G20 Eternity hot water system. Exceptionally low hot water pressure - just happened. Any ideas?

Example of service companies, since you didn't specify manufacturer and model of hot water system:

https://www.servicecentral.com.au/plumbing/repair-of-chromagen-eternity-g20-hot-water-system/406754/

My G20 hot water unit is not giving out hot water it’s been like this for a few days. There doesn’t seem to be a flame lit

hot water pressure is equal to cold water inlet pressure
check for tap restriction, hot water section free of air , ( release the pressure valve ;lever to fill and bleed out air)
if you have a hot/cold mixer at the hot water system, they can fail and block of the hot water flow
(That unit has to be fitted by law on heater installation)
have it tested first as they cost in excess of $15.00 aud to replace

Have you measured the pressure or is it a faulty valve. Are the inlet and outlet valve open. There is a safety valve for thermal relief on top of the tank.

If water is exceptionally hot, the TP will release water.
Is water hotter than normal?
Or TP will release water when TP has gone bad.
Replace TP and see what happens.
http://waterheatertimer.org/How-to-replace-TP-valve.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Before calling service technician, open following link for Rheem troubleshooting resources.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

List of causes MIGHT include one of following:
Bad household circuit breaker that powers electronics, or loose wire inside outlet where tankless plugs in.
Tankless fuses and control board.
Water flow rate too low. Minimum .8 gallon required (depending on model)
Tankless inlet filter clogged.
Household aerators clogged at faucets.
Crossover at single-handled faucet
If installed, there can be crossover at mixing valve.
Faulty household check valve, including check valve on re-circulation system if that system is installed.
Restriction in household water line, such as clog at elbow.
Unequal pressure in household hot and cold lines, caused by restriction in hot line. When single-handled faucet turns on, the cold water pressure is higher so hot does not flow and tankless will not turn on.
Loose wire connection inside tankless.
Unit temperature set too high.
Unit temperature set too low.
Water temperature is very hot coming out of tap, so more cold is added, but that reduces amount of hot water flow, so tankless turns off because flow rate is too low.
On some tankless, if water is preheated by solar or other indirect source, the burner will not activate.
High mineral content in water > delime tankless yearly
Misaligned temperature sensor
Low gas pressure. Wrong type of gas. Undersized gas line.
Too many faucets on at same time.
Faulty tankless water valve. Water valve stuck.
Fluctuating household water pressure, for example MUD and PUD district water pressure problems or water well pressure.
Faulty household pressure reducing/regulating valve if installed.
Faulty household pressure tank, if installed -or- pressure tank needs to be installed.

At a high pressure you are consuming more hot water. It sounds to me like the water heater isn't big enough for what you need. You can try turning up the heat thermostat on the water tank. If the water gets hotter you won't need as much for each shower and it will also recover faster at a higger temperature.

It could be back flowing into the hot water heater tank which is not full because of the leak. Even though the valve is closed on the cold water entry side, there is probably no valve on the exit side. That is how it is returning to the tank.

I have a triton opal shower.

I turn shower on low pressure and cold water. then no low pressure and piping hot water for a very short time then low pressure and cold water.

I've changed the solenoid and the little black plastic ball.

Help !

if you have a gate valve on the unit from the hot water tank it could be as simple as the valve on the gas system being clogged going to that bathroom,, from how it is described it sounds like that would be the problem check to make sure all valve are not stuck in the on position ans change the valve if it is frozen in place