Gas Furnace Services

Houses were airy, fuel was plentiful, and labour was available so shovel the coal in. An old coal furnace may have been under 50% efficient with a very high stack temperature. The temperature of the flue gases could be as high as 800 degrees!

NOTE: There are several definitions and types of efficiency, but the one we are concerned with is thermal efficiency. In layman's terms, the BTU of the fuel is the heat content. So if you were to burn a full cord of Maple, (in a very very very big fire) and it were to burn completely (almost impossible), it would produce 22.7 million BTUs. Essentially, that heat will go in two directions, one is to heat you and your house, the other is up up and away into the air through the chimney. Not really helping you or your family stay warm. The important efficiency for you is the amount of useful heat divided by the original amount that you get.

Let's talk gas furnace for realistic numbers. One cubic foot of natural gas has a heat capacity of 1000 BTUs. As one cubic foot of gas is burned in the high efficient gas furnace I just sold and installed for you. 970 BTUs goes to make you warm while only 30 btus go out the vent, because this is a 97% efficient furnace. That is a modern furnace - very efficient.

Back 30 - 100 years ago, the old furnaces using coal, wood, and oil a bit later, produced a large amount of BTUs per hour but had very poor efficiency which let a large amount of heat up the chimney. Chimneys had to be very large, made of fire brick or stone to withstand the heat. Whatever was left over heated you.

As homes became tighter and tighter, and furnaces more efficient, the amount of heat and temperature of the flue products dropped. As the heat up the chimney reduced, chimneys became smaller and had to become lined to reduce cooling.

Remember, hot exhaust goes up, just like hot air. Just as a burning bonfire in the backyard, the hot smoke goes up until the wee hours of the morning when the fire is almost out and the temperature of the exhaust falls until the smoke just hovers.

Another problem with cooling exhaust temperatures is the water vapour as part of it. One of the products of combustion in the burning of a fossil fuel (natural gas, oil, propane, wood, coal, gasoline, etc,) is water vapour. In a wood fire, you won't see it because the heat keeps it vapour, but as the exhaust cools, the vapour condenses. For example, first thing in the morning when you start the car and things are cold, water can drip from the exhaust pipe.

The problem back then with this is, as furnaces became more efficient and exhaust temperature dropped, the water vapor would condense into water in the chimneys, mixing with other molecules in the fuel the hot exhaust causing a mild acidic solution. Go out for a walk this afternoon in an older area of town to see the large brick buildings with chimneys. You might see, 3 or 4 feet from the top of the chimney the mortar and bricks will be deteriorating. This is the condensing line of the water vapor in the chimney.