Take shorter showers, or get a tankless, "on-demand" water heater that heats the water as you use it.
You will hear air circulating in the cold air return (or return air grille) when the heater or air conditioner is running.
Provided the heater is of a low wattage it should not affect normal running of the car
Generally no. The amount of heat is small, but you should hear a buzzing noise when the heater is running. Mark
Not unless your hot water is continuosly running.
No. Once a water heater tank leaks there is no fixing it. Just replacing the water heater. And it should probably done as soon as possible, since the potential for a bigger leak to open is very possible.
it should be on the top of the heater core box in the engine compartment, it will have a wires running to it
Heaters are usually plumbed in after the filter. The pool filter/pump is what circulates the heated water. Ans: The heater will not work or fire if the pump is not running.
After the engine has cooled to a certain temp, it should shut off. If it is running constantly, this is an indication the engine is overheating.
at certain temperature
No-it should only run for about 2 seconds, in order to pressurize the fuel delivery system. If it is constantly running, it may indicate a leak somewhere in the fuel system, or a bad fuel pump.
When cold, the unit should heat with 3-4 minutes of the engine running.
I would not use a heater of less than 100 watts for anything. The price difference is minimal between 50 watts and 300 watts. Basically it is only the comparative physical size of the tank and the heater that decides how 'big' a heater should be. My usual surmise is "bigger is better" because it will not have to work as hard or as long as a smaller wattage one.