SPACE HEATING & COOLING: Furnaces

Questions:

• How much will I save on my heating costs if I replace my furnace with a new, high-efficiency furnace?
  
• What does it mean if a furnace has sealed combustion?
  
• Several new furnaces I've looked at have power venting. What is this, and why is it needed?
  
• How do modern, high-efficiency furnaces vent without a chimney?
  
• Is the condensate from a high-efficiency furnace harmful to a septic system?
  
• Will a programmable, setback thermostat save enough energy to pay for itself? If so, what features should I look for?
  
• What services should be included in a furnace tune up?
  
• I can feel air blowing out of the ductwork joints when the furnace is running. Is there something I should do about this?
  
• Is it a good idea to turn off the pilot light on my furnace during the summer?
  
• If I close heating registers in some unused rooms, can I close too many?
  
• In extremely cold weather my furnace seems to run all the time, even though I have the setting on automatic. Will this continuous operation hurt my furnace?

How much will I save on my heating costs if I replace my furnace with a new, high-efficiency furnace?
A new high-efficiency furnace will have an efficiency of about 95 percent.
The amount saved will depend on how efficient the existing furnace is and how much is spent on heating. If the unit is more than 10 years old, it is difficult to estimate the efficiency of furnaces without an on-site inspection.

However, there are some clues to the efficiency.

First, look at the flue. If it is made of plastic (PVC) pipe, it is already a high-efficiency furnace. Low flue-gas temperatures in high-efficiency furnaces (also known as condensing furnaces) allow for the use of PVC flues. If the flue is metal, and the unit is more than 20 years old, it probably is about 65 percent efficient or less and it is possible to save about 30 percent with an upgrade to a high-efficiency furnace. If the existing furnace is between 10 and 20 years old, its efficiency is around 75 percent and a high-efficiency furnace will save about 20 percent. If the unit was built after 1990, it will have a minimum efficiency of 78 percent and savings of about 18 percent.

To estimate the savings in heating costs, total the gas bill for a year. Subtract 12 times the July gas bill to remove the amount spent on water heating. What is left is the amount spent on heating. Multiply the existing heating costs by the percentage savings possible from above to estimate the savings.

What does it mean if a furnace has sealed combustion?
Sealed combustion is a feature found on an increasing number of high-efficiency furnaces. Air for combustion is drawn from the outdoors through a small plastic pipe connected to the furnace. A furnace without sealed combustion has to draw air from surrounding rooms.
The combustion air is replaced by air leaking around doors, windows, and other openings. This can cause uncomfortable drafts and waste energy.

In very tight homes, the natural ventilation rate may not be able to support combustion, which results in inefficient burning and backdrafting.

Sealed combustion also supplies clean air that can be critical for condensing furnaces. Indoor air may contain chlorine gas from city water and laundry products, which could cause the condensate produced by the furnace to be more corrosive than the furnace is designed to handle.

Several new furnaces I've looked at have power venting. What is this, and why is it needed?
High-efficiency furnaces cannot rely on natural draft to exhaust flue gases. This is because too much heat is removed from the gases.

Since the gases are less buoyant, energy-efficient furnaces must use a power-driven fan to force the gases out of the furnace and flue.

Power-vented furnaces require less air for combustion than natural draft furnaces because the fan guarantees the air-flow rate. This, in turn, improves combustion efficiency.

How do modern, high-efficiency furnaces vent without a chimney?
Today, most high-efficiency furnaces use a small fan to exhaust flue gases to the outside.
The fan eliminates the need for a conventional chimney. The higher efficiency of the furnace reduces the temperature of the flue gases, lowering the surface temperature of the flue pipe.

Typically, furnaces that are 78 to 82 percent efficient are vented through a steel vent pipe that is run to the outside. High-efficiency furnaces, those more than 90 percent efficient, often are vented through plastic pipe.

Is the condensate from a high-efficiency furnace harmful to a septic system?
It's unlikely that a healthy septic system will be affected by the water condensed from the flue gases of a high-efficiency furnace.

A 60,000-Btu furnace operating 50 percent of the time will produce about seven gallons of condensate a day. The condensate has a pH level of about four, which is about the same as a carbonated soft drink. However, furnace condensate is not safe to drink because of trace toxic chemicals it contains.

Will a programmable, setback thermostat save enough energy to pay for itself? If so, what features should I look for?
A programmable thermostat saves energy by automatically controlling the furnace to provide heat only when needed.

The amount of energy saved will depend on how often the furnace can be set back and the amount of the setback. An automatic thermostat also can control the air conditioner in the summer.

In general, expect to save about 10 percent with a nighttime setback of 10 degrees, and an additional five percent savings if the thermostat is also set back during the day.

If a thermostat with both a night and a day setback is desired, choose one that can change the temperature at least four times each day because four changes are required for two setback periods.

Some models simply set the thermostat back by a certain number of degrees (selectable by the operator) from the normal temperature.

Other models allow the operator to select the actual temperature desired during different periods of the day. These models give more flexibility. For example, allowing a deeper setback during the day, when no one is home, than at night.

If a daytime setback is desired, but the feature isn't needed on weekends, purchase a model that allows for a separate weekend schedule. Some models allow a different schedule each day of the week.

If a heat pump is in place, a special setback thermostat designed for heat pumps is needed. This prevents unnecessary operation of the electric heating elements during the recovery period. Some studies have shown there may be little or no savings with winter heat pump setback, but automatic operation may be desirable during the cooling season. Other features are available that may add convenience but not necessarily energy savings.

Some will remind owners when to change the furnace filter or tell how many hours a furnace has operated during a particular period. Battery backup is a helpful feature that prevents the programmed schedule from being lost during a power outage.

Above all, select a model that is simple, easy to program, and use.
Condensed instructions should be printed somewhere on the thermostat, or the operation should be easily understood from the controls themselves. Thermostats that require consulting an operator's manual to change the temperature or override the schedule can cause a great deal of frustration and often end up not being used.

What services should be included in a furnace tune up?
A thorough furnace tune up should include checking of the burners, blower and motor, controls, and chimney by a trained professional.

In the burner assembly, the heat exchanger should be inspected visually for soot, corrosion, and cracks. If there is any concern about a cracked heat exchanger, additional tests should be performed to verify that it is safe.

The burners should be removed and cleaned and the air/fuel mixture adjusted if necessary. The temperature rise through the furnace should be measured to make sure it is within acceptable limits. Excessive heat rise indicates insufficient air flow, which wastes energy and may result in poor distribution of heated air.

The blower motor should be lubricated if it is designed for lubrication. The blower should be removed and cleaned by brushing. If the blower is belt driven, the belt should be checked for proper tension and replaced if it is cracked.

Inspect the furnace's filter and replace it if necessary. The fan switch should be checked for proper on-and-off temperatures, and the high-limit switch should be checked to make sure it will shut off the gas valve should the furnace overheat.

Mercury thermostats should be checked for level installation. The anticipator should be checked and adjusted if necessary for proper burner run time.

The flue should be inspected for proper draft, corrosion, or leaks.
After inspecting, cleaning, and reassembling the furnace, the technician should run an entire cycle to verify proper operation.

I can feel air blowing out of the ductwork joints when the furnace is running. Is there something I should do about this?
Leakage from ducts to unconditioned spaces reduces the heat going to the conditioned space, thereby reducing overall efficiency.

To eliminate this problem, inspect supply and return ductwork. Tape any cracks or openings with foil tape, or seal with caulking or mastic.

To detect leaks, use smoke from stick incense or a smoke pencil. With the furnace fan running, hold the smoke near suspected leak areas. If there is a leak, there will be an obvious disturbance in the smoke. A leak in a supply duct will blow the smoke away. If the leak is in a return duct, the smoke will be sucked into the duct.

Sealing the supply and return ductwork in unconditioned areas such as crawl spaces and attics is also important.

Due to normal home construction practices, the return duct is more prone to leakage than the supply is. Sheet metal is nailed over the cavity between wall studs and floor joists. Gaps between the metal and wood, plus holes for electrical wiring and plumbing, draw air into the system.

Is it a good idea to turn off the pilot light on my furnace during the summer?
Modern furnaces do not have pilot lights, but if the unit still has a pilot light, then turning the pilot light off during the summer will save energy. If the home is air conditioned, there will be more savings on the electric bill than on the gas bill.

A typical residential gas pilot light consumes about 750,000 cubic feet of gas per month. This heat energy is simply wasted when the furnace is not operating during the summer months. However, if the house is air conditioned during the summer, the pilot light contributes heat to the house that must be removed by the air conditioner.

In dollars and cents, keeping the pilot burning during the summer months costs about $6.00 per month for the gas and about $4.80 per month for the 60 kilowatt-hours the air conditioner will consume getting rid of the heat.

The pilot also creates a draft in the chimney that causes increased air infiltration through windows and around doors, further increasing the air-conditioning load.
Some people believe that turning out the pilot light in the summer will decrease the life of the furnace. Recent tests have indicated that the possibility for damaging the furnace is minimal or nonexistent.

To extinguish the pilot light, simply follow the printed instructions on the furnace. If the directions are unclear or missing, consult a service technician or the gas utility.

If I close heating registers in some unused rooms, can I close too many?
Yes. Closing too many may cause the furnace to overheat or cause other problems. Furnaces need the cooling action of air flowing through the furnace to cool the unit. Closing off too many registers will restrict the air flow and reduce the cooling action.

Furnaces are equipped with a safety device that closes the main gas valve when the furnace overheats, but it is not a good idea to use the safety switch as a controller. No more than two out of 10 registers should be closed at one time.

After closing a couple registers, let the furnace go through a long heating period. Turn the thermostat up and check for anything unusual such as the gas valve cycling off and on. If things don't seem right, open the registers.

In extremely cold weather my furnace seems to run all the time, even though I have the setting on automatic. Will this continuous operation hurt my furnace?
No. A furnace is designed to run as long as necessary to satisfy a home's heating load. In fact, the longer it runs during each cycle, the more closely it operates to its designed efficiency. Frequent cycling caused by partial loads during mild weather or by an over-sized furnace reduces the overall efficiency of the furnace.

The colder it is outside, the longer the furnace must run to provide the heat needed to maintain a home's comfort. A properly sized furnace in Kansas will maintain an indoor temperature of 70 degrees when it's 0 degrees outside.

Therefore, it is not uncommon for the furnace to operate continuously when the temperature is below zero, but this does not harm or stress the furnace.

It is critical to keep the furnace in good operating condition during cold weather. Keep filters clean, service motors annually, and check belts for proper tightness. The furnace will not provide its maximum heating potential if it is not in optimum condition.