Thanks for joining in the discussion @BonnerRon. Hopefully your suggestion will help visitors to this topic in the future. We appreciate you sharing your knowledge and experience with the community and look forward to seeing more of your posts soon.
@KingStreetReno No question is ever silly! Yes it’s highly recommended that Hot water systems are serviced 2-3 years on average. 5 years for tanks. To find out how old your unit is then check the Manufacturer label located on the unit. You’ll see the model and serial number. The month and year will also be included but not sure where on these units (I work at Rinnai Australia).
There are many factors that could be causing the hot water to run out. Common examples are faulty thermocouples, igniters/burners, thermostats or gas supply. Easiest option is to contact Rheem to organise one of their contractors to check it out - but if not under warranty then you have to pay (whether they find the cause or not). So It’s best to start with the process of elimination.
Check if the issue is localised to one outlet in the house or if it’s effecting them all.
Check the Power supply to the unit outside is working (try a charger or hair dryer to plug in an test).
Can you see any leaks around the unit? Check under the unit to see if the hot, cold and gas outlets are connected correctly. That the isolation valve in the gas line is open? The filter might need cleaning? Are your neighbours having similar issues or were there any recent works done in your area?