We don't , what we do is call the manufacturer or just replace the heater entirely so as not kill anyone over a defective installation
It is probably right next to the water heater. There should be a shutoff at the floor, if not then at the meter is the most likely shutoff.
No
It means your water heater does not have the right combustion air for the gas valve to operate correctly or your vent may be plugged
15 minutes to half an hour.
There should be a shutoff at the wall where the water line enters the house. There may be one before the water heater or there will be one at the water meter.
Inspect bottom of tank by looking through combustion chamber. If water marks or heavy rusting is noticed or if water is noticed setting in combustion chamber bottom, then the water heater needs to be replaced.
Connect the domestic coil thru a tempering valve to the hot water pipes in your house and turn off the valves to your old hot water heater. You should include a shutoff valve so you can revert to the hot water heater when you have your coal boiler shut down.
you could buy a new one ? or call a mechanic ?
The main Pressure Reducing Valve (PRV) can typically be anywhere between the City Water shutoff valve by your water meter (if external) and the first "tee" in the water supply system. Though quite often it is near the main building shutoff valve inside the home.Some fixtures or appliances have their own PRV, like a hydronic heating system.
Not sure what you are asking. The control valve on the front where you set the temperature controls the burner. The thermocouple that sits in the pilot flame let the valve open when heater needs to come on. The temperature sensor is in the tank and is on the back side of the control valve. All of these have their part to play in causing the burner to light and shut off.
Water can be a product of combustion.
It partly depends upon what sort of heater it is, but if it's a car heater, the valve alters the amount of hot water (from the car's cooling system) entering the heater matrix. This would also be true of a water-filled central heating system.