Tankless & storage hot water heaters first hour rating choice

Heating water in tankless or storage hot water heaters. How to choose the right capacity for your hot water heater

The amount of hot water that any domestic hot water heater (tankless or otherwise) can supply at peak periods is a critical choice factor and one that will often negate apparent advantages of any Tankless Hot Water Heater. It is not just efficiency that has to be considered.

If you're like me you get annoyed when suddenly there is not enough hot water when you want it ... the kids have used it all, or the washing machine is using it or the dish washer or whatever has taken over. In some systems the flow automatically reduces to cope with the increased load and in others the temperature drops.... yes you have experienced this as welI I'm sure. In the middle of a shower there is suddenly not enough water to wet a cat or the you suddenly get a cold shock.

Tankless water heaters are prone to these problems especially so take care when making a final selection.

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

Let's consider a storage tank hot water heater first. Then it means that starting with the tank full of hot water 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. In another way you can think of water coming out of the unit to satisfy all needs in that first hour ... multiple showers maybe, washing machine and other devices running etc.

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. In practice this can mean the same unit in Florida will supply hotter water in Winter than one in New York.

How Do You Calculate First Hour Rating For A Hot Water Tankless Heater

First of all you have to make some assumptions but use this table below 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 rescheduling certain activities you can of course reduce your hot water heater peak demand. This remains an important assessment when deciding upon any new hot water heater.

In the table below I have summarized the massive amount of published data on almost all makes and models of tankless and storage tank type hot water heaters. You will note significant differences between tankless hot water heaters. Choose wisely.
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