Forced Hot Water Heating System Theory

Hot water heating systems are built around the premise of radiant heat. Water is heated by a boiler and then circulated throughout a building via a series of pipes that heat radiators. The radiators distribute the heat. The three basic types of hot water heating systems are gravity flow, pumped and baseboard convection. Gravity flow is the oldest of the systems, baseboard convection is the newest of the hot water systems. Forced hot water is the most common system for houses.

The Boiler

All three systems rely on a boiler to heat the water, which then is circulated through the building. The same principles apply to large buildings. For simplicity we'll address home heating systems. The boiler can heat the water in a number of ways. In the most common method, gas or coal is burned to heat an immersion coil. The coil, in turn, heats the water.

What Makes it Work

The pump is the heart of the system. After the water is heated in the boiler, there are two ways to circulate the water. Two are by pump and one is by gravity. In the pump systems, the circulating pump forces hot water through a network of pipes at a constant flow. In the pumped system, the pipes include radiators. In the convection baseboard system, the principle is the same except the heat is delivered differently. Hot air is distributed with a fan and thin metal directional plates. In some systems, the resident turns off the pump when the house is hot enough. In other systems, the boiler becomes the variable; it shuts itself off when a thermostat tells it that the water is warm enough. Gravity feed works on a slightly differently principle. The water is heated, expands and displaces the cooler water in the piping system. In other words, it forces the water upward, displaces the cooler water in the pipes, which are then forced back into the boiler on a return line to be heated.

Close Cousin

A circulator control in the pump, run by thermostat, independently turns the boiler on and off when the water coming through the return line into the boiler hits a preset heat range. However, without the pump, the heated water goes nowhere. (The heated water does move in a gravity system, but the system is outmoded and rarely used because of its inefficiency.)

Not to Get Too Technical

There are systems in which a thermostat-controlled circulator is used, whereby the water is maintained at a constant temperature in the boiler by an aquastat, which functions like a thermometer.

One Last Method

A thermostat can also control the water temperature. It is the thing on your wall that is usually run by a mercury switch. You can turn the dial to the desired temperature and when the thermostat detects the room temperature is not at the setting you desire, it kicks the boiler and pump into action. This is, by far, the most common method of controlling a forced hot water system.

Tips

Two things should be done at least once a year, usually at the beginning of the heating season. First, use the air vent valves on the radiators to remove air that has become trapped in the radiators. Since air rises through the water in the system, the most air will collect at the highest level radiators. That includes the air pressure tank that allows for the expansion of the heated water. Open the air vents beginning at the highest point in the system and work your way down. Air gets trapped in the radiators throughout the system, not just the top radiators. Second, the water should be drained and replaced throughout the system, particularly if it is unfiltered. Minerals will build up in the pipes restricting flow and diminishing heating efficiency.

References

• The Basics
• Explanation of Hot Water Heating Systems