
Reply With Quote
  #6  
Old 04-06-2007, 04:41 PM
1010011010 1010011010 is offline
Guest
 
Join Date: Jun 1999
Required Data:
Tanks Capacity in Gallons = Vt
Heater Rating in Watts = Ph
Temperature of mains water - Water Heater Control setting = dT

(Ph/Vt)*dT = k

k has the fascinating units of gallon-seconds per Joule-Fahrenheit. If you're comparing replacement heaters to each other and to your current appliance, heaters with a larger value of k will recover faster. The actual number isn't important.

Turning k to a meaningful value of time is a matter of the specific heat capacity of water and a handful of conversation factors.
Reply With Quote
  #7  
Old 04-06-2007, 04:56 PM
Sofaspud Sofaspud is offline
Guest
 
Join Date: Jun 2006
Quote:
Originally Posted by 1010011010
Required Data:
Tanks Capacity in Gallons = Vt
Heater Rating in Watts = Ph
Temperature of mains water - Water Heater Control setting = dT

(Ph/Vt)*dT = k

k has the fascinating units of gallon-seconds per Joule-Fahrenheit. If you're comparing replacement heaters to each other and to your current appliance, heaters with a larger value of k will recover faster. The actual number isn't important.

Turning k to a meaningful value of time is a matter of the specific heat capacity of water and a handful of conversation factors.
You realize that now I'm going to have to go dig up some numbers from my heater, right? (After I get home, that is)

This is good info, actually; I'm in the process of remodeling and one thing we've been considering is a new water heater (hence the question). Now I know what to look for!

Hmm... I may have to take my laptop with me to the store, though... think they'll give me wierd looks if I'm plugging numbers into a spreadsheet while browsing?
Reply With Quote
  #8  
Old 04-06-2007, 04:59 PM
Sofaspud Sofaspud is offline
Guest
 
Join Date: Jun 2006
Quote:
Originally Posted by gotpasswords
Something else to consider is the consumption of hot water.

If you've got an old high-flow shower head, someone could run you out of hot water in 10-15 minutes. Replacing with a new low-flow head (Oh, stop whining - there are very good low-flow heads now that don't feel like someone's misting you with a plant sprayer) will work wonders at keeping hot water in the tank, rather than down the drain.
I know it's terribly eco-UNconscious of me, but I don't care too much about how MUCH water I'm getting out of the heater -- it's sufficient as-is for my needs at 55 gallons. What I'm trying to determine is, if I go shopping for another heater, what do I look at to figure out how long I have to wait after my wife takes a shower so I don't get frozen?

Put another way: even if we use low-flo heads (and I think we do, but I'm not sure), the end result will be longer showers. Nobody in my family gets out of the shower until the water runs cold, it seems.
Reply With Quote
  #9  
Old 04-06-2007, 05:36 PM
gotpasswords gotpasswords is offline
Charter Member
 
Join Date: Mar 1999
Location: San Francisco area
Posts: 14,706
I just realized another reason low-flow heads will help keep showers hot - slower draw from the heater means what? Less cold water coming in, or at least cold water coming in slower. If you can keep the consumption below the recovery rate, you will not run out of hot water. Compare this to sucking out all the hot water at once - then you're stuck with having to wait however long to heat a full tank of cold water.

I just had a look at my heater, and the First Hour rate is 81 gallons per hour - it's a 40 gallon tank, so it should take a couple seconds less than 30 minutes to heat up. A related spec is "Recovery at 90 degree Rise" or how how many gallons of water the heater can heat up by 90 degrees per hour.
Reply With Quote
  #10  
Old 04-06-2007, 06:13 PM
Lok Lok is offline
Charter Member
 
Join Date: Jul 2000
Location: Northwest Ohio, USA
Posts: 2,544
Quote:
Originally Posted by Sofaspud
You realize that now I'm going to have to go dig up some numbers from my heater, right? (After I get home, that is)

This is good info, actually; I'm in the process of remodeling and one thing we've been considering is a new water heater (hence the question). Now I know what to look for!

Hmm... I may have to take my laptop with me to the store, though... think they'll give me wierd looks if I'm plugging numbers into a spreadsheet while browsing?
Go to this US Dept. of Energy website. They have information on selecting a water heater. And be certain to check out the EnergyStar website to see if any of your home improvements can get you a tax credit for this year.
Reply With Quote
  #11  
Old 04-06-2007, 06:38 PM
1010011010 1010011010 is offline
Guest
 
Join Date: Jun 1999
Quote:
Originally Posted by Sofaspud
Put another way: even if we use low-flo heads (and I think we do, but I'm not sure), the end result will be longer showers. Nobody in my family gets out of the shower until the water runs cold, it seems.
Everyone talks about how low-flow makes your hot water last longer... but an unconsidered benefit is that it raises the steady state temperature of your hot-water after the tank is depleted. You can work out a combination of low-flow shower head and sufficiently sized water heater that the water can't run cold from a shower alone. (ninja'd by gotpasswords)

Allow me to flog my all-time-favorite low-flow high-velocity shower head...The Alson's Incredible Head power showerhead.. Model 650 or 652. All metal construction, can be completely disassembled for cleaning if you have hard water. It also costs about ten bucks.
I've never had a shower run cold since I've started using them. And lemmie tell you, I takes me some hot showers epic in both length and temperature.

Last edited by 1010011010; 04-06-2007 at 06:43 PM.. Reason: Mainly because I recently noticed we could do it. When did that change?
Reply With Quote
  #12  
Old 04-06-2007, 06:45 PM
Polycarp Polycarp is offline
Guest
 
Join Date: Aug 1999
Location: A better place to be
Posts: 26,718
From past adventures in water heaters-that-don't, I can report this: the physical structure of at least some water heaters includes two elements, both of which working together bring cold water from your mains, etc., to the desired hot-water output temperature, one of which operates to keep the heated water hot. At least two water heaters in my experience managed to burn out the heat-'em-up element while the keep-hot-water-warm element still functioned, resulting in an output that qualified as warm water but with a very slow recovery rate and nowhere near as hot as desirable. It may not be your issue, but it's at least worth reporting.
Reply With Quote
  #13  
Old 04-06-2007, 08:38 PM
Madd Maxx Madd Maxx is offline
Guest
 
Join Date: May 2004
I would like to say that IME gas water heaters recover faster than their electric counterparts.
Reply With Quote
  #14  
Old 04-07-2007, 07:30 AM
MizGrand MizGrand is offline
Guest
 
Join Date: Mar 1999
What I'd like to know is why it takes 30 seconds or more for the water in my bathroom sink (say twenty feet from heater) to get hot.
Reply With Quote
  #15  
Old 04-07-2007, 09:19 AM
Bill Door Bill Door is online now
Charter Member
 
Join Date: Nov 2003
Posts: 3,443
Quote:
Originally Posted by Polycarp
From past adventures in water heaters-that-don't, I can report this: the physical structure of at least some water heaters includes two elements, both of which working together bring cold water from your mains, etc., to the desired hot-water output temperature, one of which operates to keep the heated water hot. At least two water heaters in my experience managed to burn out the heat-'em-up element while the keep-hot-water-warm element still functioned, resulting in an output that qualified as warm water but with a very slow recovery rate and nowhere near as hot as desirable. It may not be your issue, but it's at least worth reporting.

I think its more like the lower element heats water to a warm temperature, from which the upper, higher temperature element does some trim heating to bring it the rest of the way up. If the water is as hot as desired, but runs out very quickly, the warm-up element may have failed. I've only seen this in an electric tank, there is usually a lower element set some 20 degrees or so lower than the top element.
Reply With Quote
  #16  
Old 04-07-2007, 09:43 AM
LSLGuy LSLGuy is offline
Charter Member
 
Join Date: Sep 2003
Location: Miami, Florida USA
Posts: 6,776
Quote:
Originally Posted by RSSchen
What I'd like to know is why it takes 30 seconds or more for the water in my bathroom sink (say twenty feet from heater) to get hot.
Well, the water flows out of the pipe at about 1 foot-worth of water per second and so it takes about 20 seconds for the "rod" of cooled water filling the pipes to flow down the drain. At that time the water which left the water heater when you first turned on the faucet is just arriving at the spigot.

Now that water has cooled considerably since it was trying to heat cold pipe all the way from near the heater to your faucet. It is also trying to heat the faucet itself. What you feel is whatever heat is left over after it gave up some heat along the way. Over the next few seconds you're getting water that came from the heater & flowed through pipes that were marginally warmed by the preceding water. So that water loses a little less heat along the way & is a therefore little warmer when it gets to you. This process continues as the pipes heat up & the water at the faucet keeps getting warmer & warmer.

Eventually the system arrives at steady state equilibrium. The heat flowing out of the water into the plumbing exactly matches the heat flowing out of the plumbing into the surrounding air & walls. For example, the water leaves the heater at 110 degrees. Then it spends 10 degrees of heat keeping all the plumbing warmed, finally it arrives at your faucet at 100 degress. This will continue until the water heater begins to run out of hot water.

If the faucet's flow rate is lower than the heater's regeneration rate, you'll be able to do this indefinitely. If not, the heater tank will slowly drain of hot water & be replaced with ever cooler warm, then warmish, then not-quite-cool, and finally quite cool water.

A good water heater has a tank design such that it does a real good job of delivering purely full hot water until it is very close to depletion, then the temperature falls rapidly to just a little better than the cold water temp.
__________________
The day we stopped being "citizens" and started being "consumers" was the beginning of the End of Western Civilization.
Reply With Quote
  #17  
Old 04-07-2007, 11:02 AM
raindog raindog is offline
Charter Member
 
Join Date: Mar 2004
Why not consider a tankless, on demand water heater?

The cost is roughly twice the cost of a regular tank, but should save 2 or 3 hundred dollars in gas for the average home. Which means that you'll break even in about 3 years---at which point you're banking money every year.

In the meantime, you never run out of hot water. Ever.

*Correction*
If you hire a plumber it will be around 2 to 3 times the cost of a standard water heater. Still, you'll break even in less than 5 years, and still have endless hot water.

Last edited by raindog; 04-07-2007 at 11:04 AM..
Reply With Quote
  #18  
Old 04-07-2007, 11:47 AM
Chief Pedant Chief Pedant is offline
Guest
 
Join Date: Feb 2007
Quote:
Originally Posted by Sofaspud
I have a (fairly new) water heater. Of course I know how long this particular heater takes to recover from a dishwasher/laundry/shower Trifecta of Doom that uses all the hot water, but if I hadn't timed it, how could I make a good estimate?

All I see on the label is marketing bunk -- basically, that THIS heater is better than the OTHER BRAND because it uses X less kw/h per year. Bah. This tells me nothing useful. I want to know how long I have to wait to avoid frostbite after Junior uses all the water, dangit.

I would guess that it's dependent, in part, on size. Is there a quick formula I can use to get in the right ballpark?
I'm just a practical guy that's done a lot of remodeling...

I'm sure the formulas upthread are the real answer, but here's a practical approach based on my experience if you are just looking to not run out of hot water:

1. Consider tankless on-demand, as mentioned.
2. Consider two tanks (that's what we did, but 4 women in the house...this is not as expensive as it sounds and my gas bill including dryer and heating of a 4k sq foot home is less than $150 bucks/month in Chicago.
3. There is a thermostat on the tank. Turn it up so the water is hotter and therefore lasts longer. If you are a good boy turn it up only during peak demand hours.
4. Get a bigger tank--we use twin 50-gal
5. Insulate the tank and pipes

There are numbers for efficiency and recovery rates on the tanks so it is possible to do some comparison. We have never run out of hot water.

Last edited by Chief Pedant; 04-07-2007 at 11:48 AM..
Reply With Quote
  #19  
Old 04-08-2007, 09:25 AM
MizGrand MizGrand is offline
Guest
 
Join Date: Mar 1999
Excellent and informative answer LSLGuy, thanks! Maybe now I won't be pissed at my water faucets!
Reply With Quote
Reply



Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 10:12 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright 2013 Sun-Times Media, LLC.