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Controlling the Indoor Winter Air – Furnaces & Boilers

The fundamental purpose of an architectural residential heating system is to maintain comfortable indoor air temperatures during cold weather. Most heating systems accomplish this by converting various kinds of stored energy to heat energy and then delivering it to the living spaces. Systems that burn organic fuels are called combustion appliances. Systems that convert electric energy are called electric appliances. Appliances that distribute heat by moving air through the home are called furnaces. And Appliances that distribute heat by moving warm water are called boilers.

The Standard requires that we inspect the installed heating equipment and the associated vent systems, flues, and chimneys; and that we describe the energy source and the heating system by its distinguishing characteristics. So let’s limit our discussion to installed systems – not portable or temporary space heaters. We’ll talk about vents, flues, and chimneys in conjunction with our discussion of combustion appliances. But for now, let’s explore some fundamentals of energy.


Energy takes many forms and can be stored and transported in many ways. Water stored above a dam, a battery sitting on a shelf, gasoline in an automobile gas tank, and an oil well in the Middle East are all examples of different means of storing energy.

When we allow water to fall over a turbine in a hydroelectric power plant, we convert energy stored as gravitational potential energy to mechanical energy. This mechanical energy then turns a generator and is in turn converted to electric energy. We then transfer electric energy to our homes and businesses through the power grid. Eventually, we convert this electric energy to heat energy by pushing an electric current through a resistor. We transfer the heat from this resistor, the heating element, to the air. And finally, we distribute heat energy to the living spaces through supply ducts.

Similarly, we pump oil out of a well and ship this fuel to North America where we distribute it to homes and offices, store it on site, and eventually pump it into a combustion chamber where we ignite it. Through a combustion process in the heating appliance then, we convert stored chemical energy to heat energy. At this point we transfer the heat energy to some distribution medium in order to distribute the heat to the living spaces. If this distribution medium is water, we call the appliance a boiler. If the distribution medium is air, we call it a furnace.

In our first example, where we converted potential energy to electric energy at a hydroelectric plant, we had no combustion fumes to deal with. If however, we had used an organic fuel, natural gas for instance, as our energy source, handling of combustion fumes would have been an issue. That problem though, would have been dealt with at the power plant. In our second example, where we burned the fuel in the combustion 1chamber of the appliance on site, removal of combustion fumes would have been a significant issue in the home.

There are two descriptive terms commonly used in the industry that are easily confused – draft and gravity. For our purposes, the term draft does not refer to air movements within the living spaces – a draft from an open window, for instance. Rather, we use the term draft in our discussions of combustion fumes. We remove the byproduct of combustion, the fumes, from the house by ensuring a draft up the flue. This is accomplished in a conventional natural draft appliance by natural convection – warm air rises. The term gravity also refers to a process of natural convection, but this time we’re referring to heat distribution. House air, having been warmed in a furnace will rise naturally through a system of ducts distributing heat to the living spaces. And the cooled air in the rooms will fall through a return duct, drawn by gravity, back to the appliance to be heated.

Now that’s an awful lot to put into a few short opening paragraphs, so let’s slow down and try applying some of these concepts to the real world.

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