Baseboard hot water heating systems work on the principle that heat rises. The "radiators" that bring warmth into the room are therefore installed low to the floor, typically along the baseboard. Water is heated in a boiler tank in the utility room just as water is heated for your hot water, either by gas, oil or electricity in a system separate from the water heater that supplies your plumbing system.

The hot water is then pumped by an electric motor driven pump through the house normally in a series system, that is hot water enters the first radiator in the house so that radiator is normally the hottest of any. Heat flows from the surface of that radiator so that the water that comes out of the exit end of that radiator is cooled somewhat. The water then flows to the next radiator in the system and more heat is extracted from the water an released into the room. As a result, the last room in the system receives cooler water than the first one. The water then goes back to the boiler where it is heated and recirculated again. This is a closed system. All the water in the system remains in the system. If there is a small leak somewhere in the system, there is a compensation mechanism built in. The main water supply from the house is fed into the circulation system through a pressure regulator so that any lost water is replaced instantly. If there is a significant leak in the system then you will hear water being replaced - it will sound like someone has a faucet open somewhere in the house.

One fact of physics is that anything that is heated expands. Water is no exception. Since the water is contained entirely within the circulation system there is a problem. When the water is heated it expands. If there were no place for the expanding water to go it would build up pressure in the system and eventually blow out a pipe fitting. The designers of these systems realized this and designed an expansion device into the system. It is normally a large tank installed in the system which contains air. Air is compressible, whereas water is not. So when the water expands it simply moves into the expansion tank and compresses the air. There is also a pressure relief valve installed in the system in the event that the air in the tank leaks out and/or is replaced with 100% water.

Normally the boiler temperature is controlled internally so that heat is available instantly when it is called for by the room thermostat. When the room thermostat calls for heat, a signal is sent to the circulation pump which turns on and begins circulating hot water through the house.

I mentioned in the beginning that there is normally an imbalance of temperature from the first radiator in the system to the last. Well, the engineers figured out a solution to that problem as well. There is a flow control valve on each of the radiators which can be adjusted so that a smaller amount of water flows through the first radiator, the excess flow being diverted around and on to the next radiator in the system. So if the flow rates are adjusted properly then the amount of energy coming out of each radiator will be equal. It takes a bit of time to do the adjustment. Further more, if conditions in a room change and the demand for heat in that room changes, then a rebalance of the flow rate of the system is required.

What can go wrong with a baseboard heating system?? The major failure modes are:

  1. Pump failure. Normally the bearings in the pump fail and the pump becomes noisy. Replacement is usually a fairly simple task since most systems have isolation valves on both sides of the pump so that it can be removed with little loss of water from the system. Pump seals also develop leaks.
  2. Boiler failure - just as a water heater can fail, the boiler can also develop leaks. The good news is that since the water is enclosed in the system and is typically not replenished on a regular basis, there is not a significant build-up of calcium in the tank as there is in your hot water tank which continually brings calcium into the water heater. That calcium settles in the bottom of a water heater and causes hot spots and eventual failure of the water tank - not so with the heating system's tank.
  3. Leaks in the system - normally obvious in the form of wet floors or carpets underneath the baseboard units.
  4. Air trapped in the system - causes a gurgling sound in the system when the circulator is pumping water. Most systems have a small "tire valve" at the high points in the system which can be "bled" until all the air is removed and water starts flowing from the valve.
  5. Overpressure valve blows off - caused by the air being removed from the expansion tank. Not a normal failure unless there is a leak at the top of the air (expansion) tank. The leak must be repaired and the system drained until the air is replaced in the expansion tank.
  6. Dust in the radiator units - the radiator units are typically made up of copper tubes with fine sheet metal fins attached. These fins get clogged and need to be cleaned with a vacuum cleaner.

One of the major shortfalls of a baseboard heating system is that there is no way to install central air conditioning systems, air filtering system or whole house humidifiers. The upside is that they are quiet, and clean since they don't blow air all over the house.

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