What Is The First Hour Rating For Tankless And Storage Hot Water Heaters?

When you are heating water in tankless or storage hot water heaters you have to select the perfect volume of water for your hot water heater.

The volume of hot water that any home hot water heater (whether tankless, oil or gas) can supply at peak periods is an important choice factor that usually cancels out the advantages of any Tankless Hot Water Heater. Efficiency isn't the only thing that we have to consider.... Water Heaters Use A Lot Of Energy When Heating Water...

It is quite disturbing to suddenly have cold water come out of the hot water tap when you are bathing, especially when its in winter. You go out and shout at the children for spending it all, or maybe your wife's washing machine used it all. Other water heater systems automatically lower the flow in order to cope with the high load and in others the temperature goes down.... I'm sure you know what I am talking about. In the middle of a lovely warm shower all of a sudden there is not enough water to rinse the soap off or you get a cold shock out of the blue.

Households that have tankless water heaters normally experience these problems so this might discourage you from installing a tankless water heater.

What does First Hour Rating Mean For My Hot Water Heater?

We are going to first look at a storage tank hot water heater. The storage tank usually starts off full of water already and means that the number given for a particular unit is the volume of water for which the system can supply water at the right temperature for the first hour of use. Put differently you can think of water coming out of the unit to satisfy all the requirements in that first hour  for example multiple showers, for the dish washer and other household equipment running.

look at the Energy Factor

For a tankless hot water heater it means that this is the maximum flow rate of water than can be heated to the correct temperature. This assumes a starting temperature so if the cold water is much colder than specified then the final hot water temperature will not be as hot as normal.

Practically this can mean the same unit in Florida will supply hotter water in winter when compared to one in New York....

Here is how you calculate the first hour rating for a hot water tankless heater...

Initially you have to make some assumptions. The table below serves as a guide

How To Calculate Approx Peak Hour Hot Water Demand 2 examples
Where is hot water needed? Average amount of Hot Water For This Usage measured in gallons How Many Times Used In This Peak Hour? ie frequency Total Water Required In Peak Hour Average amount of Hot Water For This Usage measured in gallons How Many Times Used In This Peak Hour? ie frequency Total Water Required In Peak Hour
For Hot Bath 20 2 40 20 1 20
For Hot Shower 25 1 25 15 1 15
For Washing Generally 3 5 15 2 3 6
For Shaving 2 1 2 1 1 1
For Hair washing 4 2 8 3 2 6
Automatic dish washer 15 1 15 12 1 12
Automatic clothes washer 35 1 35 30 1 30
Total Peak Usage     140     90

By changing the times you perform certain activities (such as taking a bath, shaving, washing clothes) you can of certainly lower your hot water heater peak demand. This is a very important assessment when deciding upon any new hot water heater.

Having Cooler Temperatures Will Help You Lower Energy Costs...

I have combined the large amount of published data on almost all makes and models of tankless and storage tank type hot water heaters. Pay special attention to the differences between tankless hot water heaters. Make the right choice.

Type Factor Parameter

First Hour Rating Gallons

Oil Storage Tank Heater MAX value             190
Oil Storage Tank Heater MIN value             100
Oil Storage Tank Heater AVG all values             136
Storage Tank Propane Gas MAX value             140
Storage Tank Propane Gas MIN value              48
Storage tank Propane Gas AVG value              81
Storage Tank Natural gas MAX value             143
Storage Tank Natural gas MIN value              48
Storage Tank Natural gas AVG all values              80
Storage Dual gas MAX value             132
Storage Dual gas MIN value              46
Storage Dual gas AVG value              77
Tankless Hot Water Heaters ... gas MAX value gpm                3
Tankless Hot Water Heaters ... gas MIN value gpm              1
Tankless Hot Water Heaters ... gas AVG all values gpm                  2.15
Storage Tank Electric MAX value             120
Storage Tank Electric MIN value               15
Storage Tank Electric AVG value               60
Tankless Hot Water Heaters Electric MAX value gpm                3
Tankless Hot Water Heaters Electric MIN value gpm gpm                1
Tankless Hot Water Heaters Electric AVG all values gpm            1.68
Note rating for Tankless hot water heaters units are galls per minute

You can read a wide range of water usage articles. They discuss the use of water in the house and home ... topics covered include filters, heaters, softeners and water in the garden.

 

 

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