How to buy the right sized gas hot water heater

and save money too  .. part 2 Capacity and On Demand Factor

The two really important factors to consider for gas water heaters are Energy Factor and First Hour Rating. These two terms are explained in detail elsewhere and you can go there by clicking the relevant links below .. this article is continued from part 1 click here for energy factor implication of your choice

hot water heaters energy factors

first hour rating for hot water heaters

Irrespective of the efficiency of the gas tankless or storage hot water heater it is pretty useless and a waste of money if it cannot satisfy the demands of the household and especially at that peak time we all know happens more than it should in theory ...

In this instance it is important to either accept or reject whether an instant, or on demand tankless hot water heater can meet the family needs.

There is no doubt as we have shown that tankless water heaters are more efficient tank storage hot water heaters by about 30%.

You will have noticed that manufacturers of tankless heaters state that their units provide an unlimited supply of hot water but this is NOT the full story ....since it only applies within the design parameters of the particular unit. A tankless heater capable of supplying 2 gallons per minute cannot possibly supply 3 gallons per minute at the same hot water temperature. The unit will either adjust the flow to allow the set temperature to be reached or it will maintain the flow rate and reduce the temperature of the "hot" water. Always bear this simplified formula in mind for a tankless gas hot water heater.... for a given unit the energy is divided as follows ...

G as Energy = flow of water x (temperature out - temperature in)

So if water flow increase the difference between in and out water temperatures must decrease in proportion. Here's a couple of examples for a gas fired hot water tankless heater of energy capacity 100,000 BTU/Hr (this is 1 therm) ...

For temperature change of 60 degrees F the maximum water flow could be about 2.7 galls per minute

For a temperature change of 40 degrees F the flow could increase by 50% to about 4.0 galls per minute

This table shows range of flows that can be expected from tankless gas water heaters operating on a temperature increase maximum of about 80 degrees F .... Note rating for Tankless hot water heaters units are galls per minute and not gallons as fro storage water heaters
Type Energy Factor Parameter First Hour Rating Galls per min
Tankless Gas Hot Water Heaters MAX value gpm 3
Tankless Gas Hot Water Heaters MIn value gpm 1
Tankless Gas Hot Water Heaters AVG value gpm 2.15

It can be seen that large units can deliver 3 gallons per minute which is not necessarily a lot at peak times. The point to bear in mind here is discover your maximum hot water demand if you intend to seriously consider a tankless hot water heater.

This table can be used to estimate your requirements
How To Calculate Approx Peak Hour Hot Water Demand gas hot water heater 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 for tankless hot water heater     140     90
Average flow tankless heater per minute   2.3333     1.5

Even here be careful though because the actual instantaneous flow through the tankless gas hot water heater can easily exceed the average shown above. The demands of most families would generally be better suited by storage heaters unless energy saving is critically important.

Here are some typical water flow rates .... For a low flow shower you need at least 1.2 gallons per minute

Low flow tap ... minimum 0.5 galls per minute

Full out shower ... about 3.5 galls per minute

Dishwashers vary around 1.5 galls per minute and if you like Whirlpool tubs then you need 4 galls per minute

Storage Gas Water Heaters

Unlike a tankless gas hot water heater so long as the hot water storage tank is full it does not matter what the actual flow rate is through 1, 2 or even more taps. The flow and temperature will not change until the tank becomes empty at which point it starts to heat up again.

I like to open the tap flat out when I wash my hands .... tankless heaters would tend to be an irritation to me. But then I never did have patience.

First Hour Rating ... this number refers to the amount of hot water the hot water heater can supply per hour ... if a storage tank system is in use the assumption is that the tank is full of hot water at the start of the first hour.