Replaced thermocouple on Bosch TF250. Now the pilot flame is so big that it heats and boils the water in the heat exchanger when hot water is not required. When we turn the tap on, we get dirty water, followed by about 10 seconds of steam, before hot water flows.

One possibility - insufficient gas flow or oxygen starvation might be making the pilot light dip when the main flame is on so the thermocouple bulb is no longer heated. This will cause the main flame to go out and restore full pressure/sufficient oxygen to the pilot.

I am not an expert but while it is possible the gas supply is insufficient to supply high heat from the burners and maintain a lively pilot flame, it is also possible the safety thermocouple is faulty or that the pilot fails to heat the bulb - perhaps the pilot flame shortens when the high flame starts, perhaps the big flames create a draught...

Hold in your pilot button, then push the spark button repeatedly to light your pilot. Once its lit, hold that pilot button in for 30 seconds to heat your thermocouple. once its heated, release the button, and slide the slider to burner position. Turn on hot water fixtures to get the unit to light

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

The thermocouple has gone bad or in this case is going bad to the point that it is useless.

The thermocouple is the pilot's, well, co-pilot! It is the electronic device that senses if the pilot flame is hot enough to ignite natural gas or propane fuel to the burner. If the thermocouple thinks it's safe, then it keeps open the main gas valve located in the pilot assembly. If the thermocouple does not sense enough heat from the pilot flame (such as when the pilot is out), then the thermocouple shuts off the gas valve to the burners.
How the Thermocouple Works
So what is this thing and how does it work? Well the thermocouple (technically called a thermocouple junction) is a device that contains two metal wires welded at the ends and placed inside a protective metal case. The thermocouple sensor is found at the business end of the pilot flame and is designed to be placed in the hottest part of the flame. The other end is connected to the pilot valve body. As the thermocouple heats up, it produces a small amount of electricity and when it gets hot enough from the pilot, send a signal to open the gas valve by using a solenoid operated by a 24 volt transformer. The thermocouple calls the shots, and by converting heat to an electrical signal, it allows the gas valve to open or close. Once the gas valve is open, gas is then constantly supplied to the pilot and as required for the gas burners (as called for by the thermostat). If the pilot goes out, then the thermocouple gets cold and produces no electric signal to open the gas valve's solenoid and the gas valve shuts off the gas supply to the pilot and burners. Neat huh?

The pilot light has a thermocouple that allows gas to the pilot light while the flame is burning. If flame goes out it shuts gas off to the pilot jet. It seems that your thermocouple is faulty and needs to be replaced if pilot light keeps going out

You may have a dirty pilot orifice. Try this. Turn off the pilot by turning the control knob to off. Remove the front panel to get access to the pilot orifice. Use a can of compressed air with attached straw (the same stuff you use to clean computer keyboards) and blow out the pilot orifice, which is inside the pilot assembly where the pilot flame burns. Blow it out from the top and from the sides through the air holes. Then relight the pilot and see if that improves the flame size and color (blue) to heat the thermocouple. Douglas

If the thermocoupler is defective the gas to everything is shut off. The coupler is the bulb that sits ib the pilot flame If it doesn't heat up it will sense no pilot and shut the gas off. They don't cost much and are easy to change. Good Luck, Ned

If the pilot light stays on, the thermocouple and pilot flame are good. If you turn the control to "on" and turn up the thermostatic control knob to call for heating and get nothing else then the control (gas valve, thermostatic control) is bad. The control can be replaced - they are mostly generic except for gas type - but they are not cheap and if the control is bad the tank is probably not far behind (changing the control could cause it to leak). Unless the water heater is less than 3-5 years old, replace the whole thing!

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