Noob question about first hour heating


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Old 12-30-18, 05:14 PM
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Noob question about first hour heating

Happy new year to all!!

We have a 6 year old Rheem Pro50-40F gas fired water heater. 50 gallons. Even w/ just my wife and I taking showers in our 2 year old redone shower with a 2.5gpm rainhead, we have to turn up the heat of the shower near the end.

With our kids home from college, I am making sure we space out our showers. Doing some googling, I read that you get about 35 gallons of hot water out of a hot water heater that's had time to bring the whole tank up to the temp setting (we have the temp control set to middle of B-C). That makes sense - cold water is coming in so even though you start with 50 gal of HOT water, the cold comes in lowering the average temp quicker than the burner can get the temp back up.

BUt then looking at Rheem's specs,

http://globalimageserver.com/fetchDo...f-e3595909a97e

they talk of 83 Gallons in the first hour? Does that sound right? Is that figured by drawing off 83 gallons slowly over 60 minutes? is that the catch? You can't shower with 1.35gpm.

Is the real question how long can a 2.5gpm shower run on a full hot water tank before the water in the shower is too cold? I;m not measuring temp while in there : )

The newer shower control valves - those are anti scald, NOT self adusting to keep the temp you want, right? As the incoming hot line drops in temp, the valve doesn't compensate to use less cold / more hot to keep the same temp?

Do those type of valves exist at a reasonable price?

How do you check the temp of shower water? Bring in a thermometer and a pail : ) ?

THANKS!
 
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Old 12-30-18, 09:52 PM
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So the real question is what is your water heater temp setting?

We have 50 gal electric heater and even after back to back showers (with smaller shower heads) we never run out of hot water!
 
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Old 12-31-18, 04:06 AM
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At 6 years old your unit is entering the last phase of it's predicted life (7 to 10 years). My first guess (and only a guess) is your water type. It may be very hard and lots of mineral deposits may be the cause. Have you flushed the unit recently? If you see lots of white chips then you have high calcium build up or one of the defective anode rods that were produced and used in almost every brand. You might try pulling your anode rod and look at it's condition.
 
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Old 12-31-18, 07:03 AM
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I do not know where that 83 gallons in the first hour came from but i think it is very gimmicky.

The heater will not kick on until enough cold water has entered to reach the thermostat (aquastat) that might be about 6 or more inches above the bottom. Then the burner produces a layer of hot water at the very bottom. This new hot water will commingle with the cold water directly above as it, being hotter, rises. So you really don't have new hot water joining the bottom the shrinking block of hot water in the upper regions of the tank at first.The only way you get more really hot water continuously is if the draw is slow enough that the developing layer of cold water near the bottom stays smal l enough that new hot water commingles with and warms up that layer reasonably quickly.
 
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Old 12-31-18, 07:42 AM
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You actually can get some pretty good shower heads that are less than 2gpm flow. Probably not a full size rain head, but there are highly efficient heads available and you can also get cheap inline valves that mount right behind the shower head to manually reduce flow. I use one of these closed to about half-way and really don't notice much difference.

The other option is to bump up the heat setting higher on the water heater so your shower valve will consume less hot water per minute. It's a safety issue if you have young children or very eldery people in the house who could accidentally scald themselves, but will all adult age people I see no problem bringing the hot temp up.
 
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Old 01-01-19, 12:33 PM
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THanks everyone!

Allan - that 83 gal in the first hour from a 50 gal hot water heater comes from the Rheem spec sheet. But yes, it doesn't say how they come up with that number - a very slow draw ? And what's considered acceptable temp / incoming water temp (does seasons change the incoming temp much?).

Not going to reinvent the wheel with another showerhead (this one cost a couple hundred dollars : )

I;m asking more of a general question - how long can the shower run at 2.5GPM and it's still going to be a hot shower when you have a 50gal water heater?

Yes, LOADS of variables.

what temp do you like it to be?
incoming cold temp
temp setting of hot water heater
Burner BTUs
etc. etc.

but a ballpark?

what size heater do you have with how many people and do you have to think how many showers just happened before you get in there to take a nice relaxing shower? Or dishwasher / clothes washer was just run? (although I hear that those are sooo efficient these days they use just a few gallons total per load
 
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Old 01-01-19, 12:46 PM
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You mention a hot relaxing shower. That is perhaps the most tell tail signal you can give. It's been my experience that one hot "relaxing" shower over 20 minutes will deplete a hot water tank in no time, rendering the shower unless for several hours until recovery can take place. In my opinion, ten minutes should be max for a shower. If you need more time, you need a bath or a hot tub. Or one of those instant tankless hot water heaters.

To answer you question, using a typical 40 gal capacity, should handle 4 people, ten minute shower consecutively and still provide enough water for dishwasher and clothes washer.
 

Last edited by Norm201; 01-01-19 at 02:02 PM.
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Old 01-01-19, 08:19 PM
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Norm.,thanks. I DON"T take really long showers.. but your comment sums up what I'm looking for - you're saying with a 40 gallon tank that's been sitting / all the water is at the setpoint, then the 2.5GPM shower head can run 40 consecutive minutes? that's 100 gallons used.. likely not all coming from the water tank (a mix of hot and some cold water). Do you have the water heater dial set low? Midrange? HOT?

Even with a 2 GPM showerhead, that's 80 gallons used. we have a 50 gallon tank. the temp control seems to need adjusting sooner than 40 minutes.

I'm admittedly not doing this systematically... could that shower control valve have issues (2 years old)? I should measure the temp in the shower and at faucets to see if it's the shower valve?

Again, these newer anti scald valves do not keep a set temp, just keep water from coming out too hot?

https://www.totousa.com/3-4-thermost...ugh-valve-only
 
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Old 01-01-19, 09:04 PM
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The spec sheet for that Rheem heater gives 38 gallons per hour recovery. So if a shower used about half the tankful then can you afford to wait 30 minutes before the next person took a shower?

The recovery will be quicker if you install a few dozen feet of finned pipe dissected from hydronic baseboard radiators in the cold water line feeding the water heater. This preheats the incoming water so more gallons will be brought to final temperature (say, 120 degrees) in a given time period.
 
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Old 01-01-19, 09:21 PM
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Allan: I can wait... just wondering how long I can expect the tank to supply hot water in that first hour. What do you think of that 83 first hour delivery also on the spec sheet?

That seems high. I have to call them and ask how they get that number. I bet it's at a very slow draw over the full hour
 
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Old 01-04-19, 08:39 AM
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Hmmm. Let's analyze the question from an English and semantics approach.

It would have to be at a draw rate over the full hour however slow.

If you wanted a higher flow, say, 5 gallons per minute, then after 16-2/3 minutes 83 gallons would have come out. Except that the heater was not spec'ed to deliver 83 gallons of hot water in about a quarter of an hour; it was spec'ed to deliver 83 gallons in one whole hour.

Also consider the recovery rate of 40 gallons per hour. After a small amouint of water has been drawn, the burner kicks on. During the rest of the first hour the burner should have produced enough heat to warm up close to 40 gallons of new water. You don't get the full 50 gallons of starting water plus 40 gallons of new water equals 90 because towards the end of the hour, the heat source is still down at the bottom of the tank and the outlet is at the top with some cold and partially heated water in between.
 

Last edited by AllanJ; 01-04-19 at 09:07 AM.
 

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