Gas water heater does not produce enough hot water

[jdsn]

We have a 50 gallon Kenmore Power Miser 9 (409.730150) that fails to produce water hot enough for two people to take a shower consecutively. Any suggestions on how to solve this problem would be appreciated.

[LazyPup]

I think the heading to the post answers the question:

"The gas water heater does not produce enough hot water."

The code maximum allowable flow rate for a shower is 3gal/min but in most jurisdictions that has now been reduced to 2.5gal/min.

One average a shower will last 10 minutes, therefore you consume 25gal per shower. Someone simultaneously taking a second shower would then increase the consumption to 50gallon.

This 50 gallon is being consumed in a 20 minute period which means in order for a tank to meet a consistant demand it would need to have a first hour recovery rate of 50 x 3 = 150gal/hr.

I checked the first hour recovery ratings for a number of water heaters both tank type and tankless from different manufacturers using electric, natural gas and propane.

The Sears tank type water heaters were rated in the top 5 of all available brands.

The first hour rating for Sears 50 gal. tank type water heaters
in domestic service with the supply temp at 50 degF and the demand temp at 125DegF: (75degF differential)

Electric = 64 gal/hr
Natural Gas = 80 gal/hr
Propane = 75 gal hr

As you can see even though the gas water heater has the highest makeup range, two simultaneous showers would consume over 60% of the first hour make up in 1/3 of that time. The end result is that water is entering the tank faster than the burner can satisfactorily heat it therefore you are getting dilution and drop in temperature.

It is argued by some that tankless water heaters can deliver a continuous flow at the point of demand, but Tankless heaters can only supply continuous full temp at the point of demand if the differential does not exceed 65 degF. and the demand temp does not exceed 120degF (Minimum temp of supply water 55degF)

In northern climates the minimum temp of supply water often drops to 40 degF during mid winter which means the maximum output temp of a tankless heater would be 105 degF. It must also be noted that tankless heaters typicaly consume about 40 to 55% more fuel than tank type heaters to heat the same volume of water.

The simple solution to the problem would be to allow about 15 to 20 minutes between showers to allow the tank to recover. (15 min. between showers is considered the industry standard design specification for residential application.)

A second solution would be to install an anti-scald mixing valve then turn the water heater up to the commercial setting of 180degF to insure a greater BTU content in the tank which would allow simultaneous showers.

[jdsn]

It make sense theoretically. Thank you very much for your advice.