Hi, I have a Rheem hot water system (315litres/6 years old) here in Sydney,Australia.

I just moved into a new (old) house and when first handed the key to the house I turned off the power to the hot water system because I have no plan to move in for a few months later.

6 months later we moved in... I turned on the power to the HWS (and waited overnight) and notice there is no hot water going to the house...

These are my observations:
- HWS is likely 'on' as the pressure release valve has very hot water when pressed.

- All hot water outlets in the house has no hot water... and the hot water outlet next to the HWS is cold when turning on any of the hot water outlets inside the house.

- I search online and some people think it's a tempering valves (mix hot/cold water to safe level) as these valves usually last around 5 years and when fail automatically close the hot water... unfortunately I don't see any tempering valves around the HWS... the only valves around the HWS is 1.Non return Isolating valve and 2. 500kPa Pressure limiting valve.

Any help is greatly appreciated as I think the HWS should last for a few more years if I maintain it (I'm planning to change the sacrificial anode next week)

My question is: will going through the manual and drain the hws and refill it help clean the hws or will I need to open the HWS and do other maintenance?

Thanks again for any help.

HWS is likely 'on' as the pressure release valve has very hot water when pressed.
That is what we in the US call a T&P valve. It is a safety device that opens if either the temperature or the water pressure gets too high. We don't recommend playing around with it too much because sediment builds up in the water heater and that sediment can cause the T&P to not seat properly and leak. Then you have to replace it.

If the water out the T&P is hot, the water heater is on and heating.

All hot water outlets in the house has no hot water.
In the US it is common practice to put a stop valve on the cold water input pipe to the water heater.

If the above mean that there is no water out the hot side of faucets, most likely the problem is that the cold water input stop valve is closed. The cold water coming in is what pushes the hot water out.
If your water heater is just for domestic hot water, not part of a home heating system, there will be no mixing valve. The water temperature is controlled by the thermostatic settings of the water heater.

Since you appear not to be familiar with this system the recommended maintenance is to periodically flush it to remove sediment that accumulates in the bottom of the tank. To do that, you simply attach a garden hose to the drain valve, point it into a sink or the outside, open the drain valve and let the water run until the water coming out is clear. Should be done about one a year, more often if you are on a well water system.

Thanks hkstroud.

There is no hot water coming out of the hot side of faucets... but there is cold water.

Update: I just did some more testing and notice I now have around 15 seconds of very, very warm water after about 10 seconds wait. This is definitely not from the sun heating the pipe as that heat is much cooler and only last about 3-5 seconds. Also the hot water outlet from the HWS is now hot-this pipe will go cold immediately if I pull on the pressure relief valve...

Any help is greatly appreciated.