Furnace Configurations
Residential furnaces come in three basic configurations here in Colorado. Upflow furnaces bring return air into the bottom of the furnace, heat it up, and send it out the top. Downflow--also known as counterflow--furnaces bring air into the top and send it out the bottom. Horizontal furnaces bring air into one end and send out the other end. Some of today's furnaces can be installed in two or even all three configurations. The furnace diagrams below show an upflow configuration but the same principles apply to all configurations.
How a Furnace Works
1. Air is pulled from the living area by the furnace blower into the return air duct system.
2. The air travels through the filter, enters the blower and is then pushed through the furnace.
3. The air passes over the outside of furnace heat exchanger and is heated by the transfer of heat from the burner flue gases.
4. Hot supply air is then distributed throughout the house.
5. Excess flue gases are released out of the house via the vent.
Operating Sequence for Today's Furnaces
Thermostat calls for heat.
1. Draft inducer blower is energized. Will usually run for 15 to 30 seconds to purge the heat exchanger.
2. Pressure switch contacts close if the draft inducer is performing properly, allowing continuation of the sequence
3. Hot surface ignitor or spark is energized.
4. Main burners are ignited.
5. Flame sensor proves ignition took place within 5 to 10 seconds and allows burners to remain on.
6. Main blower comes on in 30 to 60 seconds.
7. Thermostat becomes satisfied. Burners shut down.
8. Draft inducer is de-energized.
9. Main blower shuts down.
Tips for Savings and Comfort:
Operate your thermostat consistently. Recommended settings are 68 - 72 degrees during the daytime and 64-68 degrees during the night or at times when the house is empty for more than 5 hours. Digital setback thermostats can do this automatically for you. We highly recommend them.
Keep windows and doors shut. Some people like fresh air, but it does cost money to heat and can also add to uncomfortable drafts.
Airflow to rooms can be controlled to a certain extent by closing off some registers with the built-in louvers. Be cautious when closing off too many, because it can lead to high operating temperatures within the furnace.
Don't block off any intake louvers or grills that provide air to the furnace or duct system. Doing so can adversely affect the operation and safety of your furnace.
Be sure to keep the air filter or air cleaner clean. Once-a-month cleaning is recommended but may not be necessary for everybody. Pets, children, etc. can all contribute to dust and dirt thus requiring more frequent maintenance.
Have your furnace tuned, cleaned and inspected by a qualified technician once a year. This will help keep your furnace efficient and safe.
BTU ratings specify how much heat your furnace actually produces. BTU input reflects the potential BTU content of the gas and BTU output reflects the actual amount of heat discharged from the furnace.
Blower capacity is rated in CFM (cubic feet per minute) output. Certain number of tons of capacity may also be used. One ton is usually considered to be the equivalent of 400 CFM. Today's blowers have multispeed motors in order to adjust the airflow for comfort and proper operating performance.
Blower motors will either be a standard multi-speed motor using AC current with different taps for different speeds or a variable speed motor using DC current. Variable speed motors are more efficient, run quieter, and will automatically overcome excess duct restrictions such as dirty filters and poorly designed duct systems. They are an excellent product to help deliver and produce a more even temperature throughout the home. They can also save as much as 75% on the electrical cost when operating the furnace blower.
AFUE stands for Annual Fuel Utilization Efficiency. This is how today's furnace efficiencies are rated. This differs from older models which were rated according to bonnet capacity. Bonnet capacity is the actual amount of BTU's that you get from a furnace during the run cycle. AFUE takes into account bonnet capacity, vent loss and off-cycle loss during a period of one year.
Multi-stage furnaces are available and act as two furnaces in one. They will operate at a lesser capacity (typically about 70% of its full capacity) during mild days and when maintaining a steady temperature in the home. When demand is heavy or outdoor temperatures are frigid, then it may kick up to its full capacity. It's a superb option for the variety of weather encountered in Colorado.
Furnace efficiencies are generally in the 78-80% range or the 92-96% range. 80% furnaces are usually vented using your existing vent or chimney (some older vents or chimneys may have to be modified or lined). 90% furnaces require new PVC vents that can be installed either horizontally or vertically and also must have a means of removing condensation (floor drains, condensate pumps, etc).
Usually a new furnace can be downsized as far as BTU input ratings are concerned because of the higher efficiency of the new furnace and possible improvements in your home such as better windows and more insulation.
