# 2 - Efficiency: Why a hydronic heating system is more efficient than forced air

When most people think about their heating system's efficiency they think of that yellow sticker on the side of their boiler that shows the annual fuel utilization efficiency (AFUE) rating issued by the U.S. Department of Energy. It's an attempt by the DOE to express fuel efficiency in annual or seasonal terms. Its accuracy has been debated by much smarter people than me, and could be the subject of a future installment of Heidronics. But for now, at least, I'll spare you the mundane details concerning lab analysis of combustion performance.

In my opinion, AFUE is a convenient, high-level measure of efficiency between "like" equipment — for instance, to compare one cast-iron mid-efficiency boiler to another, or one modulating-condensing boiler to another. In real-world terms, AFUE is not a fair way to compare different heat sources like a high-efficiency, condensing furnace to a condensing boiler. There are just too many differences in how these appliances are built, used and applied to make any kind of useful comparison.

What the AFUE ratings will tell you is how much of the fuel you've purchased is converted to useful energy, and how much is wasted up the chimney. A 90% efficient boiler will (theoretically) waste 10% of its energy, or 10 cents of every dollar, up the chimney. What the AFUE ratings don't (and can't) measure is how efficiently that useful energy is delivered to the rooms in your home.

When a boiler produces heat, it transfers that heat to water and delivers it through piping to the rooms in your home. Once in the room, the water's heat is delivered through a radiator or convector to heat the people, objects and air in the room. Water is a great conductor of heat and radiation is an excellent means to deliver that heat where it's needed. (You may recall that in last week's blog post, I discussed the various ways that hydronic systems make you feel comfortable.) Very little of the heat produced by your boiler is lost in the process of delivering it to your home.

When a forced air furnace produces heat, it transfers that heat to air and delivers it through ducts to the rooms in your home. Once in the room, the air needs to move through the room to heat the people and objects. This delivery system is ripe with opportunities for inefficiency. For starters, air is a poor conductor of heat and ducts are notorious for leakage. Because heat needs to move through the room to do its work, it creates air movement across your skin, which has a natural cooling effect. The furnace's blower also pressurizes the room, forcing heated air out of any crack or seam in the walls, floors and ceilings.

A furnace may be able to produce heat with efficiencies that rival a boiler, but it just can't compete with the distribution efficiency of a well-designed and installed hydronic heating system.

Next week I'll talk about the reliability advantages of hydronic heating systems.

Heidronically yours,

Wayne