A recent episode where the 170l gas storage hot water service relieved itself through the pressure / temperature relief valve sent me on a quest to understand the problem.
At the time, a ‘full rate’ discharge of 95° water was holding the valve open, and even with the gas turned off, the flow continued for some time until I closed the water isolation valve to let the system cool down.
So, was this simply a thermostat failure, temporary or permanent, or perhaps the result of ‘stacking’.
Stacking is caused by repeated very short draws of hot water, which cool the bottom of the tank near the thermostat, triggering heat input which can cause the top of the tank to reach temperatures considerably higher than the set point of the thermostat.
Another question that was of interest in choosing a replacement if needed, was how much heat is lost from the heater, what is the running cost of heat leakage alone.
Heat leakage
The heater was allowed to reach ‘normal’ operating temperature and stabilise, and the gas valve was closed which would allow the unit to cool.
A variation on my IoT water tank telemetry project was configured to use a type K thermocouple and 4-20mA converter to provide a temperature logger, a type K thermocouple was inserted between the insulation blanket and tank at the top of the tank. The 4-20mA converter does not incorporate cold junction compensation, but the logger incorporates an ambient temperature measurement facility which will be used for approximate compensation.

Above is the improvised data logger setup. Continue reading Hot water woes
Last update: 6th August, 2022, 5:32 PM