Newly installed solar edwards hot water system and the pressure releif is pouring out hot water.The unit came with a expansion control valve 700kpa and a pressure limit valve 500kpa and my mains water pressure is 800kpa. What else do i need to fix this problem. Thank you Regards David

You need to install a home hydroneumatic sistem. You should ask a specialist. to provide the equipment

It sounds like it's doing it's job, by not letting your water heater turn into a missile, or exploding....The "T" in T&P stands for temperature. When it gets too high, the valve will open. The "P" in T&P stands for pressure. When IT gets too high, the valve will open. When they both get too high...you know the rest...The condition can be caused by many factors.
1. The house "pressure reducing valve" is letting in too much pressure. Any more than 80 lbs. is too much. 50 to 75 is good.
2. Hot water expansion in a closed loop system...no "prv" or "water meter" bypass...To fix, you can install a small "expansion tank" above the water heater. It will allow the extra bit of water to go somewhere temporarily, Letting the T&P valve stay closed. Installation is always on the hot outlet side, because high efficiency water heaters use a "check ball" in the factory installed cold water inlet nipple, which also doubles as a dip tube. (lets cold water into the heater at the bottom)
When a gas fired water heater turns on, there is a pretty fast spike in both temp. and press.
3. The gas control valve itself, has a non resetting 190 degree, "high limit" safety shut off feature. IF that were to fail, the control would run away temperature wise, but I have never heard of one failing.
regards!

just a reaction with the copper fittings

Thermocouple has 2 ends. One end attaches to gas control valve, other end sits inside the pilot flame.
Flame heats thermocouple, causing tiny electrical current that is detected inside the gas control valve and keeps the gas valve open so water heater can turn on when thermostat calls for heating.
If pilot goes out, then no electric current reaches gas control, and all gas to burner and pilot is shut off.

If pex line burst, that implies very high pressure, caused by closed system with check valve that stops expansion, or possible clog in the water line, or high local water pressure, or clogged factory-installed heat traps.
Put pressure gauge like BTG100 on the drain valve to check pressure, should not exceed 80psi, install pressure reducing valve on cold water line, install expansion tank on cold water line. Remove heat traps and install ordinary pex-lined di-electric nipples from hardware store.

If the water was super hot, then overheating could have caused the pipe explosion.
However the gas control thermostat is ECO protected, and if ECO (energy cut off) trips because of high heat detected inside the tank or combustion chamber, the gas control will trip off and no longer work, gas control needs to be replaced. Some newer electronic gas controls can be reset, but that type heater uses a thermopile instead of thermocouple.
Thermopile is basically a group of thermocouples, that product more electric current that is used to power electronics inside the gas control valve, which issues an error code on the gas control, and some day will send a text message each time out-of-range event occurs.

Copy following links for troubleshoot resources, including troubleshoot by type of gas control valve:
http://weaterheatertimer.org/How-to-troubleshoot-gas-water-heater.html
http://waterheatertimer.org/Low-hot-water-pressure.html
http://waterheatertimer.org/pdf/How-to-replace-water-heater-gas-valve.pdf

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

Usually toilets do not require hot water.
Is the plumbing divided into hot and cold water pipes?
If toilets are hooked to hot water, how many flushes and how many gallons per flush per day.
What is the expected consumption per each worker?
What is the latitude from equator, and expected solar input. Further north, and hazy atmosphere reduce input. Beijing has smog I believe.
How often is the solar panel going to cleaned of smog and dirt for maximum collection?
If the area has hard water, then the entire system is prone to sediment build-up and failure unless water is artificially softened.
Solar water heater temperatures can exceed 180 degrees at times. This temperature will damage storage tank that is not made for that temperature.
Most solar water heater systems have pressure regulating valve and expansion tank to accommodate occasional high temperature and pressure.

A good fellow to contact might be at following website:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm

Solar collectors are usually connected to a circulating pump, and then the heated water is stored in a tank.
The pump circulates water continually from solar collectors through a storage tank.
This type of system would use ordinary tank-type water heater for storage.
Most of these systems also utilize a back-up heating element inside the tank.

More complicated systems circulate special liquid through spiral tubes inside the water heater.
This type of heater would be an indirect water heater.
Indirect heaters are used for solar collectors, and used with boilers, or any outside heat source.
http://www.bockwaterheaters.com/Products/products_coil_tank_water_heaters.html
http://www.hotwater.com/products/residential/cirrex_solar.html
http://waterheatertimer.org/images/solar-water-heaters-on-roof_lbiRv_5784111-500x282.jpg

Do you mean expansion tank? To relieve pressure?
Expansion tank goes on cold water line before cold enters tank.
See image:
http://waterheatertimer.org/images/water-heater-expansion-tank.jpg

Expansion tanks are installed when TP valve is constantly drizzling hot water due to very hot temperature settings. Or when water pressure is high, etc
It's also smart to install expansion tank so plumbing lasts longer, and older tank is less likely to rupture and cause damage. Inspect tank for leaks each 6 months.
Some regions require expansion tank by local code.
Water heater can be damaged when pressure exceeds 80psi. : Test pressure using pressure gauge at water heater drain valve.

Other resources:
http://waterheatertimer.org/9-ways-to-save-with-water-heater.html
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html
http://waterheatertimer.org/images/BTG-100-Pressure-gauge.jpg

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.

Expansion tank is supposed to stop over-expansion due to pressure.
The check valve will stop over-expansion from pushing pressure back into the supply line.
The real question is what is causing the pressure.
If your TP valve is bad that could cause problem.
If the thermostat is set very high, and there is frequent short-term use of hot water, that can cause heat to build up inside tank.
Solution:
Lift TP valve and let out pressure to see if there is something lodged in the valve.
Be careful of the scalding water.
Let the TP lever down slowly.
Next step would be replace TP valve.
Turn down thermostat to 120, or set to 130-5 if objective is to kill most bacteria.
Last. You might check that your expansion tank is correct for water heater.

possible problems not knowing age of unit the
control valve could be sticking open allowing gas to continue to flow until high limit is reached and shuts down valve another possible problem with the newer style back flow preventer being install by water companys the rapid heating and expansion of the water over pressures the water system causing releif valve to pop open and reclose most water heater manufacters are recommending instalation of an expansion tank to allow the growth in volume of water somewhere to go is valve constantly leaking or only during heating cycle if you could supply additional information does unit only leak when heating
or is steam expelled when releif valve opens
t-stat not opening gas valve sticking partially open
bad high limit bad pressure releif valve no expansion tank to allow for thermal expansion if steam is present and unit is boiling water inside replacement of water heater for safety concerns,would be reccomended if only water and pops couple of times water temp not over 140 degrees after most hot water has been used out of tank and is reheating most of capacity probably lack of expansion tank as the releif valve is the final safety device on a water heater is of great concern
the temp control gas valve or high limit are supposed to shut unit done long before the safety limit of releif valve is reached

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