FURNACE OPERATION: DID YOU KNOW?

Most natural gas, propane gas and heating oil burning furnaces have heat absorbing parts in them called heat exchangers. These heat exchangers are placed in your furnace in such a position that they absorb the heat produced by the burning fuel. (The exchangers are made up of high quality carbon steel and are designed to last for many years.) This absorbing of heat happens on the inside of the heat exchangers and, because metal conducts heat, the heat travels through the metal and is radiated out through the outside of the heat exchanger. The heat radiated from the outside of the exchanger is then picked up by the air from the furnace fan and blown into various parts of your house. That, in brief, is how your forced air, natural gas burning furnace works.

PLEASE EXPLAIN!

Most furnaces will run continuously for 10-15 minutes, once the thermostat calls for heat.

The operating sequence of most furnaces is:

• The thermostat senses that the space around it is in need of warming.

• The switch in the thermostat will close causing the furnace fuel valve to open.

• The furnace pilot light or electronic ignition system will ignite the fuel supplied by the now open fuel valve.

• The exhaust gasses from this burning fuel then travel through the heat exchanger system, heating the exchangers from the inside and then carrying on up and out of your chimney.

• When the furnace warms sufficiently, the fan motor starts and begins to blow air past the outside of the heat exchangers.

• The heat being radiated from the outside of the now hot heat exchangers, is picked up by the blowing air and distributed through the house until the thermostat signals that it is warm enough.

• The switch in the thermostat then opens, cutting the power to the fuel valve.

• The fuel valve closes, cutting the fuel supply to the furnace.

• The furnace fan will continue to blow air across the heat exchangers until they are cooled to a factory designed temperature at which point the fan stops blowing and the heating sequence is finished until the space (home) cools to the point where the thermostat begins the whole process again.

THE PROBLEM:

Most furnaces are constructed in such a manner that the heat exchangers are saturated with heat after only about 1 minute and 50 seconds to 2 minutes of fuel burning. This burning produces (with natural gas) exhaust gasses in the 900-1000 F range – this is what heats the metal heat exchangers. The cooling air blowing past the outside of the heat exchangers is only at room temperature. The room air cannot possibly absorb all the heat produced by the burning fuel. Remember, with most furnaces the heat exchangers are saturated with heat after only 2 minutes of burning – but the burners will not stop burning or even slow down their burning rate at that point. Most will keep burning full blast for the next 8 to 13 minutes although the heat exchangers can only absorb a small percentage of the heat being produced. Where does this unabsorbed heat then go? The only place for it to go – up the chimney and into the outside air.

YOU MUST BE KIDDING!

Don’t take my word for it. Go to your furnace and put your hand on the furnace chimney – where it comes out of your furnace. MAKE SURE THE FURNACE IS NOT OPERATING AT THIS POINT!

Now have someone turn the thermostat up 5 to 10 degrees above the room temperature. Time yourself to see how long you can keep your hand on the chimney. BE VERY CAREFUL – you could very quickly burn your hand!

• Where do you suppose the chimney heat is coming from?

• Where do you suppose the chimney heat is going?

• Why isn’t that heat being used to heat your space, (home)?

• How can you trap that heat for use inside your space, (home)?

THE ANSWER!