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# domestic hot water heater

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Member Posts: 2
What is the definition of "first hour rating" for a domestic hot water heater?

What does an indirect water heater manufacturer mean when talking about tank recovery from cold start?

• Member Posts: 7,265
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First Hour Rating:

First hour rating is usually the volume of the tank plus the rated input or how much heat can go into the tank at some temperature I'm not sure of, It's a fudge-able number. For example, a 50 gallon water heater with a 4500 watt element will recover 18 gallons per hour. Therefore, it should be able to give you 68 gallons  of hot water at some temperature for an hour.

Its intended to be a baseline for comparison. For consumers who want to sound educated.
• Member Posts: 5,853
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Close, but no cigar....

Chris,

It is virtually impossible to draw the full tank capacity without getting into mix and dilution. Most manufacturers assume an 80% draw down capacity before M&D kick in.

First hour capacity is based on 80% of stored volume, PLUS the elements recovery capacity based on a 100 degree rise.

So, in your case, 80% of 50 gallons would be 40 gallons of stored hot water, plus the hourly recovery capacity of the elements.

4,500 watts @ 3.413 btu/watt = 15,359

15,350 divided by 8.33 divided by 100 = 18.4 GPH@100 degree F rise, hence the first hour output capacity of that set up would be 58.4 GPH.

Obviously, your mileage may vary based on the tanks insulation values, piping losses etc, but it at least strips it down to the point where you can evaluate potentials on a level playing field...

After the first hour, this set up would only be capable of 18.4 GPH capacity @ 100 degree rise.

Lets compare that to a 40 gallon, 40K btuH gas water heater,for example.

40 X .8 = 32 gallons of stored hot water available for draw down.

40K btuH @ 80% fire side efficiency = 32K btuH.

32,000 divided by 8.33 divided by 100 = 38.4 GPH, hence this set up would produce 32 + 38 for a first hour capacity of 70 GPH.

You can break it down even further, into gallons per minute by dividing the hourly capacity by 60 if you are so inclined. This will tell you whether or not your daughter can stand in the shower for hours at a time :-)

With the electric heater she'll be done sooner than she would be with the gas heater. :-)

With the shower heads of today flowing at around 2.5 GPM, and roughly 80% off that being hot water (national average shower temperature is around 115 degrees F) then the demand would be 2.5 X .8 for 2 GPM. So for the electric option, she could stand n the shower for about 1/2 an hour before exhausting the tank to the point of not having hot enough water, and in the case of the gas heater, she could stand around for 35 minutes before complete exhaustion set in.

Then there is the hourly cost of operation, and last I checked, electric is roughly 4 times as expensive as gas.

Now you know how those numbers are determined. :-)

ME

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• Member Posts: 3,231
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Great post

Thanks for this, Mark.
Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
Consulting & Troubleshooting
Heating in NYC or NJ.
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• Member Posts: 678
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Thanks for the simple explanation Mark.

I was trying to explain this to one of my techs the other day. You did a much better job of it than I did. He was more confused when I finished than he was when I started.
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You're welcome guys...

Glad to be able to contribute. Now, go use it :-)

ME

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• Member Posts: 11
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oil DHW

Hi Mark,

So if I am going to oil boiler at 73,000BTU with a 40 gallon indirect then:

40 X .8 = 32 gallons of stored hot water available for draw down.

73,000btu/hr @90 % would give me 78 GPH for the first hours for a total of 120gallon first hour right?  Then this would give my 1 hour of shower with shower head of 2.5gpm or 1/2 hour if two bathroom going at the same time.  Is this the right calculation?

One more question, the indirect said continuous rate (2nd hours) btu/hr 115,000.  Do you have to size your boiler to this rating.  I don't think we ever going to take more than 1 hours of shower at a time. 6 people in the hours.

Thanks.
• Member Posts: 11
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oil DHW

Hi Mark,

So if I am going to oil boiler at 73,000BTU with a 40 gallon indirect then:

40 X .8 = 32 gallons of stored hot water available for draw down.

73,000btu/hr @90 % would give me 78 GPH for the first hours for a total of 120gallon first hour right?  Then this would give my 1 hour of shower with shower head of 2.5gpm or 1/2 hour if two bathroom going at the same time.  Is this the right calculation?

One more question, the indirect said continuous rate (2nd hours) btu/hr 115,000.  Do you have to size your boiler to this rating.  I don't think we ever going to take more than 1 hours of shower at a time. 6 people in the hours.

Thanks.
• Member Posts: 11
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oil DHW

Hi Mark,

So if I am going to oil boiler at 73,000BTU with a 40 gallon indirect then:

40 X .8 = 32 gallons of stored hot water available for draw down.

73,000btu/hr @90 % would give me 78 GPH for the first hours for a total of 120gallon first hour right?  Then this would give my 1 hour of shower with shower head of 2.5gpm or 1/2 hour if two bathroom going at the same time.  Is this the right calculation?

One more question, the indirect said continuous rate (2nd hours) btu/hr 115,000.  Do you have to size your boiler to this rating.  I don't think we ever going to take more than 1 hours of shower at a time. 6 people in the hours.

Plus the boiler at 73,000btu/hr would be about 78 gph after that therefore the recovery period would be 40gallon/78gallon/hr * 60min/hr= 30.7 minutes.

Thanks.
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