Normally to fit in it in a small place I remove the doors or possibly remove a section of the wall
There is no filter in a hot water tank. It is very easy and cheap to fit one either side of it.
The hot water tank might be undersized for your usage.
do they help with hot water tank replacement
how long does it take for hot water after lighting water tank its gas?
No, a hot water tank heats water by itself.
So the steam can rise from the boiler to the hot water tank storage
No, a hot water tank heats water by itself.
You will get warm instead of hot water. There is a tube inside the tank on the cold side that goes to the bottom of the tank and the hot comes directly from the top. Reverse them and you are going to get the cold water that settles to the bottom of the tank. It will not hurt the tank or anything, it just makes for poor hot water.
If the hot water tank is off it is possible. It is also possible for the cold water going to the hot water tank to freeze. Insulate all pipes that are exposed - or let them drip (this is more expensive!)
it is a tank that preheats water and holds until you use it
A boiler functions by boiling water to a steam, which rises upwards. Keeping the hot water tank above the boiler helps keep the hot water hot, as well as allowing the steam to carry itself to the tank, instead of the hot water tank producing steam feeding back into the boiler.
As high as the highest setting available on the tank.