If there is not enough hot water from your gas water heater, either for the shower or dishwashing, don't immediately call the technician; use the following tips to check if the unit is operating correctly, or not. There are several reasons why there is not enough hot water, and we will use two procedures to check.
Both of them are easy to follow but are time-consuming. No need to call the expensive plumber or service personnel, go through these steps. The procedures apply to the residential water heater using the natural gas, either made by AO Smith, Bradford White, Rheem or any other manufacturer.
Let's make two assumptions here; the water heater is a gas-type, and it has a capacity of 50 gallons.
It is essential to say that according to the heater's manufacturers, the 50-gal unit does not use all 50 gallons for the shower (50-gal was used as an example). The unit will deliver approximately 70% of the tank's capacity. In our case, 70% of the 50 gallons, equal to 35 gallons (0.7*50=35 gal).
Next, if the temperature on the thermostat is set to 120 F, the temperature will be between 100 F and 120 F (approximately 20 F difference). If the capacity is unknown, the rating plate found on the heater should be looked at.
This procedure is called the Draw Test, and there are two parts of the test. The water heater should be fully recovered from the previous use.
First, discharge one cup of hot water by using the TPR valve, filling the cup at the free end of the discharge pipe. The temperature of the hot water inside the cup should be hotter than the set temperature, at least 10 F. If it is colder something is wrong.
Next, use the 5-gallon bucket to collect the hot water from a heater. Do it several times until the capacity reaches approximately 70% of the tank size. After every draw, measure the temperature of hot water. The temperature of hot water in the last draw should be within 20 F of the thermostat setting. If it is not, there is a problem.
There are also other reasons for the insufficient hot water such as the undersized water heater (buy bigger one or use the supplemental electric heater for example) or plumbing crossover. Check also the showerhead and the flow rate it has been designed for. For the water heater with the capacity of 50 gallons, and after applying the 70% rule, 35 gallons of hot water is available (thermostat is set to 120 F). The recommended showerhead flow rate is around 3 gallons per minute, which gives you approximately 10-15 minutes of shower. So, check the showerhead flow rate before calling the technician.