Water – Should I replace an old breaker or add a new one

A double-pole breaker with a "60" on each half will trip at 60 amps. The reason it's two connected breakers is to give the circuit 240V potential, by using two opposing AC phases. These do not "add up" to more current -- you're using them to double the voltage.

So if the heater requires 240V power at 120amps, you will need double-pole breakers that can provide a total of 120amps, e.g. 4 30A double-pole breakers as Tester101 suggested.

An electrician will be able to tell you if your service panel can accommodate this extra load -- it's a lot, but your 200A service might be adequate, depending on the normal load elsewhere in your home.

Hot water heat is the best heating system out there. Forced air, by its nature dries out the air as it passes through the unit, unless you have a humidifier as part of the system. The temp of the water gong through the pipes is not controlled by mixer. I don't know the temp it is supposed to be when it leaves the boiler, but when it goes through the lines and returns to the boiler it still has to be warm enough to be reheated to the same temperature as before. If it doesn't, the heat being dispersed by the pipes will loose heat gradually and the circulator pumps or the boiler will never have a break, they will run continuously, or at least more than what they need to. What I am trying to say, is a well engineered system does not need a thermostat on the water temp, just a thermostat in the room to tell the circulator pumps and boiler when to fire up.

If you are concerned about the pipes in the floor, not knowing what the material is, makes it hard to determine how long of lifespan it may have. Usually, anything buried, not exposed to environmental conditions, sunlight and such will last quite a long time. Presumably the material in the floor is not affected by direct contact to concrete either. The place where the pipes come out of the floor is where the biggest concern would be if any. Again, I have no idea what was used for the pipes or what may have been used in the 60's for this type of application.